Atomic spectrometry update. Industrial analysis: metals, chemicals and advanced materials
Simon
Carter
a,
Andy S.
Fisher
*b,
Michael W.
Hinds
c and
Steve
Lancaster
d
aHull Research & Technology Centre, BP, Saltend, East Yorkshire, HU12 8DS, UK
bSchool of Geography, Earth and Environmental Sciences, University of Plymouth, Drake Circus, Plymouth, UK PL4 8AA
cRoyal Canadian Mint, 320 Sussex Drive, Ottawa, Canada K1A 0G8
dDomino UK Ltd, Bar Hill, Cambridge, UK CB23 8TU
First published on 16th October 2012
Abstract
This review period has shown several areas of growth. The increase in popularity of LIBS continues as the problems, both real and perceived, that are associated with it (e.g., that it is capable only of qualitative analyses) are overcome. The area that appears to have seen the greatest increase in LIBS work is the nuclear industry. Presumably this is because of the stand-off ability of the technique. Another technique that is increasing in popularity is continuum source AAS. This has found substantial use in several areas of the review, notably the fuels and the organic chemicals sections. As noted in the review, the technique allows similar multi-elemental detection to ICP-OES (albeit at lower sensitivity), but at AAS running costs and is therefore likely to remain a popular technique. The necessity of causing no or minimal damage to forensic samples and for samples of archaeological or historical importance is still paramount. Therefore, micro-sampling techniques such as LIBS, LA and various X-ray-based techniques are still popular. Since the reliability of the data obtained from hand-held/portable XRF instruments has improved significantly in recent times, the use of these can be regarded as almost routine. Also noted in the review is the propensity for using multiple techniques, often simultaneously, to characterize materials more fully and more rapidly. This is the latest review covering atomic spectrometric measurements of industrial materials, metals, chemicals and advanced materials. It follows on from last year's review1 and should be read in conjunction with other reviews in the series.2–5 This year has seen the departure of Sian Shore from the writing team. Her efforts over the last few years have been very much appreciated.
1. Metals
For all types of metals and alloys, the most interesting contributions continue to centre on solid sample analysis methods. There continues to be a large number of papers in which a number of different spectroscopic methods and non-spectrometric methods were applied to the problem of interest. The overall distribution by instrumental method of papers reviewed for the metals section (as a percentage of papers received) was: AAS 4%, OES 35% (LIBS included at 12%), MS 11%, AFS 3%, SIMS 2%, XRF 14% and the use of multiple instrumental methods 30%. The papers of a fundamental nature were not cited in the review but were included in the distribution breakdown.1.1. Ferrous metals and alloys
A comparison of five analytical methods for the determination of Mg in low alloy steel was conducted by Recknagel et al.6 Spark-OES, ICP-OES, ICP-TOF-MS, ICP-QMS and GD-MS were used to measure Mg in three different samples and the average concentrations of 2.0, 2.8, and 10.3 μg g−1 were obtained. Overall, precision expressed as RSD yielded values of 20–30%, which although large, were considered to be acceptable by the authors.Tellez et al.7 recommended the combined use of SIMS, focused ion beam, XRD, white light interferometry and reflectance spectroscopy to characterize the artificial blue patina created on mild steel. These methods provided information about the metal oxide film morphology, surface composition, elemental distributions within the coloured layer and how the surface colour developed. The segregation of B (50 μg g−1) at the austenite grain boundaries in 0.5% carbon steels was studied by Seol et al.8 who used APT and nano-beam SIMS for the analysis. Boro-carbide formation was identified using SIMS and measurements from APT determined B concentration at the grain boundaries to be 1.7 ± 0.2%, which was about 70 times the concentration added to the steel.
Several authors have used LIBS for steel analysis. Wang et al.9 determined Cr and Mn in carbon steel using LIBS and, after optimization of the operating conditions, reported (in Chinese) LODs of 0.046% (Mn) and 0.005% (Cr). Two methods of calibration were compared and it was concluded that the internal standardization method was better than the traditional calibration method because the correlation coefficients were significantly higher. A LIBS sensor was developed by Anabitarte et al.10 to detect the presence of a protective coating on boron steel prior to welding. A Q-switched Nd:YAG laser operating at an energy of 600 mJ, a pulse duration of 16 ns and a repetition rate of 2 Hz was used to ablate the sample. The laser beam was focussed onto the surface of the sample using a mirror and a biconvex lens. Light emitted from the sample was focussed using a second bi-convex lens onto an optical fibre which transmitted the light to a Czerny Turner monochromator and a 2048 pixel detector capable of detection over the wavelength range 350–450 nm. A comparison of spectra obtained from regions of a steel plate that had the protective coating and from unprotected areas showed significant differences, with the unprotected areas providing complex spectra containing lines originating from Fe, Mn, Cr and, to a lesser extent, B. The spectra obtained from the areas containing the protective layer were much more simple and contained lines originating predominantly from Al. The presence of the aluminium–silicon covering on the steel during welding can form inclusions in the weld which significantly reduces the weld strength. The spectral algorithm was used with support vector machines to mark areas where aluminium was present before welding.
The elemental distribution within the sub-surface of advanced high strength steels after different annealing trials was reported by Norden et al.11 The element distribution was determined using GD-OES and adjusted to consider selective oxidation products for modelling purposes. Modelling indicated the sub-surface transformation temperature was substantially different than for the bulk and was confirmed empirically.
Karasev et al.12 described how oxide particles in alloys and steels were rapidly determined using LA-ICP-MS. The number, size and particle size distribution obtained using LA-ICP-MS were comparable to particles collected on a filter after electrolytic extraction. It was acknowledged that the comparison was limited since some of the soluble oxide particles were not collected by the filter. The method was adequate for the analysis of inclusions containing aluminium oxide, magnesium oxide etc. Small slag inclusions trapped in metal armour pieces were analysed using both LA-ICP-MS and confocal SR μ-XRF to obtain trace element information in a preliminary report by Leroy et al.13 This paper presented an analytical protocol for analysing these types of inclusions to elucidate the origin of the armour. A comparison set of data was developed by analysing inclusions in armour pieces known to be made in the Middle Ages from the Lombardy (Italy) region. The two methods were viewed as complementary. Measurements by LA-ICP-MS were limited by the small ablated volumes and confocal SR μ-XRF was limited by the few elements excited by the conventional source at approximately 20 keV.
Several papers appeared that discussed the determination of non-metallic elements in steel. Yu and Wang14 studied B in low carbon steel using SIMS and Raman spectroscopy before and after hot rolling of the sample. Originally it existed mainly as free B in the steel matrix. After hot rolling and annealing, most of the B was observed as boron nitride compounds. The B segregated to the austenite grain boundaries thereby improving the toughness of the steel. A combination of μ-XRF, μ-XANES and μ-Raman spectroscopy was applied to studying the forms of P in ancient iron samples as described by Monnier et al.15 The P was heterogeneously distributed in the corrosion products and XANES measurements indicated that P existed as phosphate complexes in iron corrosion products. Nitrogen was determined in steel using pulsed GD-TOF-MS in a paper by Ganeev et al.16 The method was successfully applied to steel samples with different concentrations of N and the LOD was estimated to be 0.03%.
A portable XRD/XRF spectrometer was used in situ by Suzumura et al.17 to determine the changes in iron oxides on railhead surfaces, which can influence wheel/rail rolling contact parameters. Studies were undertaken both in the laboratory and in the field where a test vehicle passed over a corroded section of track several times. The reduction of the oxide layer was influenced by the alignment of the rail and wheel and the vehicle running conditions (driving, freewheeling and braking).
An experiment by Ishida et al.,18 demonstrated that Mn absorbance using AAS in the vapour above molten steel correlated very well with the Mn concentration in the molten steel. The light emitted from the molten metal acts as the light source for the experiment and Mn could be measured up to a limit of 3%. This warrants more comment but the paper was published in Japanese and there were few details in the English abstract. Perhaps, these authors will publish more of this work in other journals.
1.2. Non-Ferrous metals and alloys
A literature review containing 98 references focusing on the characterization of ancient (9th–19th centuries) gilded art objects was written by Sandu et al.19 In this context, gilded was defined as the application of gold or similar metals (Ag, Cu and other metals) to art objects such as easel or mural paintings, wood pieces, leather or parchment. It was noted that the most reliable and complete information comes from using a variety of techniques (non-invasive or non-/minimally destructive): UV fluorescence photography, IR reflectography, X-radiography, thermography, SEM, TEM, AFM, μ-FTIR, μ-Raman, X-ray spectrometry (WDXRF, EDXRF, TXRF and XPS) and Ion Beam Analyses (PIXE, PIGE, RBS and SRS). The authors asserted that the microscope remains the most used and efficient tools for characterizing these materials. A review of interest was prepared by Young20 who, with the aid of 81 references, described the use of SR in archaeometallurgy. Since this type of sample is sometimes fragile and is certainly irreplaceable, minimal damage should be inflicted during analysis. Techniques discussed in the review included X-ray imaging (radiography/microscopy and tomography), XRD, XRF spectrometry and FTIR spectroscopy.Several archaeological studies have been reported. Mohamed and Darweesh21 studied ancient Egyptian black-patinated copper alloys employing optical microscopy, SEM, EDXRF, XRD and FTIR. Results indicated that the black patina consisted mostly of tenorite (CuO). There was also evidence of thermal patination and animal glue coating on the copper. A collection of 19 fragments thought to be from the same ancient bronze vessel from Sumhuram (present day Oman) were characterized by Chaivari et al.22 using SEM-EDAX, μ-Raman spectroscopy, FAAS and XRD. The alloy in each sample was identified as being leaded bronze, but three different groups were identified based on the composition and microstructure of the fragments; indicating that they came from different vessels. Electron probe microanalysis (EPMA) was used as a screening tool for identifying whether some Roman lead curse tablets were manufactured from ore bodies in Africa Proconsularis (Tunisia).23 The work by Skaggs et al. showed that Ag, As, Cu, and Sb were likely marker elements for local ores. A set of 96 tablets was screened using EPMA and 16 tablets were identified for further testing for Pb isotope ratios using TIMS. Twelve of the 16 tablets had Pb isotope ratios which matched isotope ratios of local ore bodies. This suggested that the Romans were mining local ores rather than relying only on imported lead for manufacturing.
A variety of methods were applied to the analysis of modern industrial alloys. Anawati et al.24 used a combination of SEM, TEM, XPS, SIMS and GD-OES to understand the interaction of Cu and trace concentrations of Pb in aluminium–lead–copper alloys. There was no evidence of co-segregation of Cu with Pb or any physical interaction between Pb and Cu near the metal–oxide interface, which might explain the passivating effect of Cu on aluminium–lead alloys. A study of the structure, corrosion, and hardness of zinc–nickel alloys electroplated on to 347 steel aircraft material was conducted by Gnanamuthu et al.25 The techniques used were: XRD (alloy phases), SEM (microstructural morphology), AFM (topography) and XRF (elemental composition). A layer of 88% Zn and 12% Ni formed by pulsed electrode deposition at 50 Hz and a 40% duty cycle had better corrosion resistance than the other deposited layers tested. The mechanism and kinetics of electrochemical hydriding of binary magnesium–nickel and ternary magnesium–nickel–M (where M can be cerium, lanthanum, neodymium or praseodymium) alloys were studied by Vojtech and Knotek26 through the use of GD-OES, SEM, EDXRF, XRD and MS. The experiment was conducted in a 6 M KOH solution at 20 °C and 80 °C for 480 minutes. An alloy of magnesium–nickel was also tested as an experimental control.
There were many studies where results were obtained by a single spectroscopic method, as shown in the following four paragraphs. The results of an inter-laboratory comparison were reported by Uemoto et al.,27 for the determination of metals in silver brazing alloys (silver–copper–zinc–cadmium and silver–copper–zinc–tin). Comparable results were found for all alloys by gravimetric, titrimetric and ICP-OES methods. The tin-containing alloy required dissolution with HNO3 and H2SO4 to prevent precipitation of hydrous tin(IV) oxide whereas the other alloy required only HNO3 for dissolution. Hassler et al.28 studied the determination of 22 trace elements in high purity copper using ETV-ICP-OES. The compound CHF3 was the most effective gaseous halogenated agent tested that provided sufficient analyte–matrix separation. The gas flow rates through the ETV device were set at 0.185 L min−1 for Ar and 0.0033 L min−1 for CHF3. All elements, except for Se and Te, were completely released from the copper matrix and were determined accurately in a copper CRM. Limits of detection were reported to range from 0.6 to 29 ng g−1.
Deuterium in zircaloy was determined by Marpaung et al.29 using a custom built pico-second LIBS system. Optimal operating conditions were determined to be a laser energy of 7 mJ, a helium pressure of 1.3 kPa and a measuring window lasting 50 μs. Calibration standards were made by doping zircaloy pieces with D at different concentrations. This provided a calibration curve with a zero intercept and a LOD of 10 μg g−1. This was well below the limit of hydrogen concentration that could cause damage to the alloy. An additional advantage of the method was that it produced a crater of only 25 μm diameter and therefore could be regarded as being only minimally destructive. The study was claimed to be a pre-cursor to studying hydrogen in zircaloy. A scanning LIBS system was combined with a 3 dimensional object detection system to sort recycled metal.30 The authors, Werheit et al., designed the system to accommodate single pieces moving at 3 m s−1 through a measuring volume of 600 × 600 × 100 mm3. Data, from 20 channels of a high resolution Paschen-Runge spectrometer, were processed within a few microseconds. The system was able to identify eight different Al alloy types with an average correctness of >95%. Wrought and cast aluminium pieces were classified with an accuracy of >96%, after 20% of the measurements were discarded as outliers. The authors claimed this system has the potential to sort single aluminium pieces at a rate of 4 tonnes per hour.
Wienold et al.31 investigated different calibration strategies for the determination of elements in copper metal using LA-ICP-MS utilizing an IR laser. There were differences in the measured concentrations between calibration with copper metal CRMs and calibration with liquid doped pressed copper powders. Although the pressed copper powders matched the composition and the ablated particle size distributions of CRMs, they did not match the overall ablation behaviour because of differences in the physical condition and chemical form of the analytes. It was concluded that the weak cohesion between copper particles gave rise to a higher ablation rate than ablation from solid copper metal. The opposite conclusion was reached by Gusarova et al.32 who found synthetic Cu and Zn pressed powder pins acceptable for calibrating double focusing GD-MS during the determination of 50 elements. Copper and zinc-based CRMs were used for validation. A new sample holder design accommodated both square and circular pin shapes without a change in the analytical results. It was shown that pins prepared with a maximum pressure of 95 kN cm−2 gave the best accuracy. New and more accurate relative sensitivity factors were obtained by synthetic pressed pins and, fortunately, these were relatively easy to prepare. This is especially important given the limited variety of CRMs and the time required for the production of a single CRM. The same powder sample preparation method was used by both groups as described by Matschat et al. (R. Matschat, J. Hinrichs, and H. Kipphardt, Anal. Bioanal. Chem., 2006, 386, 125–141). It would appear that differences in the sample introduction processes might explain how the two different conclusions were obtained. In the case of GD-MS, a larger surface area of the pin was sampled which averaged out small inhomogeneities within the material. However, the sampling area by LA is much smaller and subtle differences in analyte distribution and form could be revealed. In addition, sputtering erosion is a more gentle method of sampling than multiple laser shots. Therefore sputter sampling rate would be less affected by weaker adhesion between copper particles than was observed in laser sampling.
Compernolle et al.33 used a pulsed rf GD-TOF-MS instrument to determine Ag, Au, Pd, Pt and Rh in lead fire assay buttons. The concentration range covered was between 5 and 100 μg g−1 and calibration was achieved using lead buttons that contained known concentrations of each element. Accuracy was assessed (<5% bias), RSD values of <5% were determined at 10 μg g−1 (n = 3) and LOD varied from 0.020 to 0.080 μg g−1 (for Ag and Pt, respectively).
Wang et al.34 used a MC ICP-MS instrument with a collision cell to measure the isotopic composition and atomic weight of Se. Synthetic mixtures of enriched Se isotopes were used as calibration standards for the determination of the Se isotopic composition from seven natural materials. The new atomic weight of selenium was calculated to be 78.9711(9) g mol−1.
A high throughput micro-electrochemical cell was coupled with ICP-MS by Klemm et al.35 for online trace element determination. The authors claimed the apparatus provided information on the valence state of dissolved elements (which can vary with the electrolyte and applied current density), corrosion rate, and the dissolution of native oxides. Hoshi et al.36 described the dissolution of platinum–M alloys (where M = cobalt, copper, iron or nickel) in an electrochemical cell in 0.5 M H2SO4 at 25 °C and analysed the digests on-line using ICP-MS. The dissolution of M atoms occurs immediately after immersion but was quickly suppressed because the surface becomes Pt rich. This was reversed by potentiostatic polarization at 1.4 V and was enhanced with potential cycling.
The oxidation of modified nickel–aluminium intermetallic compounds implanted, separately, with yttrium and chromium at 1473 K was studied by Jedlinski et al.37 The oxidation experiment involved exposure to 18O2 which acted as a tracer for the oxidation process. Information on the tracer distribution and depth-profiling were provided by SIMS. Photoluminescence spectroscopy was employed to determine the scale phase composition. An unmodified nickel–aluminium compound was used as an experimental control. Implanted Cr accelerated the transformation of transient aluminum oxides into stable and protective α-Al2O3. Implanted Y slowed this process.
Many of the published papers utilizing XRF spectrometry deal with the analysis of ancient artefacts or objects of art. A number of the more interesting articles are noted below. Constantinescu et al.38 employed μ-SR XRF spectrometry to measure the trace elements in gold and copper Romanian archaeological items to clarify their provenance. The elemental evidence from the gold coins (Pb, Sb, Sn and Te) indicated that the gold coins with a monogram were made from refined gold and the ones without a monogram were made from native (alluvial) Transylvanian gold. For copper and bronze samples, the elements detected (Ag, As, Co and Sb) suggested that the copper originated from Serbia or northern Bulgaria. Portable XRF and XRD spectrometers and a digital microscope were used by Abe et al.39 to analyse the gold material in a folding screen art piece. The XRD measurements showed a strong preferred orientation to the (100) plane which was consistent with gold leaf. The XRF measurements of a gold checkered pattern area were found to be consistent with a double layer of gold leaf. It was not similar to a reference sample of painted gold and the overall conclusion was that the gold material was produced by gold leaf.
The techniques of synchrotron μ-beam XRF spectrometry and neutron μ-beam techniques were combined by Grolimund et al.40 to give a two dimensional (2D) chemical and crystallographic image of an early Bronze Age flanged axe found in central Europe. The internal structural details provided useful information concerning the manufacturing methods and material properties. Although the axe appeared to look similar to Greek artifacts, the spectroscopic evidence suggested that there was more influence of central European cultures than was previously believed. Figueiredo et al.41 analysed six large metallic nails, a dagger and a sickle, from a Late Bronze Age archaeological site in central Portugal, using EDXRF spectrometry, μ-EDXRF spectrometry and X-ray digital radiography. The artefacts were all made of bronze with As, Sb, and Pb impurities. The nails were most likely manufactured using the casting-on technique. Another paper by Figueiredo et al.,42 described the application of μ-EDXRF spectrometry and SEM-EDAX to analyse copper-based artefacts. The techniques were viewed as complementary: the overall composition of the metal determined using the techniques was used to analyse the microstructural inclusions within the metal matrices.
Shilstein and Shalev43 used XRF spectrometry to analyse Euro coins with denominations of 10, 20, and 50 cents from five different mints. Mass ratios of Sn–Cu varied from 0.0101 to 0.0111. The mass ratio of one coin was found to vary by a factor of 1.5 at different areas of its surface. This study served as a model for evaluating compositional changes in archaeological metal pieces where there can be much greater variance observed in the composition than in coins made with current technology. Other applications of metals analysis are summarised in Table 1.
| Element | Matrix | Technique; atomization; presentation | Comments | Ref. |
|---|---|---|---|---|
| a HG = hydride generation; L = liquid, S = solid. | ||||
| Al | Nickel super alloys | OES; LIBS; S | A low cost LIBS instrument was used for the experiment. The LOD was 0.1% by weight | 44 |
| C | Magnesium alloys | OES; GD; S | Carbon solubility in magnesium was reported for the first time. The internal atmosphere in the GD volume was a critical parameter for the measurement of C | 45 |
| Sb | Pewter metal | AA; HG; L | Sb in acetic acid leachate tests was determined using a low cost HGAAS instrument equipped with a quartz trap-and-atomizer device. The interference of Sn that is also leached was overcome by using HCl and in-atomizer collection. Calibration was achieved using the method of standard additions and the LOD for Sb was reported to be 0.03 μg L−1 | 46 |
| Se | High purity tellurium | AF; HG; L | Samples were dissolved with HNO3 and HCl. A solution of Fe3+ and citric acid was added to minimize interference from the tellurium matrix during HG. Recoveries of 96–102%, RSD of 3–4% (at 27 μg L−1 Se), and LOD 0.15 μg L−1 were reported | 47 |
| Various (10) | Gold | MS; ICP; LA | The provenance of gold inlays on a Bronze Age metal disk found in central Germany was determined by analysing a suite of elements within the inlay and comparing with results from native gold deposits known to be used in that time period. The closest match was a deposit in Cornwall, England. This suggests that there was trade between Britain and central Germany during that period | 48 |
| Various | Nickel–chromium dental alloys | MS; ICP; L | ICP-MS was used to determine the metal ion release from dental alloys when either polished or alumina particle air abraded. This was correlated with biocompatibility of oral keratinocytes | 49 |
| Various (6) | Bronze | PIXE; S | The trace elements found in the bronze artefacts were characteristic of the Portuguese and Spanish Late Bronze metallurgy which supported the theory of local production of these artefacts | 50 |
| Various (8) | High chromium cast Iron | OES; ICP; L | Samples were dissolved in aqua regia using a closed vessel microwave-assisted digester. Yttrium was used as an internal standard. The method was validated through the analysis of three Chinese steel reference materials | 51 |
| Various | Steel | MS; SIMS; S | Chemical mapping was done to correlate the effect of a hydrogen atmosphere on tensile strength | 52 |
| Various | Various | XRF; — ; S | Large historic military vehicles were analysed using XRF to determine material composition. Ultrasonic scanning was also used to assess material loss due to corrosion | 53 |
2. Chemicals
2.1. Fuels and lubricants
This was a busy area of research in this review period. A sizeable proportion of the papers published used continuum source AAS as a means of detection. Although there is a general trend of increased usage of such instrumentation, the increase in use for the analysis of fuels appears to be far greater than for many other sample types.The use of analyte pre-concentration/matrix removal can be helpful when determining analytes present at very low concentration. Rahmani and Kaykhaii55 determined methylcyclopentadienyl-manganese tricarbonyl in gasoline and water samples. The single drop micro-extraction technique described used a single drop (2.5 μL) of the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate to extract the analyte. The influence of the volume of the micro-drop, extraction time, ionic strength and volume of sample were all investigated. Detection of the analyte was achieved using ETAAS. An impressive LOD of 10 ng L−1 was obtained with an enrichment factor of 124. Precision for five replicate analyses was 4.8% RSD. Recoveries in the range 81.2–101% were obtained.
