Atomic spectrometry update. Review of advances in the analysis of metals, chemicals and functional materials
Simon
Carter
a,
Andy
Fisher
*b,
Raquel
Garcia
c,
Bridget
Gibson
d,
Steve
Lancaster
e,
John
Marshall
f and
Ian
Whiteside
g
aHull Research & Technology Centre, BP, Saltend, East Yorkshire, UK HU12 8DS
bSchool of Geography, Earth and Environmental Sciences, Plymouth University, Drake Circus, Plymouth, UK PL4 8AA. E-mail: afisher@plymouth.ac.uk
cBP FPT Technology Centre, Whitchurch Hill, Pangbourne, RG8 7QR, UK
dIntertek Sunbury Technology Centre, Shears Way, Sunbury, Middlesex, UK TW16 7EE
eDomino Printing Sciences Ltd, Bar Hill, Cambridge, UK CB23 8TU
fGlasgow Caledonian University, Glasgow, Glasgow Scotland, UK G40BA
gTata Steel, Steelmaking and Casting Department, Grangetown, Middlesborough, Cleveland, UK TS6 6US
First published on 16th September 2015
Abstract
There has been significant progress in the use of laser induced breakdown spectrometry (LIBS) as a method of monitoring steel production in real time. This has the benefits of both time and cost saving. There has been a drop in the number of papers reporting methods for the analysis of fuels. This may be a function of the drop in oil prices leading to reduced finance for research. However, the determination of S is one of the hot topics in this sample type. This is possibly a consequence of companies attempting to meet the increasingly strict regulations on S emissions. Analytical methodology, e.g. LA-ICP-MS, LIBS or XRF, which produces minimal damage to the samples is still increasingly popular. This is especially true for historical or archaeological sample types, e.g. paintings, pottery and old documents; but is also applicable to forensic materials. Often, with these sample types, the analytical data are then treated using chemometrics packages so that provenance may be determined or patterns detected. Similar methodology was used for the identification of different plastics types, facilitating sorting and re-cycling. The analysis of nanoparticles is an increasingly popular subject area. Simple introduction of samples containing nanoparticles through a standard nebuliser/spray chamber sample introduction system is often complicated by the particles agglomerating. This leads to under-estimations of the concentrations present. Single particle analysis has continued to be a popular research area with real samples now being extracted and analysed rather than standard solutions. Similarly, field flow fractionation coupled with ICP spectrometry (sometimes also in conjunction with single particle analysis) has also been used to characterize nano-particulates and to distinguish between particulate and ionic species.
Foreword
This is the latest review covering the topic of advances in the analysis of metals, chemicals and functional materials. It follows on from last year's review1 and should be read in conjunction with other reviews in the series.2–61 Metals
Within this review period, a large number of papers have been published in which established spectroscopic techniques have been used to provide a comprehensive analysis of complex metal samples. Many of these are critical to the development of advanced products and their processes and generally require a multiple spectroscopic approach to deliver the required level of characterization. Noteworthy contributions in this area are listed in Table 1, and as a general observation, most multiple method approaches rely on MS detection within the suite of techniques to provide the required selectivity and sensitivity.| Analyte | Matrix | Analytical technique | Comments | Reference |
|---|---|---|---|---|
| Ag and Cu | Bimetallic Cu–Ag superstructures (active glucose sensor surface) | SEM, TEM, XRD, XPS and ICP-OES | Identification and characterization of the Cu–Ag bimetallic structure constructed by nanoparticles | 7 |
| Al, Ti, Cr and O | Ni-based super alloys (aerospace, gas turbines) | Focused ion beam-SIMS | Evaluated the oxidation mechanism under operation load at a sensitivity, which identified the increased kinetic pathways associated with the alloy deformation under load | 8 |
| B | Steel (nano-particulates) | Focused ion beam-TOF-SIMS | Identification and quantification of boron nanoparticles around the gain boundaries with a spatial resolution of approximately 200 nm through the use of Ga focused ion beam. Ionization probability was improved by changing the surface chemistry prior to analysis via oxygen implantation | 9 |
| Cr and O | Stainless steel | TOF-SIMS | Characterization of the passive oxide surface with sufficient sensitivity to identify oxide modification by bacteria | 10 |
| Mg | Mg-rich intermetallics | XRD, XRF, SEM, and differential scanning calorimetry | The authors presented the analytical approach as a means of providing sufficient structural detail of Mg-rich intermetallics to support the experimental assessment of their efficacy as a medium for hydrogen storage | 11 |
| N and O | Martensitic stainless steel (surface modification) | XRD and GD-OES | Supported the understanding of the corrosion properties of stainless steel surfaces modified by plasma nitriding. Identified higher temperatures increases corrosion resistance | 12 |
| N and Zr | Bearing steel (surface modification) | GI-XRD, AES, XPS and TEM | Analysis identified new amorphous, microcrystalline and alloy phases formed in the surface hardened by N + Zr ion implantation | 13 |
| Nb | Transformation induced plasticity steel | Optical microscopy, SEM, XRD, TEM, and ICP-MS | Establishing the influence of Nb on the Fe allotropic phase transformation and associated micro structural properties influencing strength and ductility. TRIP steels are typically an automotive grade under development | 14 |
| Nb and Ti | Pipeline steel (nano-particulates) | TEM, ICP-MS and electron backscattering diffraction | Metallurgical characterization of crystal grain size and non-metallic precipitates related to physical tests of steel strength and durability. This facilitated the development of algorithms linked to thermomechanical controlled process of the steel ‘finishing’ process to predict final properties | 15 |
Key papers, demonstrating significant contribution to the advancement of metals analysis, are highlighted in this review to profile state-of-the-art. However, the high proportion of papers analysing solid samples influences the scope of this review along with the growing interest in LIBS applications which is by far the most popular topic in this review period.
1.1 Ferrous metals
Efforts to improve the detection sensitivity of laser induced breakdown spectrometry (LIBS) continue on many fronts, rooted in the fundamental aspects of the laser metal interaction. Hao et al.16 demonstrated the use of a ring magnet to improve the sensitivity of LIBS analysis for V and Mn. The authors reported 3 sigma detection limits of 41 ppm for V (437.92 nm) and 56 ppm for Mn (403.08 nm) in ambient conditions which improved to 11 ppm and 30 ppm, respectively, using the ring magnet. Measurements showed an increase in both plasma temperature and electron density which was attributed to spatial and magnetic confinement of the plasma.The alternative approach to improving plasma temperatures, with dual pulse lasers, has been used for several years. Pedarnig et al.17 used an orthogonal double pulse configuration to examine the effect of laser parameters on sensitivity. In this study, Cl atoms were chosen because of their high excitation energies of 9.2 eV and 10.4 eV at the emission wavelengths of 134.72 nm and 837.59 nm, respectively. Detection of Cl on Fe2O3 (powder) and Fe3O4 (ceramic), provided the complex ferrous matrix for a realistic assessment of the effect of laser parameters on the sensitivity in ferrous metals. The authors reported that the inter-pulse delay between the ablation pulse and the excitation pulse, and the wavelength of the latter were the most critical factors affecting detection sensitivity. Laser excitation at 193 nm showed significant emissions from Cl atoms indicating elevated plasma temperatures, not seen using IR excitation at 1064 nm. Inter-pulse delays of 1 μs were required with the UV excitation; detection was configured to accommodate rapidly decaying Cl emissions. In a direct comparison between single and dual pulse operation, the authors concluded that orthogonal dual pulse operation provided higher detection sensitivity because of the higher signal to background ratio.
The fundamental principles of LIBS are aligned with application to standoff analysis and therefore uniquely suitable for in-process applications. In steel production, the detection of P at low concentrations is of considerable importance but presents detection difficulties as a UV emitting element. Notable work by Li et al.18 explored the use of the P atomic emission line at 214.91 nm: a longer wavelength to the line more commonly used. The P could be detected down to 12 ppm in pig iron and 9 ppm in low alloy steels. Achieving this level of detection in air required careful optimization of the time gating to minimize interference from iron and copper. Element detection at UV wavelengths in air provides an insight into what may be achieved in a production environment with a level of engineering not complicated by gas atmosphere control. This concept was also covered by Jiang et al.19 covering C and S: two additional elements that emit in the UV region and that are critical to steel production. A range of laser and spectrometric parameters affecting the analytical sensitivity was assessed. Optimization was based on collinear, IR, double-pulsed configuration using a 200 mJ ablation with a 15 ns pulse length followed by a 665 mJ pulse at a 6 ns pulse length. The authors claimed single figure ppm levels of detection for these elements. Both Li et al. and Jiang et al. have demonstrated low detection limits for the difficult elements important in steelmaking and achieved this under ambient conditions.
Sturm et al.20 demonstrated rapid analysis of molten slag using LIBS, directly from the steelmaking process. Applying LIBS directly to slag in the ‘slag-pot’, which receives molten slag directly from the ladle, presented a pragmatic approach which operated on a 24 hour basis for 3 months within the steelmaking production process. Good agreement with the standard XRF analysis was reported for the critical oxides: CaO, SiO2, MgO and Al2O3 including total Fe losses.
Considerable engineering difficulties are associated with analytical instruments operating in this harsh environment. However, Aula et al.21 presented a novel approach based on the atomic emission data produced from the electrical arc discharge of arc furnace operation. The authors reported that emission data could be remotely measured, which represented the slag composition changing as part of the refining process. Spectroscopic analysis of the plasma conditions was derived from the Ca atom line, which indicated that an outer plasma temperature was below 7000 K. The authors proposed this region as the most sensitive for quantitative measurements.
Xu et al.22 proposed a novel approach to improve LIBS classification accuracy during the identification of the quality grades in steel bars. The authors proposed the use of model population analysis as a framework for selecting identifying variables form LIBS generated spectra. Within the model population analysis framework, accuracy influence analysis was used to identify and quantify the elements which are critical grade identifiers. The authors claimed this approach produced a robust model within the combined support vector machine network to identify nine different grades of steel bars. Conversely, work by Sheng et al.23 focused on the raw materials input side of steelmaking developing LIBS classification for iron ore samples. The random forest technique was compared with support vector machine network classification. The authors concluded that each method could adequately classify the iron ore samples analysed as unknown samples but claimed the random forest technique demonstrated a superior level of predictive accuracy.
Boue-Bigne24 reported the development of an off-line LIBS system for metallurgical assessment of cast steel products, integrated into the steelmaking process. The quantification of C segregation in cast products was achieved via spatially resolved mapping to identify cementite networks within the high carbon region. The rapid analytical capabilities and the application to C segregation offer an advantage over conventional methods used for routine segregation assessment.
The importance of element segregation in steels is also highlighted by Pickering and Holland.25 A portable XRF analyser was used to provide a rapid and quantitative analysis of segregation with respect to Cr, Mn, Mo and Ni with Si proving problematic. The authors proposed this approach for the detection of long-range variations in composition and in cast products from the steel plant.
A novel analytical approach to evaluating the multi-element dissolution of amorphous passive layers was reported by Klemm et al.26 As part of a study into the corrosion of new bulk amorphous metallic alloys the dissolution rates were assessed electrochemically in combination with on-line ICP-MS. In this work, the alloy Fe50Cr15Mo14C15B6 was analysed to evaluate this approach. This method provided an accurate measure of the dissolution rate for each of the composite elements, reflecting the passive behaviour of the metal surface and the growth of the oxide layer.
1.2 Non-ferrous metals
Papers on archaeometallurgy tend to feature more strongly in the non-ferrous metals category with several innovative publications on this topic featuring in this review period.The use of stable isotopes to resolve provenance issues in heritage artefacts is a very interesting and powerful approach. Yamazaki et al.27 demonstrated the value of the stable isotopes 124Sn and 120Sn in resolving provenance in bronze heritage artefacts. Ratios of these isotopes were examined using a multiple collector ICP-MS system applied to the analysis of ancient bronze swords. The authors demonstrated that this isotope ratio varied during high temperature processing because of the volatility of Sn and slight losses of the lighter isotope, which altered the ratio and significantly reduced identification sensitivity.
Stable isotopes evaluation offers considerable potential beyond archaeological provenance. Mineral raw materials used in primary metals production offers an opportunity to identify raw materials provenance in the final product for supplier identification. In addition, applications within metal production process could be of significant value where variation in the source of raw materials occurs. Some insight into this application can be gained from the work by Ashkenazi et al.28 who evaluated the provenance of brass artefacts from a 19th century Egyptian shipwreck. Using a combination of radiographic techniques, XRF, scanning electron microscopy-energy dispersive spectrometry (SEM-EDS) and micro-hardness, metallurgical information was provided that identified one brass artefact had been drawn to wire from a bar and the other had been made from sheet. Mass spectrometry provided data on the stable lead isotopes allowing the authors to conclude that the raw materials were made from ores from different geographical locations suggesting one of the items had been imported.
Syvilay et al.29 described a parallel approach to provenance based on the identification of trace elements in lead sheet used in historical monuments. Analysis was provided by LIBS in conjunction with laser ablation (LA)-ICP-MS. The authors claimed sufficient sensitivity to identify differences between lead sheets to allow classification to establish origins and chronological periods of production and repair.
Many LIBS applications often involve the theoretical studies using models to account for the various laser parameters and the interaction with the target metal. Models continue to be improved and progressively more variables are being added to improve comparisons with the experimental results. Shirvani-Mahdavi et al.30 further developed the concept of calibration free LIBS by addressing the complex phenomena involved in the formation of a laser initiated plasma. A peak-intensity-based model was applied to an alloy of Au, Cu and Ag to calculate the self-absorption coefficients for each element. An algorithm was proposed which avoided the need to consider the emission line broadening during the compensation for self-absorption. The authors reported a relative error of 0.34% from the certified value for Au, an order of magnitude improvement in accuracy compared with the conventional approach.
Within the metals industry, the accurate determination of O is of considerable importance especially in metal coatings or in the study of corrosion. The use of glow discharge optical emission spectrometry (GD-OES) and GD-mass spectrometry (GD-MS) features strongly in this area as a means of depth profiling. As with LIBS, an understanding of the plasma formation mechanisms is often required to progress analytical capabilities. The accuracy of O determination using GD-OES was improved by Gonzalez-Gago et al.31 who utilized new calibration standards to develop matrix specific calibrations for the aluminium, magnesium and copper matrices. It was reported that the intensity of the O 130.22 nm atom line is affected by a blue-shifted line for the Al and Mg matrices. The gas discharge pressure required to maintain the voltage and the current constant, is lower than for Cu due to the high secondary electron emission of Al and Mg. At lower gas pressures, the blue shift effect was reported to be more pronounced. This matrix effect was attributed to the Doppler effect in electronegative elements and the authors proposed that this effect is an important factor to consider for other elements. Accommodating this effect and internally referencing the oxygen to the matrix metal emission was proposed as a method of developing matrix specific calibrations to improve quantitative accuracy. Additional knowledge developed for this work allowed the instrument parameters e.g. Ar/He gas mixtures to be developed to improve sensitivity.
2 Chemicals
2.1 Fuels and lubricants
This year's crop of papers was fewer in number compared with last year's contributions. This could possibly be reflective of the global drop in crude oil prices and corresponding decrease in funding for research and commercial activity. It also yielded a large number of application-based papers rather than those based on advances in analytical techniques. Sulfur is a hot topic in this section possibly driven by the need to meet increasingly strict regulation requirements globally. A large number of contributions came from China and South America; possibly indicative of the growing markets in these regions. There are a number of papers outlining complicated alternatives to straightforward industry standard methods. This might be interesting and yield a paper for the author but is not much help to the industry where these alternative methods are not economically viable. Authors should take into account the commercial pressures within the industry when considering their research and understand the need for simple-elegant-smart solutions rather than complex time-consuming options.A method for the determination of Cl in gasoline using ICP-OES was proposed by Zhao et al.34 Samples were diluted 1![[thin space (1/6-em)]](https://www.rsc.org/images/entities/char_2009.gif)
:4 with kerosene and the Cl line at 134.724 nm was used. In order to eliminate carbon and maintain a stable plasma, oxygen was added to the auxiliary gas. The recovery for spiked gasoline samples was 96.6% and the limit of detection (LOD) was 0.27 mg L−1. The results were comparable to those obtained using microcoulometry. The proposed method had the advantages of simplicity, speed and sensitivity.
Ngila et al.35 described a method using functionalized nanometre-sized alumina supported micro-solid phase extraction coupled with ICP-MS for the preconcentration and determination of trace metal ions in gasoline samples. Nanometre-sized alumina functionalized with 3-(2-aminoethylamino)propyl trimethoxysilane was used as the packing material in the extraction device. This was coupled with ICP-MS for preconcentration and determination of Co, Cr, Mn, Ni and Ti. Limits of detection and quantification ranged from 0.2–0.7 ng L−1 to 0.7–2.3 ng L−1 respectively and a preconcentration factor of 40 was achieved. Nanoparticles were also used by Ebrahimzadeh et al.36 for the extraction of trace Cd ions and their determination in diesel oil samples. This paper describes a novel highly selective magnetic sorbent synthesized for rapid preconcentration of Cd(II) ions. A cadmium-imprinted polymer was grafted onto Fe3O4 nanoparticles to give a magnetic property to the sorbent. The retained Cd was determined using FAAS. The LOD and RSD values were 0.09 μg L−1 and 1.7%, respectively, under optimum conditions.
A paper by Chainet et al.37 described Si speciation in light petroleum products using gas chromatography coupled with an ICP-MS instrument equipped with a dynamic reaction cell. Silicon is a ‘hot topic’ in the industry at the moment and has recently gained interest in the oil and gas industry because of the significant poisoning problems caused by silicon on hydrotreatment catalysts. The coupling of gas chromatography with ICP-MS allows determination of the retention times of Si species. Hydrogen was used in the dynamic reaction cell to reduce interferences present on the 28Si isotope. The linearity was good for Si compounds and instrumental detection limits ranged from 20 to 140 μg kg−1 depending on the response of the compound.
