Open Access Article
This Open Access Article is licensed under a Creative Commons Attribution-Non Commercial 3.0 Unported Licence2013 Atomic spectrometry update—A review of advances in environmental analysis
Owen T.
Butler†
*a,
Warren R. L.
Cairns
b,
Jennifer M.
Cook
c and
Christine M.
Davidson
d
aHealth and Safety Laboratory, Harpur Hill, Buxton, UK SK17 9JN. E-mail: owen.butler@hsl.gsi.gov.uk
bCNR-IDPA, Universita Ca' Foscari, 30123 Venezia, Italy
cBritish Geological Survey, Keyworth, Nottingham, UK NG12 5GG
dUniversity of Strathclyde, Cathedral Street, Glasgow, UK G1 1XL
First published on 28th November 2013
Abstract
This is the 29th annual review of the application of atomic spectrometry to the chemical analysis of environmental samples. This Update refers to papers published approximately between September 2012 and July 2013 and continues the series of Atomic Spectrometry Updates (ASUs) in Environmental Analysis1 that should be read in conjunction with other related ASUs in the series namely: clinical and biological materials, foods and beverages;2 advances in atomic spectrometry and related techniques;3 elemental speciation;3 X-ray spectrometry4 and advances in the analysis of metals, chemicals and materials.5 In the field of air analysis, highlights within this review period include measuring the bioaccessible fraction of metals in particles and ongoing work in assessing the performance of optical and combustion techniques for the determination of the carbonaceous content of airborne particulate matter. Developments in instrumentation included new sampler designs for the collection of nanoparticles, the coupling of FFF and hydrodynamic chromatography to ICP-MS for the sizing and compositional analysis of such particles and the ongoing development of aerosol mass spectrometry. In the field of water analysis, new procedures for the detection and quantification of emerging pollutants in water such as MRI contrast agents have been developed. Instrumental developments reported include the use of molecular absorption spectrometry, by exploiting the CS-AAS technique, for measuring halogen species in water. Numerous articles involving the application of atomic spectrometry to plants, soils and related materials appeared in this review period but, as usual, most were concerned with the environmental significance of the results, rather than the methodology used to obtain them. Nevertheless, there have been some interesting developments. Both LIBS and PXRF spectrometry have been used more widely, variants such as LA-LIBS and microwave assisted LIBS have appeared, and PXRF spectrometry has been applied for the first time in the analysis of plants. Developments in geochemical analysis include the production and (re)certification of new geological RMs for bulk, isotopic and microspatial analysis. Optimisation of LA-ICP-MS techniques for the interrogation of geochemical samples continues to be reported and a number of useful instrumental review articles (AMS, ICP-MS and SIMS) have been published. Feedback on this review is most welcome and the review coordinator can be contacted using the email address provided.
1 Air analysis
1.1 Review papers
Reviews have addressed: isotopic techniques and methodologies for assessing the origins and fate of pollutants6 (71 references); on-line aerosol MS for speciation analysis7 (31 references) and the chemical analysis of single particles in ambient ultrafine aerosols8 (88 references). Consolidation of knowledge regarding emerging pollutants is most welcome, such as the review by Harrison's group9 (271 references) on nanoparticle emissions from 11 non-vehicle exhaust sources and a review on the properties, production, uses, environmental fate, toxicity and analysis of phosphorus flame retardants10 (105 references). Studies on the bioaccessibility of metals in airborne particles have increased in number in recent years so a review11 (86 references) on the approaches developed for the extraction of soluble metals was timely. The review also covered analytical techniques for the determination of dissolved species in the presence of complex sample matrix, i.e., leachates, and included a useful compilation of published results for bioaccessible trace metals in airborne particles.1.2 Sampling techniques
The development of personal workplace air samplers, used to monitor workers' exposure to airborne particles, continues to be a fertile area of research. The design of a new microtrap inertial impactor12 utilised a high-density multijet plate to direct airflow and a matched mutiwell plate to collect particles by impaction. The advantage of this system over current impactor samplers with fewer jets is a reduced pressure drop at comparable flow rates with favourable consequences for power consumption and potential for further miniaturisation. A microscale cascade-impactor using a novel soft lithography process13 included three impaction stages with 50% cut-points experimentally determined to be 1.19, 0.55 and 0.26 μm, when operating at a nominal 0.5 L min−1. Particle bounce and re-entrainment were minimised by spin-coating of silicon oil onto the impaction targets. The overall particle losses were determined to be <10% for 0.2–2.5 μm particles and <12% for particles in the 0.05–0.2 μm range.For the interrogation of nanoparticles, TEM coupled with EDS offers a powerful analytical tool for the determination of size, morphology and elemental composition. Sampling airborne particles directly onto TEM grids therefore makes sense as sample preparation time can be reduced and the potential for sample alteration or contamination minimised. Two commercially available TEM porous grids that allowed particles to be collected through filtration were evaluated.14 Collection efficiencies were ca. 18% for a nominal 30 nm size particle. A filter holder, the mini particle sampler, was specifically designed for this evaluation project to mount such grids and this new system was deemed suitable for sampling NPs in the size range 5–150 nm.
Following on from work reported in this ASU review last year on the development of field-portable FTIR systems15 for the determination of silica in coal dust, workers at NIOSH16 have now investigated the uniformity of coal dust deposition on filters and its effect on the accuracy of FTIR analysis. As with other microbeam techniques, use of FTIR analysis entails localised spot analysis and, as such, the spatial uniformity of dust deposition can affect accuracy and measurement repeatability. Three different sampler devices were evaluated using test filters loaded with silica and coal dust reference powders. The silica content was measured by FTIR spectrometry at nine locations across the face of each filter sample. Amongst a number of observations was the major conclusion that a single measurement shot at the centre of the filter was adequate for a field-based method. The average bias was ca. 10% and repeatability was ca. 15%.