Two applications have reported the determination of analytes in emulsions using ETAAS. In one,56 Cu, Fe and Pb determinations in detergent emulsions of naphtha were optimized. The optimization took into account the emulsification process and was undertaken using a three variable Doehlert design and a multiple response strategy. The three responses for the individual analytes were combined yielding a global response that was used as a dependent variable. Several variables of the emulsification process were optimized and these included the concentration of the HNO3, the concentration of Triton X-100 and/or Triton X-114 emulsifying agents and the total volume of solution. The optimized conditions for emulsification were 4 mL of sample mixed with 1 mL of 4.7% Triton X-100 in 10% HNO3 solution. The emulsions so-formed were stable for at least 250 min and the instrument used had sufficient sensitivity to determine the the analytes in samples under investigation. Method validation was by spike/recovery experiments in which recoveries in the range 88–105%, 94–118% and 95–120% were obtained for Cu, Fe and Pb respectively. The other paper that used ETAAS for the detection of analytes was prepared by Becker et al.57 who compared the performance of a transversely heated filter atomizer and a commercially available transversely heated platform atomizer during the direct determination of As in petroleum condensate, gasoline and naphtha. The filter atomizer provided a reduction in the background signal and improved the sensitivity. A matrix modifier consisting of 0.1% m/v Pd, 0.3% m/v Mg and 0.05% v/v Triton X-100 was used for both atomizers. The samples were mixed with propanol and 0.1% v/v HNO3 (in the ratio of 3.0![[thin space (1/6-em)]](https://www.rsc.org/images/entities/char_2009.gif)
:6.4:0.6) to form an emulsion which was then analysed directly. The characteristic masses obtained for the As were 13 pg for the filter atomizer and 28 pg for the commercial platform atomizer; but the LOD were approximately the same (1.9 and 1.8 μg L−1, respectively). This was attributed to the increased noise associated with the filter atomizer arising from the partial blockage of the light beam from the hollow cathode lamp passing through the very narrow tube. Method validation was by spike/recovery experiments using an assortment of inorganic and organic As species. Recoveries in the range 89 to 111% were achieved. The only advantage of the filter atomizer identified in this work was that it was approximately 20% faster because of the hot injection mode that could be used.
Two papers have reported the use of continuum source AAS as a means of coal analysis. Baysal and Akman used continuum source FAAS to determine S in coal samples following sample decomposition using a microwave-assisted digestion procedure.62 The S was measured using the C–S absorption band at 258.056 nm. Method validation was achieved using the analysis of CRMs and results were in agreement with certified values to within the 95% confidence level. Obtaining a linear calibration was not problematic because concomitant elements (other than C) were at sufficiently low concentration so as to cause no spectral interferences. Calibration standards were prepared in H2SO4. The method was described as being fast, simple, accurate and sensitive with the LOD being 0.01%. The S content of coals from around Turkey was determined and ranged from below the LOQ (0.03%) to 1.2%. The second paper to use continuum source instrumentation was by Jim and Katskov63 who used a low resolution spectrometer equipped with CCD detector and a filter furnace atomizer to determine several analytes (Al, Cu, Fe, Mg and Mn) in coal slurries. In this filter furnace, the sample vapour was passed through heated graphite into the light beam where the absorption spectra over the range of 200 to 400 nm were recorded repeatedly. The output of the CCD detector was measured within each spectrum, the atomic absorption at each wavelength measured and corrected with respect to the linearization algorithm and integrated. Resolution was 0.3 nm. Calibration was by carbon slurry impregnated with the analytes of interest and by analytes being added to the slurries as multi-element solutions. Comparison of the two calibration techniques demonstrated that the latter was the preferred option, with 60% of the measurements lying within 10% deviation of the certified reference data. The overall conclusion was that the continuum source instrument coupled with a fast CCD detector enabled the rapid, simultaneous multi-elemental analysis over a wide range of concentration. This latter attribute also enabled analysis of coal slurries without the necessity of slurry dilution.
A four stage sequential extraction protocol to determine with which phase of the sulfur-rich coal the analytes Cd, Co and Pb are associated was reported by Scaccia and Mecozzi.64 As well as the sulfur-rich coals themselves (from Sulcis, Italy), coal chars derived at 600, 750 and at 950 °C were also analysed. The actual analysis was undertaken using a slurry sampling protocol and ETAAS detection. Various parameters were optimized and these included the amount of coal slurried (between 1 and 50 mg), the volume of the slurry (1–3 mL), the dispersing medium (1% Triton X-100 or 2 M HNO3), the sonication time (5–30 min) and the pyrolysis and atomization temperatures. A matrix modifier of palladium nitrate was used for the Cd determination whereas ammonium dihydrogen phosphate was required to stabilize the Pb during the pyrolysis stage. The analysis of the coal for the Cd content required the use of the HNO3 as a dispersant because it produced a lower background signal. The coal char samples however, produced a far lower background signal and could therefore be dispersed in Triton X-100 for the determination of all of the analytes. Although Cd could be determined against acid-matched aqueous standards, the Co and Pb experienced some chemical interference effects and needed to be determined using the standard addition technique. Precision was reportedly better than 5% for all analytes (n = 5) at concentrations of 1, 10 and 15 μg L−1 for Cd, Co and Pb respectively. Under optimal conditions, a 30 mg sample mass and a slurry concentration of 30% m/v yielded a LOD for Cd of 0.001 mg kg−1. The other analytes required different conditions (50 mg of material and 50% m/v slurry concentration), but still yielded impressive LOD (both 0.01 mg kg−1). The method was validated by the analysis of the CRM BCR 182 (steam coal).
Two papers by Raeva et al. have reported a method that could measure the kinetics of vaporization of organically associated inorganic contaminants65 and inorganic matrix effects66 during coal combustion. The procedure used an ETAAS instrument as an experimental platform for temperatures of up to 2800 °C. An assortment of analytes was tested and instrumental operating conditions such as the pyrolysis and atomization temperatures, internal gas flow as well as the effects of trace element concentration were all studied. Although the degree of analyte vaporization observed at varying pyrolysis temperatures varied significantly, the vaporization activation energies were independent of the operating conditions.65 The second paper used As, Sb and Se as the analytes which were introduced to the ETAAS instrument as a water soluble form using coal-relevant matrix/trace element ratios.66 This allowed a homogeneous trace element distribution within the matrix. Calcium acetate and iron(III) nitrate increased the atomization/vaporization activation energies significantly (indicating an increased retention in the solid phase) whereas sodium aluminate did not alter the activation energy. A matrix of potassium silicate decreased the activation energy for Se, reducing its atomic absorption signal. The authors attributed this to specific interactions of anionic trace elemental species with cationic matrices.
Laser induced breakdown spectroscopy (LIBS) was used by Elnasharty et al. to monitor lubricating oil to ensure that it does not lose its lubricating properties.69 The cyanide and carbon (Swan bands) molecular emission lines were used during the monitoring process and could be related to the oil degradation as a function of mileage. The percentage decay of the CN and C2 signal intensities increased gradually with increasing mileage. The authors concluded that the method they had developed was a simple, straightforward, direct and rapid means of testing the oil to prevent engine failure.
Several papers have been published that have reported the use of plasma-based instrumentation for the analysis of oils. In one application, volatile Ni and V compounds were determined in crude oil and its fractions using GC-ICP-MS.70 The transfer line between the GC column and the injector to the torch had to be heated to at least 350 °C to prevent condensation (loss) of the analytes prior to atomization. Similarly, transfer had to be achieved using heated argon gas. The Ni and V porphyrin distributions in different oils were determined enabling a finger-printing method to be obtained. Another ICP-MS-based paper was prepared by Bettmer et al.71 These authors reported the accurate and precise measurement of several analytes (Mo, Ni, Pb and Sn) using direct micro-flow injection isotope dilution. Solutions of 97Mo, 62Ni, 206Pb and 117Sn in organic solvents were spiked into diluted oil samples and analysed directly. Although strong matrix effects affected the ICP-MS sensitivity, the isotope ratio measurements were unaffected, meaning that the results obtained were accurate. Precision was approximately 15% for peak area and peak height measurements, but this improved to approximately 2% when the isotope dilution method was used. Method validation was obtained by analysing the NIST standard reference material SRM 1084a (wear metals in lubricating oils); with results agreeing with certified values within the range 98–102%. An interesting method of trace elemental determination in diesel oil was presented by Cassella et al.72 These authors emulsified the sample with a surfactant (Triton X-114) in acid solution and then broke the emulsion using centrifugation. The result was an organic layer containing the bulk sample and the surfactant and the acidic aqueous layer containing the trace elements. This aqueous layer was then diluted, an internal standard added and analysed using ICP-MS. Parameters such as the concentration of the HNO3, the Triton X-114, extraction and collection times and the calibration strategy were all optimized. Limits of detection for the analytes Al, Cu, Mn, Ni, Sn and V were in the range 26–88 ng L−1 and the method was validated using spike/recovery experiments, with recoveries in the range 88–113% being obtained. De Souza et al. determined several (12) analytes in lubricant oil, residual fuel oil and biodiesel using ICP-OES as a means of detection.73 Samples were diluted using xylene and then an evaluation of two different μ-nebulizer types, the PFA-100 and Miramist, was undertaken. The operational conditions were optimized using factorial design. Both nebulizer types produced similar LOD (between 0.3 and 18 μg kg−1 for Mg and Ni respectively), but the PFA-100 nebulizer produced a greater sensitivity. Analysis of the NIST reference materials SRM 1643c and SRM 1085b demonstrated the accuracy of the procedure, with results of between 93% (for Pb) and 102% (for P) for the PFA-100 and 90% (for Pb) and 103% (for P) for the Miramist relative to certified values being obtained. Spiking experiments were also undertaken and yielded recovery data in the range 89–103%.
One of the official methods of determining Cl in oils is the US EPA 9075, which uses EDXRF to determine Cl over the concentration range 200 μg g−1 to percentage levels. An EDXRF method of determining Cl in crude oils that yields a much improved LOD was reported by Doyle et al.74 The sample homogenization strategy was studied to ensure accurate results were obtained. The calibration protocol used aqueous sodium chloride in glycerine. The linear range spanned from 8 μg g−1 to at least 100 μg g−1. The results obtained were comparable to those obtained using potentiometry after extraction of the Cl from the oil.
Dilution of the biodiesel samples with an alcohol was used as a means of sample preparation by two research groups. A paper by Chaves et al. described the use of an ICP-OES instrument with a Paschen-Runge spectrometer equipped with CCD detectors for the determination of the analytes Ca, Cu, Fe, K, Mg, Na, P, S and Zn in samples of biodiesel diluted with either ethanol or propanol.77 The sample introduction system (a cyclonic spray chamber) was cooled to −5 °C to decrease the amount of organic solvent entering the plasma. As well as increasing the stability of the plasma, this also decreased the spectral interferences originating from carbon compounds. The use of propanol for sample dilution was preferable to ethanol because it could be used for both biodiesel and vegetable oils, whereas the ethanol could be used for the biodiesel only. In addition, the propanol would have a lower vapour pressure, again leading to a decrease in the amount of solvent entering the plasma. Calibration of the system was achieved using inorganic standards diluted in either ethanol or propanol. Internal standardization using Y was used to increase precision and accuracy. The instrument, which could detect wavelengths over the range 130–770 nm, was also fitted with a background correction system that was capable of overcoming any remaining carbon-containing interferences. The accuracy of the method developed was demonstrated by the successful analysis of the NIST biodiesel reference materials SRM 2772 and SRM 2773. Recoveries were reportedly in the range of 87–116% for the biodiesel and 95–106% for the vegetable oils. Precision (n = 3) was typically better than 5% and LOD were at the μg g−1 level. A similar sample preparation protocol was used in a paper by Quadros et al.78 who determined Al, Cu, Fe and Mn in biodiesel samples diluted in ethanol. In this application, the detection of the analytes was achieved by using continuum source ETAAS. All analytes except Al could be determined without the need for a matrix modifier and with aqueous calibration standards. The Al required the use of a zirconium-treated tube as a permanent modifier and standardization using ethanol rather than water. The accuracy of the procedure was checked using spike/recovery experiments (with recovery values of 90–108% being achieved). Precision was typically better than 7% and LOD at the ng g−1 range were obtained for all analytes.
A procedure for the direct analysis of biodiesel samples was reported in a paper by de Campos et al.79 A continuum source ETAAS instrument was used for the determination of P in the samples. Chemometric optimization of the procedure was undertaken, with the atomization temperature and matrix modifier composition proving to be the most important factors. A mixed modifier of palladium and magnesium nitrate (30 and 20 μg respectively) along with pyrolysis and atomization temperatures of 1000 and 2700 °C were optimal. Another very important parameter was the drying stage, which had to be extremely slow to obtain good reproducibility. The authors therefore developed a five stage drying process. Calibration was against organic P standards diluted in P-free biodiesel. A LOD of 0.5 μg g−1 was obtained and participation in an inter-laboratory comparison exercise yielded results in good agreement with those expected.
2.2. Organic chemicals and solvents
Several areas of research have been very active over this review period and this paragraph is a summary of the highlights. The active areas include the analysis of valuable art and historical objects, which is very challenging and is a major user and, increasingly, a driver of developments in advanced analytical technology, as evidenced by the large number of papers dealing with this field. Also of note during this review period is the increased use of combined analytical techniques, making good use of both atomic and molecular information, together with spatial resolution. For example Cotte et al.80 described coupling WDXRF and SR sources, to assist with the drive for more spectral and spatial resolution. This is reviewed in more detail in Section 2.2.1. The use of data processing techniques to improve the quality and reliability of LIBS measurements is a significant feature of several papers although matrix effects and interferences remain significant. One very interesting advance is the development of optical catapulting as a means of removing the sample from the matrix, thereby eliminating substrate effects from the LIBS data. These developments are also reviewed in detail, in Section 2.2.2. Speciation of elements and an understanding of the molecular form of analytes features in a significant number of papers. The use of continuum source AAS for the determination of molecular P in fertilizers is a significant development and is reviewed in some detail. This reviewer predicts that this technique will be very beneficial in Africa and other developing regions where argon is expensive and scarce, thus making analysis using ICP less attractive. Phosphate determination is also of interest in African countries where phosphate availability for agriculture is becoming a problem. Other highlights in this year's literature include the increased use of high temperature liquid chromatography which requires less organic modifiers and therefore makes ICP and ESI-MS detection easier as well as ‘greening’ the analysis by minimizing organic solvent usage. A number of interesting extraction and sample introduction initiatives have been reported, including slurry nebulization with continuum source AAS detection. Finally, a TXRF procedure was developed for the determination of residual metals in active pharmaceutical ingredients, and this technique is likely to grow significantly in the near future. These developments are reviewed in more detail below and a small number of papers have been included in a table to compliment some of the work which was reviewed in more depth.In some cases, the analysis of protrusions or areas of damage is required to enable an understanding of the mechanisms of degradation with time and thereby implement new conservation strategies. Often, these protrusions are blisters or crater like holes and in Max Beckman's Pierrette and Clown these blisters and holes are filled with metallic soap aggregates.87 This paper illustrates both the importance of spatial resolution in understanding the nature of these defects and the benefits of SR sources in providing sufficient resolution. In this case, confocal SR μ-XRF spectrometry enabled both compositional and spatial distribution of the blister's components to be studied. This paper also demonstrated the benefit of molecular analysis used in conjunction with atomic techniques and Raman spectroscopy found fatty acid derivatives and lithopone-base white pigments. Similarly, a combination of techniques and various data processing approaches for the studies of pigments was demonstrated by Donais et al.88 Here, a fusion of high voltage XRF data and Raman data enabled improved differentiation of a range of pigments.
There are many parallels between the analysis of historical objects and forensic samples. In one such case,89 a bench top confocal μ-XRF instrument was employed for the depth-profiling of forensic samples, including multi-layered automotive paints and leather samples. Some heterogeneity of results was confirmed by conventional cross-sectional μ-XRF analysis, thus giving validity to the technique. In an unusual approach to the spatial determination of elements, LIPS and 3D digital microscopy were applied to encrusted stone and copper alloy sculptures and wall paintings.90 Emphasis of this paper was on the use of combined methodology for the characterization of archaeological artefacts. Near IR luminescence is a rarely used technique and has recently been reported by Thoury et al.91 for the in situ determination and elemental mapping of Cd pigments in paintings. The authors described a detailed study of the luminescence properties of Cd pigments and showed that the band edge luminescence in the visible and the deep trap luminescence in the red and infrared regions could be used successfully to assign pigment types including cadmium sulfide, cadmium zinc sulfide and cadmium sulfoselenide.
The determination of the speciation of Cr in paints using XANES was reported by Zanella et al.92 Changes on exposure to light and various chemicals implicated in environmental exposure, including sulfur dioxide and humidity, were determined in order to study the alteration of the chromate pigment zinc yellow under the above environmental conditions. Degradation was due to reduction of CrVI. Environmental SEM and confocal SR μ-XRF spectrometry were employed to obtain compositional and spatial information on the components. Analysis of closed blisters showed a high concentration of Zn in the centre with surrounding layers of Ba and Sr. Organic composition and pigments from flakes from open blisters were determined using Raman spectroscopy. In some cases this analysis revealed images of discarded paintings which were over-painted. Use of XANES in conjunction with ICP-OES and SEM-EDAX was employed for the evaluation of phytate chelating treatments on manuscripts which had been damaged by the corrosive effects of iron gall inks.93 This chelation treatment can be used to stabilize damaged manuscripts and art works by inhibiting the oxidative action of the iron. Various side effects of this treatment include changes in iron oxidation states, elemental losses and deposits. The associated mechanisms were evaluated using Fe-K-edge XANES. This paper described in detail the experimental conditions employed to record the XANES spectra and results showed the determination of Fe(II)/Fe(III) ratios with an uncertainty of ±5% absolute. The removal of Fe by the alteration of Fe(II)/Fe(III) ratios by the treatment was discussed. The degradation of Prussian blue pigments is an important parameter in preservation work and this was investigated by accelerated aging and combined molecular and atomic analysis. The techniques of UV-VIS spectroscopy, ATR-IR spectroscopy, Fe K-edge X-ray absorption and 57Fe Mossbauer spectroscopy were employed by Samain et al.94 Changes in Fe coordination number and oxidation state were confirmed.
As discussed previously, elemental mapping is of critical importance and until recently, technology to record elemental maps based upon the rapid and non-destructive measurement of a broad range of elements was rather limited. The development and improvement of three self-built, scanning, macro-XRF instruments which have addressed this issue to some extent was described by Alfield et al.,95 each instrument being improved using experience gained from the construction of its predecessor. Detection limits were discussed and the use of poly-capillary optics and pinhole collimators were employed to define the beam. In a further example of the usefulness of non-destructive or micro-destructive analysis of culturally important artefacts, a handheld XRF analyser was employed for preliminary analysis of elemental composition prior to more extensive and targeted analysis using SEM-EDAX and FTIR spectroscopy,96 thereby minimizing sample requirements and damage.
The advantages of synchrotron spectroscopies including 2D and 3D micro-imaging capabilities are becoming widely utilized in chemical analyses. Overlapping emission lines can give rise to errors in XRF analysis. The attributes of polycapillary-based synchrotron WDXRF analysis in overcoming these issues were explored by Cotte et al.80 The advantages include spectral resolution of a few tens of electron volts allowing for greatly improved separation of emission lines. Data from μ-XRF point analysis, μ-XRF mapping and μ-XANES were presented. In a further demonstration of the benefits of SR sources in the analysis of paintings, an improved radiographic method was discussed.97 Here the use of phosphor imaging plates, energy dispersive detectors and CCD cameras were described and much improved radiographic images were demonstrated. A very sensitive hybrid semiconductor pixel detector, was employed for the detection and resolution of low contrast differences in the inner composition and surface mapping of elements within a variety of art objects.98 Several X-ray transmission and emission techniques were discussed including energy-sensitive radiographic imaging procedures which enable identification and spatial resolution of elements. Ion beam analysis was usefully employed for the identification and profiling of painting layers.99,100 Here, PIXE, microbeam PIXE and optical microscopy were used to identify the pigments in different layers of paintings from an Egyptian tomb. The non-destructive determination of the provenance of prehistoric pigments101 and the characterization of the ink and paper of an Iranian manuscript was achieved using PIXE.102
Metallomics is widely used for the study of metals in biological samples. In one novel piece of work, Crotti et al.103 applied metallomic analysis to the identification of proteins in artworks. Here, model samples prepared in the laboratory were derivatized with diethylenetriamine pentaacetic acid (DTPA) and europium. The derivatization procedure was evaluated using MALDI-TOF-MS and HPLC-ICP-MS and the correct identification of various types of proteinaceous binders, typically used in artworks was achieved. This procedure was successfully extended to some unknown paint samples. The principal advantage of this was the very low sample consumption with <0.6 mg of material being required.
Coupling LIBS with multivariate analysis of the data is a successful strategy for both qualitative and quantitative analysis. In a review of this important area by De Lucia and Gottfried105 containing 91 references, the use of LIBS together with multivariate analysis to classify geological and energetic materials was discussed and the advantages of rapid scanning, and real time remote analytical capabilities were presented. These attributes make it an ideal technique for materials analysis and characterization. A further review paper, by Nevin et al.106 containing 178 references and concentrating specifically on the use of laser spectroscopies for the elemental and molecular analysis of art and archaeological objects, may be useful for the interested reader. In this review, methods were classified according to elemental analysis technique, including LIBS, LA-ICP-MS and molecular analysis, including LIF, LIDAR, time resolved absorption and laser desorption ionization mass spectrometry. Imaging applications were included as were application of THz and non-linear spectroscopy together with combinations of techniques. Further papers by Lasheras et al.107 and by Lazic et al.108 also used LIBS with data processing to identify and differentiate between different explosives. A large variability of LIBS signals of explosive materials together with a range of interferents was observed and data processing techniques were proposed to enable linearization of data sets. This enabled the differentiation of the LIBS spectra of explosives and interferents, and correlation of certain spectral features with molecular structure. The paper by Lasheras et al. also discussed the maximization of the signal to background ratio for both C and H lines by optimizing the LIBS operating parameters of laser pulse energy, delay time and integration time.
Optical catapulting in combination with LIBS is an interesting developing technique with potential for rapid and highly selective analysis of a range of materials on various substrates. It was employed recently for the analysis of explosive residues in human fingerprints by Abdelhamid et al.109 Optical catapulting works by ejecting particles from the sample using an acoustic pulse or pressure wave generated by a laser beam transmitted along the sample. The particles or aerosol liberated from the substrate are then free from interferences from the substrate when subsequently analysed using LIBS. The experimental conditions including acquisition delay, distance from the analytical surface and interpulse delay were studied and optimized for the analysis of trinitrotoluene residues and related explosive molecules in the presence of a range of potentially interfering species. This technique was able to provide chemical images thereby giving visual information on the spatial distribution of explosive residues within fingerprints.
The influence of the substrate is an area of extensive study in LIBS and methods other than optical catapulting have also been developed. Several potential biological and chemical threat simulant residues were prepared on multiple substrates in the presence of various interferents.110 Biological threats which were explored included Bacillus atrophaeus spores, Escherichia coli, MS-2 bacteriophage and α-hemolysin from Staphylococcus aureus. Chemical residues included 2-chloroethyl ethyl sulfide and dimethyl methylphosphonate. The presence of various substrates has traditionally presented a challenge for the successful determination of analytes using LIBS. In this case, this was addressed by preparing residue samples on various substrates including polycarbonate, stainless steel and aluminium foil and spectra of the blank substrates were measured. The chemometric deconvolution of the LIBS spectra was by PLS-DA with inclusion of the full LIBS spectra including the spectral contribution from the substrate. Perhaps not surprisingly, the LIBS spectrum from the stainless steel substrate was quite complex. Some analytes were quite opaque to the wavelength of the laser (1064 nm) and were not subject to significant interference from the substrate, others were transparent and this resulted in significant substrate emission. The authors described the development of appropriate chemometric models and a useful flowchart for the development and testing of PLS-DA models to discriminate residues on multiple substrates in the presence of various interferents was presented. This approach was compared with the use of selected emission line intensities and ratios. The inclusion of the full analyte and substrate spectra generally resulted in better classification performance and 98.9% correct classification was achieved on an aluminium substrate, up to 85.5% on a steel substrate and up to 85.6% on polycarbonate, although the success rate did vary according to the model used. The use of the intensity/ratio models was more successful in identifying more residue types in the presence of interferents. Substrates not included in the model were likely to result in error but this was minimized by the analysis of swabbed material. It was also demonstrated that in addition to the identification of the major components of the residue mixtures, minor components including growth media and solvents were identifiable with an appropriately designed chemometric model. This work represents a significant advance in the use of LIBS for this kind of analysis, with this approach resulting in a significant improvement in classification performance for models built using multiple substrates.