The determination of S was also represented in this section with a couple of papers proving interesting. A paper on S species in coals from the Argonne was presented by Bauer et al.40 The authors described a new direct solid sampling method for speciation of S in coals using electrothermal vaporization (ETV)-ICP-OES. Coal was decomposed in an argon atmosphere using controlled thermal decomposition to determine the different S species in addition to elemental S. The described method is time and cost effective and well suited for the fast characterization of S species in coal. In the second paper, S isotope studies of solid organics were undertaken by King et al.41 using secondary ion mass spectrometry (SIMS). They presented 34S/32S isotope compositions for nine solid organic samples, six petroleum cokes and three solid bitumens demonstrating a reliable method for complex, inhomogeneous materials. Their protocol used a homogeneous working standard and a heterogeneous matrix standard. Statistical measures identified outliers in the analyses. The authors demonstrated screening methods that minimized the impact of heterogeneities in the SIMS data. This led to a calibrated matrix correction that can facilitate determination of S isotope composition in solid organic samples. The approach is applicable to a wide range of materials and provides a means to obtain meaningful SIMS data when suitable homogeneous standards are unavailable.
de Gois et al.42 described a new method for the determination of Br and Cl in coal using ETV-ICP-MS. The procedure does not require any significant sample pre-treatment and allows simultaneous determination of both elements to be carried out. Operating parameters, including carrier gas flow-rate and radio frequency (rf) power, were optimized for maximum sensitivity and the use of modifiers/aerosol carriers Pd + Al and Pd + Ca were evaluated. The Pd + Ca was preferred because of the higher thermal stability provided by this combination. Chlorine and Br were accurately determined using calibration against solid CRM standards but Br could also be determined using calibration against aqueous standard solutions. The limits of quantification (LOQ) were 0.03 μg g−1 for Br and 7 μg g−1 for Cl. No spectral interferences were observed.
A method of quantitative chemical profiling of coal using core-scanning XRF techniques was described by Kelloway et al.43 The authors set out to test the possibility of measuring detailed quantitative profiles of different elements in exploration cores from coal seams using automated EDXRF. This approach has the potential for rapidly determining the distribution of mineral matter in a coal seam and identifying horizons at which particular elements may be concentrated. Profiling was carried out on a series of segments from a 60 mm diameter core of the Goonyella Middle seam from the northern Bowen Basin of Queensland Australia. Spectra were obtained at intervals of 200 μm along the axis of each core. A series of calibration curves derived from separately-scanned pressed pellets of reference coals was used to determine the concentration of each element allowing a set of quantitative element profiles to be created for each core segment. These profiles were evaluated in conjunction with the relevant X-radiographs and optical images to provide an integrated basis for assessing the variations in inorganic element characteristics through the core sections.
Pereira et al.45 described a method for microwave-induced combustion of light and heavy crude oil for REE determination using ultrasonic nebulization ICP-MS. Samples of crude oil up to 250 mg were inserted in polycarbonate capsules and combusted using 20 bar of oxygen and 50 μL of 6 mol L−1 ammonium nitrate. Nitric acid solutions were evaluated for analyte absorption and a reflux step applied after combustion. When 3 mol L−1 HNO3 was used and results compared with those obtained using microwave-assisted digestion combined with ultraviolet radiation and neutron activation analysis, recoveries better than 97% were obtained.
The feasibility of ICP tandem mass spectrometry (ICP-MS/MS) to overcome polyatomic interferences was investigated for the determination of P, S and Si in biodiesel, diesel and lubricating oil by Amais et al.46 This instrument arrangement comprised an octopole reaction/collision system between two quadrupole mass analysers with oxygen being used as the reaction gas in the octopole reaction/collision cell. Determinations were made ‘off mass’ at 16 atomic mass units higher than the original elemental isotope. Fuel standard reference materials were microwave digested using nitric acid and hydrogen peroxide. Adequate precision, accuracy and sensitivity were obtained when using the mass shift mode and recoveries for biodiesel, diesel and lubricating oil digests ranged from 95.0 to 113%. No significant differences were observed between the certified values and the ones obtained using ICP-MS/MS at the 95% confidence level.
An interesting paper was submitted by Desprez et al.47 who coupled gel permeation chromatography with high resolution ICP-MS to investigate the size distribution of Ni, S and V compounds in crude oils and their distillation cuts. The results show a trimodal distribution of V and Ni compounds in the crude oils, atmospheric residues and vacuum residues. For S compounds, either a mono- or bimodal distribution was produced depending upon the distillation cut considered. A correlation exists between the S fraction retention times and the temperature cuts of the distillation for temperatures below 560 °C and between the viscosity of the crude oils and the proportion of trapped S compounds in the higher boiling temperature fractions. For Ni and V the thermal treatment applied for the distillation appears to increase the aggregation of the low and medium molecular weight compounds into higher molecular weights especially when the crude oil has a high total S content. Size exclusion chromatography coupled with high resolution ICP-MS was also used by Gaulier et al.48 to investigate Hg speciation in liquid petroleum products in comparison with a modified Universal Oil Products (UOP) 938 method. The modified UOP 938 method allowed various Hg compounds to be grouped into different families, i.e. particulate, volatile, ionic and organic non-ionic Hg whereas size exclusion high resolution ICP-MS provided size distribution profiles of mercury-containing molecules. Comparison of the two different approaches was then performed with real hydrocarbon feeds, such as crude oil, condensate and straight-run gasoline samples. Elemental Hg was present for the North Sea condensate and all of them contained ionic Hg associated with molecules containing few to many tens of carbon atoms.
Osmium can be used to investigate petroleum system processes. Sen et al.49 investigated the determination of Os concentrations and187Os/188Os ratios in crude oils and source rocks using high pressure, high temperature digestion with sparging OsO4 and multi-collector ICP-MS analysis. The 187Os/188Os ratio is based on the beta decay of 187Re to 187Os with a half-life of 41.6 billion years. Despite its broad applicability to studies of hydrocarbon deposits worldwide, a suitable matrix-matched reference material for Os determination does not exist. In this study a method that enables Os isotope measurement of crude oil with in-line Os separation and purification was proposed. This method significantly reduces the total procedural time compared with conventional Carius tube digestion followed by Os separation, purification using solvent extraction, micro-distillation and negative thermal ionization mass spectrometry (TIMS) analysis. A commercially available vanadium crude oil reference material from the National Institute of Standards and Technology (NIST) (NIST 8505) was analysed and the method yielded an Os concentration of 28 ± 4 pg CI (per cubic inch) and a 187Os/188Os of 1.62 ± 0.15. The reference material NIST 8505 is homogeneous with respect to Os concentration at a test portion size of 0.2 g. Therefore, 187Os/188Os composition and Os concentration of NIST 8505 can serve as a matrix-matched reference material for Os determination.
Determination of Cd in biodiesel using micro-emulsion and electrothermal atomic absorption spectrometry (ETAAS) was investigated by Lima et al.51 The biodiesel samples were prepared using n-propanol as an emulsifier and 10% (v/v) nitric acid as the aqueous phase. The optimized conditions for micro-emulsion formation by volume were 57.6% n-propanol, 21.2% biodiesel and 21.2% of the nitric acid solution. The stability of the micro-emulsified samples was investigated using aqueous and organic standards and the samples were stable for at least 240 minutes. The applied pyrolysis and atomization temperatures were 800 and 2000 °C, respectively, and 5 μg of Al was used as the chemical modifier. The LOD and LOQ were 0.2 and 0.5 μg kg−1, respectively.
Gedik and Yurdakul52 assessed the chemical content of base oil blends called Number 10 lube, which is used as an alternative diesel fuel in Turkey. The study was conducted as a first step to assess the occurrence, spatial distribution, and potential sources of Ag, As, Ba, Bi, Cd, Cl, Co, Cr, Fe, Li, Mn, Mo, Ni, Pb, Rb, Se, Sr, Ti, V and Zn in Number 10 lube. A microwave-assisted combustion procedure was applied for the determination of metals and metalloids using ICP-MS while a commercial field test kit (Dexsil Chlor-D-Tect Q4000) was used for Cl. The range of total metallic/metalloid elements was between 0.04 and 189 μg g−1 and for Cl from below detection to 825 μg g−1. Unexpectedly, samples varied in type and levels of constituents between the western and eastern parts of the country. The enrichment of Zn, Mo and Cl suggests that some sort of waste oils, lubricating oils, chlorinated solvents, or transformer oils were mixed with the base oils. The metal emission rates derived from the annual consumption of Number 10 lube were far beyond the estimations for diesel vehicles and industrial sources. This problem, which leads to financial, environmental, and health concerns in Turkey, could possibly be experienced in other countries using similar fuels.
A method for the decomposition of biodiesel samples for elemental analysis using ICP-OES was described by Packer et al.53 Their method used a high pressure asher to digest the samples. Their aim was to remove the need to analyse these samples directly to avoid instrument interferences, carbon build up and exposure of laboratory personnel to harmful solvents. Optimum digestion conditions were 1.5 g of biodiesel, with HNO3 and H2O2, at a temperature of 300 °C and a pressure of 435 psi. Using this method Al, Ca, Cu, Fe, K, Mg, Mn, Na, Ni, P, Sr and Zn were determined in soybean, castor oil, cotton, sunflower and animal fat biodiesels by ICP-OES with the use of standard additions. Detection limits ranging from 0.05 to 0.7 mg kg−1 were achieved which are adequate to conform to worldwide biodiesel quality legislation.
Two review papers for biofuel analysis were of note this year. The first by Porizka et al.,54 containing 137 references, discussed algal biomass analysis using laser-based analytical techniques. Algal biomass has recently found many potential applications in various fields of interest. Its utilization has been advantageous in the fields of bioremediation, biofuel production and the food industry. This paper reviews recent developments in the analysis of algal biomass with the main focus on LIBS, Raman spectroscopy and LA-ICP techniques. The advantages of the selected laser-based analytical techniques were presented and their fields of use discussed in detail. The other review by Sanchez et al.,55 containing 212 references, addressed metal and metalloid determination in biodiesel and bioethanol. Biofuel quality control involves the determination of metal and metalloid content. These species play a very important role because they can modify the efficiency of biofuel production as well as the stability of these products. Some metals are toxic and generate environmental concerns whereas others are used as additives. Normally products such as biodiesel and bioethanol are mixed with conventional fossil fuels such as diesel and gasoline. Metals can come from the raw product as well as from the production and storage process and even from the added fuels. The determination of the final metal and metalloid concentrations in biofuels is a challenging subject as their concentrations are usually low and sensitive techniques are required. Several approaches have been evaluated to carry out this kind of analysis. The most often employed techniques are ICP-OES and ICP-MS, together with atomic absorption methods. The different procedures applied were discussed in the review emphasizing the most widely used ones.
2.2 Organic chemicals and solvents
The application of non-invasive spectroscopic techniques in the cultural heritage domain continues to provide valuable data, particularly in the conservation and restoration fields. Most of the papers are related to specific applications generally aimed at obtaining chemical characterization of pigments and dyes and understanding degradation processes altering the works of arts as well as identification of the provenance of cultural objects. The integrated approach of micro-techniques, μ-XRF, μ-X-ray absorption near-edge structure (μ-XANES), μ-XRD and the use of both atomic and molecular spectrometry is well established in these fields.Interesting application-based papers, including those employing techniques with synchrotron sources for the analysis of high value objects, are included in Table 2.
| Element | Matrix | Technique; atomization; presentation | Comments | Reference |
|---|---|---|---|---|
| As, S | Pigments | SR-μ-XRF, μ-XANES; —; s | Synchrotron based X-ray micro-analysis was used to reveal the distribution of arsenic sulfide throughout the multi-layered paints. The authors reported for the first time evidence of water transport within paintings | 56 |
| Fe | Iron gall inks | SR-STXM; —; s | Ink penetration at the scale of a paper fiber was investigated using synchrotron based scanning transmission X-ray microscopy (STXM). In situ mapping of the iron redox state and carbon speciation down to sub-μm scale was achieved | 57 |
| Various (6) | Paintings in Parma Cathedral | XRF; —; s; XRD; GC-MS; μ-FTIR; μ-Raman | Characterization of pigments, identification of degradation products and restoration materials in one of the dome wall paintings was carried out using different analytical techniques. Analytes determined using XRF were As, Bi, Co, Cu, Fe and Pb | 58 |
| Various (6) | Cosmetic face powders | MS; ICP; L; OES; ICP; L | The authors employed a microwave-assisted acid digestion method (HNO3–H2O2 or HNO3–HCl) for toxic metals (As, Cd, Co, Cr, Ni and Pb) determination in five commercial face-powders. Concentrations in the range of 0.06–8.0 μg g−1 were reported. Comparison was made with total digestion using HF. Method validation achieved using three CRMs. Bioavailability test also undertaken using simulated sweat | 59 |
| Various | Manuscripts | XRF; FORS, —, s | The use of fiber optic reflectance spectroscopy, with low spectral resolution enabling the creation of primary pigments maps, was demonstrated in this paper. Identification was achieved using XRF | 60 |
| Various (10) | Pigments | FTIR; Raman; XRF; —; s | Bismuth vanadate was identified in automobile finishes in situ using FT-IR, dispersive Raman spectroscopy and XRF spectrometry. The presence of other analytes (Al, Cr, Fe, Mo, Ni, Pb, Si, Ti) also proven in some pigments | 61 |
| Various (8) | Document papers | MS, ICP, L | The authors presented a quantitative method validation for determination of eight trace level metals (Al, Ba, Fe, Mg, Mn, Pb, Sr and Zn) in document paper samples | 62 |
| Various (3) | Paper | TOF-SIMS; —; s | Chronological deposition sequence of fingermarks and ink on porous paper surface was studied using TOF-SIMS determination of Ca, K and Na. Results were corroborated through inter-laboratory validation exercises | 63 |
| Various (4) | Wool carpets | HPLC-DAD-MS; MS; ICP; L | Mild extraction of wool fibers followed by high-performance liquid chromatography (HPLC) with diode array and mass spectrometry detection was carried out for natural dye compound identification. Quantification of mordants was achieved using ICP-MS. Analytes were Al, Cu, Fe and Zn | 64 |
| Various (30) | Paintings | pXRF, MS; ICP; LA | A multi-technique procedure for identification of substances materials from surface and sub-surface domains of wall paintings was presented | 65 |
| Various (16) | Pigments | SR-μ-XRF, μ-XANES; —; s | Metal inclusions and impurities in historical samples of Zn-based white pigments were investigated. Synchrotron analysis (LUCIA beamline at the SOLEIL radiation facility) enabled micrometric special resolution and sub ppm sensitivity | 66 |
| Various (10) | Paintings | MS; ICP; LA | This study presents one of the few carried out for elemental-stratigraphic analysis of paintings. Sampling damage is hardly noticeable. The system had a resolution from 1 to 2 and from 5 to 10 μm. Results for Al, Ca, Cu, Fe, Hg, Mg, Pb, S, Si and Ti were compared with those obtained using SEM | 67 |
| Various (68) | Nano-diamonds | AES; ICP; L | Various sample preparation methods were compared for analysis on micro-impurities from 20 commercial nano-diamonds samples: direct introduction of suspensions, ashing with microwave-assisted acid extraction and slurry nebulization. The authors presented a study which provides new possibilities for certifying nanodiamonds' purity | 68 |
| Various (4) | Synthetic diamonds | AES; ICP; L | Samples were burned at 1000 °C for 10 h and mixed with H2SO4, aqua regia and HClO4. Limits of detection for Co, Fe, Mn and Ni were in the range from 0.0006 to 0.0147 mg L−1. The paper was in Chinese | 69 |
Well-known examples of light-induced degradation of pigments include the discoloration of “chrome yellow”, the pigment mostly favored by van Gogh, and the darkening of vermilion (α-HgS, cinnabar). Hogan et al.70 proposed a degradation process of mercury compounds based on ab initio calculations of XANES spectra for various chlorides and sulfochlorides of mercury in combination with predictions of their colour (obtained by quasiparticle methods). Interactions of light and humidity with Cl triggers the growth of secondary structural unstable minerals that lead to the release of grey elemental Hg(0) which precipitates. This is the ultimate species responsible for the observed darkening.
The study of alteration products of chromium-based pigments in van Gogh paintings using full spectral XANES imaging with the Maia detector array was reported by Monico et al.71 The authors presented the application of a fast detector, the Maia-384 massively parallel detector, which consists of an annular array of 384 silicon diode detectors positioned in a backscatter geometry with respect to the incident X-ray beam, as a new tool to overcome limitations of the traditional EDXRF data acquisition. Experimental results from full spectra-XANES imaging data obtained at the Cr K-edge using the Maia X-ray detector and corresponding μ-XRF mapping and single point μ-XANES results were compared with those obtained using conventional silicon drift detectors. For equal dimensions of scanned areas but different pixel sizes, acquisition times were reduced by a factor of 102 to 103 with respect to similar experiments carried out using a silicon drift detector. The delivered X-ray dose was also reduced by a factor of 104 to 102 as a result of the reduced dwell time (0.5–3 ms per pixel). However, the Maia beamlines did not reveal details at the same level of definition of few micrometers that were visible in high resolution maps collected using silicon drift detector microprobe systems.
The technique of LIBS is becoming established for pigment analysis of a wide variety of objects of cultural heritage and attempts have been made to use it for the study of the composition of papers and inks. Metzinger et al.72 presented a new approach for the discrimination of paper types and prints made by digital printers by statistical evaluation of LIBS spectra. Five data evaluation methods were used including three comparative functions (linear correlation, sum of squared deviations and overlapping integral) and two advanced statistical methods: multivariate curve resolution alternating least squares (MCR-ALS) combined with classification tree and discriminant analysis (DA). The newly introduced approach, MCR-ALS-DA, provided the best classification results in the identification of paper type (96.3% accurate) and printer type (83.3% accurate). The authors highlighted the good accuracy of the novel approach considering the special and varying ablating behaviour of papers and prints and the high level of similarity of LIBS spectra. Discrimination results were collected from a single laser shot. This work illustrates the power of combining LIBS data and statistical evaluation of data to maximize the knowledge yielded from the LIBS technique.