Cascade impactor samplers are used to measure aerosol mass at different particle cuts and collected samples can further be analysed for chemical components. Concerns when using such samplers include solid particle bounce, overloading of collected particles on the impaction plate, interstage loss and clogging due to long-term or high-concentration sampling. Tests17 on a widely used system—MOUDI—showed that losses for particles <40 nm in size could be >20% and that nozzle plates with conventional step-shape nozzles could be prone to particle clogging. However, by using a lithographic technique to replace such nozzles with ones fabricated with smooth surfaces, the extent of clogging could be reduced. In addition, this new design was easier to clean between sampling exercises.
Passive samplers for collecting both gaseous and particle species from the atmosphere can be a cost-effective alternative to pumped samplers and are amenable for large-scale deployment in field sampling exercises. A disadvantage, however, is that the accuracy and precision of such devices may not meet sampler performance standards often prescribed for regulatory measurements. A new calcium-aluminium oxide-based absorbent18 was designed for the diffusive sampling of atmospheric CO2. Use of a range of techniques such as XRD analysis, electron microscopy and TGA showed that hydration of this absorbent was an essential part of the process of CO2 absorption and subsequent conversion to carbonate. The diffusion uptake rate was 47 ± 3 mL min−1 and the LOD 40 ppm for samples collected over two days at 25 °C. When the samplers were deployed in triplicate in the field, the inter-sampler precision was ca. 6%. There was good correlation (Pearson's correlation coefficient = 0.879) between results obtained by this method and those obtained using a pumped sampler employing a reference NDIRS detection system. A passive aerosol sampler19 employed electron microscopy to measure concentrations and size distributions of airborne particulate matter. Measurement precision of this sampler, determined by deploying duplicate pairs of samplers in the field, was predominately <20% for both PM2.5 >5 μg m−3 and PM10 >20 μg m−3. A new deposition sampler20 employing industry standard 37 mm diameter filters could be used in different configurations such as suspension from a support with or without a protective windshield or as a suspended dust collection plate. An XRF ready option allowed sampling onto a filter cartridge assembly which could then be loaded directly into an XRF system for analysis. Initial wind tunnel tests using test aerosols of known particle size indicated that this sampler could collect representative particle size distributions.
Generation of test particle atmospheres in a laboratory environment is difficult and it is most informative to see publications describing the development and validation of aerosol test chambers. A system21 for source characterisation and controlled human exposure to NPs generated during gas-metal arc welding had an airtight 22 m3 climate-controlled stainless steel chamber at its core. A variety of on-line measurements, e.g., using TEOM or SMPS, and offline measurements, e.g. using electron microscopy and PIXE techniques, could be made via sampling ports/samplers. Test particles were generated in a separate chamber and subsequently piped into the main chamber. Stable concentrations of atmospheric particles could be maintained for up to 6 h and concentrations could be varied from <5 μg m−3 to >1000 μg m−3 by dilution with clean filtered air. When this system was used22 to evaluate NPs generated from burning of scented candles in a simulated indoor environment, additional techniques such as IC, combustion and a TOF-MS were employed. There is increased concern that workers may be at risk of adverse health effects through exposure to CNTs which are increasingly being used in industry. Researchers at NIOSH designed a CNT aerosol respirator testing facility23 for assessing the penetration of CNTs through half-mask and filtering-facepiece respirators commonly used to protect workers.
The generation of test particles within a laboratory setting can be achieved in many different ways. The most commonly used generators, spark device generators, and the theory of their operation have been reviewed24 (60 references). These generators are already widely used in semi-conductor and materials research, and health and environmental studies and can produce high-purity particles over the entire 1–100 nm range. The high-velocity impact25 of Al or Cu cylinders of various lengths onto a steel anvil target (7.6 mm caliber, 23 or 38 mm in length, fired from a gas gun at velocities of up to 195 m s−1) resulted in a particle number distribution with a mode of 10 nm. The goal of establishing a methodology for the production of soot particles resembling those emitted from internal combustion engines has led researchers26 to evaluate the properties of particles produced under different operating conditions, using a commercially available and widely used combustion aerosol standard (CAST) generator. It was concluded that detailed and well-defined operational procedures were required if CAST systems were to be used to generate particles for instrument calibration.