In a different approach to the discrimination of organic molecules on substrates, the kinetics of the emission decay and reactions leading to the formation of emitting molecules were used to build a model which discriminated molecular and atomic emission lines successfully. Molecular emissions were longer lived, specifically molecular emissions from CN and C2.111 This was a somewhat theoretical discussion with a number of assumptions being made, although these assumptions were clearly outlined in the paper and the model did agree well with experimental data.
In an interesting variation on the LIBS theme, described by Meir and Jerby,112localized microwaves were employed for the volatilization and excitation of solid materials from various surfaces. This short letter described an experimental setup using a microwave drill applicator for applying localized microwave energy into a surface to create a localized hotspot of around 1 mm diameter and a subsequent plasma plume from which the atomic emission was detected. Detection of Al, Cu and Si was demonstrated. This technique was not capable of remote analysis and required the destruction of the sample, creating a 1 mm diameter hole, so is not suitable for explosives analysis or the analysis of valuable artefacts, but may be useful as a cheap and quick technique for the analysis of low value, low hazard materials.
Ambient sonic spray ionization mass spectrometry allows desorption, ionization, and mass spectrometry characterization of analytes directly in their natural surfaces and matrices in open atmosphere with little or no sample preparation. This was used by Romao et al.114 for the characterization of illegal drugs, including ecstasy. Ecstasy usually contains 3,4-methylenedioxymethamphetamine (MDMA) as the main active ingredient. Recently, a new molecule has been detected in street drugs sold as ecstasy tablets. This new drug was identified as meta-chlorophenylpiperazine (m-CPP), which is a piperazine-based 5-HT receptor agonist. This paper described the use of EASI as a simple technique for the analysis of street drugs, with XRF spectrometry to identify the inorganic composition and ESI-QTOF-MS, travelling wave ion mobility mass spectrometry and proton NMR as complementary techniques in molecular structure analysis. This provided unequivocal differentiation between tablets containing m-CPP and MDMA. A combination of EASI-MS with EDXRF provided reliable quantitation of m-CPP. The m-CPP was distinguished from the para and ortho-isomers using travelling wave ion mobility mass spectrometry.
High temperature liquid chromatography was coupled with ICP-MS and ESI-MS for the characterization of halogen-containing drug metabolites.115 Temperature gradients of up to 200 °C enabled the separation of a range of metabolites using mobile phases with low modifier content. This feature facilitated ease of interface to and much better responses from ICP-MS and HPLC-ESI-TOF-MS detection. Thermal degradation of the analytes was investigated as the eluent temperature reached 175 °C and was found not to be significant. The metabolites of two kinase inhibitors (SB-203580-iodo and MAPK inhibitor VIII) produced by bacterial cytochrome P450 BM3 mutants and human liver microsomes were identified based on high resolution MS(n) data and fragmentation pathways were proposed. Quantitation was achieved using normalized elemental specific response in the ICP-MS analysis. A robust ion pair reversed-phase HPLC procedure with ICP-OES detection was developed for the speciation and quantitation of the entire range of homoleptic and heteroleptic octahedral PtIV mixed halide complex anions and the corresponding PtII anions.116 Either ICP-MS or ICP-OES detection was used to determine the relative concentrations of all possible PtII and PtIV species including stereoisomers. Diode array detection was used to determine their spectra and molar absorptivities. The need for speciation and accurate quantitation of high value metals in catalysis, platinum group metals refining and medical and toxicological studies is of great importance and this work demonstrates the ability to determine the platinum complex anions species in aqueous solution unambiguously. Monitoring Br, Cl and Pt emission lines enabled the ligand to metal mole ratios to be determined for all the separated species. The experimental molar ratios agreed well with the calculated values for authentic standards and mM LOD were achieved. These relatively high LOD are likely to be because of the low emission intensities of the halides. It is believed by the reviewer that the LOD could be improved by the use of MS detection, in spite of their high ionization potentials.
Although their ‘green’ credentials are debatable, ionic liquids do exhibit a major advantage in that they have insignificant vapour pressures and do not contribute to volatile organic compound emissions. Ionic liquid dispersive liquid–liquid microextraction (IL-DLLME) was combined with HPLC-ICP-MS by Jia et al.117 for the determination of Hg species in cosmetic samples. The Hg species (Hg2+, MeHg+ and EtHg+) were complexed with APDC and the complexes extracted into the ionic liquid 1-hexyl-3-methylimidazolium hexafluorophosphate. Significant enrichment factors (760, 115 and 235 respectively) for each of the above Hg species in only 5 mL of sample were reported, with LOD in the low ng L−1 range being obtained. Spiking experiments demonstrated quantitative recovery. In an interesting if complex and quite possibly not particularly robust procedure, the selective determination of inorganic Co in nutritional supplements was achieved using ultrasound-assisted temperature controlled ionic liquid dispersive liquid phase microextraction with ETAAS detection.118 Here, inorganic Co was chelated with 1-nitroso-2-naphthol followed by ultrasound-assisted temperature-controlled ionic liquid dispersive liquid phase microextraction. The extractant was the ionic liquid, 1-hexyl-3-methylimidazolium hexafluorophosphate and ultrasound was employed to maximize the analyte recovery. Several variables were studied so that extraction of interferents was minimized. Selective extraction of the inorganic Co required careful control of pH.
The speciation of halogenated hydrocarbons is critical in certain industrial analyses and in many environmental applications. A widely reported procedure is GC-ICP-MS1 which yields species information and an element specific detector response. There is growing interest in the miniaturization of plasma sources for use in chromatographic detectors. The development of a miniaturized MIP employing a helium microstrip plasma (MSP) operated at less than 40 W and consuming less than 250 mL min−1 of helium was described.119 The optimization of the detection parameters for the determination of halogenated organic compounds using GC-MSP-OES was described. Quantitative recovery and LOD at the ng range for several chlorinated and brominated compounds were achieved. In addition, the authors calculated the rotational and excitation temperatures of the microstrip plasma and showed their dependence on analyte concentration.
Although not reliant on speciation in the traditional sense, authentication is becoming very important in many industries. In one interesting and novel paper, a dual isotope procedure for the tagging and identification/authentication of manufactured products was demonstrated by Caremes-Pasaron et al.120 The paper described the application to explosive materials. The procedure was based on the addition of an element that may already be present in the product, but which is enriched in two different isotopes (in this case 117Sn and 119Sn). This approach gave a unique fingerprint to the tagged product and its authentication can be ascertained by comparing the ratio of the mole fractions as determined experimentally (in this case by ICP-MS) with the known ratios of the added tag. The advantage of this procedure is that it relies on isotope abundances and molar fraction ratios of the tag, rather than classical isotope ratios. The mole fraction ratio is not affected by any physical, chemical or biochemical processes which the sample may undergo, nor is it affected by environmental contamination with elements containing natural isotopic abundance. These factors can create problems with single isotope spike experiments. Additionally, the use of mole fraction ratios overcomes any issues of inhomogeneity of the tag. It is likely that this simple yet highly ingenious approach to assuring authenticity will have widespread future use in many different product types.
In many analyses, the volatility of the analytes can be problematic. A very useful procedure was described for the determination of analytical error caused by volatility of Pd compounds during their trace determination in pharmaceutical preparations.123 This work illustrated that errors of up to 100% could be caused by the volatility of Pd compounds when the sample, dissolved in an organic solvent, was directly nebulized into the spray chamber during ICP-MS analysis. This procedure employed thioacetamide to convert volatile Pd compounds into their non-volatile thioacetamide complexes and this completely eliminated volatility errors regardless of the species of Pd present. Microwave induced combustion is a useful means of eliminating the matrices altogether, prior to the determination of volatile analytes. The analytes Br and Cl were determined in cigarettes by using this technique, followed by ICP-OES detection.124 The technique resulted in residual carbon of less than 1% and the analysis of CRMs yielded data that were in 98% agreement with certified values. This procedure is a potentially useful technique for the analysis of volatile elements in a range of matrices.
The analysis of untreated sample matrices by plasma techniques can often result in significant errors because of matrix effects and great care is needed if erroneous results are to be avoided. A number of studies showed that the concentration of carbon, in its various forms within a sample, can significantly affect the emission intensities of various spectral lines, and impact the robustness of the analysis. These effects can arise by influencing aerosol formation and transport properties within the sample introduction system. Matrix matching is a recognized technique for eliminating these issues but is often not possible if the matrix is not fully characterized. Standard addition calibration can be time-consuming and difficult when sample availability is limited. Internal standardization is a well tried and common technique for the minimization of matrix effects. In an important and timely recent study by Vogiatzis and Zachariadis,125 Y and Be were evaluated as internal standard elements during ICP-OES analyses. The work in this paper demonstrated that the matrix can and often does lead to large errors in analysis. This was demonstrated with 19 elements in matrices containing various organic compounds including dioxane, formic acid, glucose and sucrose at up to 50% concentration. Conventional nebulizers and spray chambers and no organic removal techniques were employed. The influence of the matrices was examined by measuring the changes in sensitivity of the calibration curves. Matrices containing 2% dioxane and 50% sucrose caused the largest changes in sensitivity. In most cases the use of internal standardization rectified the linearity of the calibration curves and beryllium was more effective that yttrium in correcting for changes in the emission intensities of many atomic lines. This work demonstrated the need for care and a good understanding of matrix effects and how to correct for them, when using matrices containing varying concentrations of organic compounds.
The very thorough development of a slurry introduction technique with continuum source ETAAS detection for the determination of Cd as a toxic impurity in fertilizer was presented by Borges et al.126 Here, the requirement for conventional acid digestion was eliminated by employing direct analysis using slurry introduction into ETAAS. The use of CCD array detection and the continuum source instrumentation removed several limitations of solids analysis that arose through spectral interferences usually associated with conventional line sources. The use of both a mixed Pd–Mg modifier (10 and 6 μg g−1 respectively) and an Ir permanent modifier (400 μg) were studied under various pyrolysis temperatures. The optimal pyrolysis and atomization temperatures were 600 °C and 1600 °C for the mixed modifier and 500 °C and 1600 °C for the permanent modifier. The particle size of the slurry was controlled by passing it through a 45 μm sieve to improve precision of the analysis. The larger, discarded particles were shown not to contain detectable Cd after the sample preparation stage and so it was valid to discard this portion of the sample. The results obtained from the analysis of the NIST reference material SRM 695 were statistically valid when either the Pd–Mg or the Ir modifier was used. However, perhaps surprisingly, the recovery of Cd from the real fertilizer samples was reduced significantly when using the iridium modifier compared with the Pd–Mg modifier. This emphasizes the requirement that recovery studies must be carried out on real samples and that good agreement for a CRM does not necessarily translate into accurate results with real samples. Recovery of Cd when conventional acid digestion was used was significantly lower than for the new slurry procedure, demonstrating losses of volatile analyte species. Ultrasonic slurry sampling for the determination of trace levels of a range of metals in drug tablets by ETV-ICP-MS was described by Lin and Jiang.127 As with the above paper, the optimization of modifiers as well as pyrolysis vaporization temperatures resulted in a procedure requiring minimal sample preparation and reduced loss of volatile elements as compared with conventional acid digestion procedures.
The technique of TXRF is potentially useful for metals determination because it is virtually unaffected by matrix effects, is very sensitive, with ppb level LOD being achieved regularly, and can require little sample preparation. In work by Antosz et al.,128 which may reduce the need for sample digestion in pharmaceutical analysis, TXRF was used for the quantitation of residual metals in active pharmaceutical ingredients and intermediates. Advantages compared with the more widely used technique of FAAS include its rapidity and multi-element capabilities and generally lower LOD. Compared with ICP-OES, the major advantages are the reduced need for acid digestions during sample preparation (solid samples can be analyzed directly) and low sample amounts required. These advantages, coupled with the very significant reduction in the matrix effects commonly seen with conventional XRF spectrometry make this an ideal technique for trace multi-element determination in a range of application areas. The technique is likely to receive increased attention for quantitative analysis of a range of samples. One caveat, however, is the need for careful application of the sample onto the sample carrier, although errors associated with sample application can be minimized by the use of a suitable internal standard. In this case, Ga was employed. Excellent linearity for a range of elements (Al, Ca, Fe, K, Ni, Pd and Sr) was demonstrated in a standard mixture, corresponding to 10–5000 ppm in the sample although the paper does not say whether this was achievable in a real sample. Recoveries of most of the metals at 0.1 μg mL−1 within the active pharmaceutical ingredient was between 87.8% and 135.3%, with the exception of Al which was much poorer. This was attributed to its low sensitivity. Results for Pd and Cu in 14 different ingredients were compared with those obtained using ICP-MS following acid digestion. Statistical analysis showed that there were no differences (95% confidence). Potential interference with the Pd determination by chlorine was noted but was minimized by using a tungsten instead of a molybdenum cathode. In an interesting development, the use of confocal μ-XRF spectrometry was employed for the chemical imaging of Cu, Fe, Mn, Ti, and Zn within pharmaceutical preparations.129 Matrix absorption effects were important and required correction and the paper showed the determination of the 2D and 3D distribution of a range of elements, to a depth of several hundred microns.
Other applications that do not provide such significant advances in knowledge are summarized in Table 2.
| Element | Matrix | Technique; atomization; presentation | Comments | Ref. |
|---|---|---|---|---|
| a L = liquid, S = solid. | ||||
| Various including isotopes and organic species | Wine | MS; ICP; L TIMS; — ; L IRMS; — ; L | The first analysis of soils and wines with correlation of data from both sample types. Data analysis was by chemometrics including generalized procrustes analysis and canonical correlation analysis | 130 |
| Various | Wine | OES; ICP; L | Multivariate statistical analysis to discriminate according to geographical origin | 131 |
| Various | Cane and beet sugars | OES; ICP, L, MS; ICP; L, TIMS; — ; — , IRMS; — ; — | Multi-element analysis and isotope ratios of Pb, Sr and C enabled the geographic origin of the sugars to be determined | 132 |
| Various | Naples yellow pigment in art works | X-ray absorption; — ; S | X-ray absorption studies determined the incorporation route of Zn and Fe ions into the pigment lattice | 133 |
| Organics | Tablet coatings | OES; LIBS; S | Determine variability of thickness of tablet coatings | 134 |
| Molecular and elemental data | Valuable and historically significant artefacts | OES; LIBS, S Raman; S | Multivariate classification of pigments and inks to establish authenticity of artworks | 135 |
2.3. Inorganic chemicals and acids
Analysis using LIBS was reported in a series of papers by Gondal et al.136–138 In the first of these applications, LIBS was used to determine S in reinforced concrete.136 The presence of S in concrete is known to cause severe structural damage with time and so its determination is of utmost importance. However, S has poor sensitivity using most techniques so a dual pulse LIBS system was devised to improve the sensitivity and LOD. Two lasers were used, one operating at 1064 nm (a standard Nd:YAG) and the other at 266 nm (the fourth harmonic of the same type). Sample was ablated by the laser operating at 266 nm (at 500 mJ pulse−1) and then the other laser was used to excite the plume of vapour (at 50 mJ pulse−1). The repetition rate was 10 Hz. The singly charged S line at 545.38 nm was used for the determination. Since a charged species was being detected, careful optimization of the time delay between the laser pulses and before the spectrograph that employed a gated intensified CCD detector was necessary to ensure that maximum signal was obtained. Once optimal time delays had been determined, a LOD of 38 μg g−1 was obtained. Inspection of the calibration graph produced gave a correlation coefficient of 0.986, but there appeared to be distinct curvature at higher levels. Despite this, there was sufficient linearity and sufficient sensitivity for the method developed to be regarded as successful. The other two applications in this series described the LIBS determination of Cl in reinforced concrete.138137 In the first of these papers, a single pulse LIBS system was employed, whereas the latter used a similar dual pulse LIBS setup to that described previously. In these cases, the Cl was monitored at 837.5 nm and at 594.8 nm (both atomic lines). The dual pulse LIBS system improved the sensitivity obtained at both lines. However, despite the latter wavelength having approximately 1000th the intensity of the former, it was the preferred line because it exhibited less self-absorption. The other paper used a single pulse LIBS system. A fourth LIBS system was described by Westlake et al.139 These authors used several techniques (LIBS, Raman micro-spectroscopy and XRD) to investigate pigments on painted plaster in Minoan, Roman and early Byzantine Crete. Both the LIBS and the Raman spectroscopy were portable instruments that provided data and an interpretation rapidly.
The other solid-sampling methodology used for these materials is XRF. The use of a benchtop WDXRF spectrometer for cement analysis was reported by Tisdale.140 Traditionally, WDXRF instruments are larger, floor-standing models. However, the development of this new commercial benchtop instrument is hoped to be ideal for the rapid quantitative analysis of cement and suchlike materials. The other relevant paper of interest was prepared by Steiner.141 Drywall is a common material used in the construction trade. However, some forms are corrosive and can damage the copper components within a building; i.e., the electrics. The presence of Sr can be used as a marker for the presence of “corrosive drywall”. This author used on-site XRF spectrometry to determine the presence of Sr in drywall as well as a supplementary laboratory test in which copper was exposed to the drywall sample.
Several research groups have used nanoparticles as matrix modifiers. Palladium nanoparticles have been used in two papers. In one, by Yi et al.,142 they were used as a modifier to stabilize As, Cd, Hg, Pb, Sb and Zn in slurries of biological samples prior to analysis using ultrasonic slurry sampling ETV-ICP-MS. Both the instrumental conditions and the preparation of the slurries were optimized. For the slurry preparation, 1 g of <150 μm material was mixed with water to a volume of 10 mL. Then, 1 mL of this was transferred to another 10 mL flask and, since standard addition was used as the calibration method, it was at this stage that known concentrations of analytes were added. The slurry was then sonicated for 5 min and aliquots (1 mL) removed for analysis whilst the slurry was being vortex mixed. Compared with the standard method of introducing the palladium as an aqueous solution, 100 ng of nanoparticles produced better signals for the analytes; with factors of improvement ranging from about one (i.e., virtually no change) for Cd through to about 10 for Pb. Analysis of the NIST reference materials SRM 1568a (rice flour) and SRM 1573a (tomato leaves) yielded data in good agreement with certified values. The data from the “real” samples were compared with those obtained using an acid dissolution followed by introduction through pneumatic nebulization. These were also in agreement. Another paper to have used palladium nanoparticles as a matrix modifier (this time used in conjunction with the permanent modifier ruthenium) was prepared by Resano and Florez,143 who determined S in an assortment of solid samples using continuum source graphite furnace molecular absorption spectrometry. No sample pre-treatment was necessary. The sample was simply cut with a ceramic knife and placed on a platform that had been pre-treated with the ruthenium modifier. The palladium nanoparticles were added and the platform transferred to the graphite furnace. Calibration was achieved using aqueous standards (10 μL) mixed with the palladium nanoparticles (40 μg). The analyte was determined as the C–S molecule at 257.958 nm. The nanoparticles were a more efficient method of thermally stabilizing all of the S-containing species analysed when compared with a conventional palladium solution that was capable of stabilizing non-volatile inorganic species such (e.g. sodium sulfate) but had a much poorer efficiency with organic species such as thiourea. Numerous materials were analysed using the procedure developed, including biological samples, petroleum coke, polyethylene and steel CRMs. The LOD was 9 ng and the characteristic mass 14 ng, but these values could be improved to 3 ng for both by combining data from the main six C–S lines. Gunduz et al.144 used gold nanoparticles to thermally stabilize the analytes As and Sb up to a temperature of 1100 °C during conventional ETAAS analysis of salt solutions. The optimal amount of modifier was identified and the protocol developed was validated using certified materials and spiked seawater samples. Precision was better than 10% RSD and the LOD for the analysis of seawater were 2.3 and 3.0 μg L−1 for As and Sb, respectively.
Several applications of permanent matrix modifiers have been published. A paper by da Silva et al.145 described the determination of Cd in rice using ETAAS with an aluminium permanent modifier. Samples were digested using HNO3 and H2O2 in a digester block. A full two level factorial design was used to optimize the operating conditions and led to the following conditions being used: aluminium mass of 400 μg, pyrolyis temperature of 400 °C and a time of 20 s and an atomization temperature of 1800 °C. Calibration could be achieved using aqueous standards. Under optimal conditions, the LOD was 2 ng g−1, characteristic mass was 1.3 pg and precision (at a concentration of 41.3 ng g−1) was 1.67% RSD. The method was validated using a Japanese rice flour CRM. Ferreira et al.146 also used aluminium as a permanent modifier; this time during the determination of Pb in sugar cane spirits. Optimal conditions were: pyrolysis temperature of 800 °C, pyrolysis time of 20 s, atomization temperature of 1800 °C and an aluminium mass of 3 μg. Calibration could again be achieved using aqueous standards and the optimal conditions yielded a LOD of 0.14 μg L−1 and a characteristic mass of 24 pg. Since no CRM for sugar cane spirits exists, the authors resorted to validating the method using an orchard leaves reference material. Further validation was achieved by comparing the data obtained using the proposed method with those obtained using ICP-MS. No significant difference between the data sets was observed. A comparison of permanent modifiers was made for the determination of Sb in slurries of sediment and soil samples.147 The paper, by Dobrowolski et al. compared the performance of a mixture of iridium/niobium with that of iridium/tungsten. Slurries were prepared by grinding the material so that >80% was <25 μm in diameter, then adding concentrated HF (100 μL), and waiting for 20 min. A slurry of PTFE (100 μL of 60%) was added followed by 800 μL of pure water. The sample was then shaken for 2 min and vortex mixed prior to analysis. The aim of the HF and PTFE addition was to remove the silica during the pyrolysis stage (at 900 °C) which would otherwise have produced a large background signal during the atomization stage. The integrated absorbance results were interesting in that a mixture of iridium/tungsten led to a decrease of absorbance of approximately 30% compared with that obtained using iridium alone, whereas the mixture of iridium/niobium led to a relative increase of 30%. The optimal amount of iridium/niobium was 30 μg and 40 μg, respectively. The procedure was validated by the successful analysis of the CRMs PACS-1 and SL-1 sediment and NIST SRM 2709 (San Joaquin soil) and SRM 2711 (Montana soil). The iridium/niobium mix allowed modification for up to 180 heating cycles. A LOD of 0.04 μg g−1 was achieved, calibration ranged from 2.5–50 μg L−1, characteristic mass was 10.6 pg and precision was typically better than 6% RSD.
Not all matrix modifiers are designed to thermally stabilize the analytes since some deliberately make the analyte more volatile and/or prevent it from forming refractory compounds, i.e., carbides, with the graphite atomizer. Chen et al. have published two papers that report such volatilization aids for the speciation analysis of inorganic Cr.148,149 In one148 diethyldithiocarbamate was used as both an extractant and as a modifier. The CrVI – diethyldithiocarbamate chelate is volatile and could therefore be volatilized into the ICP-MS detector at a much lower temperature than the CrIII. By using a simple temperature program, the two species could effectively be separated and detected. The LOD for the CrVI was 0.045 ng mL−1 and precision for 1 ng mL−1 was 4.6%. The linear range spanned three orders of magnitude and the method was applied to the analysis of water samples. Recoveries were reportedly 94.3–103%. The second paper149 was similar in design, but used the compound thenoyltrifluoroacetone as the modifier to form a volatile chelate with CrIII. By using a temperature program, the volatile CrIII could effectively be separated from the CrVI. The LOD for CrIII was 0.008 ng mL−1 and precision was again 4.6% RSD for a concentration of 1 ng mL−1. Recoveries for the analysis of water samples were 93.5–104%.