Gaona et al.74 presented a novel strategy for standoff recognition of explosive fingerprints on solid surfaces using a supervised learning method and LIBS. In standoff instruments, the distance between the target and the sensing platform plays an important role in determining the signal intensity and the noise of the information collected. The emission response from the investigated substance it is governed by external conditions. The authors used, for the first time, an algorithm based on the adaptation of a supervised classification model to changing experimental situations, handling the inherently multifactorial nature of the standoff LIBS analytical response. Residues of ordinary materials (olive oil, fuel oil, motor oils, gasoline, car wax and hand cream) hardly caused confusion in alerting the presence of an explosive (DNT, TNT, RDX or PETN) when tested from arrange of 30 to 50 m and with laser irradiance changing between 8.2 and 1.3 GW cm−2. The authors acknowledged that a universal strategy coping with all possible changes (light propagation through the atmosphere, weather conditions, etc.) is difficult to accomplish and reflected on future developments to improve the consistency of the strategy to significant changes in environmental conditions. The technique's capabilities are being enhanced significantly by work such as this. Laser plasmas are very reactive chemical environments because of their high electron density and elevated ionic temperatures. Atoms, molecules, ions and electrons interact and therefore interpretation of molecular emission spectra is difficult, in particular for species containing C, N, O or H. Delgado et al.75 carried out experiments to address the question of whether the LIBS spectrum mimics the structure of the molecule or the molecular information obtained corresponds to recombination of atoms and molecular fragments with ambient constituents. The influence of the surrounding atmosphere, from gas type (H2, N2) to pressure (from atmospheric to 10−5 mbar) on the spectra of energetic nitro-compounds (TNT and PETN) was evaluated with simultaneous detection at high vacuum of both emission and mass spectra originated from the same laser event. The authors proposed probable fragmentation pathways for these compounds, both containing NO2 groups in their structure as a native source of N. The explosive TNT is aromatic and therefore has carbon–carbon double bonds. The suggested routes were supported by LIBS measurements where many non-emitting, reactive species present in the plasma were identified. Pressure was identified as being a critical factor affecting the dynamics of the plasma. In a nitrogen atmosphere, C2 emission was strongly related to molecular structure whereas CN was mostly produced by chemical reactions. Results in a hydrogen atmosphere suggested that the hydrogen altered the formation pathways for molecular species by reducing the CN and C2 emissions and favouring the formation of NH, CH and OH. In the absence of air molecular emissions were found to be very weak.
The reader may find a review paper with 50 references76 exploring laser-induced decomposition of different explosives by various spectroscopic techniques including LIBS, MS, FTIR, UV-Vis, XPS, XRF, etc., useful. The review covered three main groups of explosives (nitrate esters, aromatic nitro compounds and nitramines). The initial decomposition steps were similar for similar explosives. Various spectroscopic methods played an important role in detecting transient intermediates and only few studies provided detailed information on the whole process of decomposition. Another review with 111 references was produced by El Haddad et al.77on good practices in LIBS analysis. The review presented analytical results obtained using LIBS for identification and classification of samples as well as results for concentration measurement based on calibration, discussing both univariate and multivariate approaches. Good practices for both classification and concentration measurements were also presented.
Sreedhar et al.78 illustrated the power of the use of multivariate statistical data processing techniques in maximizing the knowledge generated from LIBS measurements. The authors proposed a methodology to classify three high energy materials with diverse composition using nanosecond LIBS data. Elemental peak ratios for O/N, N/H, and O/H were calculated using a ratiometric method and used to construct 1D, 2D and 3D classification models.
Direct elemental determination of inorganic contaminants in APIs using LA-ICP-MS presents an analytical challenge, mainly due to lack of matrix matched reference materials for calibration and validation purposes. Rudovica et al.80 developed laboratory made matrix calibration standards for quantification of six elements (Cd, Co, Cu, Mn, Ni and Pb) in the API arbidol tablets using LA-ICP-MS. Various operating conditions of the laser ablation system such as laser radiation energy, pulse rate and spot size were optimized and the homogeneity of the elemental distribution in the standards evaluated. Mass concentration in the standards was in the range 1 to 150 μg g−1. Calibration graphs with regression coefficients better than 0.997 were achieved. The authors reflected on the fact that further evaluation of long-term stability of prepared standards would be required.
The control of residual genotoxic impurities is a critical factor in the development of APIs. Genotoxic impurities form a special case that poses significant risk, even at low concentration, because they may be mutagenic and are therefore potentially damaging to DNA. As a result they can lead to mutations or cause cancer. Analysis of genotoxic impurities can be very challenging because they must be controlled at levels significantly lower than 0.01–0.03%. Therefore sensitive and selective methods for these impurities at sub-ppm levels are required. Some significant analytical developments in this field are reviewed, including two papers which reported novel methods for trace-level determination of genotoxic impurities in APIs by LC-ICP-MS employing iodo-derivatization. Harigaya et al.81 presented a novel sensitive determination of 4-chloro-1-butanol, a genotoxic alkylating agent. This compound originates from tetrahydrofuran, which is frequently used as a solvent in API synthesis, by reaction with hydrofluoric acid. When using tetrahydrofuran and HCl in API synthesis, residual amounts of 4-chloro-1-butanol must be monitored. The authors exploited the selectivity and sensitivity of two-dimensional liquid chromatography (LC) hyphenated with of ICP-MS for impurity determination by derivatization with 3-iodobenzoyl chloride. The linearity of the method was observed in the range from 0.5 to 50 μg g−1 of API. The LOD and LOQ were 0.2 and 0.5 μg g−1, respectively, values well above the thresholds specified in the guidelines. In a later study,82 the same investigation group expanded the range of options of iodo-derivatization for the determination of phenylhydrazine in antipyrine using the same technique and 2,3,5-triiodobenzoyl chloride as a derivatization reagent. Five different commercially available lots of antipyrine were analysed. Phenylhydrazine results were below the LOD (0.06 μg g−1) in all the lots.
Platinum Group Metals (PGM) are used widely as catalysts in the synthesis of APIs but may represent a health risk if they remain in the target product. According to the United States Pharmacopeia convention (USP), the concentration limit for Pd, Pt and Rh has been set at 10 μg g−1 in APIs. The determination of PGM at low levels (μg g−1 to ng g−1) is an analytical challenge because of the difficulty in bringing them in solution and occurrence of interferences when using spectroscopic techniques for analysis. Resano et al.83 investigated the potential of high-resolution continuum source (CS)-GFAAS for direct determination of Pd, Pt and Rh in two different types of samples: used automobile catalysts and APIs. The authors observed that the matrix in the API, consisting of organic compounds, was removed efficiently during the pyrolysis step. Stop flow conditions were used during atomization. Both Pt and Rh were monitored simultaneously; using 244.006 nm and 244.034 nm for Pt and Rh, respectively while for Pd another line was selected (360.955 nm). No modifier was added since the signals obtained were unimodal and very well defined. Limits of detection were calculated to be 0.08 μg g−1 for Pd, 0.15 μg g−1 for Pt and 0.10 μg g−1 for Rh for a mass of 5 mg, enabling the quantification of PGM at the limits set by regulations. This work presents an important step forward in PGM determination overcoming spectral interferences encountered by spectroscopic techniques such as ICP-OES.
A digestion method using a single reaction chamber (SRC) microwave system for determination of elemental impurities in API in compliance with USP requirements was evaluated by Muller et al.84 The system allowed temperatures and pressures up to 300 °C and 199 bar respectively. High digestion efficiency of up to 500 mg of sample with a carbon content in the digest lower than 250 mg L−1 was obtained. Digestion of API was also performed using conventional high-pressure microwave-assisted digestion system to compare efficiencies. The determination of elements was carried out using ICP-MS in standard mode and dynamic reaction cell (DRC) mode with ammonia as reaction gas in order to solve polyatomic ion interferences on 51V, 52Cr, 53Cr, 63Cu and 65Cu. The determination of As and Hg was undertaken using a homemade flow injection chemical vapour generation (FI-CVG) unit coupled with ICP-MS. Using this procedure masses of 500 mg were decomposed with 6 mL of concentrated nitric acid for As, Ad, Cr, Cu, Hg, Mo, Ni, Pb and V determination. A mass of 250 mg was decomposed with 4.5 mL of concentrated nitric acid and 1.5 mL of concentrated hydrochloric acid for determination of Ir, Os, Pd, Pt, Rh and Ru. The procedure was not suitable for Os determination due to memory effects. The method was validated in accordance to USP requirements. Spike recoveries were in the range of 94% to 108% except for Os where recoveries higher than 106% were obtained. Precision was evaluated using repeatability and intermediate precision studies ranging from 2.0 to 6.5%. Limits of quantification were calculated using 500 mg or 250 mg of sample and a final volume of 25 mL and values obtained were in agreement with USP requirements.
A review with 117 references was produced by Timerbaev85 on recent progress of ICP-MS in the development of metal-based drugs and diagnostic agents. Inorganic compounds, particularly those containing metal atoms, are highly promising as therapeutic drugs. In this review, the current status of ICP-MS to aid the discovery and development of metal-based drugs is assessed in terms of method's current capabilities and shortcomings as well as development trends.
The separation of four cobalamins (hydroxo-; adenosyl-; cyano-; and methylcobalamin) was described by Bednarik et al.86 by coupling thin layer chromatography (TLC) to diode laser thermal vaporization (DLTV) ICP-MS as an alternative to HPLC or capillary electrophoresis (CE) coupled to ICP-MS. The technique of DLTV was used by the same authors in previous studies as a sensitive and low cost technique of sub-microliter dried droplet analysis. The technique uses a 808 nm continuous wave diode laser to induce pyrolysis of the cellulose stationary phase on TLC sheets overprinted with black ink. The aerosol generated is carried into the ICP. The entire TLC lanes were then analysed using DLTV in a 17 cm long glass tubular cell. Laser positioning and focus as well as speed of the line scan were optimized to provide the maximum integrated signal to noise ratio for 59Co. The 8 cm long lanes were scanned in ∼35 s. Under optimized conditions, a LOD of 2 pg for each analysed cobalamine sample and 15% repeatability were reported. The different cobalamines were found to provide different analytical responses; therefore species-matched standards were used for calibration. The method was applied to quantitative determination of cobalamines present in a vitamin supplement, in which only cyanocobalamin (as opposed to the cyano-, hydroxyl- or methyl-analogues) was detected at a concentration of 130 ± 12 μg. This was in excellent agreement with results from a more traditional analysis method.
High levels of Cr were discovered in edible gelatin capsules used in pharmaceutical products in China. Chromium is known as carcinogen and can be toxic if ingested in large quantities. Lin et al.87 presented a novel method for the determination of Cr(III) and Cr(VI) in gelatin capsules by separation using CE and ICP-MS determination with a collision reaction cell. The authors used a commercial fused silica capillary for Cr species separation and the migration of ions was induced by electrophoretic and electroosmotic flow. Optimum conditions were achieved using 20 mM buffer NaH2PO4− 5 mM Na2B4O7 at pH 6 with an applied voltage of 15 kV. Chromium species were extracted into a buffer 5 mM 1,2-cyclohexane-diaminetetraacetic acid mixture. Rh was selected as internal standard for the ICP-MS analysis and addition of 5% methanol proved to improve ionization efficiency. At a peristaltic pump rate of 7 rpm the chromium species were baseline separated within 10 min. In order to overcome polyatomic Cr interferences, hydrogen was used as cell gas. 40Ar12C+ interference was suppressed when the flow rate of hydrogen was about 4.0 mL min−1. For capsules spiked with 0.3 mg kg−1 and 30.0 mg kg−1 of Cr(III) and Cr(VI) respectively, recoveries of 87% and 95% were obtained. This work presents a simple analytical method for Cr(III) and Cr(VI) determination which does not require derivatization.
The use of a direct current glow discharge micro-plasma combined with a mass spectrometer for organophosphate nerve agent detection was reported by Wang et al.89 The micro-plasma source provided sufficient energy to fragment and excite the targeted nerve agent. A small, low temperature, atmospheric pressure, inert gas supported plasma enabled the simultaneous detection of two parts of the nerve agent component, the elemental P and the organic CH radical. The influence of operational parameters such as discharge current and gas flow rates on the P signal were investigated. To compare the analytical ability of the micro-plasma, a microwave plasma was introduced. From a security point of view, a nerve agent stimulant, triethylphosphate was used and the detection limit was 5 ppm by volume. The problem of carbon deposition, common for hydrocarbon detection using plasma-based detectors, was avoided. The authors reflected on the possibilities of extending this methodology to screening S-containing chemicals.
Direct determination of trace elements in organic solvents by ICP poses a challenge because of problems associated with carbon loading of the ICP including plasma instability and spectral interferences by carbon and soot formation on the torch or injector. A novel online combustion system was developed by Wiltsche et al.90 for the quantification of metals (Ag, Cd, Cr, Fe, La, Li, Mg, Ni, Pb, Y, Zn and Zr) in acetone, methyl isobutyl ketone, chloroform, dichloromethane, tetrachloromethane and trichloro-trifluoroethane. The authors exploited the removal of carbon prior to introduction into the ICP by burning the sample in the presence of oxygen. A commercial carbon analyser was placed between the aerosol exit of the spray chamber and the torch of the ICP-OES. The sample aerosol was mixed with additional argon and oxygen in the carbon analyser oven where the combustion took place at 1050 °C effectively burning all carbon to CO2. The flow of argon and oxygen added to the aerosol as well as combustion conditions were optimized. The authors reflected on possibilities for improvement when the proposed method was applied to specific samples. Even if the ICP was stable, the whole analytical performed was degraded when compared with the introduction of aqueous samples, mainly because of the presence of 1% oxygen in the gas stream and analyte-specific losses in the combustion and desolvation system. Another problem encountered was the instability of peristaltic pump tubes towards various organic solvents. However, the proposed method presented advantages over other approaches. These included stable operation of the ICP even when introducing highly volatile samples that would normally extinguish the plasma. Analyte signals in halogenated solvents were higher than in non-halogenated ones. Non-linear calibration functions were observed for all investigated analytes below 5 mg kg−1.
The development of a new method for determination of C, H and O by ICP-OES in organic compounds such as glucosamine was undertaken by Odenigbo et al.91 A long demountable torch, with the outer tube being 2.5 cm longer than the regular torch, was used to reduce N emission background arising from ambient air. Operating conditions were optimized while monitoring the N 174.273 nm atomic emission line to maximize the signal-to-background ratio. Standard solutions of tris(hydroxymethyl)aminomethane were used for external calibration and D-glucosamine hydrochloride was used as a test solution. Internal standardization was not necessary. Samples were aspirated directly into the plasma without any pre-treatment other than dilution in double de-ionised water. Measured concentrations for C, H and O were in agreement with expected values, but N was not. The authors noted that the source of discrepancy for N was still under investigation.
The use of a micro-hollow glow discharge plasma in pulse mode was used for the first time by Vander Wal et al.93 for compound identification of solid materials in the form of dry powders. An alternative removable metal disk device was used as surrogate cathode for solid sample support. Samples ranging cellular/biological organics, fertilizer and inorganic samples were analysed. The acquired emission spectra, over the region covering 180–480 nm, provided molecular and elemental information. High-resolution spectra were useful in resolving and identifying atomic transitions such as Mg, Ca, Fe and Si for the inorganic materials. The greatest utility of this detector system would be its portability for field-based measurements. The same research group94 also exploited the analytical utility of micro-hollow cathode glow discharge plasma for detection of acetone, ethanol, heptane, nitrobenzene and toluene. Linear relationships between optical emission spectroscopy emission lines and parent compound concentrations over a wide range enabled detection limits of species of interest down to ppb levels.
2.3 Inorganic chemical, catalysts and acids
This year has seen an increase in the number of papers detailing the application of atomic spectrometry for the analysis of fertilizers and this has, therefore, been included in a new section. There was also increased activity in the field of catalyst characterization as well as continuation of popularity of atomic spectrometry techniques for the forensic analysis of gunshot residues and the analysis of building materials. A small number of papers which the reader may find useful, which deal with interesting applications are included in Table 3.| Element | Matrix | Technique | Comments | Reference |
|---|---|---|---|---|
| Various (21) | High-purity tantalum pentoxide | ICP-MS | Determination of 21 elements without the separation of sample matrix from analytes. Solutions with tantalum concentrations higher than 30 mg L−1 suffered significant matrix interference | 129 |
| Various (26) | High purity lithium hexafluorophosphate | ICP-MS | Sample was dissolved using absolute ethanol prior to determination of 26 impurity elements in the solution using ICP-MS. The LOD were in the range of 6–32 ng L−1 | 130 |
| Various (7) | High-purity germanium dioxide | ETAAS | Separation of analytes (Cd, Co, Cu, Cr, Mn, Ni, and Pb) from the matrix through reactive evaporation in the form of germanium tetrachloride | 131 |
| Various (5) | Sodium chromate | ICP-OES | Standard addition method was used to eliminate matrix effects. The LOD of analyte (Al, Ca, Fe, Mg and Si) impurities were in the range of 0.013 to 0.028 mg L−1. Sample recoveries between 97% and 107% were achieved. The paper was in Chinese | 132 |
| Various (9) | Black crust deposits on carbonate building materials in three Italian cities | MS, ICP, LA | A study of the impact of air pollution on the formation of black crust deposits on carbonate building materials on the Cathedral of Milan, the Cathedral of St. Maria del Fiore in Florence, and the Vittoriano Monument in Rome. All deposits showed general enrichment (of As, Cr, Cu, Ni, Pb, Sb, Sn, V and Zn) with respect to the substrate. The deposits from Milan were richest in heavy metals, particularly Pb and Zn, reflecting the severe air pollution. Deposits from the Cathedral of St. Maria del Fiore in Florence, facing a pedestrian area, showed little enrichment in heavy metals, whist those from the Vittoriano Monument displayed variable enrichment, attributable to mobile emission sources | 133 |
| Various (11) | Mortar samples from the Abbey of St John, Mustair, Switzerland | SR-XRF; —; s; XANES | Samples were taken from the Carolingian plaster and late Gothic plaster used at different periods in the building history. Analytes determined were Al, Ca, Cl, Fe, K, Mg, Mn, S, Si, Sr and Ti. The most abundant elements in all areas were Si, Ca and Fe, with Mg and Sr also prevalent. Spatial correlations indicated chemical bonding between them. The Ca K-edge XANES spectra were collected on areas of light and dark spots. Calcite was the dominant form of Ca in the samples. However, both Ca and Fe demonstrated different forms in the light and dark areas (vaterite and hydroxyapatite and Fe3O4 and FeO in the light and dark areas, respectively). The mortar was derived from sand from the local river bed. No significant differences were observed between the different time periods | 134 |
A new analytical method for the collection and quantification of GSR using ICP-OES was proposed by Vanini et al.97 Optimization of the spectrometer operating parameters, detailed in a later paper,98 allowed the determination of Ba, Pb and Sb with detection limits of 4.79, 0.15 and 1.49 μg L−1 respectively. The investigation focused on three important factors in sample collection: the collection region of the hand; tape-type versus sample swab; and concentration variations between left and right hands. Swab collectors provided greater collection efficiency compared with the tape-type collectors. Additionally, the extraction of analytes from tape-type collectors was performed in a microwave using a solution of nitric acid, hydrogen peroxide and water, whereas the extraction of analytes from swab collectors could be simply performed in an ultrasonic bath. One interesting observation was that male shooters had higher concentrations of Pb in the GSR at the back of the hand than found on women shooters. This phenomenon was thought to occur because male hands have more and thicker hair, allowing higher adhesion of GSR. The same group of authors published a further paper employing the optimized method for the characterization of GSR from a range of pistol types.99
A LA-ICP-MS method was reported for the detection and identification of compounds consistent with GSR from lead-free ammunition.100 Particles were collected using commercially available tape lift kits, designed for SEM-EDX analysis. Laser ablation of the carbon adhesive was performed using a 266 nm pulsed Nd:YAG laser with a 10 Hz repetition frequency. Ablated material was transported through a polyurethane tube to the ICP-MS instrument. Ablation stability was monitored using the 13C signal for the adhesive, whilst plasma stability was monitored using Th standard solution. The analysis of 20 elements allowed the detection of several aggregate particles related to the ammunition: Al–Ti, Cu–Zn, Cu–Zn–Sn and Sr–Zr. In addition, the presence of Pb–Sb–Ba particles was also detected, indicating a memory effect in the firearm from previous ammunition.