1.3 Sample preparation
A closed-vessel microwave-assisted digestion method27 was evaluated for the ICP-MS determination of trace and major elements in atmospheric aerosol samples collected on commonly used quartz-fibre filters. The use of HF was a prerequisite in dissolving the filter media but a consequence of its use was the potential for precipitation of insoluble fluoride compounds. Three approaches were examined for minimising this unwanted reaction: evaporation of excess HF using a microwave-assisted evaporation accessory; addition of boric acid to complex free fluoride ions; and a mixture of these two procedures. The hybrid approach was deemed to be the most successful based upon recoveries of 79–113% for elements in NIST SRM 1633b (constituent elements in coal fly ash) and 80–98% for elements in INCT CRM FFA-1 (elements in fine fly ash). Alternatively, a simple ultrasound-assisted extraction28 involving a HNO3–HF acid mixture has been proposed. Recoveries were 80–120% for a range of elements (Ag, Al, As, Ba, Be, Cd, Co, Cr, Cu, Fe, Mn, Mo, Ni, Pb, Sb, Se, Sr, U, V and Zn) in four RMs, namely NIST SRMs 1633b (constituent elements in coal fly ash), 1648 (urban particulate matter), 2584 (trace elements in indoor dust) and 2710 (Montana soil). Similar recoveries were obtained when the study was replicated in a different laboratory indicating that the method was both reliable and reproducible. As a further indicator of data quality, indoor air samples collected on filters were analysed initially by EDXRF spectrometry and then subsequently by ICP-MS using this extraction method. For the six elements detectable by EDXRF spectrometry (Cr, Cu, Fe, Ni, Pb and Zn), there were no significant differences in values between the two measurement approaches (paired t-test, p > 0.2, 95% CI). In summary, the authors concluded that this method was suitable for the high throughput analysis of lightly loaded air filter samples.The bioaccessible metal fraction in urban aerosol particles may be a more informative metric than the total metal composition in assessng the potential health effects of exposure to inhaled particles. The bioaccessibility of nine elements (As, Cd, Co, Cr, Cu, Mn, Ni, Pb and Zn) in both TSP and PM2.5 filter samples were measured using a SBET in vitro extraction procedure.29 Filter aliquots were extracted (1 h, 37 °C) in 0.4 M glycine (pH 1.5). Total concentrations were determined following HNO3–H2O2 digestion of separate filter sections using a closed-vessel microwave-assisted procedure. The mean bioaccessible fractions were 11% (Cr) to 65% (Cd) for both size fractions. The bioaccessible fractions of PGEs in airborne particulates were evaluated in two separate studies. In one,30 a procedure using a synthetic gastric juice extractant was compared with closed-vessel microwave-assisted digestion with HCl–HF–HNO3. The Pd and Pt bioaccesible fractions were 41 ± 17% and 27 ± 17%, respectively, in TSP filter samples and 26 ± 17% and 34 ± 17%, respectively, in PM10 filter samples. The second study31 was similar but simulated human lung fluids, ALF and Gamble's solution, were used. Significant amounts of up to 29, 22 and 51% of Pd, Pt and Rh, respectively, were mobilised by ALF within 24 hours. The corresponding figures for exposure to Gamble's solution were 17, 18 and 44%. Interestingly, when sample leaching testing was performed on NIST SRM 2557 (used auto catalyst) test samples, much lower bioaccessible values were obtained. These results underline the danger of using RMs for leaching experiments, in which, despite best intentions, matrix matching between RMs and real world samples may not be as complete as believed.
1.4 Instrumental analysis
New AES instrumentation included: a LIBS system37 employing a low-pressure rf-plasma discharge that had the potential for enhancing sensitivities over those obtained with plasmas operating at atmospheric pressure; a LIBS system38 that used a triggering system to discharge the laser only when a particle was expected in the focal zone thereby improving the hit rate over current systems which commonly operate pulsed lasers at constant repetition rates; and a new portable dual-mode (OES and CRS) plasma spectrometer39 which had the potential advantages of a large dynamic measurement range and an ability to cross-calibrate between the two modes of operation.
1.4.3.1 Inductively coupled plasma mass spectrometry. The multi-element capabilities of ICP-MS were exploited in a newly developed DRC-ICP-MS method40 employing a microwave-assisted digestion (HNO3–HCl–HF mixture) for the determination of 45 elements in road dust samples including PGEs (Pd, Pt and Rh), 13 main group elements, 15 transition metals and 14 lanthanoids. The recoveries of PGEs from IRMM BCR CRM 723 (road dust) and NIST SRM 2556 (used auto catalyst) were 105–111%. The recoveries of main group and transition elements from NIST SRM 1648a (urban particulate matter) were 85–115%. There was excellent correlation between analysis of digests using DRC–ICP-MS and re-analysis by SF-ICP-MS (r2 = 0.9–0.99 for RMs and 0.99–1.00 for road dust samples). The authors concluded that large element sets were needed to provide the necessary data to understand better anthropogenic influences on the wider environment. One source of metals in the atmosphere is the combustion of fuels in road vehicles. Because European national emission inventories have previously shown a very high variability in elemental emission factors (mass released per kg fuel burnt), a new survey41 analysed representative fuel samples, both diesel and petrol, from service stations in 9 nation states. Test samples were analysed using a collision cell ICP-MS system employing a HEN (ESI Apex Q). Use of a self-aspirating nebuliser with additional O2 make-up gas ensured complete combustion and thus minimised potential for carbon deposits on the cones. Although diesel samples could be analysed undiluted, petrol samples required dilution with Conostan Premisolv™ solvent. Organometallic oil standards were used for calibration and NIST SRMs 1634c (trace elements in fuel oil) and 1084a (wear metals in lubricating oil) used as QC materials but the recoveries were not reported. Marked differences of up to two orders of magnitude in concentrations between samples remained but the proposed revised emission factors were generally lower than previously published values.