Several other applications have also been reported. The determination of Hg in compact fluorescent lamps was described by Singhvi et al.154 The cap and electronic parts were removed carefully, ensuring that the bulb itself was not broken. The bulb was then placed in a polypropylene bottle containing glass marbles or stoppers as well as a mixture of nitric acid and bromine monochloride solution. The bottle was shaken to break the lamp. The Hg released was absorbed completely by the bromine monochloride solution. This was then analysed using CV-AAS. Recovery was high (90–110%) and repeatability was good. Zhang et al.155 reported the determination of some REEs (Ho, La and Tb) in marine sediments and in high purity REE oxide materials.155 Since the analytes are at such low concentration and the determination was done using microwave plasma torch OES, a pre-concentration step was necessary to obtain sufficient analyte to be determined reliably. The authors used a micro-column packed with a titanium dioxide–graphene composite to pre-concentrate the analytes. The high purity REE oxide material was dissolved in HNO3 and the sediment was prepared by acid digestion using HF, HNO3 and HClO4. Sample was flowed through the column at a rate of 2 mL min−1 for 1.5 min and then HNO3 (1.5 M) was used to elute the retained analytes directly into the detector. All of the usual parameters, e.g. sample and eluent flow rates, pH of sample, concentration of eluent, etc., were optimized during the study. The LOD were 2.8, 2.2 and 1.6 μg L−1 for Ho, La and Tb respectively; enrichment factors ranged from 10.2 to 17.1. Precision (n = 7) for the target analytes was also good, being between 1.3 and 3.6% RSD. The authors validated their method by analysing the reference material GBW07313 (marine sediment).
Two papers reported the analysis of gaseous samples. Gerdes and Carter156 described a method by which aqueous calibration standards could be used for the direct analysis of gas phase samples, albeit in a semi-quantitative manner. Empirical factors relating ion generation and transmission efficiencies to standard operating parameters were generated that were similar to the instrument's own semi-quantitative software algorithms. Equations were produced that enabled the prediction of relative ionization efficiencies and isotopic transmission which allowed for the adjustment of limited parameters between liquid and gas injection modes. Using the method developed, the authors analysed a single liquid sample containing 45 elements using 133Cs as an internal standard. The concentrations of most of the other analytes were quantified to within 50% of the real value. The study was expanded to predict the concentration of Hg in a liquid sample (correct to within 12% of the actual concentration) and its determination in a gaseous sample (correct to within 28%). Although clearly only semi-quantitative, the technique does offer the advantage of speed of analysis compared with a standard method of air analysis. The other example of the analysis of gas was prepared by Hoffmann et al.157 who analysed the gaseous reaction products of wet chemical silicon etching using conventional direct current GD-OES. The mechanism of the chemical etching of silicon is not fully understood. In this study, different ratios of HNO3–HF were used and the H evolved was monitored at 121.5 and 656.3 nm using a commercial GD-OES instrument with a standard Grimm type GD chamber. A small amount of the gas evolved during the etching of silicon in a PTFE container was transported in a stream of argon through a drying column and into the plasma produced by the continuous GD sputtering of an iron sample. Although other elements, e.g., B, N, O and Si were also detected, it was only the H that was monitored closely. Results indicated that the amount of H released was inversely related with the amount of HNO3 present. A study of temperature (between −10 and 35 °C) of the etch solution demonstrated that this parameter had a negligible effect on the H generation, but did affect the amount of nitrous gases evolved. The study enabled the authors to elucidate the mechanism of the etch process, although some questions remained unanswered.
2.3.5.1. Reviews and CRMs. Several reviews of different aspects of the analysis of nano-materials have been produced during this review period. One entitled “Application of plasma spectrometry for the analysis of engineered nanoparticles in suspension and products”, by Krystek et al.,158 contained 325 references and discussed the assorted methods of analysis of both pure products and in materials containing them. Also discussed were acid digestion approaches. The most interesting parts of the paper from this review's perspective, is that is also discussed some of the most recent approaches used to characterize the materials. These include the introduction of single particles and FFF or size exclusion chromatography coupled with the plasma-based detectors. A similar review by Jimenez et al.159 contained 96 references. This review concentrated on methods of nanoparticle characterization such as PAGE, different types of FFF and size exclusion chromatography coupled with detection using ICP-MS. Also discussed were the papers that had undertaken single particle detection. A review (188 references) that concentrated on flow FFF for the analysis and characterization of natural colloids and engineered nanoparticles in environmental systems was prepared by Baalousha et al.160 Although FFF is a powerful tool on its own for the high resolution characterization of particle size distribution, when it is coupled with other detectors such as ICP-MS, TEM, AFM, fluorescence and UV-VIS absorbance, it can provide a wealth of information on the composition and shape of the particles. The review was designed to give its readers the theoretical principles and considerations of flow FFF and the range of analytical tools that may be coupled with it to characterize the particles more fully. In addition, how its results compare with other techniques and a range of applications were also given. For those capable of reading Chinese, mention should also be made of a review by Qu et al.,161 who discussed, with the aid of 76 references, the techniques of synchrotron radiation XRF, XAFS, XANES, EXAFS, synchrotron radiation circular dichroism spectroscopy, ICP-MS and NAA methods of analyzing nanoparticles. A review by da Silva et al.,162 containing 73 references, discussed the analytical chemistry of metallic nanoparticles in the environment. The authors stated why determination of the particles size is important and that traditionally, methods such as micro-, nano- and cross-flow-filtration as well as ultra-centrifugation have been used to determine it. Similarly, the chemical characterization has traditionally been undertaken using techniques such as TEM and SEM (often combined with EDAX) and XRD. The review emphasized the limitations of such traditional techniques and described how coupling together of different techniques with powerful detectors such as ICP-MS can lead to more information being obtained. Also discussed in the review were methods of isolating the nanoparticles from environmental samples.
2.3.5.2. Field flow fractionation. The different types of FFF have been used for several years now and represent powerful methods of analysis of particulate matter. As well as the reviews cited above that discussed the techniques and how they may be coupled with atomic spectrometric detectors, other individual papers have also been produced. Songsilawat et al.163 described the use of flow FFF with off-line detection using ETAAS to characterize Ag nanoparticles stabilized in citrate, pectin and alginate. It is known that nanoparticles can aggregate forming much larger collections of materials and that these can settle in solution and hence not present a representative sample when introduced through a standard nebulizer/spray chamber assembly of an atomic spectrometry instrument. The citrate-stabilized nanoparticles were prepared by using sodium citrate as a capping agent on Ag nanoparticles produced by sodium borohydride reduction of silver nitrate. The pectin and alginate-stabilized materials were prepared from Ag nanoparticles prepared from the ascorbic acid reduction of silver nitrate. The three types of material were characterized using flow FFF, zeta potentiometry and TEM. The mean particle size differed markedly between the three types, with diameters of 9, 19 and 45 nm for the citrate-stabilized, pectin-stabilized and alginate-stabilized materials respectively. The flow FFF-ETAAS approach to determining the different types of nanoparticle in different water systems demonstrated that the citrate-stabilized nanoparticles aggregated more rapidly than the other materials. This was attributed to the other types being stabilized sterically as well as chemically. The presence of humic acid in the waters did help prolong the stability of the citrate-based materials though.
Another paper that described a study into the effect of natural organic matter and ionic strength on the stability of Ag nanoparticles in the aqueous phase was presented by Delay et al.164 Asymmetric flow FFF was coupled with both UV-VIS and ICP-MS detection systems for the analysis. After preparation using sodium borohydride reduction of silver nitrate, the particles were suspended in different aqueous solutions of differing ionic strength and natural organic matter content. The presence of natural organic matter helped stabilize the materials and this was attributed to them coating the particle, thereby stabilizing them sterically. The organic matter went some way to overcoming the de-stabilizing effects of increased ionic strength. Poda et al.165 also coupled FFF with ICP-MS to characterize Ag nanoparticles. The primary particle size was first characterized using TEM and the hydrodynamic size measurements obtained using FFF were comparable to those obtained using dynamic light scattering. Detection of the materials using ICP-MS was, as expected, extremely sensitive with standards of mixed Ag nanoparticles with concentrations at the μg L−1 level being determined readily. Once optimized, the technique was applied to the determination of Ag nanoparticles in a biological system. The fresh water, sediment-dwelling oligochaete Lumbriculus variegatus was exposed to a sample of well-characterized Ag nanoparticles and then the post-exposure tissues were extracted and analysed. The Ag nanoparticles increased in size significantly during the exposure. This would have implications for the overall toxicity of the material.
Size characterization and quantification of Ag nanoparticles was also studied by Bolea et al.166 who used asymmetric flow FFF coupled with ICP-MS for the determination. The authors went to great lengths to avoid aggregation processes, alteration of the original size distributions of the particles and losses from sorption processes onto the channel membrane. The mobile phase composition, injection, focussing and fractionation conditions were all optimized in terms of their influence on both the separation resolution and recovery. The ionic strength, pH and the presence of ionic and non-ionic surfactants had a strong influence. Better results were obtained when conditions were used that favoured charge repulsion with the membrane. Commercially available Ag nanoparticles were use for the size calibration of the FFF channel and direct injection of ionic Ag standards was used to calibrate the ICP-MS instrument. Integration of the peaks and interpolation of the fractogram area was used for the quantification. Of the consumer products analysed, recoveries were 83 ± 8% and 93 ± 4% with respect to the content of the Ag nanoparticles. Results obtained were comparable to those obtained using photon correlation spectroscopy and TEM images. The prospects and limitations of asymmetric flow FFF used in combination with a multi-stage detection system consisting of UV-VIS spectrometry, light scattering and ICP-MS with respect to the characterization of Au nanoparticles were discussed by Hagendorfer et al.167 Use of a citrate-based dispersant ensured that the ICP-MS signal obtained for the nanoparticles was the same as for ionic Au standards for concentrations between 1 and 50 μg L−1 and for particle sizes ranging from 5 to 80 nm. The FFF membrane and carrier flow conditions were optimized, but particle interaction with the membrane could be problematic in some cases, which led to retention time shifts and losses of nanoparticles. Consequently, the authors used ultra-centrifugation followed by ICP-MS detection to distinguish between ionic and particulate Au species. Despite this, the results obtained using the multi-stage detection system were comparable to those obtained using batch dynamic light scattering and using TEM. The method was validated further by analysis of the standard Au nanoparticle materials NIST 8011, 8012 and 8013; with results being in excellent agreement with certified values.
2.3.5.3. Single particle analysis. Single particle analysis is an emerging technique that enables the determination of both the size and the number of metal-containing particles entering the instrument. It also has the benefit of being capable of characterizing nanoparticles at concentrations relevant to the environment. Mitrano et al.168 demonstrated the process using ICP-MS as the detection system in a paper that described the analysis of a sample containing a well-characterized sample of Ag nanoparticles and a health food sample that also contained Ag nanoparticles. Serial dilution, filtration and acidification confirmed that the method detected particles. The authors applied their method to the analysis of wastewaters and demonstrated that Ag nanoparticles at 100–200 ng L−1 could clearly be differentiated from Ag ions at a concentration of 50–500 ng L−1. A paper by Pace et al.169 presented a practical guide on achieving single particle analysis. One of the most important factors involved is the transport (i.e. nebulization) efficiency and several methods for calculating this was demonstrated. Well-characterized monodisperse Ag nanoparticles demonstrated the importance of knowing the transport efficiency accurately when calculating both the particle number concentration and the particle size.
Laborda et al.170 used the different behaviours of dissolved Ag and Ag nanoparticles to differentiate between the two in aqueous samples. Using ICP-MS with a time resolution of 5 ms as a means of detection, they introduced both Ag nanoparticle suspensions at a number concentration of <109 L−1 and Ag ions through a conventional pneumatic nebulizer. The Ag ion solutions produced pulses of averaged constant intensity. Conversely, the Ag nanoparticles were converted by the plasma into a packet of ions that produced discrete pulses, the intensities of which were proportional to the number of Ag atoms in the particle, i.e. the size. Frequency plots with respect to the intensity measured for each pulse showed independent distributions for dissolved Ag ions and nanoparticles (Poisson and lognormal, respectively). Interestingly, the size limitation for the Ag nanoparticle was 18 nm (approximately 32 ag). This particle size is significantly smaller than the 2–3 μm limit normally associated with the introduction of suspended solids through a conventional nebulizer/spray chamber assembly.
The advantages of using micro-droplet generators rather than conventional nebulizers to introduce nanoparticles to ICP-MS instruments have been demonstrated by Gschwind et al.171 A droplet size of between 30 and 40 μm could be reproduced accurately and this led to 100% transport efficiency to the plasma. Analyses were conducted using standard ion solutions containing 1, 2, 5 or 10 mg L−1 (for Ag, Au, Au and Cu respectively) and with highly diluted suspensions of Ag and Au nanoparticles of <110 nm in size. Detection efficiency was reportedly 10−6 counts per atom, but the LOQ for the nanoparticles were size-related and were 21 nm and 33 nm for Au and Ag respectively. Helium was chosen as the aerosol transport gas since this enabled the aerosol to be desolvated, hence preventing their agglomeration/precipitation. The authors also used an oscilloscope to collect the data at shorter dwell times (500 μs) than the instrument software would allow (a minimum of 10 ms). This decreased the possibility of more than one particle being detected simultaneously.
Kapellios and Pergantis172 have combined the use of ion mobility spectrometry with off-line ICP-MS detection for determining nanoparticles in aqueous solutions. The size and metal content enabled the determination of the concentration of the nanoparticles. The resolving power of the ion mobility spectrometry and the detection power of the ICP-MS instrument as well as the single particle mode it offers were the main features the authors quoted as being attractive for the analysis of nanoparticles. The nanoparticles were separated according to size by setting a metal collection cup with an appropriate voltage (up to a maximum of −10 kV). Small nanoparticles required a low voltage whereas larger ones required high voltages. The particles collected in that cup were then dispersed in water and analysed using ICP-MS through conventional pneumatic nebulization. In this example Au nanoparticles obtained from NIST that had a nominal diameter of 60 nm were tested. The authors used a collection voltage of −9.5 kV to collect the material and then dispersed them using 1 mL of deionized water prior to subsequent ICP-MS determination using a rapid dwell time of 10 ms. The authors admitted that the possibility of multiple particles being detected during the same 10 ms dwell time could not be totally excluded, despite the large dilution that had occurred.
2.3.5.4. Other methods of analysis. To overcome problems associated with the introduction of nanoparticles to ICP instruments some authors have resorted to converting them to ions through a digestion procedure. Khosravi et al.173 used ammonium persulfate to digest titanium dioxide nanoparticles present in aqueous suspensions and biosolids. The aqueous samples contained 6 mg L−1 of Ti present as 10 mg L−1 suspensions of rutile, anatase or a mixture of nano-sized crystals. Only 1 g of the fusing agent was required for the rapid destruction of the particles yielding a recovery of 95%. This figure was higher than that obtained using a three component acid digestion on a hotplate and was comparable to that obtained using the same acid mixture in a microwave oven. A study of the effects of cations and anions present in the range 10 ng to 110 mg indicated that there were no significant interferences. The LOD obtained using ICP-MS as a means of detection was 0.06 ng mL−1. The digestion procedure was applied to the analysis of treated and untreated wastewater samples and biosolids collected from wastewater treatment plants. Some metal particles are encapsulated in a silica shell to decrease bio-toxicity. A paper by Wang et al.174 described a method involving microwave digestion and ICP-OES detection that was capable of determining this coating efficiency. Three acid digestion mixtures were tested for this. Aqua regia was capable of dissolving those metal particles not coated with the silica and a mixture of HNO3/HF was better at digesting those particles that had got the coating. The coating efficiency could therefore be calculated. Analysis of nanoparticles produced during a welding process in which tetramethylsilane had been added to the shielding gas indicated that, depending on the conditions used, between 14 and 39% of the fume particulates were encapsulated in the silica casing.
A method of distinguishing between ionic Ag in solution and Ag nanoparticles present in environmental waters and commercial anti-bacterial materials was described by Chao et al.175 Nanoparticulate Ag was determined after Triton X-114 was used for the cloud point extraction of the samples and, after a microwave assisted acid digestion of the organic-rich extracts, the Ag was determined using ICP-MS. Total Ag was determined using ICP-MS following an acid digestion of the unextracted samples. The LOQ was 0.4 μg kg−1 for Ag associated with the nanoparticles and 0.2 μg kg−1 for total Ag, although the authors did note that reliable determination occurred only at total Ag concentrations of higher than 3 μg kg−1. Spike/recovery experiments conducted on commercial anti-bacterial products yielded slightly variable data, with recoveries in the range 71.7–103% being achieved. Despite the manufacturers claiming the presence of Ag nanoparticles in their products, analysis indicated that three of the six products tested did not contain them at detectable levels.
Quantitative measurement of Au and Fe in ferromagnetic nanoparticles using LIBS was reported by Borowik et al.176 Direct analysis of the powders using LIBS led to spattering of the samples. The authors therefore developed a rapid, reliable and convenient method to avoid this which involved fixing the powders (or water suspensions of them) in a polymer gel. This procedure allowed any compositional changes during the production process to be evaluated and also enabled the final product to be characterized. The data obtained were “successfully correlated” with those obtained using an ICP-based technique.
X-ray-based methods have been used in an application of note by Choi et al.177 who developed a non-destructive WDXRF method that was capable of determining the water content of silica nanopowders. The very low fluorescence yields of H and O mean that the water cannot be determined directly. Instead, the authors took advantage of the attenuation of the Si fluorescence yield by the presence of water to determine the water content indirectly. This is possible because the extent of the signal attenuation is directly proportional to the water content, with linear regression equations of > 0.9 being obtained. In addition, both Compton and Rayleigh scattering of the X-ray tube lines Rh K-L(2,3) and Rh K-M(2,3) was also proportional to the water content. Good linearity of this latter method was obtained over the water content range 0–61.5%. One problem was noted though and this was that the porosity of the material had a significant influence, especially at low levels of water content (up to 7.5%). The sensitivity for a silica nanopowder with well-defined mesopores (3 nm in diameter) decreased to 0.40 kcps/wt% compared with 0.99 kcps/wt% for non-porous materials. The morphology of cobalt nanoparticles embedded in silica and then subjected to heavy-ion irradiation was investigated over a wide energy and fluence range by Sprouster et al.178 Using TEM and small angle X-ray scattering as means of detection, nanoparticles below a certain diameter remained spherical in shape but became smaller under irradiation. This was attributed to their gradual dissolution. However, larger particles became rod-like in appearance with their longer dimension being parallel to the incident ion direction. The state of the Co present in the nanoparticles was determined using XANES. Prior to irradiation, only Co atoms were present. However, after irradiation, both atomic Co and oxidized Co were present; an indication, according to the authors, of dissolution. The structure of the nanoparticles was then examined using EXAFS. Increased fluence of the heavy ion irradiation led to an increase in disorder, a decrease in bond length and asymmetric deviation from a Gaussian inter-atomic distance distribution.
Gold nanoparticles have been characterized by Helfrich and Bettmer,179 who used liquid chromatography and gel electrophoresis coupled with ICP-MS and complementary ESI-MS techniques for the analysis. The liquid chromatography-ICP-MS approach was optimized for monitoring the formation of citrate-stabilized materials. Two reversed phase columns with differing pore sizes were used to separate different sized nanoparticles using a size exclusion type mechanism. The gel electrophoresis-ICP-MS was ideal for determining the Au–S ratios in materials covered by mercaptosuccinic acid. The instrument was operated in medium resolution mode to overcome the polyatomic interference arising from the O dimer on the 32S isotope. The gel composition was between 2 and 2.4% agaraose, the voltage applied was 400 V and the electrode buffers were 1 mM NaH2PO4 and 1 mM Na2HPO4 (pH = 7.3). The atomizing nature of ICP-MS does not yield structural information and so the authors utilized the complementary technology of ESI-MS to elicit molecular information. Such an approach is common in speciation studies, but has, until now, not been used for nanoparticle analysis. Various complexes of mercaptosuccinic acid and Au were observed.
2.4. Nuclear materials
The number of applications reporting the analysis of nuclear materials or the determination of radioisotopes has again been very high in this review period. In general, those applications that described the determination of radioisotopes in environmental samples have largely been ignored, whereas those that described methods of overcoming interferences or the analysis of nuclear materials were favoured. There was a notable increase in the number of LIBS applications and this is probably a reflection of the technique's ability to achieve “stand-off” analyses successfully.The results of an inter-laboratory study determining36Cl at ultra-trace levels using AMS were reported by Merchel et al.181 Eight laboratories from Europe, USA, Australia and Japan participated in the study which involved the analysis of three synthetic silver chloride samples which had 36Cl/Cl ratios of 10−11, 10−12 and 10−13 respectively. Initial statistical inspection of the data indicated that there was no significant difference between the data obtained from the laboratories. However, closer inspection demonstrated that there was actually evidence of inter-laboratory bias and underestimation of uncertainties by some laboratories. Despite re-measurement and further data manipulation, data from two independent laboratories could differ by as much as 17%. The conclusion was that further work had to be undertaken to ensure harmonization of the analytical methodology which would ideally lead to an improvement in reliability of data.
Two reviews of different aspects of the analysis of nuclear materials and/or the determination of radioisotopes have been presented. Litherland et al.182 reviewed (with approximately 300 references) the use of AMS. The lengthy review covered the historical development of the technique and then went on to discuss the instrumentation currently available (including different ion sources) and possible future developments. Some of the different application types were also described. The authors illustrated their discussions with examples such as the determination of 14C (for dating purposes), 10Be and 129I. Some of the difficulties of the technique, e.g., of obtaining large anion currents with low electron affinities and of isobar separation and how this has prompted the use of molecular anions and the search for alternative isobar separation methods was also discussed. The other review of interest was presented by Shi et al.,183 who described (with the aid of 134 references) methods for the determination of 99Tc in environmental samples. This analyte is regarded as being one of the most important radionuclides in the safety assessment of environmental radioactivity as well as nuclear waste management. It is also an important tracer for oceanographic research because of the high solubility of the pertechnetate ion (TcO4−) in seawater. In general, many methods in the literature have used a chemical separation/pre-concentration prior to either a radiometric or mass spectrometric means of analysis. The review covered methods of pre-concentration and matrix separation (to avoid interferences), e.g., co-precipitation, solvent extraction, anion chromatography, extraction chromatography; as well as the different methods of detection, e.g., ICP-MS (including different methods of sample introduction, e.g., ETV, nebulization etc.), AMS, RIMS and TIMS etc. Also included were examples of on-line FI methods that achieve both pre-concentration and detection. A comparison of the different methodologies was made which indicated their relative advantages and capabilities.
Cremers et al.186used LIBS to monitor H, Li and U and their isotopes. Usually, a low resolution spectrometer suffices for LIBS applications, but a much higher resolution spectrometer is required for isotopic work because the difference in wavelength between different isotopes is extremely small (25 pm for U and 13 pm for Pu). However, to be of real use, the spectrometer must be sufficiently small to be deployable in field-based scenarios. The authors reported the use of a hand-held LIBS probe that was used in combination with two new spectrometers that had a resolution (lambda/delta lambda) of 75000 in one and 44000 in the other. These instruments were significantly smaller and lighter than previous high resolution spectrometers. The authors applied their instrumentation to the rapid characterization of the isotopic composition of U and H and highly enriched samples of 6Li and 7Li. This system is likely to be of considerable use in numerous other areas of research, potentially including the analysis of chemical or biological warfare agents.
The other paper relevant to this section was presented by Aitkenhead et al.187 These authors developed artificial neural networks for use as a potential “information barrier” technology in the verification of arms control treaty accountable items. Data obtained using over 400 γ ray spectra were used to create the database. The 239Pu/240Pu isotope ratio, the amount of Pu present and the age of the material were used as the attributes to form the network. Several methods of data collection were tested and the most reliable was one which discounted low energy regions that were susceptible to shielding effects. Once trained, the network was capable of identifying samples that contained Pu, estimating the 239Pu content and distinguishing between spectra of plutonium samples of different ages.