Cloud point extraction has gained popularity as a pre-concentration method of low level PGM because of its low cost and simple procedures. However, samples that require nitric acid for dissolution often suffer from interferences with the complexing agent, causing problems in the subsequent extraction stage. Suoranta et al.102 discussed the use of sulfamic acid as a means of eliminating the nitric-related problems for the preparation of a used ACC material, NIST SRM 2557, for subsequent analysis using ICP-MS. It was suggested that sulfamic acid acts as a nitrous acid scavenger, preventing the unwanted formation of nitrosyl complexes of PGM that can cause low recoveries during extraction. The optimized method involved the microwave-assisted extraction of 40 mL sample solutions at 90 °C in the presence of 0.5 g of sulfamic acid, 3 mL of 10% Triton X-100 and 4 mL of 1% 2-mercaptobenzothiazole with an extraction time of 75 minutes. Recoveries of Au, Pd, Pt, Rh and Ru obtained in solutions were between 80 and 100%. However, the recovery of Ir did not exceed 40%. Two alternative PGM pre-concentration methods were reported by Tokalıoğlu and colleagues at Erciyes University, Turkey. The first study used graphite oxide as an adsorbent and 2,6-diaminopyridyne as a complexing agent for the removal of Ag and Pd from aqueous solutions.103 Analytes were eluted using 2 mol L−1 HCl with their quantification being achieved using FAAS. The graphite oxide was reusable for 150 cycles and the LOD of Ag and Pd were 0.39 μg L−1 and 0.94 μg L−1, respectively. The second paper suggested the use of a Mn2O3 nano-sponge as a new adsorbent for the pre-concentration of Pd(II) and Rh(III) ions in a range of materials prior to FAAS determinations.104 A 30 s contact time was enough for both adsorption and elution and a pre-concentration factor of 100 was obtained using 100 mg of the adsorbent. The method was validated by analysing the NIST standard reference material, SRM 2556 (used auto catalyst pellets) and spiked real samples. The LOD were 1.0 μg L−1 for Pd(II) and 0.371 μg L−1 for Rh(III) and there was no significant loss in performance of the material after 120 procedures.
The direct solid-state analysis of metal impurities in a PGM catalyst has been achieved by means of AES with direct current arc discharge.105 The optimized analytical system was evaluated for the determination of Al, Fe, Ni, Si and Ti in a palladium oxide loaded carbon catalyst. Synthetic standards, prepared from graphite, palladium oxide and pure oxides of interest, were used as calibrants. The working range for element impurities was from 1% to 0.0003%, and the LOQ varied in the range from 0.002% (Ti) to 0.0038% (Si).
The in situ study of catalyst materials can give a valuable insight into their behaviour under reaction conditions. The combined application of in situ XRF and XAS analysis allowed for an advanced study of the atomic layer deposition-based synthesis of Pt catalysts.106 Both XAS and XRF spectra were recorded simultaneously at different catalyst loadings during deposition. Measurements were performed at the SAMBA beam line of the 2.75 GeV SOLEIL synchrotron (Saint-Aubin, France) using a top-up electron beam of 430 mA. Analysis of the combined in situ data yielded a quantitative picture of the evolution of the diameter, shape, lattice packing and density of the deposited Pt clusters. Additionally, the degree of oxidation at the cluster surface after ozone pulses, could be monitored. An in-depth understanding of the chemical state and spatial distribution of polypyrrole and Co in electrochemically synthesized oxygen reduction reaction electrocatalysts was realised using combined STXM, XRF mapping and μXAS measurements at the TwinMic beamline of Elettra synchrotron facility (Trieste, Italy).107 The photon beam was focused to a microprobe using zone plate optics and the imaging measurements were performed by simultaneous detection of transmitted and emitted (fluorescence) photons raster-scanning the sample with respect to the microprobe. Correlation of Co and N XRF maps showed evidence of bimodal Co distributions as micro-grains on a background of nano-grains and co-nucleation of PPy and Co. The co-location of O in the XRF maps and the Co XAS spectra taken in different locations of deposits formed under different conditions also revealed that Co was always present as an oxide.
The elemental and crystalline distributions of FeCrAlloy foams, coated with Rh containing layered hydrotalcite-type compounds active toward the partial oxidation of methane, were investigated.108 Combined μ-XRF and μ-X-ray powder diffraction (μ-XRPD) measurements in both 2D scanning and tomographic mode were performed at the MicroXAS beam line of the Swiss Light Source (Villigen, Switzerland). Together with data from catalytic testing, the results highlighted the interaction between the elements of the catalytic film and those of the metallic support. It is the nature of the crystalline phase and the distribution of Rh on the coating that play a key role in its properties. Results showed that a chemical reaction between Al, coming from the foam support, and Mg in the coating occurred during calcination at high temperature. This led to the formation of spinal phases in which Rh is involved, together with Rh2O3 and elemental Rh.
A series of interesting papers on the use of ICP-MS to gain an insight into Pt dissolution from Pt-based nanomaterial for proton exchange membrane fuels cells was reported by Jovanovic et al.109 A newly designed micro-electrochemical flow cell was coupled with an ICP-MS instrument via Tygon® tubing. The electrolyte was pumped through the microcell and over the working catalyst and subsequently introduced into the ICP-MS system, equipped with a cyclonic spray chamber and a Meinhard nebulizer, to measure traces (ppb levels) of Pt. The amount of dissolved Pt increased with increasing upper potential and with the dwell time within the oxidizing potentials. Notably, ICP-MS had a superior sensitivity over classical cyclic voltammetry, in which the dissolution current is usually masked by large capacitive currents. The same team also investigated Pt/C fuel cell catalyst corrosion as a function of chloride concentration using the same set up.110 The Pt corrosion mechanism changed significantly: the anodic corrosion was much enhanced compared with the cathodic corrosion that prevails in electrolytes without chloride. The effect of catalyst particle size on dissolution was also reported.
Romero et al.111 presented a paper detailing the preparation and characterization of palladium nanocatalysts for the dissociative chemisorption of arsine as a platform for TXRF analysis. The initial affinity of the catalysts to absorb arsine was demonstrated using ETAAS. Freshly prepared catalysts were deposited on a graphite tube and dried at 150 °C for 40 s. Then arsine was generated using a continuous flow system and flushed onto the catalysts in the graphite tube for 10 min. Finally, ETAAS measurements at an atomization temperature of 2250 °C were performed. Following this step, water samples containing arsenic were mixed with a reducing agent (0.5% m/v sodium tetrahydroborate) in a gas–liquid separator and arsine was produced. The volatile hydride was separated and carried by an argon stream to a quartz substrate containing immobilized catalyst nanoparticles. For analysis using TXRF, a Ga internal standard was added and measurements performed over 500 s. The combination of concentration on the absorbing catalyst and sensitivity of TXRF yielded a LOD of 0.08 μg L−1 of arsenic in natural water. Furthermore, once absorbed on to the substrate, the samples were stable for several months.
Hofmann et al.112 discussed the recent advances in SIMS and its application for solid acid catalyst characterization. Techniques based on SIMS were explored in the 1980s and 1990s to study porous catalyst materials but, because of their limited spectral and spatio-temporal resolution, there was no real major breakthrough at that time. The technical advancements in SIMS instruments, namely improved ion gun design and new mass analyser concepts, allow for a much more detailed analysis of surface species relevant to catalytic action. Imaging with high mass and lateral resolution, determination of fragment ion patterns, novel sputter ion concepts as well as new mass analysers (e.g. TOF, Fourier transform ion cyclotron resonance-SIMS) are just a few novelties, that could lead to new fundamental insights of heterogeneous catalysts from SIMS analysis.
Besides the essential the nutrients N, P and K, fertilizers can also be a source of trace levels of toxic heavy metals, which can potentially make their way in to the food chain. The ability to determine the concentration of these is therefore essential. Coelho and Rezende115 described a simple ultrasound-assisted extraction procedure for the determination of Cd, Cr and Pb using ETAAS. Extractions were performed using 20 mL of a 1 mol L−1 HNO3 and 1 mol L−1 HCl solution with sonication for 7.5 min. Under optimized conditions the LOQ were 0.11, 0.90 and 0.63 μg L−1 for Cd, Pb and Cr respectively. The same elements were determined using LIBS by Nunes et al.116 Fertilizer samples were homogenized by cryogenic milling and pressed into pellets prior to analysis. Excitation was achieved using a Q-switched Nd:YAG, 1064 nm laser with a 10 Hz repetition rate. Intensity signals from Cd 214.441 nm, Cr 267.716 nm and Pb 220.353 nm emission lines were measured using an intensified CCD detector. Analysis of 30 fertilizer samples was undertaken and no significant differences in data were seen for samples measured using the proposed LIBS method and for those using ICP-OES after microwave-assisted acid digestion (AOAC 2006.03 Official Method). The LOD were 1, 2 and 15 mg kg−1 for Cd, Cr and Pb respectively.
An optimized procedure for the determination of total As and AsIIIin phosphate fertilizers using HG-AAS following ultrasound-assisted extraction was reported.117 The determination of AsIII was performed through the simple control of solution pH with a 0.5 M citric acid–sodium citrate buffer solution at pH 4.5. Total As was determined after a pre-reduction reaction using 1.0% (w/v) thiourea. Ultrasound-assisted acid extraction was performed and extracts separated by centrifugation. Limits of detection for AsIII and total As were 0.029 and 0.022 μg L−1 respectively and the accuracy of the method was confirmed using CRMs. Ultrasound-assistance was utilized by Lima et al.118 for the extraction of Hg in inorganic fertilizers prior to determination using CV-AAS and MIP-OES, using a 10% (w/v) SnCl2 solution as a reducing agent. A multi-mode sample introduction system was coupled to the MIP-OES instrument to make possible the generation of Hg vapour in the nebulizer chamber. The optimized protocol involved the treatment of 150 mg of fertilizer with 4 mL of 30% (v/v) HCl inside glass tubes using a conventional ultrasonic bath for 5 min. Comparable results were obtained for both CV-AAS and MIP-OES, with lower detection limits being obtained for the latter technique.
The simultaneous determination of macro-nutrients (Ca, Mg, Na and P), micro-nutrients (Cu, Fe, Mn and Zn) and trace elements (Al, As, Cd, Pb and V) in mineral fertilizers using ICP-OES was described by Souza et al.119 Two and three point factorial designs were used to optimize the plasma and digestion conditions to allow the highest accuracy, whilst using minimal reagents. Approximately 0.2 g of dried sample was digested in a solution of 2.0 mol L−1 HNO3 and 3.0% H2O2 and the resulting solution was analysed directly. Limits of detection were <10 mg kg−1 for most elements. Accuracy was evaluated through analysis of a multi-element fertilizer standard (NIST 695) and two phosphate rocks (NIST 694, NIST 120). Recoveries were in the region of 80–120% across the element range.
The analytical technique of LIBS has become a widely used tool for material element detection. However, the instrument set up can vastly affect the quality of the results. Wang et al.122 explored the impact of parameters such as laser frequency, measurement time, and measurement delay on the intensity of several elements in cement. A 2 Hz change in laser frequency increased the intensities of analytes such as Al, K, Mn and Na by as much as 80%. The fast classification of bricks using LIBS spectra together with principal component analysis (PCA) was reported.123 Two instruments were described; a bench top system based on a Nd:YAG 532 nm laser with a CCD detector, and a stand-off instrument using the same setup in conjunction with a three-lens Galilean focusing telescope and a Newtonian telescope for emission collection. The latter allowed on-site analysis at a distance of ∼10 meters. A sample set of 29 bricks, from 7 different localities was divided into training and test samples. The method was able to accurately predict the locality of the test bricks for this sample set.
The trace-element fingerprints of 84 emeralds, from five different localities, were characterized using LA-ICP-MS, with the aim of predicting a stones origin.126 Laser ablation conditions consisted of a 7 Hz Nd:YAG, 213 nm laser with a 40 μm spot size and a fluence of 10 ± 1 J cm−2. Ablated material was transported to the ICP-MS instrument using a 0.78 L min−1 He carrier gas. log–log plots of paired element concentrations, such as Li versus Cs, Fe versus K and Fe versus Ga proved useful distinguishing features. A similar study was performed by Agrosì et al.127 using collinear double-pulse LIBS to determine the provenance of synthetic emeralds. The relative concentrations of traces of V, Cr and Fe proved useful for the corrected attribution of the stones manufacturer.
Hanson et al.128 investigated the effect of sample temperature and physical state on LIBS spectra of molten and solid salts. Samples were composed of a LiCl–KCl eutectic salt, a MnCl2 internal standard, and varying concentrations of CeCl3. The influence of sample temperature and physical state on self-absorption, plasma temperature, electron density and local thermal equilibrium were considered by observing changes in the Ce calibration. The analysis of salts in their molten form was preferred as plasma plumes from molten samples experienced less self-absorption, less variability in plasma temperature, and higher clearance of the minimum electron number density required for local thermal equilibrium. These differences were attributed to plasma dynamics as a result of phase changes. Spectral reproducibility was also better in the molten state because of better sample homogeneity.
2.4 Nuclear materials
A review of flow analysis applications for the determination of selected radionuclides (241Am, 239Pu, 249Pu, 99Sr and 99Tc) was produced by Kolacinska and Trojanowicz.137 It is well known that flow analysis simplifies procedures and instrumentation, shortens analysis time, is easily automated and can often improve precision. These advantages are illustrated in the review with the assistance of 84 references. The review was not specific to atomic spectrometry and also included spectrophotometric and radiometric methods. Also included were on-line and off-line methods of preparation and the schematics of several of the flow systems reviewed.
The results of an inter-laboratory comparison of the analysis of uranium ore materials were reported by Burger et al.138 Six laboratories were tasked with determining 69 impurities. The majority of the labs used ICP-MS with matrix matched external calibration for the analysis. The study highlighted the current state of practice and identified previously unaccounted for polyatomic interferences. Also identified were issues related to sample dissolution, blank correction and calibration and estimating measurement uncertainty. The study yielded two materials that were suitable for use as standards.
Two papers by Krachler et al. reported the use of ICP-OES for the analysis of spent nuclear fuel.141,142 In the first of these, 237Np was determined without prior isolation from other matrix components. A total of 13 Np wavelengths were tested in terms of sensitivity, peak shape, interferences and background signal. Some had very broad peaks (up to 35 pm) and others were split into multiplets. Despite these problems, LODs were an order of magnitude lower than in previous studies. The lowest LOD (3.1 μg kg−1) was obtained at 382.92 nm, but this had a severe spectral interference from Nd at 382.914 nm. Unfortunately, since Nd is present in many spent fuels, this line was of limited use. A study of interferences found that Nd, Pu, Th and U all caused interferences at some Np lines. Three wavelengths (410.84 nm, 429.09 nm and 456.04 nm) were identified as being useful analytically for this sample type. Results from the ICP-OES analysis of deposit samples were cross-validated by those from a SF-ICP-MS instrument. The second paper142 was similar to the first, but with Am as the analyte of interest. This time, six Am wavelengths were tested with 283.226 nm providing the lowest LOD (0.07 μg g−1), but with 408.929 nm yielding the most accurate results. Calibration using standard additions and external calibration yielded similar Am results. A homemade stock solution of Am was tested against a calibration curve run on a SF-ICP-MS instrument prepared using the signals from 238U and 232Th and corrected for mass bias. Results were similar, demonstrating the validity of the calibration method. The ICP-OES data from the analysis of one thorium/plutonium oxide spent fuel and two uranium/thorium fuels were again cross-validated using a SF-ICP-MS instrument.
The determination of Nd isotope ratios in irradiated nuclear fuel is fundamental for the validation of neutronic calculation codes, especially the burn-up qualification. Unfortunately, there are potentially several spectral and non-spectral interferences that can confound the calculations. Gueguen et al.143 have overcome these problems by coupling ion exchange chromatography on-line with a MC-ICP-MS instrument. The paper was split into two main sections. The first investigated the causes of the isotope ratio drifts so often seen during transient signal acquisitions, finding that there were two main causes. These were the mass-dependent isotopic fractionation on the chromatographic column and the time lags between the amplifier responses for the Faraday cup detectors. The second part of the paper described the development of a new approach for the correction of mass bias. The authors termed their method “Intra-injection Sample Standard Bracketing”. This was based on the on-line injection of the standard through the chromatographic system before and after the analyte peak. The method was, according to the authors, particularly suited to the analysis of nuclear materials. Results from the method were comparable to those obtained using off-line measurements and from a traditional sample –standard bracketing technique.