A technique42 for the detection and characterisation of metal-containing NPs coupled HDC, suitable for sizing NPs in the 5–300 nm range, to ICP-MS to provide a sensitive and selective analytical tool. Three calibration functions were required to measure NP size, number concentration and metal content, simultaneously. Reference Au NPs of various sizes were used as the model system. For water samples spiked with 60 nm sized Au NPs, the LOD was 2.2 ng L−1 (600 NPs mL−1). A working prototype43 for the real-time size discrimination and elemental analysis of Au NPs consisted of a customised electrospray source and a differential mobility analyser to provide upstream particle size discrimination in real-time and CPC and ICP-MS instruments as downstream detectors to provide information on number density and elemental composition, respectively. A key design component in this system was the gas-exchange device for converting the air flow from the electrospray to the argon flow required to sustain the ICP-MS plasma. The Japanese research group led by Naoki Furuta,44 continued their research into monitoring of elements in airborne particulates by direct introduction into an ICP-MS instrument. Calibrants were prepared using a desolvating USN to provide particle standards. Comparison of the results obtained with this real-time approach with those obtained for time-integrated samples collected using a more conventional filter sampling methodology gave good agreement for those elements (e.g., As and Pb) with oxides that possess low melting points. In contrast, the real-time data were lower for elements (e.g., Cr and Ni) with oxides with high melting points, indicating that such particles were not fully vaporised and ionised within the plasma. The authors suggested that this real-time approach could still be applicable if a correction factor based upon the ratio of real-time data to filter sampling data were used.
The use of quadrupole ICP-MS for isotopic measurements included the determination45 of B isotopes in aerosol samples as tracers of emissions from coal burning. Water-soluble B was leached from filter samples, purified and concentrated using a B-specific resin and analysed using a bracketing calibration approach with normalisation to NIST SRM 951 (boric acid standard). The mean reproducibility (1σ) of 3.0‰ for δ11B was sufficient for B to be used as a discriminator. Isotopic data46 for Nd, Pb and Sr were used in conjunction with elemental ternary diagrams to assess the relative contribution of particulate pollution from industrial point sources, such as an incinerator, a steel mill and a thermal power plant, in the air of two European cities (Kehl and Strasbourg). Suspended dust samples collected in passive samplers were digested in HNO3–HF and elemental analysis undertaken using ICP-AES and ICP-MS. For isotope ratio measurements, MC-ICP-MS was used under dry plasma conditions employing a HEN equipped with membrane desolvator. This combination of isotope tracers data with element triangle plots made it possible to identify specific point sources of pollution. The direct measurement47 of U isotope ratios in single 10–20 μm U-doped glass particles used ns LA coupled to MC-ICP-MS. The analysis of 28 glass reference particles, measured under optimised conditions, yielded an average bias of <0.6% from certified values for 234U/238U and 235U/238U. Results for the 236U/238U isotope ratio deviated by <2.5% from certified values. The calculated expanded measurement uncertainties (k = 2) were 2.6, 1.4 and 5.8% for 234U/238U, 235U/238U and 236U/238U, respectively.
1.4.3.2 Other mass spectrometry techniques. New measurement tools for nuclear safeguards will require new RMs to allow comparability of measurements by commonly used techniques such as LA-ICP-MS, SIMS and TIMS. To address this, IRMM, in collaboration with ITU, initiated a study48 to investigate the feasibility of preparing and characterising a U-particle RM, certified for isotopic abudances and for U-mass per particle. To assist this process an improved ID-TIMS methodology was developed by IRMM to quantify the U-mass in single particles. Monodisperse U-oxide test particles were prepared by ITU using an aerosol-generation technique capable of producing particles of well-characterised size and isotopic composition. Experimental results demonstrated that it was possible to measure the U-mass per particle to a relative expanded uncertainity of ca. 10% (k = 2). The authors concluded that this enhanced method will be a valuable tool to assist in the certification of future RMs. It was possible49 to distinguish between different valences states such as UO2, U3O8 and UO3 on surface layers and through depth profiles of U-oxide particles by interrogating the ion distributions measured by TOF-SIMS.
Developments and improvements in gas-phase MS included a new isotope ratio method50 for the high-precision measurement of 17O/16O and 18O/16O in CO2 to assist in tropospheric studies. The method was based on isotopic exchange equilibrium between H2O and CO2 in sealed glass ampoules followed by water fluorination to produce O2. Dual inlet IRMS measurements of the δ17O and δ18O of O2 in ca. 70 μmol of CO2 allowed δ17O values to be obtained with very high precision (0.01–0.3‰). In a rapid extractive ESI-MS method51 for the quantification of trace radioactive 129I in ambient air, gaseous 129I2 was initially converted into 129I− by passing it through an excess of Na2SO3. By adding an excess of 127I2 into the solution, this 129I− was coverted into a tri-iodate ion complex (I3−). Quantification was based upon measurement of the characteristic 129I− fragment. The calibration curve had good linearity over a relatively wide range of 0.01–1000 ppbv (r2 = 0.991), the LOD was 4.5 ppt and the RSD was 4–13%. The method was successfully applied to the detection of trace amounts of 129I2 released in a simulated nuclear leakage accident.