During quadrupole-based mass spectrometric analysis, these interferences may be polyatomic, isobaric or non-spectral (physical) in nature. A paper by Amr and Abdel-Lateef188 compared the capability of collision/reaction cell quadrupole-based instrumentation with that of SF-ICP-MS instruments for the determination of 137Cs, 226Ra and 90Sr. The 90Sr is overlapped by 90Zr on quadrupole instruments and therefore direct determination is complicated. The use of oxygen in the collision reaction cell converted the 90Zr to ZrO whilst leaving the 90Sr unreacted. A similar approach was adopted during the determination of 137Cs in the presence of 137Ba. This time, nitrous oxide was used as the collision/reaction cell gas, with the Ba forming BaO and BaOH, leaving the unreacted 137Cs available for measurement. Helium and hydrogen overcame interferences on 226Ra. The authors applied their methods to the determination of the analytes in a range of materials including pottery and plants. The SF instrument required high resolution to overcome some of the polyatomic/isobaric interferences and hence the sensitivity was diminished. The digestion methods for the samples were described, as were the resin-based protocols for matrix elimination and/or pre-concentration required for the SF-ICP-MS determinations. The conclusion was that the quadrupole instrument could overcome the interferences in an efficient manner resulting in the rapid determination of the analytes and was therefore preferable to the longer methods required by the SF instrument. Further method validation was achieved by analysing the samples using gamma ray spectrometry. Another example of interference removal was presented by Fujiwara et al.189 who overcame interference effects from 129Xe+ (present as an impurity in the argon plasma gas) and 127I1H2+ on 129I+. The former was removed by using oxygen as the gas introduced to the dynamic reaction cell. The latter was removed by ensuring that all traces of hydrogen and water vapour were removed from the reaction gas and by careful optimization of the instrumental operating conditions; especially the ion optics. A deviation from linearity was observed on the signal obtained at m/z 129, but the authors corrected this by usıng a Rh internal standard. Even in the presence of 1 mg L−1 127I, the LOD for 129I was 15.2 pg L−1. The protocol developed was applied to the determination of 129I in soils around a Japanese nuclear fuel re-processing plant.
Sector field ICP-MS instruments may also be used to overcome some polyatomic interferences. A paper by Pointurier et al.190 discussed how PbO2+, ArHg+ and IrO3+ may interfere during the determination of Pu at either m/z 239 or 240 in dust swipe samples taken from a nuclear plant. Although the counts per second originating from these molecules is usually very low, it is still above the baseline observed during aspiration of dilute acid and could, therefore, constitute a potential interference. The acid leach process and the ion exchange methodology used to separate the U, Th and Pu fractions were described. The anion exchange purification goes a long way to interference removal since it removes most of the heavy elements found in such samples. However, as with all sample manipulation procedures, it is prone to contamination from reagents or from glassware and it is these contaminants that may lead to over-estimations of Pu – especially when measuring at the fg level. The prior determination of the level of heavy elements present in the sample was necessary to estimate the possible extent of the problem. Careful centering of the peaks of both the analyte and the heavy metals was also necessary to prevent bias of the count rates. Another paper by the same researchers191 described how the use of medium mass resolution combined with desolvation of the sample enabled the determination of Pu at the fg mL−1 level. A mass resolution of 4000 was sufficient to separate the Pu signals from those originating from the polyatomic interferences (as well as from the tail of the larger U signal), but this resulted in a nine-fold loss of ion transmission, and hence a similar drop in sensitivity. Sample desolvation was achieved using a commercial device comprising two spray chambers, the first being heated to evaporate the aerosol and the second being condensing to remove the liquid. Desolvation of the sample partially overcame the signal lost through the use of medium mass resolution since it resulted in a five-fold enhancement of sensitivity. The authors explained at length that the medium mass resolution had to be used in conjunction with the desolvation device, since use of the latter alone also led to an increase in signal originating from the polyatomic interferences. This was because of the increased transport efficiency of the interfering ions to the plasma. Similar samples and sample preparation protocols discussed in the previous paper were also described here. Although not as sensitive as many other methods described in the literature for the determination of Pu, the authors stressed that their method was robust and was reliable even in the presence of significant levels of potentially interfering elements.
Krachler and Wegen192 have investigated the determination of 233U using high resolution ICP-OES at the wavelength 385.950 nm. This wavelength has been used for many years, but the authors contended that it is neither specific for this isotope nor reliable. Spectral overlap by Th and by other U isotopes may occur (since 235U and 238U are only 5 and 8 pm away) and the resolution required to split the lines is higher than that provided by any commercial instrument. Instead, the authors used lines around 411.6 nm and 424.4 nm. Unfortunately, although the latter has the largest difference in wavelength between the different U isotopes, it suffers from a strong spectral overlap from Th. The region around the 411.6 nm line, although not having as clearly resolved U isotope peaks as the 424.4 nm line, did not suffer the Th interference to the same extent. Not only did the Th line at 411.577 nm have an intensity approximately a tenth of that at 424.393 nm, but it also appeared at a wavelength 8 pm away from the 233U line, a sufficient distance for a high resolution spectrometer to distinguish between the two.
A dual pulse application of LIBS was reported by Lee et al.,200 who determined B and Li in aqueous solutions. The procedure developed used two Nd:YAG lasers operating at 532 nm, one to vaporize the surface of a laminar water jet and the second to excite the analytes within the vapour causing them to emit light. Optimization of the timings between the two laser pulses was required to obtain optimal sensitivity. The time gap was < 3 ms for the B but approximately 10 ms for the Li. Using these optimal conditions, LOD for B and Li were 0.8 ppm and 0.8 ppb respectively. Although the LOD for B, being 0.8 ppm, appears not to be terribly impressive, it represented an improvement of approximately two orders of magnitude compared with other research reported in the literature. The method developed was applied to the determination of the analytes in boric acid and lithium hydroxide which act as the neutron absorber and pH controller in the primary coolant water of pressurized water reactors.
Lasers have also been used for techniques other than LIBS. Pointurier et al.201 used LA-ICP-MS with a quadrupole-based instrument for the isotopic analysis of single submicrometer-sized uranium particles. The particles were extracted from their original matrix using ethanol and placed on a polycarbonate disk where they were fixed using an organic compound. The particle size for the majority of the particles was confirmed to be <1 μm using SEM prior to analysis. A nanosecond UV laser (213 nm) was used for the ablation process. The particles contained 235U at the femtogram range, but the authors still managed to determine the 235U/238U ratio successfully. Although the results obtained exhibited much poorer precision than those obtained using SIMS and FT-TIMS, they were in good agreement. Agreement was also obtained with certified values when analysing CRMs.
Several other applications have been described during this review period. Two of the interesting ones are by Grate et al.202 who developed extraction chromatographic methods prior to the TIMS determination of Pu isotopes and Raeder et al.203 who reported a three-step excitation and ionization scheme for the ultra-trace determination of 237Np using RIMS. The former of these (ref. 202) described the use of TEVA and DGA resins to separate the analytes of interest from matrix concomitants and/or to pre-concentrate them. Recoveries of Pu from TEVA and DGA were 87 and 86% respectively. Many of the other applications are discussed in Tabular form (Table 3) because, although interesting, they offer less significant advances in knowledge.
| Element | Matrix | Technique; atomization; presentation | Comments | Ref. |
|---|---|---|---|---|
| a L = liquid, S = solid. | ||||
| Am | Solid waste containing uranium, plutonium and silver | OES; ICP; L | A combination of methods was used to separate 241Am from 15 L of waste material. Ion exchange on Dowex 1 × 4 anion resin was used to separate Pu. A 30% tri-butyl phosphate-kerosene extraction was used to remove U. The Am analyte was separated from the rest of the components by fluoride precipitation followed by conversion to the nitrate form. Following all of these procedures, the recovery of the Am was 90% | 204 |
| Ga and In | Plutonium oxide | OES; ICP; L | Plutonium matrix removed using extraction with 30% tri-butyl phosphate in carbon tetrachloride. Quantitative recovery of the analytes was confirmed using radiometric tracer studies. Determination of In and Ga at 303.396 nm and 403.298 nm respectively indicated that residual Pu could still cause spectral interferences – especially when the analytes were at very low concentration. Analytical ranges were 0.1–1000 μg mL−1 and 0.05–500 μg mL−1 for Ga and In respectively | 205 |
| Pu | Synthetic aqueous solutions | OES; ICP; L | Inter-element effects (uranium, thorium, REE and other common metallic elements) on Pu determination studied using a high resolution instrument. Correction factors required to obtain accurate data. Precision of 10–20% RSD at very low concentration of Pu and ∼5% RSD at higher concentration levels | 206 |
| Pu | Nanoparticles downstream of a six stage high efficiency particulate air filter system | MS; ICP; aerosol | Aerosols from a plutonium reprocessing facility were analysed directly and others passed through the filter system and then analysed directly using two different ICP-MS systems. The LOD were 5.0 × 10−2 Bq m−3 and 5.5 × 10−4 Bq m−3. Some plutonium nanoparticles passed through the filter system. These particles were <10 nm and were PuO2 | 207 |
| 241Pu | Nuclear waste slurries | MS; ICP; L | Results obtained using ICP-MS and liquid scintillation counting were compared. Lack of suitable CRM forced the authors to analyse the certified sediments IAEA-300 and IAEA-384. Data from both techniques were in good agreement. Data from CRM analysis were in good agreement with certified values | 208 |
| Pu and U | Uranium and plutonium mixtures | TIMS; — ; | Isotope ratios measured using a continuous heating method TIMS. The probe underwent a ramped temperature programme and so the analytes were vaporized at different temperatures. The 239Pu and 240Pu/239Pu were measured down to 0.1 fg and 0.4 fg respectively. The 235U/238U and 240Pu/239Pu were measured with precisions of 4 and 2% respectively | 209 |
| P and U | Individual Pu and mixed Pu–U mixed oxide particles | TIMS; — ; S | Similar paper to that above | 210 |
| Th and U | Uranium and thorium mixed oxides | EDXRF; — ; S | Studies on the matrix effects of measuring the analytes in samples of their mixed oxides. A comparison of results obtained using rhodium X-ray tube and 109Cd radio-isotope source. Samples were ground, had yttrium internal standard added, mixed, and pelletized using boric acid as binder. Samples rich in Th required more grinding because it is more refractory | 211 |
| Various (19) | Uranium-based fuel materials | OES; ICP; L | Sample (1 g) was dissolved in HNO3 and then diluted to 10 mL. Uranium was then removed using four repeat extractions by 1.1 M di-n-hexyl-octanamide and dodecane. Analytes then determined using ICP-OES. The LOD obtained were sufficiently low to reach required specification limits of 5 ppm for common ions and 1 ppm for critical ions | 212 |
3. Advanced materials
3.1. Polymeric materials and composites
Method validation is required to ensure the quality of the data being produced. The production of certified reference materials (CRM) is therefore very welcome. Ostermann et al.214 discussed the homogeneity and stability of a new material called BAM-H010, which consists of an acrylonitrile–butadiene–styrene terpolymer doped with different contents of Br, Cd, Cr, Hg and Pb. The determination of these analytes in such a material is a requirement under the EU directive 2002/95/EG, which is commonly known as the Restriction of Hazardous Substances (RoHS) and applies to waste materials produced by the electronics industry. Analysis of the material using XRF spectrometry demonstrated that any uncertainty associated with the long-term thermal stability was negligible compared with that from either the method of analysis or the batch inhomogeneity. The material degraded though, forming long and short chain products after exposure to hard X-rays. A similar paper by Lee et al.215 also discussed the stability and homogeneity of reference materials. Various concentrations of Cd and Pb were spiked into epoxy moulding compounds made by mixing silica powders and epoxy resin. The materials were stable over at least a year, with no concentration changes observed. The analytical techniques used for the certification were ICP-OES, LA-ICP-MS and XRF spectrometry.
Another laser-based technique that was used for the study of polymers and composites is laser induced fluorescence (LIF). This technique was used for the multi-elemental analysis of both ceramics and polymers by Chu et al.222 Using an ArF laser the fluorescence signals for Al, Ca, Co, Cr, Cu, Fe, In, Mg, Mn, Na, Pb, Si and Sn were several orders of magnitude higher than the emission signals obtained using LIBS. Fluorescence intensities varied with the fluence and timing of the laser pulse, which was attributed to the presence of particulates within the plume of the ablated material. The authors applied the technique to four different problems. These were: the analysis of paint for Pb content when the concentration of Pb was at the μg g−1 range; the analysis of valuable potteries when two similar specimens were differentiated; the elemental analysis of inks where lines written by two different pens could be discriminated and the analysis of electrode–plastic interfaces. All of the applications were virtually non-destructive with damage being hard to identify even under a microscope. For the last application, the sensitivity was reportedly comparable to that obtained using SIMS.
Two papers described the use of TOF-SIMS for the analysis of surfaces of polymers. One of these papers was by Min et al.224 who studied the deuterium–hydrogen exchange induced by exposure of polyethylene and polypropylene to a low pressure ammonia plasma. Exchange of hydrogen between the polymer surface and the deuterated ammonia plasma was studied and polypropylene was more susceptible to hydrogen exchange than polyethylene. This was because of the side-chain methyl groups. However, similar N-containing fragments were obtained from both sample types. An interesting development was that isotopic effects were observed in the plasma process when comparing ammonia containing H and the deuterated polyethylene results with those from deuterated ammonia and H-containing polyethylene. The other application was by Awaja et al.225 who studied the surface molecular degradation of some high performance polymer composites under simulated low earth orbit environmental conditions. The samples were polymer composites reinforced with carbon fibre, carbon nanotube, nano-clay and 3D-glass and these were examined using the techniques of TOF-SIMS and XPS. The TOF-SIMS data indicated that the extent of decomposition was related to the increase in intensity of O-containing ions whereas the XPS results indicated that an increase in duration of surface treatment led to an increase in O and a decrease in C concentrations. Of all of the samples analysed, the polymer that showed least degradation was the one reinforced using carbon fibre (an O content of 15.6%).
Sample preparation is always an important factor in a successful analysis and several papers have reported methods that have utilized acid digestions, either with the assistance of microwaves or without. A paper by Matoso and Cadore229 used an acid digestion in a closed microwave vessel system as a means of sample destruction prior to the ICP-OES determination of As, Cd, Co, Cr, Cu, Ni, Pb and Sb in polyamide raw materials and textiles used for manufacturing sports T-shirts. The analysis of the raw material (polyamide pellets) was validated using the reference materials ERM-BCR 680 and ERM-BCR 681 with good results reportedly being obtained. Recovery factors were in the range 94.4–105.7 % with the precision typically being 0.5–2.2 % RSD. Once the method had been perfected for the raw materials, it was applied to the textiles. One textile in particular, a black fabric, contained 901 mg kg−1 Cr. The authors tested this material further by undertaking lixiviation tests using synthetic sweat and temperature. However, only 0.3% of the Cr leached into the solution. A comparison of microwave assisted acid digestion and microwave induced combustion in closed quartz vessels as sample preparation techniques for polymer digestion and trace elemental determination using ICP-MS and ICP-OES was reported by Pereira et al.230 Fifteen analytes of interest were determined in sample types such as high and low density polyethylenes, polypropylene, polystyrene, PET (both colourless and green samples), PEEK and nylon 6,6. A mixture of 4 M HNO3 and 4 M HCl was suitable for the microwave induced combustion. Concentrated acids were used for the microwave assisted digestion. The authors concluded that both sample preparation methods were suitable for the analysis but recommended the microwave induced combustion because it used more dilute acids, allowed higher sample mass to be digested and therefore yielded better LOD. The microwave induced combustion technique was also more rapid with eight samples being prepared in 25 minutes compared with more than 50 minutes for the acid digestion protocol. The procedures were validated by the analysis of CRMs and by comparison with data obtained using NAA. A third application that utilized a microwave assisted acid digestion was reported by Lin et al.231 who determined Ag in food packaging materials containing nanoparticles. The microwave digestion protocol was optimized, with several different acid mixtures being tested. A mixture of 6 mL HNO3 and 2 mL H2SO4 was optimal when compared with HNO3 alone (8 mL) or 6 mL HNO3 and 2 mL of H2O2. According to the authors, both ICP-OES and ICP-MS provided adequate results, although the LOD were not impressive (25 μg g−1 for ICP-OES and 0.75 μg g−1 for ICP-MS). This was presumably related to the fact that 0.1 g of sample was digested and then diluted to 50 mL prior to analysis. In addition, spiking of blank sample solutions resulted in recoveries of between 82.53 and 87.6% for ICP-OES and 78.09 and 92.72% for ICP-MS.
Two papers by Becker et al.232,233 reported the use of a prototype storing matter technique instrument for the analysis of polymers. The storing matter technique is where the sputtering of the specimen is decoupled from the subsequent analysis step. The sample is sputtered by an ion beam and the emitted particles are deposited at the sub-monolayer level on a dedicated collector. The second step consists of the monolayer of deposited sample being analysed using SIMS in either the static or dynamic mode. The nature of the collector is chosen to enhance the subsequent SIMS analysis. In the first of the applications,232 the collector surface was silver and the samples were PVC, polystyrene and polymethyl methacrylate. The prototype instrument enabled the secondary ions to have higher intensities compared with conventional static SIMS, especially in the higher mass range. The technique enabled a clear identification of the different polymers. In the second of the applications,233 PVC was collected on both gold and silver materials. The analysis using gold was not successful, since identification of the polymer was impossible. This was attributed to the gold having a higher degree of organic contamination than the silver collector.
3.2. Semiconductors, superconductors and electronic materials
As has been the trend over the last few years, the number of papers published in this area of research was very high, especially for the depth-profile analysis of materials and for the analysis of thin films. This particular topic will be discussed in later Sections (3.2.4) and it is here that any fundamental papers describing improvements in resolution will be commented upon. These first sections will concentrate on the analysis of electronic components and other such materials; although mention may also be made of more routine depth-profiling techniques.Dill and Rossiger237 compared the performance of XRF spectrometry with different detector types (proportional counter, positive intrinsic negative and silicon drift) for the determination of Au and Pd thickness measurements on printed circuit boards. The aim was to provide a reference material for such samples. Other techniques, e.g. RBS and gravimetric analysis, were also used for the certification. The instruments with the proportional counter detectors were not very suitable for the measurement of thin films because of their poor energy resolution. However, the systems with the semiconductor detectors provided results that were far more reliable and accurate. The composition of the base material had to be taken into account and so a variety of background correction systems were compared. A global base subtraction was performed and this led to better repeatability, but could also cause incorrect absolute values. When using the semiconductor-based detectors, the use of polycapillary optics during the analysis of small (∼150 μm) spots improved the sensitivity significantly.
Nanoscale defects in large area solar cells lead to decreased device efficiency and increased cost. However, the non-destructive analysis of such defects is problematic because most techniques are designed to measure just the surface layers and struggle to analyze material below the surface with sub μm resolution. Therefore, Bertoni et al.238 used synchrotron-based nanoprobe XRF characterization of a recombination-active intragranular defect and managed to improve resolution to approximately 80 nm. Their technique also managed to make a clear distinction between benign and deleterious dislocations in the solar cell; recombination-active dislocations being observed to contain a high level of nanoscale Fe and Cu whereas the recombination-inactive dislocations appeared clean. The high resolution measurements would facilitate optimization of the industrial solar cell production process.
The last paper to be reviewed in this section used LIBS for the analysis of the insulating materials surrounding high voltage electrical cables.241 The presence of impurities, e.g., Mg and Na in the insulating material means that when the cables are buried underground, the impurities react with water forming chlorides. This can form a “water tree” that can short-circuit the metallic conductor and the earth. In this work, LIBS employing a Nd:YAG laser operating at 1064 nm, was used to determine these impurities. The instrument used comprised four spectrometers covering the visible and near UV regions of the spectrum and an intensified CCD detector. The results were comparable to those obtained using conventional ICP-OES. The maximum concentrations found in the cable ranged from 1.6 mg kg−1 (for Cr) to 148.4 mg kg−1 (for Fe).
3.2.3.1. Reviews, comparisons and inter-laboratory comparisons. ‘The present and future of GD-TOF-MS in analytical chemistry’ is the title of an overview with 109 references presented by Pereiro et al.216 The instrumental developments in recent years and the applications that are now achievable using the instrumentation were discussed; these included the multi-elemental, high resolution depth-profiling of layered structures. Other areas of the review highlighted the identification of polymers and the analysis of gases. Wilke et al.242 gave an overview (containing 48 references) of GD-OES for the accurate and well resolved analysis of coatings and thin films. After a brief overview of the fundamental aspects of the technique, the review went on to discuss the new developments in instrumental design and applications of the determination of very light elements (e.g., C, H, N and O), depth-profiling and the analysis of multi-layered systems.
The other paper to be discussed in the section was by Abou-Ras et al.243 who reported a comprehensive comparison of various techniques (electron microscopic techniques, SIMS, TOF-SIMS, SNMS, GD-OES, GD-MS, Auger emission spectrometry, RBS, Raman depth-profiling, ERDA and GI-XRD) for the analysis of elemental distribution in copper indium gallium (di)selenide thin films. Although the techniques were compared in terms of their spatial and depth resolution, measuring speed, availability and LOD, the authors did stress that each of the techniques had their own advantages and disadvantages, for instance, that some were ion sputtering techniques, i.e., that they left craters in the surface and these craters had roughened edges which can affect the measurement significantly. In such cases, it is necessary to adjust for these by rotating or cooling the sample or by processing the data post-analysis. Since the sputtering process and extent may differ between sample types, these techniques often require careful optimization to obtain credible data. The analytical capabilities were summarized in tabular form and the table also specified whether the technique was capable of depth-profiling or was purely a surface analysis method and whether or not it required standards for calibration purposes. Other techniques that were not included in the comparison (e.g., GI-XRF, cathodoluminescence, atom probe tomography and electron backscatter diffraction) were also discussed in terms of their relative advantages and disadvantages. The authors stressed that of all the techniques they used for the comparison, there was not one that could fulfil all requirements and that a combination of at least two was required to obtain all the relevant information.
3.2.3.2. Laser-based techniques. Reviews in previous years have commented on how LA can be used to “drill” through the thin layers by repeat firing onto the same spot, with detection using ICP-MS or ICP-OES. This year is somewhat different with the papers of interest being LIBS-based. Mercadier et al.244 used LIBS for the depth-profile analysis of samples made of tungsten–molybdenum or tungsten–carbon layers on titanium substrates. Laser pulses produced by a Nd:YAG laser at several different wavelengths (1064, 532, 355 and 266 nm) were used for the ablation. Also tested, were the effects of laser beam shaping and fluence as well as both the nature and pressure of the gas mixture inside the sample cell (with air, argon and helium all being used). The results of the depth-profiling were comparable to those obtained using GD-OES. However, it was noted that the LIBS technique led to a mixing of the layers and this was attributed to diffusion through the melted material and to non-homogeneity of the laser beam spatial distribution. The ablation depth increased with increasing laser wavelength. Similarly, the LIBS signal and the ablation depth also increased with laser fluence. A compromise had to be made, therefore, between increased depth resolution and sufficient LIBS signal. Argon was the most suitable surrounding gas since it limited signal fluctuations and enhanced the signal to noise ratio. Overall, the most accurate data were achieved using a wavelength of 355 nm, a fluence of 18 J cm−2 and argon at a pressure of 1 atmosphere. The other paper of interest in this section was prepared by Yuan et al.245 who used LIBS to determine the film thickness and elemental content of an aluminium zinc oxide film. Several Al and Zn atom lines were monitored to study the influence of the delay time, pulse energy, concentration of the elements and film thickness on signal intensity. A Q-switched Nd:YAG laser operating at 1064 nm and with a pulse duration of 10 ns was used to irradiate the sample in an air environment at atmospheric pressure. Light emitted from the ablated sample was detected through a bundle of seven optical fibres using a linear CCD array detector. The pulse energy governed the temperature of the excited plasma and the intensity of the emission lines.