Using synchrotron μ-XRF, the uniformity of uranium-thorium oxide pellets prepared using two different processes (powder metallurgical compaction and coated agglomerate pelletization) was determined by Misra et al.135 The U distribution in pellets prepared using powder metallurgical compaction was uniform, whereas those prepared using coated agglomerate pelletization was not. The actual preparation of the sample for analysis was far simpler using the μ-XRF technology than for SEM. These fuel pellets are used in advanced heavy water reactors and the authors' discovery that the uniformity of the U varies between preparation methods is worth considering. It is unfortunate that the coated agglomerate pelletization methodology is the safer of the two procedures in that there is less exposure of personnel to high radiation doses.
The determination of Kr in irradiated uranium dioxide fuel was achieved using XRF and XAS by Degueldre et al.144 The main focus of the study was the analytical challenge of the sample and sub-sample production given the restricted conditions required by radio-protection regulations. The paper also dealt with the potential interferences in both techniques, including the effects from other dissolved gases. The most problematic of these was Xe, whereas residual Ar, He and H were negligible and did not cause a problem.
Another paper by the same research group147 determined 240Pu/236U in individual plutonium particles using ICP-MS without a prior chemical separation. Individual particles were placed on a silicon wafer using SEM and a micro-manipulator. The wafer was then placed in a beaker, 2 mL of water added and after ultra-sonication, the wafer was removed. After evaporation of the water, the particle remaining in the beaker was acid digested using 1 mL of nitric and 1 mL of hydrofluoric acids, on a hotplate at 200 °C. The digest was evaporated to dryness, and the process was repeated a further two times. The residue was then taken up in 1.22 mL of 2 M nitric acid and 1.78 mL of water. The 238Pu/234U, 239Pu/235U and 240Pu/236U ratios were then determined using ICP-MS. However, the 238Pu/234U and 239Pu/235U were not suitable for age determination because of contamination from the natural U.
Hubert et al.148used femtosecond LA-ICP-MS to determine the isotopic composition of uranium micrometer-sized particles and compared the results with those obtained using fission track – TIMS and SIMS. The LA unit was equipped with a very high frequency source that operated at up to 10 kHz. After optimization of the operating conditions, the typical accuracy and precision was better than 4% for 235U/238U for a transient signal lasting 15 s originating from between 10 and 200 pg of uranium. The LOD was 350 attogram for the 235U isotope, indicating that a particle of approximately 220 nm in diameter could be analysed. The precision of 4% was thought to be acceptable given that a quadrupole-based instrument was used.
A different method of chronometric determinations was developed by Kurosaki et al.149 who quantified Am, Cm and Pu using a sector field ICP-MS instrument. In particular, the isotope ratio of 240Pu and 244Cm was determined. Three 244Cm samples were analysed. One was obtained from the Savannah River site and had been stored for decades. There was no information on the last separation date or the isotopic composition. A second sample was SRM 4320A, obtained from NIST, where the last separation date was well known but the reported quantity of 240Pu had a 50% uncertainty value. This material is no longer available and so the Pu value could not be re-measured. The third sample was obtained from a commercial company and the isotopic composition was well known, but was not certified. Despite the lack of certification, the third sample acted as a useful reference sample. As an alternative technique, a chemical separation using TRU resin followed by alpha spectrometry was used for validation. Although the precision obtained using the ICP-MS method was poorer than that from the radiometric method, the authors stated that it was sufficiently good for their particular application.
Another paper to use LIBS to analyse a tokamak window was presented by Maurya et al.152 The three layers of the window were analysed with the first layer (the impurity layer) yielding signals from C, Cr, Cu, Fe, Mn, Mo, Ni and O, most of which were thought to originate from the stainless steel used for the construction of the tokamak. The second layer (anti-reflection coating layer) gave signals from Ca and Mg and the inner layer emitted light from Al, B and Si. Principal component analysis of the LIBS data facilitated the differentiation between the three different layers. High resolution spatial mapping of the deposited layer on a tokamak mirror was also reported by Xiao et al.153 These authors used LA microprobe TOF-MS for the analysis with data acquisition using LabVIEW software. This enabled a depth resolution of 20 nm and a surface resolution of 500 μm to be obtained. The three-dimensional information obtained using this approach enabled both the composition of the layer to be determined and also the removal efficiency of the species of concern.
Another paper to use LIBS for the analysis of tokamak components is by Karhunen et al.,154 who analysed the deposited layers in beryllium–tungsten mixtures and compared the results of the calibration free LIBS with standard ion beam methods. Li et al.,155 also used LIBS to determine the deuterium retention on lithiated tungsten exposed to high-flux deuterium plasma. Results indicated that after exposure, the deuterium was saturated in the lithium layer and that it was chemically bound to the lithium. The lithiation inhibited the blistering of the tungsten surface.
One final paper of relevance discussed the use of LIBS for the determination of Ce through a shielding window.156 The effect of the optical transmittance of the window on the analytical performance of the LIBS system when analyzing a mixture of cerium dioxide and potassium chloride was determined. A comparison was made with the signals obtained when no window was in the way. The pulsed laser beam (Q-switched Nd:YAG at 532 nm) was focused onto the sample surface which was 1.45 m away from the LIBS device. Light emitted from the sample was passed through a Schmidt–Cassegrain telescope and transmitted through a Czerny–Turner monochromator to an intensified CCD detector. Both univariate and multivariate analyses were used. The univariate analysis of both singly and doubly charged Ce species yielded LOD of 0.046 and 0.061% for open path and through the window analysis, respectively. The multivariate approach using partial least squares regression improved the quality of the calibration.
3 Functional materials
3.1 Ceramics and refractories
Ceramics are a class of sample that is very difficult to dissolve and therefore many workers resort to using techniques capable of analysing solids directly. This helps avoid the use of lengthy dissolution procedures that frequently use hazardous acids. This means that techniques such as XRF, LA-ICP-MS, LIBS, INAA and even slurry nebulization are used.A report by Hunt et al.157 described the use of EDXRF, WDXRF and INAA for the analysis of three fireclay reference materials manufactured by Mittal Steel Ostrava a.s. and distributed by Brammer Standard Company Inc. Numerous analytes were determined including the major elements, which were reported as their oxides (Na2O, MgO, Al2O3, SiO2, P2O5, K2O, CaO, TiO2, MnO and Fe2O3) and 30 other trace analytes.
It has been known for many years that the technique of slurry nebulization has many potential advantages. These include the ability to calibrate using aqueous standards, not having the requirement of acid dissolution, simplicity, rapidity and low risk of analyte loss. However, it is also well known that several precautions have to be taken to ensure accuracy and precision and these include ensuring that the particle size is sufficiently small to be transported by the nebulizer gas flow to the plasma in a representative way. This usually means that the material must be crushed and then ground into a fine powder and then dispersed in a suitable solvent. The grinding process can be achieved in many ways and by using different materials. The choice of material used for the grinding depends on the analyte of interest and on the hardness of the material to be analysed and on the grinding medium. Slurry nebulization for the analysis of advanced materials was reviewed (with 90 references) by Wang and Yang.158 Included in the review were sample preparation and sample characterization methods, modifications to instrumentation that may be necessary and calibration strategies. An outlook for the technique was also provided. An application paper by the same research group described the determination of impurities (12 analytes) in high purity silicon nitride using axially viewed ICP-OES.159 The use of polyacrylate amine as a dispersant was tested using zeta potential and slurry stability measurements. A concentration of 0.8% was optimal for micron sized particulates, but this increased to 1.2% for nano-particulate silicon nitride. Results for the slurry nebulization ICP-OES were compared with those obtained using a high pressure acid decomposition of the material followed by conventional nebulization and were in good agreement. The exceptions were K and Na which provided better results using the slurry technique because of contamination issues with the acid decomposition. Limits of detection were in the range 8–250 ng g−1, which were superior to those obtained using conventional ICP-OES.
The storing matter technique to reduce matrix effects during SIMS analyses was reported in two papers by Kasel and Wirtz.160,161 The storing matter technique first vaporizes the sample and then collects it on another, well-defined, surface. The collected sample then undergoes SIMS analysis. This methodology helps overcome the matrix effects that affect SIMS analyses. The first paper focused on the Ti sputtered from titanium, titanium boride, titanium carbide, titanium nitride and titanium dioxide. The effect of the addition of oxygen on the yield was also determined. The oxygen enhanced the Ti signal on the tantalum collector by a factor of 3, but the other collector materials yielded no enhancement. The storing matter SIMS operating conditions were significantly different to those for standard SIMS analysis. The impact energy was 1 keV compared with 4 keV and the primary current was 2–3 nA rather than 9–10 nA for SIMS. The latter paper determined Si yields from silicon, silicon carbide, silicon nitride and silicon dioxide. In both cases, the effect of the collector material (silver, gold, copper or tantalum) on the yield was tested. In both papers, the analyte could be determined with fewer interferences in all matrices than when using SIMS alone.
Wang et al.162 described the determination of Ag, Ca, Cu, Fe, K, Li, Mg, Na and Pb in titanium dioxide powders using Solution Cathode (SC)-GD-AES. The system's optimal conditions used 0.1 M nitric acid, a voltage of 1060 V with a flow rate of 2 mL min−1 and a sample concentration of 10 mg mL−1. The samples were also prepared using a high temperature acid dissolution method and then analysed using axial ICP-OES. The results from the two methods were in good agreement. In addition, the CRM NIST 154c was analysed to give extra method validation. In the SC-GD-AES method the Ti emission intensity was very low. This meant that the matrix exerted fewer interferences during the determination of the analytes. Detection limits ranged from 0.02 μg g−1 for Li and Mg to 5 μg g−1 for Fe. An investigation into improving the sensitivity of Pb was also undertaken. The LOD in solution improved by a factor of 6.5 to give a value of 2 ng mL−1 in the presence of 3% formic acid.
A review (with 54 references) of the use of LIBS in archaeometry was published by Spizzichino and Fantoni.163 The review gives a brief overview of the principles, goes on to discuss the use of LIBS in conjunction with other techniques, e.g. LIF and Raman and then discusses the use of chemometric tools such as PCA for obtaining maximal information from the data. The final sections of the review concentrate on giving examples of the analysis of ceramics, glasses, metal alloys, paintings and gems.
As in previous years, the review of the atomic spectrometric analysis of archaeological samples will be tabulated (Table 4). This is because most of the applications are very much orientated towards the archaeology rather than the analytical chemistry and often, there is a shortage of experimental detail.
| Analyte | Matrix | Technique; atomization; presentation | Comments | Reference |
|---|---|---|---|---|
| Fe | A vessel from the 5th century BC | μ-XRF; —; s; full field transmission XANES; —; s | A detailed description of the experimental setups was given explaining how and where the data were acquired. Samples from the interior and exterior surfaces were analysed. Results of the Fe K-edge indicated that several firings had been necessary, since one surface contained hematite (requiring a temperature of 950 °C) and the other surface contained maghemite (requiring less than 800 °C) | 168 |
| Various (13) | Chinese porcelain glazes | μ-XRF; —; s | μ-XRF determination of Al, Ca, Co, Fe, K, Mg, Mn, Na, Ni, Rh, S, Si and Ti followed by PCA interrogation of the data enabled the glazes from Kangxi (1661–1722) and Qianlong (1735–1795) periods to be distinguished | 164 |
| Various (9) | Faience beads from the Western Zhou Dynasty | μ-probe EDXRF; —; s | μ-probe EDXRF and synchrotron radiation μ-computed tomography used to analyse 16 beads. The instrument had a molybdenum tube and a 125 μm beryllium window, which was calibrated using appropriate primary standards. The detector was a liquid-nitrogen cooled Si(Li) crystal. The operation conditions of the X-ray tube included an accelerating voltage of 50 kV and current of 800 mA. The X-ray beam spot was set at 0.3 mm. Precision for the analytes Al, Ca, Cu, Fe, K, Mg, Na, P and Si (expressed as their oxides), was 1–3% for analyte concentrations > 1% m/m but worsened to 10% RSD for analytes that were less than 0.1% m/m. It was possible to distinguish between glazed beads and glassy beads | 165 |
| Various (28) | Majolica ceramics from Toledo, Spain | XRF; —; s | Thirty two ceramic fragments from two separate sources were analysed using XRF, XRD, differential scanning calorimetry and thermogravimetric analysis. Cluster analysis was used to investigate the analytical data. Differences in chemical composition between the two production centres were identifiable. The other techniques enabled an estimate of firing temperature to be made | 166 |
| Various (>30) | African Red Slip ware | XRF; —; s | 61 potsherds of slipware from three sites and the local clays were analysed using XRF. Data were treated using cluster analysis and PCA. Materials from Henchir el Guellel were very different to those from the other two sites. In addition, materials from Sidi Khalifa were found to be homogeneous whereas those from Oudhna comprised two distinct groups. Neutron activation analysis used for some samples to provide comparative data | 167 |
| Various (>20) | 14th–16th century Vietnamese and Chinese ceramic shards | Portable XRF; —; s | Portable XRF and a transportable Raman instrument were used to analyse 13 ceramic shards from the Omani coast. Cluster analysis, PCA and binary scatter diagrams used to treat the data. Distinction between Vietnamese and Chinese ware was achieved through determination of Rb, Ti and Zr as well as K:Ca ratio in the glaze/body of the ceramic. Other analytes (Co, Mn and REE) helped identify production places |
169 |
| Various | Ceramics, glass and concrete in an underwater archaeological site | LIBS; —; s | A prototype instrument used to analyse materials from a shipwreck. The instrument was capable of both single pulse and multiple pulse excitation and the laser light (Q-switched Nd:YAG operating at 1064 nm) was transmitted through a 55 m fiber optic to a hand-held probe which focuses the light into a 450 μm spot. The same optical fiber was used to transmit the emitted light to the detector. The spectrometer was of Czerny–Turner design with 1200 grooves mm−1. The multi-pulse system yielded a sensitivity enhancement of 15 when compared with single pulse when using the same irradiance value (1.89 GW cm−2). The sample probe also supplied a gas that displaced the water between sample and the probe which also enabled an improvement in signal | 170 |
| Various (19) | Terra Sigillata Hispanica Roman pottery | WDXRF; —; s | A series of 38 samples were characterized using WDXRF, SEM-EDX and X-ray powder diffraction. Small pieces of sample were crushed and then fused into a glass bead prior to WDXRF analysis. Concentrations of major analytes (Al, Ca, Fe, K, Mg, Mn, Na, P, S, Si and Ti) were expressed as their oxide whereas the trace analytes (Ba, Cr, Ni, Rb, Sr, Y, Zn and Zr) were expressed as the elemental concentration. Data were assessed using PCA. A biplot of the first two principal components obtained from the PCA analysis of the WDXRF data indicated three main groups, comprising Alameda, Andujar and Antikaria Teba samples | 171 |
| Various (18) | Cooking ware from Pompeii | MS; ICP; LA | Ablation using Nd:YAG laser operating at 213 nm, with a constant laser repetition rate of 10 Hz and fluence of ∼20 J cm−2 producing a 50 μm spot. Data treated with PCA and binary plots for REE, Sr, V, Y and Zr. Comparison of data from ceramic ware with products from Latium and Campania volcanic districts distinguished provenance areas | 172 |
| Various (12) | Thin-walled pottery from Herculaneum and Pompeii | MS; ICP; LA | Numerous techniques (optical microscopy, SEM-EDX, XRD and LA-ICP-MS) used to characterise pottery for the analytes Al, Ca, Cr, Fe, K, Mg, Mn, Na, Rb, Si, Sr and Ti. Calibration achieved using National Bureau of Standards material 679 and NIST 610. Standardized analytical data (by subtracting the average value (per variable) and by dividing by standard deviation (per variable)) were treated using PCA. LOD ranged from 4% for Si down to 0.05 μg g−1 for heavier elements. Data supported the hypothesis that raw materials came from Campanian region | 173 |
3.2 Semiconductor materials and devices
Developments in the analysis of semiconductor wafers and materials doped with impurity elements, thin films and multilayer materials for use in electronics applications and in the characterization of solar cells and other electronic components and devices are described in this section of the review. The use of analytical techniques to support the development of processes for the fabrication and recycling of such materials is also considered. The accompanying table (Table 5) summarises details of analytical applications concerning these materials that are not specifically highlighted in the relevant sections of the text.| Element | Matrix | Technique | Sample treatment/comments | Reference |
|---|---|---|---|---|
| Al | Cathodes from lithium ion battery cells | ICP-OES | Detection of impurities resulting from a recycling process by destruction of cathode with sulfuric acid and separation from binder and carbonaceous conductor material | 202 |
| Al | Field effect transistors | GI-XRF | Determination of AlN interlayer thickness in multilayer transistor structures. X-ray reflectometry and high resolution XRD also used | 203 |
| Ag | Nano-crystalline thermoelectric thin films | XPS | Depth profiling of laser ablation-grown, Ag blended-PbTe thin films of 200 nm thickness | 204 |
| B | Glasses | ICP-OES and EPMA | Comparison of technique bias, linearity of response, and precision in the determination of B2O3 content at the level of 0.5% by weight | 205 |
| B | Solar grade silicon | ICP-OES | Under optimum digestion conditions using HNO3, NH4+ ions generated prevented the loss of B and avoided need to use complexing agents | 206 |
| Cd and Pb | Food contact glassware articles | FAAS, ETAAS, ICP-OES and ICP-MS | Report of the results of an inter-laboratory trial for the determination of Cd and Pb in food contact glass | 207 |
| Cu | Barium phosphate laser glass | ICP-MS | Digestion and extraction procedure for the determination of ultra-low levels of Cu with a recovery of 94.3%. A LOD of 2.5 ng g−1 and an RSD of 3.3% (n = 6) were reported in Chinese | 208 |
| Cu | Flexible printed circuit boards | ICP-OES | Measurement of Cu bio-leachate from circuit boards by Acidithiobacillus ferrooxidans | 209 |
| Cu | N-type mono-crystalline silicon | GD-MS | A depth resolution of 0.5 μm within the overall sputter depth profile of 8–30 μm was achieved. Accuracy of analysis using RSFs could be affected by Cu diffusion to the surface | 210 |
| O | Organo-photovoltaic layered materials | SIMS | Detection of selected ions fragments (SO−, NO−, SO2−, NO2− and CO−) caused by atmospheric oxygen diffusion through the photo-active layer | 211 |
| P | Silver pastes for photovoltaic applications | LA-ICP-MS | Direct detection of cationic phosphonium dispersants used in solar cells | 212 |
| Ta and Ti | Surface acoustic wave substrates coated with Ta and Ti thin films | XPS | Study of layered structures containing Ta and Ti at surface of lithium niobate substrates using angle-resolved technique | 213 |
| U | Barium borosilicate waste glass | LIBS | A portable LIBS instrument was used. Two U emission lines were used for calibration. The effect of laser plasma operation in argon was investigated and was found to improve signal intensities by up to 5-fold with improved precision | 214 |
| Various (4) | CIGS solar cell thin films | LA-ICP-MS | Reference data from ICP-OES were used to calibrate LA-ICP-MS response which varied according to fs laser parameters used. Quantification accuracy in the range 95–97% was reported for major components (Cu, In, Ga and Se) | 215 |
| Various (15) | Germanium dioxide | ICP-MS, ICP-OES and ETAAS | Interference-free determination of 15 impurity elements by prior removal of GeCl4 matrix by open vessel volatilization | 216 |
| Various (4) | Glasses | LIBS | Analysis of laboratory prepared fused beads used as reference standards (characterized previously by XRF) and glass samples for the determination of Ba, Ca, Cr and Ti. A calibration range from ppm to several percent levels was achieved | 217 |
| Various (11) | Heavy flint and crown barite glasses | LIBS | Direct LA of samples in an argon atmosphere for calibration-free standardless determination of Ba, Ca, H, K, Na, O, Na, Si, Sr, Ti, Zn and Zr using an iterative plasma LTE model for intensity calculations. Comparison with XPS and EDX methods | 218 |
| Various (5) | Kesterite nanoparticles and solar cell thin films | EDXRF | The composition of nanoparticles of the generic type Cu2ZnSnS4 and selenised thin films of the type Cu2ZnSnSe4 were determined | 219 |
| Various (54) | Lanthanum gallium silicate | LA-ICP-MS | Direct determination of elemental composition. The LODs were reported in the range n × 10−5% (for Cr, Cu, Mg, Mn, Ni, Ti, V and Zn) to 2 × 10−7% by weight (for U). The ratio of bulk component elements (Ga, La, Si) was investigated | 220 |
| Various (31) | Printed circuit boards | ICP-MS, ICP-OES, INAA and ED-XRF | Twenty nine elements were determined after pyrolysis using ICP-MS and ICP-OES. The content for P (28 mg g−1) was determined using XRF and Br (60 mg g−1) was estimated using INAA | 221 |
| Various (3) | Silicate glasses | LA-ICP-MS | An on-line ID method using the same isotope post-cell aerosol spike in sample and a certified abundance material was used in the determination of Pb, Rb Sr in glasses and NIST glass SRMs (610, 612 and 614) | 222 |
| Various (5) | Silicon dioxide and nitride films | ICP-MS and SIMS | Detection of transition metal (Fe, Cr, Cu, Ni and W) diffusion through thin films into silicon substrate using etching process | 223 |
| Various (16) | Solar cell silicon | GD-MS | Determination of impurity or dopant elements (Al, B, Ca, Co, Cr, Cu, Fe, Mn, Mo, Ni, P, Pb, Sn, Ti and W). Quantification was achieved using relative sensitivity factors. Limits of detection of <1 ppb were reported for all elements except for Al, B, Ca, P and Pb | 224 |
| Various (4) | Waste from personal computers | ICP-MS | Detection of toxic metals (Cd, Cr, Hg and Pb). Results were compared with those set by EC Directive RoHS 2002/95/EC | 225 |
| Various | Window glass | LIBS | Study of Australian glass samples (14 laminated, 6 non-laminated). Statistical comparisons showed a glass-type discrimination power for the method of >97% at the 95% confidence level. Analysis validated using SRMs NIST 610, 612 and 1831 | 226 |
The analysis of complex semiconductor materials has always presented challenges in relation to achieving reliable quantification. The ISO Technical Committee 201 on Surface Chemical Analysis has produced an authoritative update report describing the development of standards for reliable surface analysis.174 Topics discussed included a description of the work of the expert Technical Committee; the process for prioritization of standard development, the approach taken in inter-laboratory comparisons, and standards for reporting. A table listing the available ISO standards and technical documents in seven categories (general, Auger-electron spectroscopy and XPS, GDS, scanning probe microscopy, SIMS, sputter depth profiling, and X-ray reflectance and T-XRF), was presented. A specific review (38 references, in Chinese) of LA-ICP-MS to the surface microanalysis of materials including semiconductors is worthy of some attention in this context.175 It was recognised that LA-ICP-MS has now matured as a complementary tool to other forms of surface analysis citing high sensitivity and minimal sample preparation as key advantages over other competing techniques.