Real-time measurements using aerosol MS, aided by the increased availability of new instrumentation, are providing more information on the origins and fate of particles in the atmosphere. Particulates containing trace elements emitted from vehicles are the focus of increasingly stringent air quality regulations but, to date, the emission rates and physiochemical properties of such particles are not well characterised, largely as a result of difficulties in making such measurements. A newly developed instrument,52 the soot-particle aerosol mass spectrometer, employed a continuous 1064 nm laser beam to vaporise all particles, including the refractory ones, which were subsequently ionised by electron impact and analysed by HR-aTOF-MS. In contrast, commercially available aerosol MS systems that employ thermal desorption and electron impact ionisation methods can only quantify species that readily vaporise at up to 600 °C. The new soot-particle instrument also had a conical vaporiser used in more established aerosol MS systems so that only non-refractory components could be measured if the laser were switched off. The new instrument was considered a powerful tool for meeting future requirements for measurements at increasingly lower levels. The limitation53 of a HR-aTOF-MS system that employed thermal desorption and electron impact ionisation for the on-line near-real-time detection of trace elements in airborne particles was that only elements with low melting and boiling points (e.g., As, Cu, Zn) could be detected. With a nominal 5 minute sampling interval, the average LOD was ca. 0.3 ng m−3 (3σ). In order to assess whether aerosol MS could be used as a quantitative tool, concentration data obtained were compared to those obtained by offline analysis of particles collected at the same times and locations using alternative methodologies. Two offline approaches were used: 12 h time-integrated PM2.5 filter samples collected using a high volume air sampler, subsequently digested for ICP-AES and ICP-MS analysis; and 6 h time-integrated PM0.07–0.34, PM0.34–1.15, PM1.15–2.5 filter samples collected using the IMPROVE DRUM sampler for PIXE analysis. Not unsurprisingly, the degree of correlation and agreement between the three measurement approaches varied depending upon the element in question, reflecting inherent limitations in each of the analytical approaches. The authors suggested that further intercomparison exercises, under controlled conditions, would be required to understand more fully the differences but that aerosol MS showed promise for the real-time detection of trace elements in air.
Novel but intriguingly simplistic applications of XRF for analysis of airborne particulates included the use56 of a field-portable XRF spectrometer to determine if desert varnish rock samples contained a record of recent air pollution. A major component of desert varnish, a coloured coating found on rocks in arid environments, is windblown clay. As the concentrations of As, Cr, Pb and Zn, elements commonly found in fly ash, were significantly higher in samples collected downwind from two power plants than in unvarnished substrate rock, it was concluded that desert varnish could indeed be utilised as a passive environmental monitor to investigate recent air pollution (over the last 20–30 years). The potential57 of using commercially available carbon adhesive tabs, typically used to mount samples for SEM analysis, in TXRF analysis was tested by sprinkling small quantities of particles (ca. 0.5 mg) onto these carbon substrates mounted on quartz reflectors. Good agreement with certified values was obtained for a number of powdered CRMs including NIST SRM 1648 (urban particulate matter). For quantification, a chosen certified element was taken as an internal standard. For unknown particulate samples, elements were normalised to the most abundant element so only elemental concentration ratios and not absolute concentrations could be measured. This procedure was advocated for the analysis of atmospheric fall-out particulate matter by swabbing surfaces to collect settled dust.
Solid-state speciation analysis gives an insight into the chemical composition of airborne particles. The XAS analysis58 of airborne particulate matter, vehicle brake lining and brake pad wear residues showed that brake pads contain appreciable levels of Sb, typically 1–2% (m/m), mostly as SbIII derived from the use of stibnite (Sb2S3) as a lubricant to reduce vibrations and to improve friction stability. The other major form was SbIII derived from Sb2O3. However, the finding that the majority of wear residues, airborne particles and suspended road dust contained Sb2O4, an admixture of SbIII and SbV oxides, supported the findings from thermogravimetic experiments that during braking brake pads may indeed reach temperatures high enough to induce oxidation. The authors concluded that Sb could be used as a tracer of motor vehicle emissions in source apportionment studies. The XAS technique is indeed powerful and is increasingly being used in diverse air pollution applications such as the speciation and bioaccumulation of S in camphor tree leaves59 and Zn speciation in flue dust arising from the generation of waste during the production of carbon steel in blast furnaces.60 The HR-PIXE analysis61 of aerosols collected in Budapest clearly identified the major Cl component as NaCl. Readers are directed to our companion Update for further information on developments and applications in reviews of X-ray spectrometry.4
Evaluation of the analytical steps within TOT methods is another fertile area of interest. A step-wise solvent extraction method64 for characterising carbonaceous aerosols allowed organic material to be separated into distinct fractions. Three punches taken from each filter were extracted with solvents of increasing polarity. The first was extracted in hexane (100%), the second in hexane–methylene chloride (1![[thin space (1/6-em)]](https://www.rsc.org/images/entities/char_2009.gif)
:1 by volume) and the third by a mixture of hexane–methylene chloride–acetone (1:1:1 by volume). In summary, the method fractionated organic aerosol into extractable and non-extractable OC and then further separated the extractable OC into three fractions defined as non-polar, low-polar and high-polar OC. It is hoped that this new methodology could assist atmospheric scientists to gain a better insight into the formation of secondary organic aerosols for which knowledge of precursors, production pathways and atmospheric evolution is lacking. The outcome of a study65 on the effects of different thermal treatments on radiocarbon measurements of OC and EC fractions in the atmospheric aerosol was suggested improvements to existing thermal protocols. One such improvement was the addition of a thermal step at high temperature in a helium atmosphere, after the traditional O2 precombustion step, to remove more fully the refractory fraction of OC. By doing so, the potential for carry over of charred OC into the EC determination step was minimised. A study66 was undertaken to ascertain the effect of increasing Fe content in laboratory-produced soot aerosol on its compostion, structure and thermo-chemical properties. The temperature-programmed oxidation profiles showed a main peak ascribed to the evolution of CO2 and a shoulder peak which was assigned to the combustion of iron carbides which became more pronounced as the concentration of Fe increased. The temperature of maximum CO2 emission was found to decrease exponentially with increasing Fe content. Such a finding casts doubt on the feasibility of commonly used thermo-optical technniques for the quantification of OC and EC in carbonaceous particles if they are substantially contaminated with inorganic compounds, given that the distinction between the carbon forms using thermal techniques requires an operationally defined combustion temperature split-point.