3.2.3.3. Glow discharge techniques. A review of GD-TOF-MS was presented by Pereiro et al.216 This review was described previously, in Section 3.2.3.1. Other applications of GD-TOF-MS have also been published. Pisonero et al.246 described the use of a pulsed rf GD-TOF-MS instrument to obtain information regarding the presence of minor elements and to evaluate segregation/diffusion processes at the interfaces of a series of ultra-thin Nb/Al(1−x)Cox (where x = 0, 0.015, 0.035, 0.045, 0.09 or 0.35) layers deposited on silicon wafers by DC magnetron sputtering. A rf generator operating at 13.56 MHz in pulse mode (0.8 ms pulse width, 3 ms period and 27% duty cycle), with a constant power (45 W) and pressure (650 Pa) was used for the analysis. The instrument was used in dual beam mode, i.e., the erosion and analysis beams were different (C60+ ions at 10 keV at an incidence angle of 45° and Bi+ primary beam at 25 keV respectively). The data indicated that there was segregation of the Al and Co and that there was diffusion of the Co into the silicon substrate; the extent of which was dependent on the Co concentration in the AlCo layer. Results obtained for the depth-profiling were compared with those obtained using TOF-SIMS. In general, the results were in good agreement, although the TOF-SIMS had better depth resolution. The GD-TOF-MS was faster, had lower cost and did not require the ultra-high vacuum conditions required for SIMS.
A new μs pulsed GD assembly using a fast flow high power source that was capable of time resolved measurements and of thin film analysis was described by Churchill et al.247 The impact of pulse width (over the range 12–75 μs), pulse voltage (600–1000 V) and argon flow rate (175–700 standard cm3 min−1) on the glow current, matrix ion and argon ion intensities were evaluated. Also discussed were the effects these parameters had on the flight time of Cu ions from the target material (a copper indium gallium (di)selenide thin layer) and the Penning ionization process. A description of the fast flow source and of the pulse counting detector dead time and correction and automatic detector switching issues was also provided. The intensity of ions produced from the sample were directly proportional to the pulse voltage (as were the ions generated from the plasma itself), however, other ions of secondary origin, e.g., 40Ar2+, 40Ar1H+etc., had no correlation. Very low carrier gas flow rates led to incomplete flushing of the sample plume to the detector before the next pulse arrived. The current of the GD increased from 3 to 20 mA as the gas flow rate increased from 175 to 700 standard cm3 min−1. Over the same flow range, the intensity for the 65Cu+ first increased to a maximum at approximately 475 standard cm3 min−1 and then decreased again. The new assembly enabled depth-profile measurements of layers of sub-micron thickness to be achieved with improved resolution and increased signal to background ratios. A paper by Sanchez et al.248 used similar instrumentation for the rapid and highly sensitive characterization of layered photovoltaic devices. The presence of H during the analysis produces the “hydrogen effect”, i.e., it can affect the ion signal intensities and sputtering rates significantly. The authors compared two modes of H addition (exogenous H added as 0.2% H2 in argon as the discharge gas and endogenous H sputtered as a sample constituent) and their effects on the spectral interferences and depth resolution. Non-hydrogenated material (containing B, P and Si) and three types of Si:H thin films were used for the comparison. The exogenous H produced a significant influence on the pulse profiles of the analytes whereas the affects from the endogenous H were less marked. Judicious selection of the after-peak region was required to obtain optimum mass spectra that had high sensitivity and were free from interferences.
Several applications of GD-OES have also been reported. An overview paper of GD-OES by Wilke et al.242 was described previously in Section 3.2.3.1 and will not be discussed further here. A paper similar in design to the one described above (ref. 248) that discussed the effects of endogenous and exogenous hydrogen during the analysis of thin films was presented by Sanchez et al.249 This application discussed the use of GD-OES as a means of detection rather than GD-TOF-MS. Using a pressure of 600 Pa and a power of 50 W, the other operating conditions were optimized for the analysis of both a conducting material and a silicon wafer. A 10 kHz and 25% duty cycle was optimal. Enhanced emission signals were observed for most analytes in the presence of hydrogen (especially for Si), even though the conditions used reduced the sputtering rate. A paper by Gamez et al.250 highlighted how GD-OES has historically been regarded as being poor for laterally resolved analysis, i.e., it cannot easily be used to analyse the surface distribution of analytes. These authors described a method entitled “Push-broom hyperspectral imaging” that could maintain depth resolution measurements below 10 nm, but could also undertake lateral measurements. The system comprised a 140 mm focal length spectrograph equipped with a 1200 grooves mm−1 grating blazed at 250 nm, an intensified CCD detector containing 1024 × 1024 pixels and collection optics that consisted of two triplet lenses to minimize chromatic aberrations and a broadband metallic mirror with reflectivity >90% over the wavelength range 200–650 nm. The system developed enabled a much higher light throughput enabling faster acquisition times. However, it did have the disadvantage of suffering from distortions, some of which could be significant. These could largely be overcome by decreasing the aperture and/or implementing image correction routines.
3.2.3.4. Thin films and depth profiling – X-ray-based techniques. A large number of different X-ray based techniques are available to analysts specialising in the analysis of thin films and layered samples. It should be emphasized that many of the applications in this area report new and exciting materials but many of the techniques used to characterize them can be regarded as relatively routine and so will not be discussed further in this review.
The use of GI-XRF is still attracting substantial attention because of its improved resolution. There was, however, less groundbreaking research published during this review period. Unterumsberger et al.251 used the complementary techniques of conventional XRF and GI-XRF for the characterization of buried boron-carbon layers with nominal thicknesses of 1, 3 and 5 nm. The techniques were compared with respect to quantification reliability and elemental sensitivity. Although the sensitivity and resolution were different, the techniques showed good agreement when the layers were characterized. Another paper to have used GI-XRF was presented by Torres et al.252 who also used XANES to investigate the electronic structure of zinc iron oxide thin films. During this study, the non-equilibrium cation site occupancy as a function of depth and oxygen pressure during deposition was monitored. Low deposition pressures (<10−3 mbar) led to Fe super-occupation of tetrahedral sites without Zn2+ inversion. This resulted in an ordered magnetic phase with high room temperature magnetic moment.
Other grazing incidence techniques have also been described. These included GI-SAXS spectrometry which was used in conjunction with XRF by Devloo-Casier et al.253 to study the initial growth of hafnium dioxide thin films formed using atomic layer deposition with tetrakis(ethylmethylamino)hafnium and water on both oxidized and H-terminated silicon and germanium surfaces. The XRF spectrometry was used to quantify the amount of material deposited during each atomic layer deposition cycle and the GI-SAXS was used to monitor the film roughness. The XRF data showed that there was no inhibition period observed during film preparation when oxidized substrates were used, but that the H-terminated materials did exhibit one. A correlation between the inhibition period and the onset of surface roughness was identified. Grazing incidence X-ray reflectivity along with angular dependent XRF was used by Arac et al.254 to analyse the structural and compositional changes in nickel iron/gold bilayers induced by irradiation with a focussed 30 keV Ga+ ion beam. A low dose (<15.6 × 1014 ions cm−2) had an influence on the interface of the two layers whereas at high dose sputtering of the surface and ion implantation was observed. In both cases, inter-mixing of the layers was observed. The authors confirmed the usefulness of the combined use of the techniques to study layer inter-mixing. High resolution grazing exit XRF using SR excitation and a high resolution detector was reported by Kayser et al.255 who described the depth-profile measurements of Al-implanted silicon wafers. The advantages of the technique were discussed and results indicated that results with nm scale precision could be achieved.
An interesting paper by Yoshida and Sato256 described a new experimental method of XPS that was capable of mapping core-energy levels as a function of depth from the surface of the film. Layers (of thickness 5, 10 and 15 nm) of the organic semiconductor bathocuproine were deposited on polycrystalline metal surfaces at a rate of 1 nm min−1. The films then underwent XPS analysis using a magnesium K-α X-ray source and a detection angle of between 5 and 65°. By changing the detection angle, a series of XPS spectra could be obtained as a function of depth. The spectra at each depth were then analysed using the mathematical algorithm of Taylor expansion and the coefficients required for the core energy levels were determined using target factor analysis. The paper described the mathematics behind the analysis and further details were provided in an appendix. The overall result was that profiling of the interface between the semiconductor and the metal surface was achieved. The authors concluded that the technique could provide information on the electronic structures that existing methods have never accessed.
There are two methods of depth-profile analysis available when using standard XRF instrumentation. One is the fundamental parameter method that uses multiple angle measurements and a mathematical algorithm to help calculate the concentrations at each depth. The other method is confocal XRF which uses an X-ray lens to excite the sample at a specific depth and a second lens to focus emitted X-rays to the detector. A paper by Gherase and Fleming257 discussed a third option in which a portable XRF spectrometer was used with a layered calibration method. The approach required two assumptions to be made: X-ray attenuation properties of the unknown sample can be reproduced reliably by a layered phantom and that the elemental concentration values sampled at equally spaced depths can be related by a mathematical function. A single XRF measurement of the sample was required and the concentration present at each of the layers could be estimated using this one measurement, the slope of the calibration lines and an a priori knowledge of the concentration behaviour as a function of depth. As well as presenting the theory behind their approach, the authors also validated the protocol using 10 polyester disks (of 0.5 mm thickness and 35 mm diameter), some of which contained As at concentrations of 5, 10, 20, 30 and 40 μg g−1, whereas others contained no As. Four disks were then stacked together for analytical measurements. The approach was reported to work satisfactorily, but more accurate results were obtained using higher As concentrations. The advantages were that it circumvented the necessity for precise alignment of components as needed by the confocal method and also that it did not require the variable geometries required by the fundamental parameter method. In addition, only one measurement was required and so the exposure to ionizing radiation was minimal. The disadvantage, as mentioned previously, is that it required an a priori knowledge of the sample. The authors suggested that the approach could be of particular use for the determination of As and Se in skin samples, although this work has not yet been undertaken.
Another paper to rely on mathematical algorithms was prepared by Luhl et al.258 who combined the use of 3D μ-XRF and conventional XAFS to enable depth-profiled chemical speciation data to be obtained. The exciting X-ray beam was focussed onto the stratified sample (layers of copper foil, laquered copper(I) oxide and copper(II) oxide) using a polycapillary half lens, and the emitted X-rays were focussed using a second polycapillary half lens to the detector. The measurements were collected in fluorescence mode and this led to distortions of the spectra caused by absorption effects. A reliable algorithm was therefore constructed to calculate the attenuation coefficients of the analyte in order to overcome this distortion. The result was a new spectroscopic tool for three-dimensionally resolved, non-destructive chemical speciation.
Several other papers were reported that used different X-ray methods. This included one by Mainz and Klenk259 who used synchrotron XRF and EDXRD simultaneously to investigate the synthesis of copper indium sulfide thin films using the copper–indium precursors and then annealing in a sulfur-rich atmosphere. The EDXRD monitored the time evolution of the phases whereas the synchrotron XRF spectrometry measured the relative concentrations of the analytes in each phase. Another example is a paper presented by Vrielink et al.260 who compared the usefulness of XRF depth-profiling with profilometry and with images taken using SEM. In general, film thickness results obtained using the different techniques were in good agreement but the XRF did not suffer the drawbacks of being affected by different thicknesses or hardness of the different metal layers. It also enabled discrimination between different sub-layers in multi-layered films, density estimations to be made and, in addition, it enabled compositional analysis.
3.2.3.5. SIMS and TOF-SIMS applications. It is now well established that the use of low energy primary beams during SIMS analysis enables better depth resolution to be achieved and minimizes the damage inflicted on the sample. Any roughness caused by the bombardment leads to the degradation of the depth resolution. Mansilla et al.261 have addressed this problem by studying the effects of a 1 keV Cs+ ion bombardment on the surface of (1 0 0) silicon. After bombardment, the surface was examined using AFM. The formation of ripples perpendicular to the direction of the ion beam was observed and their wavelength and the roughness caused were evaluated as a function of different ion flux (over the range 1 × 1014 to 1 × 1015 ions per cm2) and fluence at a constant flux of 1.0 × 1014 (7 × 1014 to 1.5 × 1018 ions per cm2, corresponding to sputtering times of between 7 s and 7500 s). Both the roughness and the wavelength of the ripples increased with increasing ion fluence, whereas the ion flux showed little effect. Sample rotation had a profound effect on the formation of ripples, which ceased to be formed, and surface roughness, which decreased significantly.
The storing matter technique was developed recently as a means of overcoming matrix effects that can be encountered during SIMS analyses. After bombardment with a focussed ion beam, the emitted material is collected as a monolayer on a virgin, well-characterized collector material. Since the collector constantly rotates, different layers of original sample will be collected on different areas of the collector. Analysis of the material on the collector using SIMS is then undertaken and, since the collector material is well-characterized, it gives a constant background signal which may easily be subtracted from that obtained from the different layers of the sample material. Mansilla and Wirtz262 described the use of this technique for the characterization of boron-doped and boron-implanted silica/silicon. The materials were first sputtered onto the collector (a polished germanium wafer) using an argon ion beam (at 10 keV and with an ion flux of 1014 ions per cm2 s). Low energy O2+ (at 1 keV and at an angle of 61°) was then used as the primary ion beam during SIMS analysis of the material on the collector. Results obtained were compared with those obtained using standard SIMS (under the operating conditions of an O2+ primary ion beam at an energy of 3 keV and at an angle of 63°). The standard SIMS analysis suffered from interferences (especially at the near-surface layers), whereas the storing matter-SIMS analysis did not. The concentration of 11B at a depth of 130 nm was 40% using traditional SIMS whereas the storing matter technique of SIMS found a concentration of 37% at a depth of 131 nm. The FWHM value was also very different for the storage matter technique, being 170 nm compared with the 140 nm for traditional SIMS.
Demenev et al.263 described the combined use of ultra-low energy SIMS and MEIS for the analysis of arsenic shallow implants in silicon. The depth resolution of MEIS (<1 nm) is far superior to that obtained using SIMS, but it suffers from relatively poor sensitivity. The SIMS analysis was achieved using a Cs+ primary ion beam of 300 eV and at an angle of 45° with the negative ions 18O−, 30Si− and 28Si75As− being detected. The MEIS operating conditions were obtained using a 100 keV He+ ion beam in double alignment configuration. The data obtained using SIMS could be aligned with the high resolution data obtained using MEIS providing a powerful approach to the calibration correction of SIMS profiles.
The method of extended full spectrum TOF-SIMS was described in two papers by Py et al.264,265 Both SIMS and TOF-SIMS can suffer from non-linearity of relative sensitivity factors when profiling through films with various compositions and this can especially be the case when the material under investigation is silicon germanide layers on silica. The use of the extended full spectrum method helps minimize these interferences. One study (ref. 264) described its use during the quantitative depth-profiling of both matrix elements (Ge, O and Si) and of contaminants (C) in Si(0.82)Ge(0.16)C(0.02) materials annealed in an oxidizing atmosphere. The accuracy and precision of the data obtained using the method were compared with those obtained using another technique (conventional SIMS) and were both superior, even under the optimal SIMS operating conditions. Depth resolution and signal to noise ratio were also improved. The improvements in performance were most marked at the interface regions, which enabled better understanding of the behaviour of the material under oxidizing anneal conditions. This, in turn would allow the fabrication of well-controlled materials.
The surface sensitivity of TOF-SIMS using different primary ions (Binq+ (where n = 1 or 3 and q = 1 or 2) or C60q+ (where q = 1 or 2)) in organic films of molecular trehalose or polymeric tetraglyme was explored by Muramoto et al.266 The impact crater depth, implantation depth and molecular escape depth were all monitored. Using a dose of 1 × 1012 ions per cm2, the primary ion Bi+ at 25 keV was the most surface sensitive, producing a molecular escape depth of 1.8 nm for the protein films. However, they also had the deepest implantation depth (18 and 26 nm for trehalose and tetraglyme respectively). Primary ions of C602+ was the second most surface sensitive, providing a molecular escape depth of 2.3 nm. The most important factor governing the surface sensitivity was the impact crater depth (the amount of surface erosion). The most sensitive primary ions created craters with depths of 0.3–1 nm whereas other primary ions that created craters of 1.8 nm depth provided a molecular escape depth of 4.7 nm.
3.3. Glasses
The development of CRMs is necessary whatever the sample type to ensure that valid data are being produced. This review period has seen a number of papers re-visiting the analysis of CRMs in an attempt to either re-certify them or to ensure that the data obtained in the first instance were correct. A series of CRMs (four materials labelled A, B, C and D) that have been designed to simulate archaeological glasses have been re-analysed by Wagner et al.271 who used ICP-MS to analyse the materials with sample introduction using either a nanosecond pulse duration LA system (at 193 or 266 nm) or a femtosecond system (at 800 nm). The concentrations of 26 analytes were determined and the data obtained were compared with values found in the literature. Numerous discrepancies were found, the largest of which was for Cr2O3 in material A, Li2O, B2O3 and Cr2O3 in material B and MnO, Sb2O5, Cr2O3 and Bi2O3 in material C. In general, the data obtained during the analysis of material D were in closest agreement with certified values, but even then, significant deviances were observed. A similar paper, by Jochum et al.,272 re-analysed the NIST materials SRM 610 to SRM 617. The analysis of the materials was undertaken according to the ISO guidelines and the protocol recommended by the International Association of Geoanalysts. Data quality was checked by the application of the Horwitz function and by careful investigation of the analytical procedures. Elemental inhomogeneities were identified when different test portion masses (1, 0.1 and 0.02 μg) were analysed. These included moderate inhomogeneities of several chalcophile/siderophile elements and gross inhomogeneities of Ni, Pd, Pt and Se at small test portion masses. In general, the results were in agreement with certified values (with the exception of Mn in NIST SRM 610).
Laser ablation was the most commonly used of the laser-based techniques in this review period. Konz et al.275 used LA as a means of sample introduction to a double focussing, SF ICP-MS instrument in an attempt to identify local defects in glasses coated with metallic and oxide films in the low nm range. The low LOD and high mass-resolving power of the instrument enabled the analytes P, S and Cl to be determined; which are analytes that, under normal circumstances, suffer from severe polyatomic interferences (from 15N16P+, 16O2+etc.). The data was sufficiently accurate to enable the authors to identify three different types of defect, which were characterized by the impurities identified. This enabled the authors to identify, unambiguously, the contamination source that produced the defects. A paper by Weis et al.276 described the use of LA-ICP-MS to prove (or disprove) the common origin of two or more sources of glass. This forensic application was optimized to give very low rates of both false negative errors and false positive errors. Two sets of float glass (fragments from the same pane in one set and fragments originating from different float glass of global origin in the other) were analysed for 18 analytes and the analytical data tested using different statistical methods. The best match was achieved using a modified four sigma match criterion based on fixed RSD or, using t-tests modified analogously by applying the same fixed RSD. The overall protocol was reported to have been used successfully in over 70 court cases. High spatial resolution trace elemental analysis, using LA-ICP-MS with an ablation cell designed for multiple or large samples, was reported by Fricker et al.277 The dimensions of the cell were 230 × 34 × 16 mm. The performance of the cell was tested using the CRMs NIST SRM 610 glass and JK-2D steel. To obtain high spatial resolution, the cell must enable a rapid removal of the aerosol to ensure that particles produced from different laser pulses of material from different locations do not mix. The gas flow patterns within the cell were modelled using computational fluid dynamics. Therefore, despite the increased size of the cell, rapid removal of the aerosol was achieved, with a washout time of 2.6 s being required to remove 99.9% of the signal. This represented a 70% improvement on the cell used previously by these workers.
It has been known for some time that the presence of carbon in the plasma increases the sensitivity of some analytes during ICP-MS determinations. A paper by Fliegel et al.278 described how this was extended to the use of carbon in the form of methane–argon or methanol–water, to the LA-ICP-MS determination of several analytes. Addition of methane at a rate of between 0.6 and 1.4 mL min−1 during the ablation of NIST SRM 610 glass led to a sensitivity increase by at least a factor of two, but for select elements (As, I, Se and Te) the increase was by an order of magnitude. Introduction of the carbon in the form of a methanol/water mixture through the cooled spray chamber of the instrument with the simultaneous introduction of the laser ablated material led to sensitivity increases of up to 20 fold. The buildup of carbon on the interface cones which can lead to signal drift and inaccuracy was avoided by use of a fast cleaning procedure.
An interesting paper by Douglas et al.279 described the new technique of LASIL. In this technique a micro-droplet (25 μL) is dispensed onto the surface of a material and the laser ablates the material into the droplet. The particulate matter within the droplet can then be analysed either on-line directly or off-line using aqueous calibration standards. The authors described their technique as robust and easy to implement. It also enabled containment of the ablated particles, eased the generation of suspended solids in solution from insoluble materials and allowed easy control over the dissolution and dilution to obtain measurable concentrations of analyte. The authors demonstrated the use of their technique using the CRM NIST SRM 610. The particle size distribution and the shapes of the particles produced were investigated and were found to vary with laser fluence; with higher fluence producing wider particle size distributions with particles also being irregular in shape. Several types of particle were found. These included jagged particles of 1–2 μm in diameter (which the authors stated were produced by micro-jet impingement), spherical nanometer-sized particles (from vapour condensation melt ejection) and thin, string-like particles produced by particle agglomeration or liquid jet fragmentation. Lower laser fluences tended to lead to the spherical particulates being formed. The small size of these particles (typically 250 nm) ensured that the particulates had a very slow settling rate once in the liquid. This enabled the samples to be stable, even over a time period of several days.