Argon cluster ion beam sputtering has been used increasingly in SIMS in recent years because of the minimal sample damage caused during depth profiling in comparison with that resulting from the use of monatomic ions. However, while this approach has seen great success in characterizing polymer and organic sample types, its use has been less well documented in the semiconductor field. The use of Ar cluster ion beam sputtering has been applied to the study of five compound semiconductors (CdTe, GaAs, GaP, InAs and ZnSe) and considered with particular reference to sample damage.176 The expectation prior to the study was that inorganic materials would be unlikely to be damaged by use of cluster ions and this was found to be the case for most of the materials studied. However, surprisingly, it was reported that damage to InAs materials using this approach could be very high resulting, in some cases, in the formation of visible metallic In. Argon ion cluster sputtering has also been investigated in relation to InSb by a separate group.177 In contrast to the observation for InAs above, it was reported that the approach was effective for sputter depth profiling of InSb. The maximum depth resolution was obtained at an incident angle of zero degrees using energy of 20 keV. However, it was also observed that while surface roughness was lower for Ar cluster ion sputtering compared with Ar+ irradiation at 3 keV, under some conditions a ripple formation could be observed which affected the depth resolution that could be achieved. Clearly, the use of cluster ions sputtering in application to semiconductor materials should be treated with some caution until the possible effects of damage are better understood.
Depth profiling of semiconductor materials remains a key area of methodological development. For example a combined SIMS and angle-resolved XPS approach has been proposed to minimize sputtering derived artefacts and matrix effects affecting depth distribution in implanted silicon.178 Thus single crystal Si (100) wafers were implanted using a 15 keV energy beam and ion fluences of 4 × 1015 cm−2 (high dose) and 5 × 1014 cm−2 (low dose) conditions. For B, the wafers were implanted using a 7 keV energy and 5 × 1015 cm−2 (high dose) and 5 × 1014 cm−2 (low dose) conditions. For SIMS determination of P, a Cs+ ion beam was used, whereas for B an O2+ primary beam was employed, both operated at an energy of 2 keV and at the normal angle of incidence. The XPS technique was used in the variable angle mode to determine the native oxide thickness and relative positions of implants in the material and then using this to calibrate SIMS response. The impurity doses in native oxide and Si substrates were subsequently satisfactorily quantified under a range of thermal treatments, helping to explain the related diffusion properties of the doped materials.
Contamination of ultra large scale integration devices, such as wafers, by Cu, is problematic because the element is highly soluble in silicon and can diffuse through the material even at room temperature. Such contaminants are usually removed by oxygen precipitated in the wafers via a process known as ‘gettering’. While highly sensitive bulk analysis techniques such as ICP-MS and ETAAS can determine Cu at very low levels, these methods are destructive and time consuming. An alternative approach has been proposed for the measurement of gettering efficiency based on TOF-SIMS.179 Samples (1 cm by 1.3 cm in size) were initially treated with HF because the presence of native oxide affects Cu measurements. Contamination was introduced by dipping the samples in Cu solution for 5 min. A relative sensitivity factor for Cu was used to calibrate the SIMS response to account for matrix variability (e.g. due to oxide levels). When the sample was treated with HF, SIMS matrix effects were minimized in comparison with measurement of samples that were either annealed to create an oxide layer or subjected to conventional hydrophilization via exposure to a NH4OH–H2O2–H2O (ratio 1:1:5) mixture. The method thus developed was used to form the basis of a reliable 7 day getter test.
Titanium nitride is usually applied to silicon wafers to provide a diffusion barrier to maintain electrical conductivity. The material is extremely hard and exhibits strong adhesion to the substrate. An investigation has been undertaken to examine the factors affecting the performance of LA-ICP-MS in depth profiling titanium nitride thin films.180 The coatings were deposited on the substrate using an arc ion-plating technique and analysed using an ICP-MS instrument equipped with a commercially available Nd:YAG laser (output wavelength: 213 nm). Samples were ablated under helium (1 L min−1) using an applied power of 1.3 mJ (30%) and a 25 μm spot size. The ablation craters produced in the coated material were examined using SEM. As might be anticipated, the TiN coating was subject to less thermal degradation than the Si substrate although differences in behaviour were noted between the centre and edge of the ablation crater. However the penetration depth was found to vary inversely with laser pulse repetition rate (i.e. greater depth was achieved at low repetition rates down to 1 Hz). Thus, a 1 μm thick TiN coating took 26 shots to ablate at an average rate of 40 nm per shot. The increase in penetration depth at low laser pulse frequency was attributed to the low thermal conductivity of the TiN film which resulted in greater material residual stress.
The thickness of films or layers is an important parameter that affects the functionality of semiconductor materials. The location of interfaces in SiTi multilayer films has been examined using SIMS depth profiling.181 A Cs+ ion beam was used to sputter the material to produce SIMS depth profiles which were related to concentration using RSFs for Si and Ti derived from a previously characterized alloy of these elements. The interface location was identified by relative composition (50 at% definition). Thicknesses of the Si and Ti layers were measured from the location of the interfaces.
Matrix effects are often problematic in the analysis of complex hetero-structures using SIMS. Methods for minimizing such effects have been investigated for the quantitative determination of Ge in GexSi1−x layers.182 The TOF-SIMS instrument set-up utilized secondary ion yields derived from sputtering with Cs+ ions. A linear dependence of the Ge:Si ion concentration ratio on GexSi1−x was demonstrated. Two further linear calibrations for Ge concentration as a function of Ge2− secondary ion yield were identified. It was claimed that the use of Ge and Si negative secondary ions for calibration yielded optimum depth resolution. In a related publication183 the calibration dependencies of the yield for atomic Ge and Ge2 secondary cluster ions in the characterization of nano-islands in GexSi1−x hetero-structures was described. The secondary Ge2 cluster ions yields followed a non-linear quadratic relation calibration model that could be used to distinguish between planar layers and island structures without requiring prior information about the sample. Interestingly, it was also suggested that the evolution of the formation of the islands could be studied using this approach.
Chalcopyrite thin film hetero-structures of the general type Cu(In1−xGax)Se2 (also known as CIGS) are now increasingly seen as viable alternatives to silicon-based materials in photo-voltaic applications. Consequently there has been continued interest in the year under review concerning methods for the characterization of this type of thin film. South Korean researchers,186–188 in a series of related publications, explored the application of SIMS to the characterization of CIGS materials. The primary developmental focus of this work was the demonstration of the effectiveness of using MCs+ cluster ions (in which M is an element derived from the sample matrix) for detection. The emission of cluster ions resulting from Cs+ ion bombardment was monitored using TOF-SIMS and magnetic sector SIMS.186 Sample matrix effects were suppressed when MCs+ ions were used for measurement purposes. Quantitative results, obtained using the SIMS cluster ion approach, were in good agreement with those derived from ICP-OES analysis. Depth profiles achieved using both SIMS methods were similar. In a related study it was shown that the cluster ion method also yielded depth profiles consistent with those obtained using Auger electron spectrometry and XPS.188 In a third paper, the performance of ICP-AES, XRF and EPMA were compared for the quantitative determination of the major element components of CIGS.187 A comparison of methods for depth profiling including XPS, Auger electron spectroscopy and the SIMS approaches described above was also made which confirmed the satisfactory performance of the latter.
As noted in last year's ASU, laser ablation techniques have been utilized for the analysis of photovoltaic thin films, and reports have continued to appear in the period under review. For example, LIBS has been used both for direct quantitative analysis and depth profiling of CIGS materials.189 While matrix interferences can be a problem in LIBS, a calibration model that took into account compositional grading with depth was evaluated with eight test samples. The average concentration of the major CIGS components could be predicted with an RMS error below 1% and an RSD of 1% or less. Results for LIBS depth profiling using a mean sample penetration resolution of 100 nm per pulse agreed well with those derived from SIMS and Auger electron spectroscopy. LA-ICP-MS has also been applied to quantitative depth profiling of multi-layer cadmium telluride devices.190 Thus, laser fluence and repetition rate were optimised to minimize mixing between ablations and to deliver good resolution. The morphology and shape of ablation profiles generated were surveyed using AFM. A material layer 100 nm thick could be profiled at a depth of 3 μm. The results achieved by LA-ICP-MS in quantitative depth profiling of major and minor elements were comparable to those obtained using SIMS and GD-TOF-MS.
The degradation in performance and recycling of lithium ion battery components and materials have remained the subject of study since the previous ASU review.1 A twenty three page review of the development of combined spectroscopy and microscopy techniques for in situ characterization of these devices has also been published.191 Topics examined included the application of XRF microscopy, Raman, microscopy, transmission X-ray microscopy, SEM and TEM to the study of metal and metal oxide-based anode materials and layered oxy-sulfite, metal fluoride and Li based cathode materials.
The characterization of highly porous cathodic materials used in Li–S batteries was investigated.192 Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy was used to achieve S speciation inside the cathode material. The study of the S K-shell absorption edge at 2472 eV requires the use of low photon energies. Consequently, in a first step towards in situ monitoring, a new sealed argon cell fitted with 1 μm thick Si3N4 windows was designed which allowed NEXAFS measurements to be made under ultra-high vacuum conditions. The sample was kept under an argon atmosphere at all times. The X-ray beam was passed through vacuum which facilitated the detection of low-energy S X-ray emission. Using this arrangement, the presence of Li polysulfides was identified in the cathode films.
The European Directive (RoSH 2011/65/EU) on the restriction of the use of certain hazardous substances in electrical and electronic equipment, specifies that plastics with brominated flame retardants (BFRs) with content above 0.1% are identified and removed from the recycling process. This represents a major challenge for the electronics industry, because such materials may be incorporated in waste electrical equipment from both inside and outside the EU. For this reason a rapid sorting procedure for the detection of Br in such materials has been developed using XRF.193 Raman spectroscopy was then employed to confirm the presence of brominated flame retardants. This technique combination was shown to be suitable as a quality control method in recycling processes.
The rate of change in the mobile phone industry is dramatic leading to a high volume of discarded devices that need to be recycled. However, in order to effect as strategy for dealing with such waste, it is important to determine the elemental composition of a multitude of component parts. The development of the application of LIBS in combination with chemometrics strategies for identifying the metal composition of a printed circuit board from a mobile phone is therefore of direct relevance to improving processes for recycling.194 A segment (area 30 mm × 40 mm) of a mobile phone printed circuit board manufactured in 2011 was analysed using LIBS using a 1200 point ablation sampling matrix with 10 pulses at each point. Spectra were recorded for each ablation event over the wavelength range 186 to 1040 nm. Following a PCA study of the data, emission lines were selected for analysis for 18 elements. The relative intensities obtained were then used to normalise the response and a further PCA carried out. As a result it was confirmed, unsurprisingly, that Au was principally located in the electrical contacts, whereas Si was distributed within the bulk material of the board. The approach was also applied to a printed circuit board from a computer mouse for the determination of Pb using 5 separate wavelengths for measurement. The Pb content of solder material was of the order of 25% w/w.
The effective spatial resolution achieved in elemental profile scanning of electronic materials and components using LA-ICP-MS is affected by carry-over from one sample to the next. One practical means of dealing with this problem using existing instrumentation is to minimise the ablation repetition rate so that washout is complete prior to the next ablation event but this greatly increases the analysis time. However, in an important new development, a new low-dispersion ablation cell was designed that achieves a 99% washout time of only 6 ms.195 This system was able to deliver separated pulse responses of up to 200–300 Hz, overcoming washout peak broadening effects. Additionally, an iterative Richardson–Lucy algorithm was used for deconvolution of overlapping ablation positions post acquisition. This approach achieved lateral resolutions (ca. 0.3 μm) that were less than the laser beam diameter. The effectiveness of this new approach was demonstrated in application to the spatial mapping of a high-capacitance multilayer ceramic capacitor.
3.3 Glasses
Methodologies for determining the bulk composition of glasses continue to appear, despite the fact that well-established techniques such as ICP-OES and XRF are well suited to this application. Indeed the advocates of alternative techniques often make this point by default by using one or other as a reference method. A summary of reports concerning the analysis of glass is provided in the accompanying table.Developments in the analysis of glasses fall into three distinct categories: (a) those involving direct sampling of the glass without the need for chemical digestion, (b) those involving analysis with spatial or depth resolution and (c) those where particular improvements in sensitivity, accuracy and or precision are required to fulfil the needs of the application. The advantages of LA techniques for the rapid analysis of glass have been explored for one or more of these purposes. Thus, the application of LA-ICP-MS using different laser sampling systems has been investigated for the quantitative multi-element analysis of glasses.196 Femtosecond and nanosecond lasers with operating wavelengths of 193 nm, 365 nm and 795 nm were evaluated for the determination of major, minor and trace elements in glass CRMs (NIST SRM 612, BCR-2G, GSE-1G (both available from United States Geological Survey) and BAM-S005A). An internal standard element was used to calibrate response. It was reported that similar performance could be achieved for all three laser sampling systems when Ca was adopted as the internal standard. However, if Si was used instead for this purpose, the fs laser operated at 795 nm produced deviations of up to 20% under the operating conditions selected. Fractionation calculations indicated that these differences arose from changes in the composition and particle size distribution of aerosols formed during sampling which were, in turn, affected by laser fluence and spot size.