1.5 Data analysis and quality
Evaluation of methodologies and instrumental techniques through interlaboratory comparisons is recognised as a valuable tool amongst measurement scientists. The divergent sampling techniques for respirable dust and the analyses for crystalline silica are an important issue amongst industrial occupational hygienists. In a comprehensive comparison study,67 the workplace atmosphere multi-sampler, a rotating device in which 12 samplers could be exposed to the same dust environment concurrently, was used to compare the performance of six respirable dust samplers. The samplers were exposed to airborne dusts in four workplace settings (enamel production, sand extraction, foundry and brickworks) and the resultant filter samples analysed in seven participating laboratories using FTIR or XRD spectrometries.It is possible for particles to deposit on surfaces in air samplers rather than fully on the filters. An acid-soluble internal capsule digested with the filter has been proposed as a possible solution to the incomplete sampling of airborne particles. An interlaboratory study68 with 10 participants evaluated the suitability of cellulosic capsule inserts in the determination of trace elements in airborne samples. Prototype inserts were spiked with: aqueous solutions of Pb, Pb-containing soil RMs or aerosol samples (As, Cd, Co, Cr, Cu, Fe, Pb, Mn and Ni) generated using a desolvated nebuliser system. For these elements evaluated, the interlaboratory precision and mean recoveries were similar to those that could be expected if only filter samples were digested and analysed. As such this extra cellulose material should not present undue analytical complications but it remains to be seen whether such inserts can be produced commercially.
In a simple method69 for generating matched filter sets with known OC and EC contents, carbon black particles were dispersed in water and nebulised, dried and desolvated in a test chamber to provide a reference EC particle aerosol. A saturated alginic acid solution was used as the starting material for preparing an OC-containing aerosol. Analysis in seven participating laboratories using the NIOSH 5040 combustion method gave a laboratory repeatability of <10%, 11% and 12% RSD for TC, OC and EC, respectively. Within-laboratory repeatability was <12%. This relatively simple generation system was considered suitable for method-performance studies.
Comparisons of instrumental techniques for measuring the same chemical moieties are interesting to read. Aerosols of ultrafine TiO2 particles70 with an agglomerate mean diameter of 67 nm were loaded (3–578 μg) onto PTFE filters and analysed independently using FTIR, LIBS and portable XRF procedures. The LODs (3σ) were 108, 0.03 and 11.8 μg, respectively, for the three procedures. The greatest dynamic range was given by the portable XRF instrumentation. In summary, XRF spectrometry could be a useful field measurement tool but further tests are required to test its performance on real matrix-containing samples. Robust methods to detect and characterise engineered NPs in environmental samples are urgently needed. The combination of ICP-MS with a size separation method holds promise in this context. Hence, the comparison71 of the HDC and AF4 techniques coupled to ICP-MS for detection, quantification and characterisation of NPs was welcome. The LODs, resolution and recoveries for both techniques were determined using Au NPs. The AF4 system was capable of separating mixtures of 5, 20, 50 and 100 nm particles with greater resolution than the HDC system, particularly in the smaller size range. Higher sample recoveries were noted for the HDC system (77–96%) than for the AF4 system (4–89%). The LOD for both techniques was ca. 5 μg L−1. An additional benefit of HDC over A4F was that it has a capability of separating a dissolved-sample signal from a NP signal.
2 Water analysis
2.1 Sample preparation
As noted in last year's review,1 the determination of Hg in natural waters remains a challenge due to preservation and stability problems. In order to investigate the stability and behaviour of Hg2+ spiked at natural levels in water samples, Guevara and Horvat72 irradiated an enriched 196Hg2+ solution to produce a 197Hg2+ radiotracer solution that was used to spike filtered and unfiltered marine, coastal lagoon, lake, river and rain waters at 3 to 13 ng L−1 with and without acidification (1% v/v HCl). Subsequent storage was at room temperature or at 5 °C. The 197Hg2+ was stable for over 10 days in unfiltered and unacidified marine and lake waters but 20% of the spike was lost, probably to the walls of the container (the authors did not specify), from the particulate and dissolved phases of river and lagoon waters. In refrigerated samples, the 197Hg2+ partitioned between the dissolved and solid fractions as above but reached equilibrium more slowly. The 197Hg2+ spike in samples acidified to 1% v/v HCl was stable at room temperature and at 5 °C when stored in PTFE or borosilicate glass bottles, but significant (20–30%) losses occurred at all temperatures when the samples were stored in polyethylene bottles. The authors did not speculate on the fate of the lost mercury and did not carry out speciation analysis, so unfortunately we do not know if species inter-conversion occurred.2.2 Sample preconcentration and extraction
Review articles published this year examined preconcentration methods from environmental matrices including waters. Of particular interest was a review73 (52 references) of recent developments in SPE of elemental species from aqueous solutions. Two reviews focussed on the use of CNTs as SPE sorbents. One concentrated on their use prior to atomic spectrometric detection of trace elements74 (140 references) and the other more specifically on the chemical modification of CNTs75 (78 references). Given the explosion of interest in liquid microextraction techniques, a review of DLLME coupled with atomic absorption spectrometry76 (126 references) was timely. Tables 1 and 2 summarise the most significant developments for analyte preconcentration prior to water analysis in this Update period.| Analyte(s) | Matrix | Method | Reagent(s) | Detector | Attributes LOD (μg L−1) unless otherwise stated | CRMs or other validation approaches | Reference |