| Element | Matrix | Technique; atomization; presentation | Comments | Ref. |
|---|---|---|---|---|
| a L = liquid, S = solid. | ||||
| Mn | Glass from stained glass windows | XANES; — ; S, XRF; — ; S | In stained glass, Mn should be present in the (II) or (III) oxidation state. With age, sometimes it reaches the (IV) state and changes to a brown/black colour. During restoration, hydroxylamine hydrochloride is used to reduce the Mn back to the (II) or (III) state. The analytical techniques were used to investigate the effectiveness and kinetics of the reaction. Although effective, the reducing agent caused some unwanted effects. The analytical findings enabled optimization of the restoration process | 280 |
| Pb and Sb | Ancient and modern opaque glass | XANES; — ; S | A synchrotron μ-XANES instrument, along with TEM, SEM and μ-Raman was used to study lead antimonite based opacifiers in glasses with age spanning from the second century BC through to 18th century AD. High spatial and high energy resolution techniques were used to investigate the materials. Information on the raw materials used and the technological processes employed to produce the opacifiers was obtained | 281 |
| Various | Historical glass and ceramic objects | MS; ICP; LA | Home made open ablation cells of volume 60, 9 and 4.5 cm3 were used. The open cells enabled direct analysis of the materials whilst increasing the sample aerosol washout by a factor of three when compared with a standard cell | 282 |
| Various | Two glass artefacts, possible fragments of conglomerate glass bowls | MS; ICP; LA | Rastering over the surface of the samples enabled assorted metal oxide maps to be determined. Quantification was by the sum normalization procedure in which the concentrations of 54 elements (as their oxides) at each rastering point were summed to 100% without use of an internal standard. NIST 610 was used to validate the procedure | 283 |
| Various | 15 blue, turquoise and dark green beads dating from 7th and 8th century BC from excavations in Bologna. | MS; ICP; LA, EPMA | Major and minor elements determined using EPMA, trace elements determined using LA-ICP-MS. Chemical data identified several different classes of beads | 284 |
| Various | Gold-enclosed tesserae of palaeo-Christian glass mosaic in Padova | MS; ICP; LA, EPMA | The aim of the study was to identify the base composition of the glass and to reconstruct the production techniques. Most tesserae were natrion-based. Some soda ash tesserae were identified as medieval restorations. The extent of re-cycling used for these 6th century AD samples was also determined | 285 |
| Various | Islamic glass specimens of Sasanian production | μ-XRF; — ; S | A commercial μ-XRF instrument was used for the analysis. Results were compared with those obtained previously and indicated that μ-XRF was better than EPMA but inferior to LA-ICP-MS in terms of sensitivity. The XRF technique gave a bias from the expected values of better than 5% for the major analytes, approximately 5% if the analytes were at a concentration higher than 100 ppm and about 10% if they were <100 ppm | 286 |
| Various | 31 glass samples from Pergamon, Turkey dating from 4th to 14th century | MS; ICP; LA, EPMA | Chemical analysis and statistical methods identified at least three different glass production technologies. A new glass production group characterized by mineral soda and high alumina was identified. The author hypothesized that the base soda used came from a different source to the traditional Egyptian soda and that this was indicative of regional Byzantine primary glass production in Asia Minor | 287 |
| Various | Tesserae from San Marco Basilica, Venice | PIGE, PIXE, Portable XRF, XPS | Non-destructive techniques used for analysis. Green tesserae and the “cartellina” (both have low chromophore amounts) were difficult to characterize using portable instrumentation. Red tesserae were easier to analyse and, although similar under a microscope, had differing composition | 288 |
| Various | Ancient glass samples from Xinjiang, Guangxi and Jiangsu provinces, China | Portable XRF; — ; S | The influence of the surface conditions on the portable XRF analysis of glasses was examined by comparing a weathered surface with the inside of ancient glass samples. Also studied were the effects of distance between the sample and the reference plane and the curved shape of the sample. Three calibration methods were compared with the normalizing method proving to be best | 289 |
| Various | Ancient glass vessel fragments from Xinjiang, China | XRF; — ; S | An EDXRF instrument was used to analyse 58 fragments of glass dating from 202 BC through to 1368 AD. Outer, weathered surface and inner, fresh surfaces were compared. Three classes of glass were identified using cluster analysis. Each type was prepared using a different recipe. Provenance and the techniques used to prepare the samples were discussed | 290 |
| Various | 81 samples from 68 glass beads from South western Poland | MS; ICP; LA, EPMA | Most analyses undertaken using EPMA, but LA-ICP-MS used to validate results in 18 samples. Several different classes of glass were observed | 291 |
| Various (32) | Iron age glass beads from North East Scotland | MS; ICP; LA | Beads had good homogeneity of major and minor elemental composition. Imported natron glass formulation indicated the glass was typical of Roman origin | 292 |
| Various | Mycenaean glass | XRF; — ; S | Portable XRF instrument used to analyse glass beads in an attempt to identify the technology used for manufacture and their source. For blue beads coloured using Co and one coloured using Cu were analysed. The Ti and Zr compositions were consistent with the glass originating in Egypt. The Ti and Zr in other beads indicated that their origin was Mesopotamia. The indications were that the glass was imported and then worked locally | 293 |
3.4. Ceramics and refractories
Several applications have been described that have used lasers as a means of solid sampling. A paper by Wagner and Jedral282 reported the use of LA-ICP-MS with an open ablation cell for the analysis of historic objects. Cell volumes of 60, 9 and 4.5 cm3 were tested and the open cells had aerosol rinse-out times that were shorter by a factor of three compared with standard cells. The systems were tested on both glass and ceramic objects, with validation achieved by using the NIST SRMs 610 glass and SRM 679 brick clay in the form of a pressed pellet. The technique of LA-ICP-MS was also used by Zaoralkova et al.294 (along with LA-ICP-OES and EPMA) to investigate multi-layered silicate ceramics. Single spot laser drilling using a Nd; YAG laser operating at 1064 nm gave a spot diameter of 1 mm. The detection system used for this was ICP-OES. Transverse scans across the surface using an ArF laser (operating at 193 nm) and with detection using ICP-MS were also performed. The results for LA-ICP-OES and LA-ICP-MS were verified using EPMA and two dimensional back-scattered electron images. The LA-ICP-OES system was less dependent on sample surface and layer irregularities, but the small laser spot size produced during the LA-ICP-MS enabled an insight into the micro-morphology of the individual layer.
Several authors have reported the use of LIBS for ceramics analysis. Cowpe et al.295 discussed the use of LIBS for the determination of hardness of bio-ceramics. The relationship between the sample's hardness and the LIBS plasma properties were investigated. Measuring the plasma excitation temperature, by monitoring the line to continuum ratio of the Si 288.16 nm line, enabled a linear relationship between hardness and plasma temperature to be noted. The results were compared with conventional Vickers hardness measurements and offered greater reproducibility than the standard method. A portable LIBS instrument was designed and constructed by ElSayed et al.296 and then applied to the analysis of archaeological pottery samples. A passive, Q-switched Nd:YAG laser operating at 1064 nm was used with a pulse width of 20–30 ns and with between one and 6 pulses per laser shot. Total output energy of the system was 170 mJ and the entire duration of the output pulse train was less than 300 μs. The multi-pulse nature of the laser shots enhanced the LIBS signal. Another paper297 described the use of collinear double pulse LIBS for the trace element determination (Al, B, Ca, Cr, Cu, K, Mn, Ni, Si and Ti) in sintered iron oxide ceramics. A Nd:YAG laser operating at 532 nm and with a 6 ns pulse duration was used to ablate the material and to form the plasma, whilst the spectra produced by the samples were recorded using an Echelle spectrometer equipped with an intensified CCD detector. Atomic and ionic emission intensities were measured for different inter-pulse delay times (between 100 ns and 50 μs) and gate delays after the second pulse. Similarly, the energy partition between the two pulses was also varied and the effects on signal intensity observed. The LIBS signals obtained using double pulse LIBS were higher than for the single pulse mode, but the extent of improvement was dependent on the inter-pulse delay time, the energy partition between the pulses and the spot size. Detection limits below 10 ppm were obtained for most of the analytes.
Chu et al.222 described the use of an ArF laser for the multi-elemental analysis of ceramics (and polymers) using the technique of laser induced fluorescence (LIF). This paper was discussed in detail at the end of Section 3.1.3 and so the reader is referred to there for further information.
Several other applications that were not laser-based have also been reported. Amberger et al.298 reported the direct determination of trace elements in boron carbide powders using direct current arc atomic emission spectrometry. Twelve analytes were determined in three well-characterized materials, with LOD between 0.2 μg g−1 (for Mg) and 25 μg g−1 (for Na) being obtained. Precision for nine replicate measurements of 5 ± 0.3 mg of material varied from 6.2 to 27% RSD (for Al and Cu respectively). Analysis of the CRM ERM (R)-ED102 yielded results within 89 and 116% of the accepted values. Other sample types analysed included aluminium oxide, boron nitride, silicon carbide, coal fly ash, graphite and obsidian rock.
A solid sampling method using ETAAS detection was reported by de Mattos et al.299 who determined the analytes Cr, Cu, Fe, K, Mn, Sb and Zn in aluminium nitride powders. The instrument used featured an inverse-Zeeman effect background correction system that had a variable magnetic field which enabled measurements in two sensitivity modes. This led to a linear range spanning three orders of magnitude being obtained. The measurement sensitivity could be altered depending on the concentration of analyte present in the sample. Calibration was achieved using aqueous standards and no matrix modifier was necessary. Method validation was obtained by comparison of data with those obtained using instrumental and radiological NAA. With the exception of K, results between the different techniques were in good agreement; with no significant differences being observed at the 95% confidence level using the t-test. Limits of detection ranged from 0.05 ng g−1 (for Zn) to 80 ng g−1 (for Fe) and precision was typically better than 11% RSD. The proposed method enabled up to 10 measurements to be made per hour. A second method using ETAAS for the analysis of refractory samples (lithium niobate and bismuth tellurite) was reported by Gyorgy et al.300 The objective of the study was to remove/diminish the memory and carry over effects of the refractory analytes Er, Nb and Nd. The cleaning step of a normal program was replaced with a halogenation cycle in which 20 μL of carbon tetrachloride was dispensed using a conventional autosampler, dried at 80 °C under an internal gas flow of 40 mL min−1 argon and the residue decomposed by fast furnace heating to approximately 2000 °C under interrupted internal flow gas conditions. A clean out stage at 2100 °C then followed using the maximum gas flow. The technique had the advantage of not requiring any modification of the gas supply system. One drawback noted was that the analytical sensitivity decreased. This was attributed to the accelerated rate of tube surface deterioration. This required the mathematical correction (re-sloping) of the calibration curves. Calibration curves were linear to 1.5 and 10 μmol mole−1 for Er and Nd respectively. Characteristic masses were 18 and 241 pg and LOD were 0.017 and 0.27 μmol mole−1 for Er and Nd respectively. Results were compared with those obtained using a conventional ETAAS method and were validated using the completely independent technique of XRF spectrometry. In general, data between the different techniques were in good agreement; especially for the Er.
Not quite all of the methods reported in the literature used solid sampling techniques. On occasions, the dissolution of the sample can have beneficial effects. Souza et al.301 described a method in which alumina (250 mg) was dissolved using HCl (5 mL), H2SO4 (1.5 mL) and water (1.5 mL) with microwave assistance and then diluted to 20 mL. The aluminium in the digest was then precipitated by bubbling ammonia through it until the pH reached 8, (approximately 10 min). The analytes Ca, Fe, Ga, Na, Si and Zn were then determined using ICP-OES. The use of a multitude of internal standards was necessary to obtain precise and accurate results. The internal standards used were Be (for Fe), Dy (for Ga), In (for Zn), and Sc (for Na). The advantage of the procedure was the removal of aluminium from the sample, which reduced spectral interferences on other analytes. At very high concentrations the aluminium can react with the silicon of the torch forming aluminosilicates which drastically reduces torch lifetime. The removal of the aluminium therefore also prevented this. The method was validated by the successful analysis of NIST SRM 699.
| Element | Matrix | Technique; atomization; presentation | Comments | Ref. |
|---|---|---|---|---|
| a L = liquid, S = solid. | ||||
| Ba | Ceramics from the north coast of Papua New Guinea. | TOF-MS; ICP; LA | Elevated Ba concentrations in sections of the ceramics indicated that it originated from post depositional enrichment (post-burial uptake). The authors identified the formation of pronounced concentration gradients relative to shards with low Ba content. | 302 |
| Co | Blue and white porcelains from Jingdezhen kiln | XAFS; — ; S, XANES; — ; S | K-edge XAFS and XANES were used to characterize the oxidation state and environment of Co in the blue decoration of the samples. The Co(II)/Co(III) ratio was similar for all samples regardless of the historical time period. The XAFS demonstrated that the Co occupied both tetrahedral and octahedral sites | 303 |
| Various (11) | Ancient ceramics from the Quartaia site, Tuscany | TOF-SIMS, AAS | Depurata and non-depurata ceramics were analysed for both elements and for organic components in an attempt to determine what was adsorbed during the firing process. The TOF-SIMS imaging identified some elements (Na, Ca, Fe, K and Mg) that were inhomogeneous in non-depurata ceramics. These appeared to be concentrated inside the inclusions | 304 |
| Various | White earthenware from Lorraine | XRF; — ; S | XRF as well as XRD and back-scattering SEM-EDAX were used to determine provenance and manufacturing technique. Bulk, major and minor elements were determined. Two types of ceramic were identified, CaO rich and CaO poor. The CaO rich materials were obtained by mixing an imported white firing clay, a temper (calcined flint or sand) and a flux (chalk). The CaO poor material was the same but without the flux. The glazes also differed, with the CaO rich having either a Sn or Pb – rich glaze whereas the CaO poor material had only the Pb-rich glaze | 305 |
| Various | Portuguese polychrome glazed ceramics | XRF; — ; S | Both 3D and 2D XRF as well as SEM-EDAX were used for the analysis with the aim to advance the knowledge of manufacturing techniques. The 3D XRF analysis with μm resolution was instrumental in determining the analyte distribution. This was dependent on the glaze used, the firing temperature and the pigments | 306 |
| Various | White porcelains from the Ding kiln site | OES; ICP; LA | Porcelain shards (69) spanning five dynasties were analysed using LA-ICP-OES. Different chemical compositions of the porcelain and of the glazes were identified. The P2O5 content of the glazes indicated that wood ash may have been used as the source of the calcium oxide | 307 |
| Various | Amphorae shards from Portugal | XRF; — ; S | XRF, XRD and SEM-EDAX were used to characterize 70 samples from Castro do Vieito and samples found inside kilns from Baetican and Lusitanian provinces of the Roman empire. The samples from Castro do Vieito were homogeneous chemically (with virtually no Na and an Al content of 9%), whereas those from inside the kilns were not. The Ca content was about 1% in the Vieito shards, whereas the content was between 1 and 10% in the other materials | 308 |
| Various | Mimbres pottery from the American South West | XRF; — ; S | A comparison of capabilities of sourcing ceramics from the results obtained using a portable XRF instrument with those obtained using instrumental NAA. It was concluded that Instrumental NAA was the better option simply because it could determine a larger number of analytes which could be used for the discrimination. It also offered superior accuracy and precision compared with the portable XRF instrument. | 309 |
| Various | Ancient glazed pottery shards from Khirbet Faris, Jordan | MS; ICP | Chemometric analysis of experimental data using PCA and hierarchical cluster analysis with the Bray–Curtis statistical tool to identify similarity indicated that the pottery had at least three different sources of origin | 310 |
| Various | Chinese terracotta polychrome structures | XRF; — ; S | The benchtop instrument used had capillary optics which normally have the disadvantage of having low transmission of high energy X-rays. However, this configuration was used specifically for the analysis of pigments of over-glaze porcelain that contains high amounts of lead which can cause severe problems. The colorants were Cu, Fe and Mn | 311 |
| Various | Terracotta polychrome sculptures before and after conservation | XRF; — ; S | A portable XRF instrument, a portable Raman instrument, an optical microscope and fibre optics reflectance spectroscopy were used for the analysis. Results were comparable or better than those obtained using some micro-destructive techniques | 312,313 |
| Various | Ancient ceramic samples from Al Fustat excavation, Cairo | OES; LIBS; S | Design, construction and optimization of a LIBS system | 296 |
| Various | Archaeological ceramics from Turkey | XRF; — ; S | Portable instrument used for the analysis. The effects of the surface morphology, organic surface coating and the grain size and morphology were all determined. The authors concluded that with appropriate methodology, the analysis could be undertaken successfully | 314 |
| Various | Pre-colonial pottery from Brazil | XRF; — ; S | Portable instrument used to determine elemental composition of 68 pottery fragments. The presence of engobe in 43 of the fragments was identified. Results from hierarchical cluster analysis (Ward method) and 2D graphics were in agreement. Samples could be grouped into five classes | 315 |
| Various | Enamel of 20 Champleve objects in Belgian museums | XRF; — ; S | Portable XRF, μ-XRF, vacuum μ-XRF and EPMA used for the characterization. Three groups of glass were identified. One consisted of soda-lime silica with low K content and was opacified using calcium antimonite crystals (made only in 12th century). A second was made with soda-lime silica with high content (from plant material) and was opacified using tin oxide crystals. This was made from 13th century onwards. The third group was made from synthetic materials and therefore had a completely different composition. This group of materials came from the 19th century | 316 |
| Various | Ancient ceramic from Southern Gaul and Italy | XRF; — ; S, XANES; — ; S | XRF microprobe analysis of materials used in an attempt to elucidate firing protocols during their construction. Chemical mapping of Fe and Fe K-edge XANES also used. Pieces from different workshops showed significant differences in starting materials, clay conditioning and firing conditions. Shards from the same workshop showed more subtle differences | 317 |
3.5. Catalysts
As usual, there was a huge range of materials reported for a similarly wide range of reactions over this review period. However, although these papers use atomic spectroscopy to characterize their products, the analytical work was usually presented with limited detail. Therefore, although interesting in terms of the catalytic application, these papers were largely ignored for the purposes of this review. Two papers with a specific analytical focus were reported by Hussain and Siddiqa318 and Pouzar et al.319 The former described the effect of particle distribution in LIBS analysis of mesporous vanadium–silica, whilst the latter detailed XPS, SIMS, TGA, FTIR and SEM studies of nickel–copper–aluminium modified supported nanocatalysts. The remainder of this review has categorized papers on catalysts and catalyst function as follows: petroleum/petroleum products, automotive catalysts, alternative fuels/fuel cells, photocatalysts and oxidative reactions.![[double bond, length as m-dash]](https://www.rsc.org/images/entities/char_e001.gif)
O bonds and at 710 cm−1 to P–O–P bonding. Phosphorus was incorporated in the mesoporous silica, forming medium strength PO4 acid sites, which aided isomerization and desulfurization. The chemical state and surface composition of freshly sulfided catalysts were also studied using XPS. An alternative scheme of selective adsorption of dibenzothiophenes was discussed by Sarda et al.321 However, the only analytical aspect of the study reported was total S determination using XRF spectrometry.
Suo et al.322 reported the use of monometallic gold and palladium and bimetallic gold–palladium catalysts supported in silica for the hydrodesulfurization of thiophene. Catalysts were characterized using a standard suite of techniques including: AAS, N2 BET measurement, XRD and TEM. The study also used XANES to look at the interaction of Au and Pd in the bimetallic catalyst. Spectra at the Au–L3 edge were collected using a 2.5 GeV positron beam energy and 250 mA stored current. The signal at 11.92 KeV was assigned to cationic Au and higher energy signals to metallic Au. In the monometallic gold–silica catalyst the presence of metallic Au was clear in the spectra. However, the addition of Pd led to a noticeable shift of the Au L3 edge resonance to a lower energy indicating fewer features of metallic Au in the catalyst, demonstrating electron interactions between the Pd and Au. The formation of this alloy improved resistance to poisoning by preventing the formation of inactive Pd4S.
Other approaches to hydrodesulfurization catalysis that the reader may find interesting include: micro- and nano-structured molybdenum trioxide,323 hollow metal phosphides,324 unsupported gallium sulfide incorporated into tungsten sulfide325 and aluminium–SBA-15 supported nickel–tungsten catalysts.326 The last of these papers utilized the 27Al NMR chemical shift of Al atoms in tetrahedral and octahedral coordination to determine the amount of framework and extra framework Al loaded onto the support.
The hydrogenation of lower grade petroleum fractions to premium fuel grades is an important refining process, as a high aromatic content significantly reduces the combustion characteristics. Hydrogenation using ZSM-5–alumina supported ruthenium–nickel catalysts was reported by Masalska.327 The effect of the nature of the Ru precursor on the composition of the ruthenium–nickel catalyst surface was measured using XPS and TOF-SIMS. Results from the Ni 2p3/2 region showed that a catalyst prepared using a Ru3(CO)12 precursor gave rise to a high proportion of reduced Ni, whilst preparation using [Ru(NH3)6]Cl3 and (NH4)2[RuCl6] precursors gave rise to Ni–Al oxide spinals on the surface. Positive TOF-SIMS spectra showed that Ni+, Ru+ and traces of NiO+, NiOH+ and NiOH2+ dominate the surface. The intensity of the Ni species was weaker in the [Ru(NH3)6]Cl3 and (NH4)2[RuCl6] precursor catalysts, which correlates with higher hydrogenation activity. Negative spectra of these precursors also identified Cl− on the surface, implying incomplete removal of Cl during the preparation. A similar study of hydrogenation over zinc–nickel and molybdenum–nickel catalysts may also be of interest.328 The reverse reaction, oxidative dehydrogenation of alkanes over mesoporous silica supported vanadium oxides, was also suggested as an alternative to steam or fluid catalyst cracking for production of olefins. Separate publications, using the same catalyst, covered its use for both n-butane329 and propane330 dehydrogenation. Characterization was achieved using the usual suite of techniques, including XRF spectrometry, N2 BET measurements, XRD, SEM and TPR.
Light olefin metathesis is attracting particular interest for the petrochemical industry as it enables the conversion of olefins as a function of market demand. Debecker et al.331–334 produced a detailed investigation, over four papers, on the preparation of molybdenum trioxide supported on silica–alumina catalysts for olefin metathesis. The most interesting, from an analytical perspective, reported the use of a combination of XPS and TOF-SIMS to gain insight into the distribution and interaction of Mo species on the surface of flame made catalysts.334 The Mo 3D XPS data, collected using a monochromatic Al source, showed a distinct doublet directly attributable to Mo6+. Its binding energy tended to shift at higher loadings – closer to that of pure molybdenum trioxide. The lower binding energy indicated a strong interaction between the Mo species and the support material, whereas the increased binding energy suggested pure molybdenum trioxide built up on the surface with increased loading. This theory was supported by comparing Mo/Si + Al ratios at the surface to the bulk measurements obtained using ICP-OES. Data collected using TOF-SIMS, in the negative spectrum mode with a Bi+ ion source, showed that the nature of the Mo species on the surface was highly dependent on the molybdenum trioxide loading. Samples with low loading levels (<2%) had fragments containing a single Mo atom, MoOx−. However, increasing loading levels gave rise to the detection of poly-Mo species; e.g., Mo2O6−, Mo2O7−, Mo3O9−. Such clusters imply the presence of Mo–O–Mo bridges, i.e., crystalline molybdenum trioxide. This was confirmed using XRD. Other techniques employed included N2 BET measurements, Raman spectroscopy, TEM and 27Al MAS-NMR.
The recovery and recycling of PGM from spent automotive catalysts is an important process, because of their high value and demand. Fornalczyk and Saternus336 presented a brief, but interesting, review of the available recycling technologies for Pt. The high value of these metals imparts a need for a fast and accurate means of analysis of materials to be recycled. Compernolle et al.337 reported the use of LA-ICP-OES as a means of detecting PGM in lead fire assay buttons. A typical analysis involved lead fire assay followed by cupellation and digestion of the metal residue in acids for spectroscopic measurements. The presented method reduced the number of steps, thus increasing sample throughput. Measurements were performed using a 266 nm Nd:YAG LA unit coupled with an ICP-OES spectrometer equipped with 32 linear CCD detectors in a Paschen-Runge mount. Calibration was achieved using Pb buttons prepared from pure alloy materials and the accuracy was assessed against reference catalysts. Detection limits ranged from 2.5 to 12 μg g−1 in the lead button and the procedure had a precision of better than 5% RSD.
Stringent legislation limiting the levels of nitrogen oxides (NOx) and particulate soot matter emitted from diesel engines has led to the development of removal technologies such as NOxtraps and particle filters. Oliveira et al.338 investigated the effects of preparation and structure of cerium–zirconium mixed oxides on diesel soot combustion. Measurements using EDXRF spectrometry in a polypropylene cell and fundamental parameters calibration were used to determine the Ce and Zr content. Structural information was gathered using XRD, FTIR, BET measurements and Raman spectroscopy. A Ce0.8Zr0.2O2 catalyst was the most active for soot combustion. Hydrothermal and vehicle aging of commercial NOx traps was characterized using XRF spectrometry, XRD, SEM-EDAX, TEM, BET measurements and operando FTIR by Benramdhane et al.339 The purpose of operando FTIR was to study the storage of NOx and carbonates under running conditions to determine the impact on aging. Samples were pressed into self-supporting wafers and were placed in a quartz reactor with KBr windows which enabled analysis using FTIR, allowing measurement of surface adsorption. A combined NOx and soot combustion catalyst based on copper–magnesium–aluminium hydrotalcites was also proposed by Wang et al.340 A standard suite of XRD, N2 adsorption–desorption, H2-TPR and ICP-OES were used to characterize the materials. Other papers that may be of interest include: modelling of P poisoning,341 SSZ-33 molecular sieve automobile tailpipe hydrocarbon traps342 and gold-based thin film alternatives to platinum, palladium and rhodium based catalysts.343
The reaction of CO and H2 (syngas) in the Fischer–Tropsch process to produce liquid hydrocarbons is considered to be a possible alternative to diesel fuel. Cronauer et al.344 explored the use of Ir as a promoter for a typical cobalt–alumina Fischer–Tropsch catalyst. Temperature programmed experiments with in situ EXAFS/XANES monitoring of Co oxidation states showed that Ir was an effective promoter for facilitating cobalt oxide reduction to the active species (CoO to Co). The effect of K as a promoter for iron-based Fischer–Tropsch catalysts was studied by Farias et al.345 Catalysts were characterized using N2 adsorption, H2-TPR, XRD, XRF spectrometry, TGA and SEM. A strong interaction was observed between Fe and K, which inhibited the reduction of Fe catalyst. Addition of potassium increased the production of heavy hydrocarbons (C20+).