Differences in elemental fractionation in the analysis of glasses by LA-ICP-MS have also been investigated by a separate group of researchers.197 An excimer ns laser and a fs laser operated at 193 nm and 257 nm respectively were used to sample NIST SRM 610 and GSE-1G (USGS) silicate glasses. In contrast to the previous study, the most significant fractionation occurred when using the ns laser and was eliminated when using the fs laser at 257 nm. Laser fluence was only found to contribute to fractionation of Bi, Pb and Sb out of 17 elements studied. A smaller spot size (24 μm) was recommended for analytical measurements in order to reduce laser-induced fractionation effects. Under optimum conditions selected for fs LA-ICP-MS, agreement with reference values was better than 10% for most of the elements in MPI-DING, USGS, and NIST glasses. It is worth commenting that these two reports are not inconsistent as both indicate that good results can be achieved using appropriate internal standard response normalization strategies. However, it does indicate the importance of careful evaluation of laser selection and operating conditions in the direct analysis of glasses.
Isotope dilution provides a convenient means of improving both accuracy and precision in solution-based ICP-MS. However the application of on-line ID in the direct analysis of glasses by LA-ICP-MS represents a more complex challenge because of the potential for poor mixing of the spike sample aerosol.198 In order to obtain the good isotope equilibrium needed for ID-LA-ICP-MS, four different mixing devices (including pre- and post-cell introduction of the isotope spike) were designed and evaluated for application to the determination of Pb in silicate glasses. A pre-cell connection mode provided optimum performance in respect of consistency of the blended Pb isotopes, delivered good precision in the measurement of the 208Pb/207Pb isotope ratio, and exhibited significantly less loss of sensitivity than in post cell mode. Good agreement was achieved with certified values for Pb in NIST SRM 619 glass.
LIBS is often applied to the direct determination of the composition of glasses. Reports of this type continued in the year under review, citing advantages of rapid analysis times and the possibilities for in situ measurement. However, while challenges undoubtedly remain in quantification related to calibration strategies, its application to depth- or laterally-resolved characterization of functional glasses is less well explored. A ns UV Nd:YAG laser was employed in a LIBS system to investigate the homogeneity of vitrified Mn-doped glasses.199 Samples were ablated in air at atmospheric pressure using a laser pulse irradiance of 1.7 × 109 W cm−2. The emission from the laser plasma thus generated was integrated spatially and imaged on the spectrometer slit using a fibre-optic collection device. The homogeneity of the glass was verified by sampling at different sites on the glass surface and capturing the emission generated. The results achieved correlated well with those obtained from surface elemental mapping using SEM-EDX. The concentrations of Mn determined by the method were in good agreement with certified values for the bulk material, suggesting that interference effects in the LIBS analysis were minimal.
X-ray fluorescence is a technique that is well known to be ideally suited to the analysis of glasses because of its ability to examine solid samples rapidly and non-destructively on a quantitative or qualitative basis. However, the development of field portable XRF equipment has opened up new opportunities in the forensic analysis of glasses that has so far been under-exploited.200 In this study, a portable XRF instrument was used to rapidly scan 25 glass samples for major (Al, Ca, Na, O, and Si) and trace (Fe, K, Rb, Sn, Sr and Zr) elements. The technique identified certain elements that showed variation between samples (Fe, K, Sr, Zr). The results could be used to discriminate 98.3% of 7500 pair-wise comparisons of glass samples indicating that the technique has significant potential in forensic applications.
Finally, the direct analysis of glasses is not normally associated with ETAAS because of the refractory nature of the material which is difficult to atomise and the high background signals generated in the vaporization of solids. In a novel development, a solid-sampling CS-ETAAS instrument has been used in application to the determination of Pb.201 A very small amount of sample (30–100 μg) was introduced directly to the atomiser. The method employed a Pd(NO3)2 + Mg(NO3)2 modifier and either aqueous standard solutions or a NIST glass SRM 612 for calibration. Several wavelengths were evaluated for the purposes of measurement and the most sensitive line (283.3 nm) was selected for the analysis. For the Pb determination under optimal conditions, the LOD reported was 11.2 pg. The method was validated for the analysis of glass samples, and it was concluded that solid sampling CS-ETAAS was a reliable technique for this application.
3.4 Nano-structures
The use of engineered nanoparticles is still increasing rapidly with applications as diverse as catalysts, drug delivery vehicles, nano-electronic components, anti-microbial agents and sun-block agents, amongst others. The focus of this review is advances in atomic spectroscopy and therefore papers that described methods of formation of the materials and their applications will not be included; irrespective of the novelty. Instead, the review will focus purely on their characterization or their determination in varying sample types using atomic spectrometry. Subjects that will be discussed at length include their characterization using the various forms of field flow fractionation coupled with atomic spectroscopy, chromatography coupled with atomic spectroscopy, single particle analysis and problems associated with the introduction of particles to instrumentation.Nanoparticle analysis has now reached the point where some research workers are considering the production of certified materials. Two papers have discussed this topic.227,228 The first discussed the use of polystyrene nanoparticles doped with elements that are low in abundance in natural samples and that have a high response using ICP-MS, i.e. Dy, Gd and Nd. The work produced materials that were in the size range 33–193 nm, were highly monodisperse and could be detected using all of the standard techniques, i.e. TEM, ICP-MS and DLS. The second paper228 discussed the preparation of reference materials for the detection and size determination of silica nanoparticles in tomato soup. This represented a proof of principle exercise for food analysis. The neat silica suspensions were first tested for homogeneity and stability and then characterized for total Si content. The particle diameter was then determined using DLS, SEM, FFF-ICP-MS and gas phase electrophoretic molecular mobility analysis. Tomato soup was then spiked at two concentration levels with the silica nanoparticles. The food sample was then also analysed using electron microscopy and FFF-ICP-MS. Although no actual material that could be called a CRM was produced, the authors did say that it was the first step towards the production of one.
The determination of Ag nanoparticles using FFF-ICP-MS was addressed by two research groups. Wimuktiwan et al.231 investigated the association between proteins (bovine serum albumin, globulin and fibrinogen) and different sized Ag nanoparticles. The authors discussed how the particles were made and how the pH determined the particle size. They then incubated solutions of nanoparticles with known concentrations of protein at 37 °C for different times. The binding stoichiometry between BSA and Ag nanoparticles was approximately 1:5 × 10−7 (i.e. 2 × 106 molecules of BSA were attached to a single nanoparticle). The binding stoichiometry with the other proteins was not determined. The other paper, by Ramos et al.,232 described how Ag nanoparticles in nutraceuticals and beverages were quantified. After optimization of the operating conditions of the FFF separation, a wide dynamic range spanning 10–1000 μg L−1 was obtained with a LOD of 28 ng L−1. Silver nanoparticles are renown for agglomeration and, since this can confound the FFF separation, the authors resorted to using an ultrasonic probe for 90 seconds (45 pulses of 2 s duration) to ensure disaggregation. Total concentration data were in good agreement with those obtained using ultracentrifugation followed by acid digestion. The particle size distribution was also in agreement with that observed using TEM.
Gold nanoparticles have also received attention in this review period. Meisterjahn et al.233 used a combination of detectors to determine the interaction between Au nanoparticles and naturally occurring ones because Au particles interfered with the light scattering detection that is usually used. Use of both ICP-MS and light absorbance detectors enabled distinction between heteroaggregation and homoaggregation. Addition of natural organic matter did not change the particle size distribution but did appear to stabilize the Au nanoparticles. However, fractograms of samples containing natural nanoparticles and Au particles but no organic matter showed a large change in particle size distribution. This led the authors to conclude that there were interactions between Au and natural nanoparticles. A second paper to discuss Au nanoparticle detection using FFF-ICP-MS was published by Mudalige et al.234 These authors pointed out the problem of nanoparticles causing fouling of the FFF membrane which leads to low analyte recoveries and decreased lifetime of the membrane. They tackled this issue by modifying the Au nanoparticles by functionalizing it with a phosphine molecule and by coating the surface of the membrane with a negatively charged polystyrenesulfonate polymer. The result was that improved separation of 10, 30 and 60 nm particles (obtained from NIST) and recoveries of 99.1% were achieved. A LOD of 6 μg kg−1 was obtained.
Silica nanoparticles have also been investigated this year. A paper by Heroult et al.235 discussed the on-line coupling of FFF with both ICP-MS and multi-angle light scattering detectors as well as the off-line detection with EDAX for the detection of silica particles in food matrices. Coffee creamer was used as a model sample. The effects of sample de-fatting using organic solvents and sample preparation protocols that mimic the cooking process, were studied. The de-fatting with hexane had no effect on particle size or the particle size distribution. Similarly, raising the temperature to 60 °C also had no effect. Unspiked and sample spiked with food grade silicon dioxide were extracted and analysed. Detection using ICP-MS and post FFF calibration with ordinary aqueous standards enabled the elemental composition of the different size fractions to be quantified. Off-line fractionation with filtration followed by ICP-MS and TEM/EDAX detection provided reliable information of nanoparticle size in the food matrix. Barahona et al.236 analysed suspensions of silica nanoparticles with diameter, 20, 40, 60, 80, 100 and 150 nm using FFF-ICP-MS. The LOD was 0.16 to 0.3 mg L−1, depending on the particle size (with larger particles having greater LOD). Other techniques, e.g. TEM, centrifugal liquid separation and dynamic light scattering, were also used.
Several other types of nanoparticle have been analysed using FFF-ICP-MS. These include titanium dioxide,237 selenium238 and cadmium selenide/zinc selenide quantum dots.239 The titanium dioxide particles were determined in food products (sugar glass and coffee cream) and in cosmetics (a moisturizing cream).237 The particles were separated using a carrier solution of 0.2% SDS and 6% (v/v) methanol at pH 8.7. Size calibration was achieved using latex beads of known diameter and TEM. Quantification of the Ti was achieved by using on-line calibration using rutile particles. This method suppressed the non-specific interactions between nanoparticles and the membrane and also overcame errors associated with calibration using Ti ions. No significant differences were found in Ti concentration when the moisturizing cream was analysed using FI-ICP-MS (3770 ± 24 mg kg−1), in the sample prior to mineralization (3865 ± 139 mg kg−1) and using FFF-ICP-MS with the on-line calibration protocol (3699 ± 145 mg kg−1). The Se-based nanoparticles were stabilized in a range of food-like materials (pectin, a mixture of pectin/alginate, ovalbumin and beta-lactoglobulin) and then incubated under gastro-intestinal conditions prior to FFF-ICP-MS analysis.238 Over 90% of the Se was still in the nm range after the digestion process and the particle size results from FFF-ICP-MS were in good agreement with those obtained using TEM.
Several papers have still worked on the theory of single particle analysis, with the dwell time being one of the most crucial factors. Two papers discussed the effect of dwell time on single particle analysis.241,242 Both papers agreed that detection of a nanoparticle is an event with duration of approximately 0.5 ms. This can be problematic when many commercial instrument have a default dwell time of 10 ms. Hineman and Stephan241 removed the settling time of the quadrupole mass spectrometer, enabling dwell times as low as 10 μs to be used. Such short dwell times decreased the background signal by three orders of magnitude compared with that at 10 ms. The NIST reference materials 8012 and 8013 gold nanoparticles were used as test materials for the experiments determining the effect of dwell time on particle sizing and the sample concentration. The paper by Montano et al.,243 decreased the dwell time to 100 μs using commercially available hardware and software. They concluded that such a dwell time enabled clear distinction between signals from nanoparticles and ionic Ag in solution even when the latter was in ten-fold excess. They also concluded that low dwell times offer the possibility of determining more than one element or isotope in a single nanoparticle. A third paper discussed the SP-ICP-MS analysis of gold nanoparticles over the range 10–200 nm.244 A dwell time of 10 ms was optimal to limit split particle events, particle coincidences and false positives. Using optimal sensitivity, the linear range was 10–70 nm. However, this could be extended to 200 nm by decreasing the sensitivity.
Single particle analysis is not rapid and the data manipulation stage after collection adds significantly to the time required. Bi et al.245 used the K-means clustering algorithm to process single particle data. This improved discrimination of particle signals from background signals and provided a statically-based method to resolve different sized groups of nanoparticles. As an application Au nanoparticles with a nominal size of 80 nm, but with “impurities” of diameter 20 nm, 50 nm or 100 nm, comprising 2% by mass of the sample, were analysed. The results were in good agreement with those obtained using dynamic light scattering, but offered significantly better resolution. The process was then also applied to the analysis of cerium dioxide nanoparticles.
Isotope dilution analysis (IDA) with single particle ICP-MS was described by Telgmann et al.246 Under normal circumstances, size discrimination and concentration determination requires external calibration. Using the ID methodology, internal calibration was achieved, leading to a more rapid analysis. Silver nanoparticles with diameter 50 nm coated with citrate and 100 nm coated with polyvinylpyrrolidone were analysed using SP-ICP-MS with a solution enriched with 109Ag being introduced via a T-piece. Dwell times of 1, 5 and 10 ms were tested and 5 ms yielded high intensity and supplied data that could easily be fitted into a Gaussian function. According to the authors, the 1 ms dwell time was insufficient to record an entire particle event and the 10 ms led to coincident particles being detected. The paper provided the equations that were used to determine the mass per particle using IDA data and, helpfully, took the reader through an example calculation. The LOD with respect to the particle size was 40 nm.
There are numerous sources of error in single particle analysis. A paper by Tuoriniemi et al.247 discussed one of these sources, the nebulization efficiency. It is well known that most conventional nebulizers have a transport efficiency of sample to the plasma of significantly less than 20% (often less than 2%), with the finer particles being transported in preference to larger ones. Using citrate coated silver nanoparticles (nominal diameter 80 nm) and citrate coated gold particles (50, 100 and 250 nm) the authors tested the effects of the nebulization efficiency on the accuracy of the particle sizing. The waste collection method was used to calculate the nebulization efficiency. This involved collecting the waste from the spray chamber and weighing it and using a flow meter to measure the amount reaching the plasma. The accuracy of the particle size measurements was dependent on the uncertainty of the nebulization efficiency. This uncertainty was improved by correcting the efficiency for the partitioning effects. Once this was done, the particle size measurements achieved using single particle ICP-MS were in good agreement with those obtained using SEM. Other factors investigated included the duration of the particle events and the dwell time.
There have been several papers that have applied single particle ICP-MS to the analysis of real samples. Included in this number is a paper by Peters et al. who determined Ag nanoparticles in chicken meat.248 The material was digested using an enzyme rather than acid to ensure that the nanoparticles remained intact. After dilution, the analysis of the material which, contained up to 25 mg kg−1 of Ag nanoparticles of 60 nm diameter, led to good accuracy for particle size (98–99%) and for concentration (91–101%). The repeatability for the sizing was also good (better than 2%) but the repeatability for the concentration determination was less good (better than 11%). Reproducibility was less impressive with values for size and concentration being 6% and 16% respectively. A LOD of 0.1 mg kg−1 was obtained which, given the circumstances, was impressive. However, the authors noted that the diluted Ag nanoparticles were not stable since they had a tendency to dissolve and then form silver sulfide. Another paper to determine Ag nanoparticles in foodstuffs (actually, food simulants) was prepared by Linsinger et al.249 This paper described an inter-laboratory study involving 23 labs throughout Europe, USA and Canada. The labs were sent three concentrated suspensions of Ag nanoparticles with nominal diameter 20, 40 and 100 nm and they then prepared eight more dilute suspensions in two food simulants (distilled water and 10% ethanol). Single particle ICP-MS was then used to analyse the suspensions. The average standard deviations for the repeatability and reproducibility for the median particle diameter were 1 and 14 nm respectively. The precision was worse for the concentration measurements (11% and 78% respectively). The authors stated that further improvements could be made if shorter dwell times were used, nebulizer efficiency measurements were made etc., and therefore, the conclusions were largely in agreement with those made in papers discussed already in this review.
The problem of silver nanoparticle dissolution in natural, laboratory and processed waters is a subject tackled by Mitrano et al.242 Silver nanoparticles of diameter 60 nm and 100 nm and coated with either citrate, tannic acid or polyvinylpyrrolidone were placed in water and their size monitored using SP-ICP-MS for 24 hours and for one week. The water chemistry played a critical role in the dissolution process. Chloride and sulfide at 1 mg L−1 and a dissolved organic carbon content of 20 mg L−1 resulted in negligible dissolution after 24 hours, whereas lower concentrations led to a decrease in particle diameter of approximately 10%. Interestingly, chlorinated tap water led to total dissolution within a few hours. The different coatings had a variable effect on dissolution. The single particle approach for determining particle diameter was claimed to be simpler than attempting to determine the ionic content.
It is difficult to distinguish signals arising from particles from those from dissolved ions. Hadioui et al.250 attempted to resolve this problem by coupling ion exchange resins with SP-ICP-MS. The efficiency of the method was tested by using silver nanoparticles in the presence of various concentrations of silver ions as well as other major ions (calcium, chloride, magnesium, potassium and sodium). The ion exchange resins removed 1–4 μg L−1 spikes of the ionic Ag, facilitating the particle size determination, particularly of particles less than 60 nm in diameter. It was noted that although the proof of principle study was focused on Ag, it could be used for several particle types.
The detection limit is normally associated with a concentration, but with SP-ICP-MS it can also apply to the smallest size of a particle that can be determined. The LOD for 40 elements that form nanoparticles, or have the potential to form nanoparticles in the future, were calculated by Lee et al.251 This theoretical approach was validated by comparing the LOD calculated with those in the literature. Also, SP-ICP-MS was used for three elements. The experimental results were in good agreement with the theoretical ones. The authors split the analytes into four categories: <10 nm, 11–20 nm, 21–80 nm and >200 nm.
Capillary electrophoresis was coupled with ICP-MS to enable rapid, high resolution speciation and characterization of Au, Pd and Pt nanoparticles in dietary supplements.254 The separation was optimized in terms of the surfactant added, the pH and applied voltage. The anionic surfactant sodium dodecyl benzenesulfonate in the buffer system improved the separation. Quantum dots were used as mobility markers to eliminate run-to run variation. The electrophoretic mobility of the particles was linearly related to the diameter of the particles and so a size distribution could be elicited. The accuracy of the method was demonstrated by analysis of 10 and 30 nm gold nanoparticle reference materials. Results from the analysis of extracts from dietary supplements were in good agreement with measurements obtained using TEM. A second paper to use CE-ICP-MS was published by Liu et al.255 These authors also reported that chemical compositions, size distributions and ionic species could all be determined in a single run.