|---|---|---|---|---|---|---|---|
| Al, Bi, Cd, Co, Cu, Fe, Ga, In, Ni, Pb, Tl and Zn | Water | Ultrasonic LLME | APDC, CCl4 | ICP-AES | LODs 0.13 (Al) to 0.52 (Tl), Enrichment factor 17–20 | NRCC SLRS-4 (riverine water), HPS TMDW-500 (drinking water) | 123 |
| Au, Tl | Water | SFODME | 1-Undecanol, benzyldimethyltetradecyl ammonium chloride dihydrate | ETAAS | Enrichment factors 441 (Au) and 443 (Tl); LODs 0.66 (Au) and 4.67 (Tl) ng L−1 | NIST SRM 1643 d (trace elements in water) | 124 |
| B | Water, seawater | DLLME | Aliquat® 336, trichloromethane | ICP-MS | Preconcentration factor 18, LOD 0.3 | Spike recovery and comparison with ISO 9390:1990; azomethine-H method | 125 |
| Cd, Co, Mn, Ni | Water | Ultrasonic LLME | Tetrachloroethene, NaCl | ICP-AES | Sample pH12, LODs 0.13 (Co) to 0.28 (Ni) | NRCC SLRS-4 (river water), HPS TMDW-500 (drinking water) | 126 |
| Cd, Cr, Cu, Pb | Water | CPE | APDC, Triton X-114® | ID-ETV-ICP-MS | Preconcentration factor 14, LODs 1 (Pb) to 4 (Cr) pg mL−1 | NRCC SLRS-4 (river water) and SLEW-3 (estuarine water) | 127 |
| Cd, Co, Cu, Ni, Pb and Zn | Water | CPE | 8-Hydroxyquinoline, Triton X-114®, back extraction into aqueous phase | ICP-AES | Enrichment factors 9.95–22.16, LODs 0.01 (Co, Pb) to 0.34 (Zn) | Spike recovery | 128 |
| Cd, Cu, Mn, Ni, Pb | Water and effluent | CPE | APDC, Triton X-114® | ETAAS | Preconcentration factor 10, LODs 5 (Cu) to (80) Pb pg mL−1 | NIST 1640a (trace elements in natural water), IRMM BCR-713 (effluent waste water), BCR-715 (industrial effluent waste water) and BCR-609 (ground water) | 129 |
| Cd, Hg, Sb | Water | CPE | APDC, Triton X-114® | FI-VG-ID-ICP-MS | 10 mL of sample, LODs 0.0006 (Sb) to 0.002 (Cd) ng mL−1 | NRCC SLRS-4 (river water) and SLEW-3 (estuarine water) | 130 |
| Co, Cu, Fe, Se, Pb and Zn | Water | DLLME | DDTC, CCl4 | EDXRF | 5 mL sample, LODs 1.5 (Co) to 3.9 (Pb) ng mL−1 | Spike recovery and comparison with FAAS and ETAAS results | 117 |
| Cu, Pb | Water | DLLME | APDC, MIBK | FAAS | Enrichment factors 310 (Cu) and 187 (Pb), LODs 0.12 (Cu) and 1.2 (Pb). | NIST SRM 1643 e (trace elements in water) | 131 |
| Analyte(s) | Matrix | Substrate | Substrate coating or modifying agent | Detector | Attributes LOD (μg L−1) unless otherwise stated | CRMs or other validation approaches | Reference |
|---|---|---|---|---|---|---|---|
| Al, Be, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, Ti | Water | Porous silica | ICP-AES | LODs 0.03 (Be) to 0.48 (Fe), 15 mL sample | NIST SRM 1643 e (trace elements in water) | 132 | |
| Cd and Pb | Water | Oxidised MWCNT | APDC | TXRF | LODs 1.0 (Cd) and 2.1 (Pb) ng mL−1, 20 mL sample | Spike recovery | 133 |
| Cd, Co, Cu, Pb, Zn | Seawater and urine | PVC packed beads | ICP-MS | LOD 0.7 (Co) to 7.0 (Pb) ng L−1, Enrichment factor of 10 | NRCC SLEW-3 (estuarine water), NASS-2 (seawater) and NIST SRM 2670a (human urine) | 134 | |
| Cd, Cu, and Pb | Natural waters and mussel tissue | Bond Elut® Plexa™ PCX polymer resin | FAAS | Enrichment factors of 90–95, LODs of 0.1 (Cd) to 0.5 (Pb) | NIST SRM 1643e (trace elements in water) and IRMM BCR 278r (mussel tissue) | 135 | |
| Co, Cr, Cu, Mn, Ni and Zn | Water | Oxidised MWCNT | Electrochemically assisted sorption | EDXRF | LODs 1 (Cr and Cu) to 8 (Zn) ng mL−1 100 mL sample | Spike recovery, comparison with ICP-AES | 136 |
| Cs | Estuarine and coastal waters | Separation using an AG50W-8X ion exchange resin | Adsorption onto ammonium molybdophosphate | ICP-MS | LOD 1.0 ng L−1, 20 mL sample volume | NRCC SLEW-3 (estuarine water), CASS-4 (near shore seawater) and NASS-5 (seawater) even though not certified for Cs | 137 |
| Hg2+ | Drinking water | Activated carbon | Aliquat® 336 | XRF | 1700-fold enrichment factor, LOD 1 ng L−1 | Spike recovery of 98.5–100.6% at unrealistic spike levels (μg L−1 level) | 138 |
| Ir, Pd and Pt | Sediments, water | Aminopropyl-controlled pore glass | 1,5-bis(2-pyridyl)-3-sulfophenyl methylene]thiocarbonohydrazide | ICP-MS | Enrichment factors of 43 (Ir) to 2.3 (Pd), LODs 0.1 (Ir) to 78.5 (Pt) ng L−1 | NIST SRM 2557 (autocatalyst) | 139 |
| REEs | Seawater | TSK™ Polymer beads | MWCNT | ICP-MS | LOD 0.01 (Lu) −0.16 (Ce) pg mL−1, 100 mL sample | Spike recovery and comparison with another method | 140 |
| 99Tc | Seawater | Eichrom Teva | Fe(OH)2 coprecipitation | ICP-MS | LOD 11.5 pg m−3 for a 200 L sample (equivalent to 7.5 mBq m−3) | Comparison with standard method | 141 |
2.3 Speciation
A review on the speciation of As, Cr, Hg, Sb, Se and Sn by coupled techniques77 (79 references) included analysis of air, drinking water and soil matrices.In a field–separation–based method for the speciation analysis of arsenic,78 AsV was selectively retained on an anion-exchange resin and the eluent containing other cationic or neutral species collected. The AsV was then eluted with 5% HNO3 and the As content of both eluents determined by ICP-SFMS operating in medium resolution mode. The method accuracy was assessed by analysis of NIST SRM 1640 (natural water). In another offline method,79 the AsIII in drinking water and snow was complexed with APDC and retained on polymeric microbeads. The adsorbed As was eluted with 0.25 M HNO3 and quantified by ETAAS. The enrichment factor was 86 and the LOD for AsIII 10 ng L−1. The method was validated by carrying out spiking experiments (recovery varied from 96–100%) and analysis of water RM SEM 2011 from a Turkish national QA scheme.