The production of syngas from biomass gasification is seen as an important step towards renewable energy. Two papers describing laboratory- and pilot-scale upgrading of biomass fuel gas by reforming over nickel–magnesium oxide–γ alumina catalysts were reported by Wang et al.346,347 However, the papers only briefly described the analytical procedures used. Ammendola et al.348 examined the effect of S on the performance of rhodium–lanthanum–cobalt oxide based catalyst for biomass conversion to syngas. Characterization of catalyst materials was achieved using ICP-MS and BET analyses. The poisoning effect of S was studied by in situ DRIFT analysis of CO adsorbed catalysts. Doublet bands at 2090 and 2016 cm−1 corresponded to dicarbonyl species isolated on Rh sites. As S was introduced the bands reduced before completely disappearing after 120 min, showing selective poisoning at the isolated Rh reaction sites. The synthesis, characterization and catalytic performance of cerium–SBA-15 supported nickel catalysts for methane dry reforming to syngas was discussed by Wang et al.349 Data obtained using both XPS and XRD showed how Ce became incorporated into the SBA framework. Textural and morphological properties were characterized using N2 BET measurements and TEM. Chemical compositions were determined using ICP-OES after acid digestion.
Dimethyl ether (DME) was suggested as a substitute for conventional petroleum-derived diesel because of its low ignition temperature, low toxic emissions on combustion and reduced noise. The compound is traditionally produced by a two-step process involving synthesis of methanol from syngas using a copper-based catalyst followed by methanol dehydration over a solid catalyst to produce the DME. A one-step, syngas to DME, route using hybrid copper zinc aluminium–HZSM-5 catalysts was studied by García-Trenco et al.350 The interactions of the components within the catalysts were studied using a range of techniques including: ICP-OES, XRD, N2 physisorption, H2 TPR, 27Al MAS-NMR, pyridine-FTIR and EPR spectroscopy. Details of analytical procedures were provided.
The rapid increase in biodiesel production from triglycerides, such as vegetable oil and animal fats, has led to a large surplus of glycerol on the market. This is because for every tonne of biodiesel produced 100 kg of glycerol is also made. This has resulted in an increased interest in transforming glycerol into higher value materials. Glycerol carbonate is a valuable glycerol derivative, which is used in a number of industries. The calcium oxide catalysed transesterification of dimethyl carbonate with glycerol to glycerol carbonate was examined by Sinmanjuntak et al.351 using TOF-SIMS to identify the active Ca species. After reaction in a round bottom flask, acetonitrile was added to precipitate the dissolved calcium complexes. These precipitated complexes were then washed and dried ready for analysis. The TOF-SIMS measurements were made using a pulsed Bi+ ion gun operated at 25 kV with a sample surface bias of 3 kV with respect to the grounded extraction electrode for positive and negative collection mode. Below the mass range of 100 m/z, Ca species such as Ca, CaH, CaO and CaOH were detected; most of which were formed from the ionization process. Above 100 m/z, major peaks detected appeared to be related to a Ca species with both glycerol and methyl carbonate moieties. Combined with elemental analysis the active species was determined to be Ca(C3H7O3)(OCO2CH3). An alternative system of oxidative carbonylation of glycerol to glycerol carbonate using a palladium chloride (1,10-phenanthroline) catalyst was reported by Hu et al.352 The heterogeneous catalyst's structure and composition were characterized using AAS, CHN analysis, N2 sorption, XRD, FTIR, solid-state NMR and XPS.
Oxidation of glycerol to α-hydroxy acids and dicarboxylic acids is another method of adding value to the by-product. Reactions over platinum and palladium nanoparticles353 and gold supported on activated carbon, graphite and carbon nanofibres354 were discussed. Dehydroxylation to propylene glycol over copper–zinc oxide–alumina355 and dehydration over zirconium and titanium modified montmorillonites356 were also reported. In each case a combination of AAS, ICP-OES, XRF spectrometry and CHN analysis was used for elemental characterization whilst morphology and structural characterization was determined using SEM, TEM and XRD.
Fuel cells are recognized as being clean, energy-converting devices because of their high efficiency and low emissions. In particular, the PEMFC is a promising power source that is expected to become used as a high efficiency, low operating temperature and low pollution method for transportation and residential applications. In the short term, cells containing significant amounts of Pt catalyst may be practical, but in the longer term a lower cost alternative will be needed. An alternative carbon-supported hafnium oxynitride (HfOxNy–C) material prepared by heating carbon-supported hafnium oxide under NH3 gas was investigated by Chisaka et al.357 They used XRD and TEM to determine the crystal structure and morphology of prepared catalysts together with XPS for the determination of surface composition and chemical states. The chemical stability was assessed using ICP-OES by measuring the mass of Hf dissolved in a 0.1 M H2SO4 solution at 303 K over various times. Electrochemical measurements for the oxygen reduction reaction were promising, but short of the current Pt catalysts. A carbon-supported cobalt oxide nanoparticle alternative was also examined by Huang et al.358 Prepared by thermal decomposition of cobalt–ethylenediamine complexes on a carbon black support and characterized using a full suite of XRD, high resolution TEM, XPS and ICP-OES, the cobalt oxide catalysts had a larger power density than that of commercial 20 wt% Pt/C material.
Esmaeilifar et al.359 examined the use of multi-walled carbon nanotubes (MWCNT) as a support for highly dispersed Pt nanoparticles as a means of increasing catalyst activity. Optimization of a hydrothermal preparation method was achieved by measuring the nanoparticle size with a combination of XRD and TEM and determining the elemental content using XRF spectrometry. Optimized conditions resulted in a catalyst with specific surface area of 99 m2 g−1 and superior performance to a commercially available platinum–carbon catalyst.
Catalyst lifetime is another major challenge for commercialization of PEMFCs as this limits the power and efficiency of the cell and hence its useable lifespan. Electrochemical oxidation and dissolution of platinum can be a significant cause of cell degradation. Kim and Meyers360 used a combination of a rotating ring-disk electrode, an electrochemical quartz crystal nanobalance (EQCN) and ICP-MS to determine the influence of hydrogen- and cation-underpotential deposition on Pt dissolution whilst potential cycling in 0.5 M H2SO4 solution. Small mass changes during tests were measured using the EQCN, whilst total dissolved Pt was measured using ICP-MS. Electrochemical dissolution of the carbon catalyst support is also a cause of cell degradation. Senevirathne et al.361 discussed the use of niobium dioxide and titanium suboxide (Ti4O7) nanofibres as alternative support materials. Synthesized nanofibres were characterized using XRD, SEM, TEM, BET measurements, ICP-MS and XPS. Thermal, chemical and electrochemical stabilities of the supports were evaluated. In addition, both oxygen reduction reaction activity and stability in acidic solutions were also evaluated. Measurement of fresh and treated samples using XPS suggested the formation of electronically insulating surface oxides, which reduce the activity and stability of the catalysts.
| Analyte | Matrix | Technique, atomization, presentation | Comments | Ref. |
|---|---|---|---|---|
| Au, Ag | Au, Ag loaded Zeolite | OES; ICP; L, μ-XRF, — ; S | Comparison of ICP-OES and μ-XRF results for Au determination agreed well. However, ICP-OES results for Ag loading were consistently low compared with those obtained using μ-XRF because of precipitation of Ag salts during digestion. μ-XRF was also used to map the surface composition of both metals | 370 |
| Au, Ti | Aqueous Au nanoparticles with a titanium dioxide suspension | AAS; — ; L | LC-MS-MS used to calculate 15 degradation products of 3,4-dichlorophenylurea | 371 |
| Cr, Ti | MCM-41 molecular sieves, Aqueous Acid Orange 7 | EDXRF; — ; S, OES; ICP; L | Loadings on solid MCM-41 catalysts determined using EDXRF. Leaching of Cr into the reaction solution was measured using ICP-OES. Cr species inferred by UV-VIS DRS | 372 |
| Fe, Zr | Alumina support | OES; ICP; L | Catalysts fully characterized using XRD, TEM, SEM and XPS. Comparison of XPS data obtained from fresh and used catalysts indicated no change in the surface composition during reactions. The extent of surface change is used as a measure of stability | 373 |
| P, Ti | P doped titanium dioxide | OES; ICP; L | P determination was also performed using the ascorbic acid reduction UV-VIS method; the results from which agreed well with the theoretical and ICP-OES values | 374 |
| Zr, Ti | Titanium dioxide hollow nanospheres | WDXRF; — ; S | Bulk analysis using WDXRF compared with surface analysis using XPS to determine enrichment of Zr at the catalyst surface. Morphology determined using SEM and TEM | 375 |
| Various (6) | Thin films on stainless steel | EDXRF; — ; S | Calibration using fundamental parameters method | 376 |
The compositional analysis of catalyst surfaces using XPS continues to be a key analytical tool for understanding reactions and preparation methods. The ability to measure elemental surface ratios, in comparison to bulk ratios, was utilized by Liu et al.366 to understand the location of lanthanum and boron on co-doped titanium dioxide. A Combination of XPS and ICP-OES analyses revealed La gathered predominantly on the surface, whereas B penetrated into the titanium dioxide support. Analysis of the B 1s spectra indicated that B became incorporated into the titanium dioxide framework through substitution of O sites to form B–Ti–O bonding, which appeared to cause a narrowing of the adsorption band gap and enhanced photocatalytic ability. The C, N and S environments of tri-doped titanium dioxide prepared using a sol–gel method were studied by Wang et al.367 Analysis over the C 1s, N 1s, S 2p, O 1s and Ti 2p regions using XPS indicated that C substituted some of titanium dioxide's oxygen sites to form Ti–C bonds, N was interstitially and substitutionally doped into the titanium dioxide lattice to form Ti–N–O, Ti–O–N and O–Ti–N bonds, whilst S6+ substituted the lattice Ti4+ resulting in cationic S-doping. A thiourea to Ti molar ratio of 0.05:1 was an optimum doping level for tetracycline degradation under solar irradiation. The incorporation of N doped into sodium tantalate nano-cubic structures for azo dye degradation was studied using XPS.368 The N 1s spectra of N-doped samples contained a peak at 396–397 eV, which was characteristic of N3− in Ta–N bonds. Diffraction patterns collected by XRD suggested the replacement of O with N in the sodium tantalate lattice, which does not cause any significant structural changes and resulted in a NaTaO3−xNx structure. Degradation products of methyl orange were extensively characterized using GC–MS.
An interesting paper on radiation-induced synthesis of Fe-doped titanium dioxide for photocatalytic destruction of sodium dodecylbenzensulfonate was presented by Bzdon et al.369 Catalysts were prepared by wet impregnation of titanium dioxide with iron nitrate solution, then irradiated with γ-rays or an electron beam with a total dose in the range of 30–980 kGy. Hydrogen TPR, XRD and TOF-SIMS analyses showed that the radiation-induced synthesis results in the formation of Fe2O3 and FeTiO3 phases. Surface mapping of Fe species using TOF-SIMS also showed that irradiation caused a higher dispersion of Fe compared with catalysts prepared by calcination.
Sulfated vanadium pentoxide–titanium dioxide mesoporous catalysts were prepared by Gannoun et al.379 using the sol–gel technique for the oxidation of chlorinated volatile organic carbon (VOC) compounds, particularly chlorobenzene. Elemental composition of calcined materials was measured using ICP-OES. Comparison of measured and theoretical compositions indicated a significant loss of S during the calcination process. Analysis using XPS of catalyst powders pressed into homemade holders was performed under monochromatic Al-Kα radiation. Comparison of bulk and surface composition showed that S species mainly existed on the surface of the catalyst. Structural and catalytic analyses were also performed. A similar study presented by Delaigle et al.380 incorporated Ag into the same catalysts. The catalytic oxidation of non-chlorinated VOC (such as light alcohols and toluene) using Cu, Mn, Mg, Al and Co mixed oxides381 and chromium–yttrium zeolite382 was presented. The latter was prepared by biosorption of chromium onto sodium yttrium zeolite followed by calcination to remove the bacterium organic matter. Measurement using XPS of the binding energies for peaks in the Cr 2p region, from sample before and after calcination, showed a change from a single peak at 577.5 eV to two peaks at 578.2 eV and 587.7 eV. Comparison with reference materials indicated that upon calcination in air, exchanged Cr3+ compounds suffered oxidation to Cr6+ species.
The oxidation of olefins to give value added compounds such as alcohols, aldehydes, ketones and acids are important reactions in both chemical and pharmaceutical industries. Khare and Chokhare reported the synthesis and characterization of Cu (ref. 383) and Fe (ref. 384) N,N′-bis(salicylidene)-ethylenediamine complexes inter-calcined with α-zirconium phosphate for cyclohexene oxidation. The electronic and coordination environments were measured using EPR for Cu and Mössbauer spectroscopy for Fe centres; together with peak shifts in UV-VIS spectra for ligand formation confirmation. Structural and composition analyses were also reported. The combination of EPR and UV-VIS analyses was also used by Selvaraj et al.385 who determined the extent of incorporation of Ce into mesoporous SBA-15 for the same reaction. Measurement of catalysts prepared by insipient wetness gave rise to two peaks (at 300 and 400 nm) in the UV-VIS spectra which were assigned to framework tetra-coordinated and extra-framework hexa-coordinated Ce4+ respectively. However, catalysts prepared by hydrothermal methods had only a single peak at 300 nm, indicating that all Ce was incorporated into the SBA-15 framework. This was supported by 29Si MAS NMR analyses that demonstrated a reduction in Si(OSi)4 environments in these samples. Elemental composition was determined using ICP-OES, following digestion with HF. Similar characterization procedures were performed for iron incorporated into ZSM5 (ref. 386) and MWCNT anchored oxovanadium complex387 catalysts.
The synthesis, characterization and catalytic oxidation activity of chromium containing MCM-41 sieves was reported by Robles-Dutenhefner et al.388 Prepared materials were largely amorphous leading to no defined peaks in the XRD spectra. Small-Angle X-ray Scattering (SAXS) experiments, performed using a Huber-423 3-circle diffractometer indicated a long range, highly ordered, hexagonal structure. The loading of Cr was determined using ICP-OES after digestion of the material with HF/HNO3. Hydrogen-TPR measurements showed an absence of low temperature reduction peaks which would be assigned to extra-framework Cr species, thus indicating all Cr was incorporated into the framework.
Preferential CO oxidation has become a research topic of interest over the last decade. This is because of increased regulation of automotive exhaust gases, air cleaning applications and depletion from the inlet streams of PEMFC. A number of papers were presented on the application of gold catalysts for CO oxidation, including gold–cerium dioxide389 and silica supported nano-sized gold catalysts loaded onto magnesium oxide.390 Pulsed laser deposition of FeOx, TiO2 and CeOx onto Au films and nanoparticles were reported by Frey et al.343 Large spot static TOF-SIMS data, collected using a 3 mm pulsed beam of 5 keV Ar+ ions, was used to estimate the film thickness of each material. Data obtained using XPS, collected using non-monochromatic Mg-Kα radiation, were used to characterize the composition and chemical state of the catalyst surface. The XPS analysis of gold–lanthanum–manganate catalysts was used by Jia et al.391 to understand catalytic deactivation during CO oxidation reactions. Measurement of fresh and used catalysts revealed the Au was present as Au3+, Au+ and metallic Au; however, there was a larger percentage of metallic Au in used catalysts suggesting transformation of ionic Au to bulk Au as the main cause of deactivation. The presence of highly dispersed metallic Au nanoparticles on mesoporous gold–titanium dioxide catalysts was confirmed using XPS by Roos et al.392 As synthesized, the catalysts possessed an amorphous network structure comprising mesopores. The long range mesoscopic ordering of the pore system was confirmed using SAXS. Upon calcination the material crystallized and anatase nanocrystallites were also formed. This was confirmed using XRD.
In situ IR experiments have been utilized to study the CO adsorption and reaction mechanisms of a number of catalysts. The CO oxidation over copper–copper oxıde–titanium dioxide was investigated by Wu et al.393 A strong band at 2105 cm−1 corresponding to Cu+–CO and a weak one at 2060 cm−1 corresponding to Cu–CO together with broad bands centred at 2170 cm−1 corresponding to Cu+–(CO)x were observed as key intermediates during reaction conditions. Catalysts prepared by wet impregnation and photo-deposition were also compared using XPS and Auger emission spectrometry. The Cu 2p spectra obtained using XPS analysis of wet impregnated catalysts showed binding energies of 932.5 and 952.3 eV associated with Cu and Cu2O together with AES kinetic energies of 913 and 914.5 eV associated with Cu2O–CuO. Meanwhile, binding energies of 932.2, 934.4, 952.3 and 954.0 eV were seen in the XPS spectra for photo-deposited catalysts, suggesting the presence of CuO and Cu–Cu2O. However, the absence of a 914.5 eV peak in the Auger emission spectra eliminated the existence of zero valent Cu. Catalysts prepared by photo-deposition consisted of copper species in the form of Cu2O and CuO. A detailed in situ DRIFT spectrometry study of the silica supported Pt nanoparticle used for the catalytic oxidation of CO was presented by Wang et al.394 Spectra were collected using a FTIR spectrometer with a KBr window and high temperature reaction chamber. Temperatures were varied from 303 to 493 K with a 1% CO gas flow. The main feature of the spectra was a band in the 2000–2100 cm−1 range, which corresponded to linearly adsorbed CO on Pt atoms. Bulk and surface Ag and Pt content were determined using XRF and XPS. In situ DRIFT spectrometry was also used by Romero-Sarria et al.395 to study CO oxidation over a gold–cerium phosphate catalyst. The techniques of XRF spectrometry, XRD, TEM and N2 adsorption–desorption were also employed to characterize the material further.
The selective oxidation of alcohols to aldehydes and ketones is a significant transformation in organic chemistry with recognized industrial importance. A wide range of catalyst types was reported including a graphene supported oxo-vanadium Schiff base,396 titania supported cobalt–manganese–aluminium oxide,397 SWCNT supported gold nanoparticles398 and various palladium materials.399,400 However, it is difficult to find literature with an analytical focus.
4. Glossary of terms
| 2D | Two dimensional |
| 3D | Three dimensional |
| AAS | Atomic absorption spectrometry |
| AFS | Atomic fluorescence spectrometry |
| AFM | Atomic force microscopy |
| AMS | Accelerator mass spectrometry |
| ANOVA | Analysis of variance |
| APDC | Ammonium pyrrolidine dithiocarbamate |
| APT | Atom probe tomography |
| ATR | Attenuated total reflection |
| BCR | Community Bureau of Reference |
| BET | Brunauer, Emmett and Teller |
| CCD | Charge coupled device |
| CRM | Certified reference material |
| CV | Cold vapour |
| DME | Dimethyl ether |
| DTPA | Diethylenetriaminepentaacetic acid |
| DRIFT | Diffuse reflectance infrared Fourier transform |
| DRS | Diffuse reflectance spectroscopy |
| EASI | Easy ambient sonic-spray ionization |
| EDAX | Energy dispersive X-ray analysis |
| EDXRD | Energy dispersive X-ray diffraction |
| EDXRF | Energy dispersive X-ray fluorescence |
| EPA | Environmental protection agency |
| EPMA | Electron probe microanalysis |
| EPR | Electron paramagnetic resonance |
| EQCN | Electrochemical quartz crystal nanobalance |
| ERDA | Elastic recoil detection analysis |
| ESI-MS | Electrospray ionization mass spectrometry |
| ETAAS | Electrothermal atomic absorption spectrometry |
| ETV | Electrothermal vaporization |
| EU | European Union |
| EXAFS | Extended X-ray absorption fine structure |
| FAAS | Flame atomic absorption spectrometry |
| FFF | Field flow fractionation |
| FI | Flow injection |
| FTIR | Fourier transform infrared |
| FWHM | Full width at half maximum |
| GC | Gas chromatography |
| GD-MS | Glow discharge mass spectrometry |
| GD-OES | Glow discharge optical emission spectrometry |
| GI-SAXS | Grazing incidence small angle X-ray scattering |
| GI-XRD | Grazing incidence X-ray diffraction |
| GI-XRF | Grazing incidence X-ray fluorescence |
| HG | Hydride generation |
| HPLC | High performance liquid chromatography |
| IAEA | International Atomic Energy Agency |
| ICP | Inductively coupled plasma |
| ICP-MS | Inductively coupled plasma mass spectrometry |
| ICP-QMS | Inductively coupled plasma quadrupole mass spectrometry |
| ICP-TOF-MS | Inductively coupled plasma–Time-of-flight mass spectrometry |
| IL-DLLME | Ionic liquid–dispersive liquid liquid microextraction |
| IRMS | Isotope ratio mass spectrometry |
| ISO | International organization for standardization |
| LA | Laser ablation |
| LASIL | Laser ablation of sample in liquid |
| LIBS | Laser induced breakdown spectrometry |
| LIDAR | Light detection and ranging |
| LIF | Laser induced fluorescence |
| LIPS | Laser induced plasma spectrosocopy |
| LOD | Limit of detection |
| LOQ | Limit of quantification |
| MALDI-TOF | Matrix assisted laser desorption ionization time-of-flight |
| MAS | Magic angle spinning |
| MC | Multicollector |
| MEIS | Medium energy ion scattering |
| m-CPP | meta-Chloro phenlypiperazine |
| MDMA | 3,4-Methylenedioxy methamphetamine |
| MIP | Microwave induced plasma |
| MS | Mass spectrometry |
| MSP | Microstrip plasma |
| MWCNT | Multi-walled carbon nanotube |
| NAA | Neutron activation analysis |
| Nd:YAG | Neodymium doped:yttrium aluminium garnet |
| NMR | Nuclear magnetic resonance |
| OES | Optical emission spectrometry |
| NIST | National institute of standards and technology |
| PAGE | Polyacrylamide gel electrophoresis |
| PCA | Principal component analysis |
| PEEK | Poly ether ether ketone |
| PEMFC | Polymer electrolyte membrane fuel cell |
| PET | Polyethyleneterephthalate |
| PGM | Platinum group metals |
| PIGE | Particle-induced gamma ray emission |
| PIXE | Particle-induced X-ray emission |
| PLS | Partial least squares |
| PLS-DA | Partial least squares discriminant analysis |
| ppb | Part per billion |
| ppm | Part per million |
| PTFE | Poly(tetrafluoroethylene) |
| RBS | Rutherford backscattering spectrometry |
| RDA | Regularized discriminant analysis |
| REE | Rare earth element |
| rf | Radiofrequency |
| RIMS | Resonance ionization mass spectrometry |
| RoHS | Restriction of hazardous substances |
| RSD | Relative standard deviation |
| SEM | Scanning electron microscopy |
| SF | Sector field |
| SIMCA | Soft independent modelling of class analogy |
| SIMS | Secondary ion mass spectrometry |
| SNMS | Secondary neutral mass spectrometry |
| SR | Synchrotron radiation |
| SRM | Standard reference material |
| SRS | Synchrotron radiation source |
| SWCNT | Single-walled carbon nanotube |
| TEM | Transmission electron microscopy |
| TGA | Thermogravimetric analysis |
| TIMS | Thermal ionization mass spectrometry |
| TPR | Temperature programmed reduction |
| TXRF | Total reflection X-ray fluorescence |
| UV-VIS | Ultraviolet-visible |
| VPD–DC | Vapour phase decomposition–droplet collection |
| VOC | Volatile organic carbon |
| WDXRF | Wavelength dispersive X-ray fluorescence |
| XAFS | X-ray absorption fine structure spectrometry |
| XANES | X-ray absorption near-edge structure |
| XAS | X-ray absorption spectroscopy |
| XPS | X-ray photoelectron spectroscopy |
| XRD | X-ray diffraction |
| XRF | X-ray fluorescence |
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