Franze and Engelhard256 used micellar electrokinetic chromatography, a technique using the same instrumentation as CE, coupled with ICP-MS to obtain a rapid separation, characterization and speciation of gold and silver nanoparticles and their ionic counterparts. The particles are usually coated with a compound to prevent agglomeration. This often gives a charge which can be utilized analytically. The optimal separation conditions with respect to resolution, peak shape, and migration time were 60 mM sodium dodecyl sulfate and 10 mM N-cyclohexyl-3-aminopropanesulfonic acid containing 10 μg L−1 Cs to monitor electroosmotic flow, which is a measure for stable analyte transport from the CE capillary to the ICP-MS instrument. The migration times were precise (4–6%) improving to 1.4% with the use of the internal standard 127I (in the form of iodine-containing sodium diatrizoate). Recoveries from the capillaries were in the range 72–100%, depending on the species and nanoparticle size. Gold nanoparticles of 5, 20 and 50 nm eluted after 415, 450 and 530 s, respectively and with a recovery of 72–80%. The Au ions eluted first and with a recovery of 100%. The presence of a chelating agent (penicillamine) was necessary to elute the Au ions in a single peak. Limits of detection for all species were sub-μg L−1. The procedure was applied to the determination of Au particles of nominally 6 nm in a dietary supplement.
Two papers reported the use of liquid chromatography coupled with ICP-MS. The first, by Hanley et al.,257 separated Ag ions and starch-modified Ag nanoparticles using cation exchange chromatography and 0.5 M nitric acid as the eluent. Rhodium was used as an internal standard and was introduced via a T-piece, on-line. Detection was achieved using UV-Vis and ICP-MS in tandem. The Ag+ was retained on the column and had to be eluted with 0.1% ethanolamine. The nanoparticulate Ag was eluted prior to the ions. The ICP-MS response was linear over four orders or magnitude and the LOD was 0.04 mg kg−1. The authors did undertake a stability study of the nanoparticles in the acid eluent to determine whether there was any dissolution. Although there was evidence that dissolution did occur (measured after 30 min), the amount over the five minute chromatographic separation was judged to be minimal. The other paper to use liquid chromatography was published by Zhou et al.258 These workers used a 500 Angstrom pore size amino-column and an aqueous mobile phase containing 0.1% FL-70 (a surfactant) and 2 mM sodium thiosulfate at a flow rate of 0.7 mL min−1. The nano-particulates were separated as one peak and the Ag+ as a baseline-resolved second peak. The method was applied to speciation of Ag in anti-bacterial products and natural waters. Good precision of retention time (better than 0.6%) and in peak area (better than 2%) was achieved. Spike recovery experiments indicated a recovery of 84.7–102.7% for Ag+ and 81.3–106.3% for nano-particulate Ag.
The simultaneous mass quantification of nanoparticles of different composition in a mixture using a micro-droplet generator ICP-TOF-MS instrument was reported by Borovinskaya et al.260 A test system comprising certified Au nanoparticles, well-characterized Ag nanoparticles and core–shell nanoparticles consisting of a Au core and a Ag shell was employed. The rapid simultaneous detection and the full spectral coverage capabilities of the instrument enabled the composition of individual particles to be determined. Individual Ag nanoparticles could be differentiated from the core–shell nanoparticles even though they contained the same amount of Ag. Use of the micro-droplet generator enabled monodisperse droplets of calibration standards to be produced. This meant that calibration for mass quantification was achieved without recourse to certified nanoparticle materials enabling particle size measurements to be made with an accuracy of 7–12%. The LOD for the size quantification were 13 nm and 16 nm for Au and Ag nanoparticles, respectively.
The use of LIBS as a means to determine the size and the concentration of nanoparticles was discussed by Fedotova et al.261 As the laser hits individual particles, a plasma event occurs. If a calibration curve of breakdown energy is prepared as a function of laser energy, it is possible to extract information on the size and concentration of the nanoparticles. This study looked into the effects of matrix components, e.g. surfactants that are often used to avoid agglomeration of the particles, on the breakdown signals produced. A series of experiments was undertaken in which the energy calibration curves were prepared using different concentrations of nanoparticles and different surfactants, also at different concentrations. Increasing concentration of surfactant led to a decrease in the energies. The size determinations were less affected by the surfactant than the concentration measurements, with the latter yielding data that deviated 300% from the expected value. The surfactants had the greatest effect when the number of particles was low (at the 2 × 107 mL−1 level).
3.5 Polymers and composites
This has been a relatively quiet area of research in this review period. Previous years have yielded numerous papers that used LIBS to identify polymers. This has continued into this year. Another area of research was the analysis of waste electrical and electronic equipment (WEEE). This includes items such as television sets and computers. Another area of research during this review period was the analysis of leachates from plastics, especially when the leachates were food simulants. In reality, this is not actually the analysis of the polymers and hence a decision was made to omit most of these papers from the review. However, some of the more important ones have been included for completeness.A LIBS system comprising a Nd:YAG laser operating at 1064 nm, with a pulse energy of 80 mJ and for a duration of 7 ns and a spectrometer equipped with a CCD detector was used by Huber et al.263 to identify polymers containing Cl at an industrial waste sorting plant. The plastic pieces were placed on a conveyor belt moving at a rate of 2 m s−1 and the laser beam, which gave a focus spot of approximately 1 mm in diameter, excited any Cl present causing it to emit light at 837.6 nm. The detection system also had software for quasi-real-time evaluation of the spectra produced. All measurements were made in air and on samples that had arrived at the plant and that had not been treated in any way and consisted of items such as bags, pipes, bottles, cups and packaging materials. The results obtained using the LIBS system were compared with those obtained using a near infra-red reflectance detector and were, in general, in good agreement. The infra-red system did miss some of the dark PVC samples, indicating an advantage of the LIBS system.
The other study relevant to this section was provided by Cushen et al.,265 who reported on the determination of Ag leached from commercially available zeolite filler polyethylene composites. Three commercial materials containing 0.5, 1 and 2% Ag ion filler were extracted using either 3% acetic acid or distilled water, according to EU Commission Regulation 10/2011. Some polyethylene materials made in-house containing 0.1 and 0.5% Ag nanoparticles were also tested. Results obtained using ICP-OES were compared with those from spectrophotometry. During the ICP-OES analysis, the authors took the precaution of using chloride-free Viton tubing rather than the standard PVC tubing to minimise the Ag interaction with chloride. However, they appear not to have used any sonication or even vortex mixing immediately prior to analysis which, given silver nanoparticles' proclivity to agglomerate, may have been an error.
000 mg kg−1. However, nearly 40% of plastics from computers contained over 50000 mg kg−1 of Br. Another application analysing WEEE products was reported by Aquino and Pereira,267 who used LIBS to analyse 144 fragments from 50 obsolete mobile phones. Data for 16 different analytes (some metallic, others being C, CN etc.) were obtained using a Nd:YAG laser operating at a wavelength of 1064 nm, a power of 100 mJ, a duration of 8 ns and a frequency of 10 Hz and a CCD spectrometer capable of detecting wavelengths between 186 and 1042 nm. Once the data were obtained, the authors used a range of chemometric tools (PCA, SIMCA, partial least squares-discriminant analysis (PLS-DA) and K-nearest neighbour) to classify the samples. A model was first constructed using some samples of known origin and then the chemometric tools were used to try and identify the manufacturer of other samples. The K-nearest neighbour technique was the most successful with correct identification close to 100% for all sample types. Slightly less successful was SIMCA, where correct identification was obtained in >95% of samples for the white polymers and >82% for the black polymers. The PLS-DA was the least successful, with correct identification rates of 16–32% being achieved. Most easily identifiable were the coated polymers, which tended to have significant quantities of Ag, whereas the uncoated polymers tended to have higher levels of Al, K, Na, Si and Ti.
Three papers described the determination of hexavalent Cr in polymers. Ohata and Matsubayashi268 used XAFS and XANES to determine CrVI in the plastic CRMs NMIJ 8106a and NMIJ 8106a(02) (which are both acrylonitrile–butadiene–styrene) and NMIJ 8136a – polypropylene; which have been developed by the National Metrology Institute of Japan. The first material was expected to have a CrVI content of 100% of the total Cr because it had been added as lead chromate. The last two materials were expected to have a CrVI content of approximately 25% of the total Cr because it was added as lead chromate and chromium acetylacetonate (CrIII). Calibration for the XANES was accomplished by pressing pellets of plastic, methyl cellulose binder and lead chromate and/or chromium acetylacetonate. The last two components could have their proportions changed to obtain 0, 5, 10, 15, 20, 25, 30, 50, 75, 90, 95 and 100% CrVI. The absorption of the pre-edge peak of Cr K-edge XANES spectra were summed and normalized and yielded straight calibrations, even though measurements were conducted at different beam time and beam line. The XAFS was achieved using 2.5 GeV and 300–450 mA. Results from the analysis of the materials were in agreement with the expected values. The authors concluded that the procedure they had developed could be useful for measurements associated with the EU's directive Restriction on the use of Hazardous Substances (RoHS) in electrical and electronic equipment. The second paper, by Oki et al.269 described the use of XAFS to determine CrVI and ICP-OES for the determination of total Cr in plastic and chromate conversion coating. The pre-edge peak intensity of the Cr K-edge increased linearly with the CrVI amount. Results from the analysis were in good agreement with those obtained using a wet chemical analysis method. The third paper to have determined CrVI in polymers was reported by Kim et al.270 The determination of CrVI in the presence of SbIII has been problematic for analysts implementing RoHS because the Cr is reduced to its trivalent state. The study by Kim et al. reported a method that overcomes the problem. This involved the dissolution of the sample using alkaline tetrahydrofuran (THF) which formed an SbIII–THF adduct that was identified using MALDI-TOF, XRD and NMR as having the formula Sb2O5Cl5(THF)3. The polymers acrylonitrile–butadiene–styrene (ABS), a heat resistant polystyrene and polycarbonate were mixed with appropriate amounts of lead chromate and antimony trioxide, a dispersing agent and an extruder, heated to 200 °C and extruded repeatedly to ensure homogeneity. These in-house reference materials were then extracted using the protocol developed and the extracts analysed using ICP-OES. The CrVI data obtained were approximately 90% of the expected value for the ABS and the polycarbonate. However, the polystyrene material was less successfully analysed, yielding an extraction efficiency of only ∼72%.
Two papers have developed methods for the determination of Br in polymers. Ohata and Miura271 developed an ID-ICP-MS system for the determination of Br in polypropylene pellets that are to be used by the National Metrology Institute of Japan to develop a reference material. The pellets and a spike of 81Br were digested using nitric acid in a microwave oven. Two microwave units were tested, one of which used PTFE digestion bombs and the other a quartz vessel. The latter was the favoured option since the blank levels were lower. The analysis using ICP-MS was undertaken using samples diluted with water and others diluted with ammonia solution so that the final pH was approximately 9. The samples diluted using water led to severe memory effects, whereas those diluted in ammonia solution were washed out much more efficiently. Results from these analyses were compared with those obtained using INAA, with both techniques yielding results close to the expected value of 300 mg kg−1. The pellets were analysed using LA-ICP-MS to determine the Br homogeneity. The other paper to determine Br in polymers was reported by Gorewoda et al.,272 who used WDXRF for the determination, even though the polymer samples were thinner than the critical thickness required for Br. This was especially true for cellulose and polyethylene samples that have a high Br critical thickness and yet come as thin sheets. Four different methods were developed to overcome this potential problem. The method that provided the best accuracy and longest linear range for calibration was slightly more costly and time-consuming than the others because it required the samples to be ground and for other chemicals to be added. In this procedure, 1 g of sample was ground with 0.5 g of the aluminium oxide abrasive and 0.2 g of the barium carbonate (a compound with a high absorption coefficient) and 1 mL of ethanol until the particle size was less than 0.2 mm. The mixture was then pressed into a disk. Calibration was achieved by preparing reference materials in the same way. The second method involved the pressing of the sample into disks. This method was rapid and could be used for routine analysis. However, it required careful optimization of the tube-detector geometry and mathematical manipulation of the data. The other methods used the Rayleigh line intensities from the tube as an internal standard for the Br Kα or Br Lβ line intensities. These two methods were the fastest, but were less precise and accurate than the others. They also had shorter linear ranges.
The use of SIMS or TOF-SIMS was reported by Bailey et al.273 who used argon cluster sputtering and TOF-SIMS to obtain a three-dimensional characterization of multi-layer plastic films comprising alternating layers of polystyrene and polyvinylpyrrolidone. Uniform sputter rates were obtained even over depths of more than 15 μm. The use of the argon clusters for sputtering enabled measuring layer homogeneity, thickness and interface widths to be characterized far better than when C60n+ was used. The data obtained for the layer thickness were in good agreement with those obtained using ellipsometry. According to the authors, “the quality of the data allowed a detailed analysis of the chemical structure of these systems, revealing some minor imperfections within the polymer layers”. This system could therefore be used to inform future systems manufacturing and development.
4 Glossary of terms
| 3D | Three dimensional |
| AAS | Atomic absorption spectrometry |
| ACC | Automotive catalytic convertors |
| AFFF | Asymmetric field flow fractionation |
| A4F | Asymmetric flow field flow fractionation |
| AFS | Atomic fluorescence spectrometry |
| AFM | Atomic force microscopy |
| AMS | Accelerator mass spectrometry |
| API | Active pharmaceutical impurities |
| APT | Atom probe tomography |
| ASTM | American Society for Testing and Materials |
| BCR | Community Bureau of Reference |
| CCD | Charge coupled device |
| CE | Capillary electrophoresis |
| CRM | Certified reference material |
| CPFAAS | Collinear photofragmentation atomic absorption spectrometry |
| CS | Continuum source |
| CV | Cold vapour |
| DA | Discriminant analysis |
| DLS | Dynamic light scattering |
| DLTV | Diode laser thermal vaporization |
| DRC | Dynamic reaction cell |
| DSC | Differential scanning calorimetry |
| EBA | Ion beam analysis |
| EBS | Elastic back scattering spectroscopy |
| EDAX | Energy dispersive X-ray analysis |
| EDTA | Ethylenediamine tetra-acetic acid |
| EDXRD | Energy dispersive X-ray diffraction |
| EDXRF | Energy dispersive X-ray fluorescence |
| ERDA | Elastic recoil detection analysis |
| ESI-MS | Electrospray ionization mass spectrometry |
| ETAAS | Electrothermal atomic absorption spectrometry |
| ETV | Electrothermal vaporization |
| EXAFS | Extended X-ray absorption fine structure |
| FAAS | Flame atomic absorption spectrometry |
| FFF | Field flow fractionation |
| FI | Flow injection |
| FI-CVG | Flow injection chemical vapour generation |
| FTIR | Fourier transform infrared |
| FWHM | Full width at half maximum |
| GA-XRD | Grazing angle X-ray diffraction |
| 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 |
| GSR | Gunshot residue |
| HG | Hydride generation |
| HPLC | High performance liquid chromatography |
| HR-CS-AAS | High resolution continuum source atomic absorption spectrometry |
| hTISIS | Heated torch integrated sample introduction system |
| IAEA | International atomic energy agency |
| IBA | Ion beam analysis |
| 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 |
| ID | Isotope dilution |
| 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 |
| LC | Liquid chromatography |
| LEIS | Low energy ion scattering |
| LIBS | Laser induced breakdown spectrometry |
| LIF | Laser induced fluorescence |
| LIPS | Laser induced plasma spectroscopy |
| LOD | Limit of detection |
| LOQ | Limit of quantification |
| LTE | Local thermodynamic equilibrium |
| MALDI-TOF | Matrix assisted laser desorption ionization time-of-flight |
| MC | Multicollector |
| MEIS | Medium energy ion scattering |
| MHGD | Micro hollow glow discharge |
| MIP | Microwave induced plasma |
| MS | Mass spectrometry |
| MCR-ALS | Multivariate curve resolution-alternating least squares |
| MWCNT | Multi-walled carbon nanotube |
| MWTN | Microwave thermal nebuliser |
| Nd:YAG | Neodymium doped:yttrium aluminium garnet |
| NMR | Nuclear magnetic resonance |
| NRA | Nuclear reaction analysis |
| OES | Optical emission spectrometry |
| ND | Neutron diffraction |
| ND-YAG | Neodymium doped yttrium aluminium garnet |
| NIST | National Institute of Standards and Technology |
| PARCI | Plasma-assisted reaction chemical ionization |
| PCA | Principal component analysis |
| PGAA | Prompt gamma neutron activation analysis |
| PGM | Platinum group metals |
| PIGE | Particle-induced gamma ray emission |
| PIXE | Particle-induced X-ray emission |
| PL-PSDA | Polymer labs particle size distribution analysis |
| PLS | Partial least squares |
| PLS-DA | Partial least squares discriminant analysis |
| ppb | Part per billion |
| ppm | Part per million |
| PGM | Platinum group metal |
| PVG | Photochemical vapour generation |
| RBS | Rutherford backscattering spectrometry |
| RDA | Regularized discriminant analysis |
| REE | Rare earth element |
| rf | Radiofrequency |
| RIMS | Resonance ionization mass spectrometry |
| RSD | Relative standard deviation |
| SEM | Scanning electron microscopy |
| SEM-EDS | Scanning electron microscopy-energy dispersive spectrometry |
| SF | Sector field |
| SIMCA | Soft independent modelling of class analogy |
| SIMS | Secondary ion mass spectrometry |
| SP | Single particle |
| SR | Synchrotron radiation |
| SRM | Standard reference material |
| SRS | Synchrotron radiation source |
| SXRF | Synchrotron X-ray fluorescence |
| STXM | Scanning transmission X-ray microscopy |
| SWCNT | Single-walled carbon nanotube |
| TE | Trace element |
| TEM | Transmission electron microscopy |
| TGA | Thermogravimetric analysis |
| TIMS | Thermal ionization mass spectrometry |
| TLC | Thin layer chromatography |
| TPR | Temperature programmed reduction |
| TXRF | Total reflection X-ray fluorescence |
| USGS | United States Geological Survey |
| UV-VIS | Ultraviolet-visible |
| VOC | Volatile organic carbon |
| VUV | Vacuum ultra-violet |
| 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 |
| XRPD | X-ray powder diffraction |
| XRR | X-ray reflectometry |
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