Non-chromatographic methods80 for the speciation analysis of Cr in natural waters have been reviewed (87 references). Addition81 of the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate ([C4MIM]BF4) to water samples improved the DDTC extraction of CrVI complexes during hollow-fibre liquid-phase-extraction. The CrVI in the eluate was determined by FAAS. The improvement in enrichment factor from 50 in the absence of ionic liquids to 175 resulted in a LOD of 0.7 ng mL−1. Results for IERM CRM GSBZ5027-94 (environmental water) were in good agreement with the certified values. The dye dimethyl indocarbocyanide, an ion pairing agent82 that complexes CrVI under acid conditions, was used in a colorimetric method for the determination of CrVI by ETAAS. The complex was back extracted into toluene from the sample and the Cr determined by ETAAS. The LOD was 0.25 μg L−1 for a 3 mL water sample. Both coprecipitation and DLLME were used83 in the determination of total Cr and CrVI in water, effluent and soils. The total Cr concentration was determined by FAAS after oxidation with Ce(SO4)2 in H2SO4 and coprecipitation with APDC. The CrVI was complexed with APDC and extracted by DLLME before quantification and the CrIII was determined by difference. The enrichment factor for a 40 mL sample was 400 for the DLLME extraction and 100 for the coprecipitation approach. The LODs were 0.037 (CrVI) and 2.13 μg L−1 (total Cr). The method was validated by recovery of a spike from drinking water and the determination of total Cr in the NRCCRM CRM GBW-07309 (stream sediment).
Chromatographic methods for the determination of halogen species continue to receive attention. A method for fluorine speciation analysis84 used molecular absorption spectrometry (CS-AAS) to detect F via the stable GaF molecule at 211.248 nm. Offline RP LC with a C8 column and a water–methanol (1 + 1) mobile phase separated the two model compounds trifluorobenzoic acid and 5-fluoroindol-5-carboxyilic acid. The method LOD was ca. 1 ng mL−1 (100 pg absolute) and species identification was verified by ESI-MS. The authors postulated that this method approach could be suitable for the identification and quantification of unknown fluorinated organic compounds in environmental water samples. A new method85 using the ion pairing agent tetraethylammonium hydroxide allowed the separation of the iodine species I−, IO3−, 3-iodotyrosine and 3,5-diiodotyrosine in a single chromatographic run and was tested on seawater and seaweed samples. The LODs using an ICP-MS detector were 0.061 (IO3−) to 0.24 (3,5-diiodotyrosine) μg L−1 and the recoveries 90–110%. The coupling of ion exchange chromatography with ICP-MS for the inorganic speciation analysis of Br and I in only 0.3 mL of Antarctic ice86 gave LODs of 5–9 pg g−1 for the individual species.
The determination of gadolinium-based MRI contrast agents in biological and environmental samples remains a hot topic with a review87(79 references) and two new significant articles being published recently. Detection of three unknown species in sewage sludge and waste water88 indicated that species transformation can occur during anaerobic sludge treatment. The presence of the proprietary MRI agents89 DotaremA® and GadovistaA® in water samples and river plants 5 km downstream of treatment works near Berlin was confirmed by coupling HILIC with ICP-MS and comparing retention times with those of standards. This finding supports concerns that certain emerging pollutants may not be fully captured in traditional water treatment processes.
The fractionation of trace elements in water has become more feasible with the ever increasing proliferation of SEC and FFF instruments. The transport and fate of REEs90 and 16 other trace elements91 in Alaskan rivers and the extent of their binding to organic and iron-rich nanocolloids was studied by directly coupling FFF to ICP-MS. It was noted that the distribution of these elements within these fractions was dependent on seasonal variations. The trace elements associated with marine dissolved organic matter have been characterised92 by SEC followed by AEC. Using SEC it was demonstrated that marine dissolved organic matter, following ultra-filtration with a 10000 Da cut off filter, ranged from 6.5–16 kDa. The AES-UV analysis (absorbance at 205 nm) of this mass fraction revealed the presence of 4 sub-fractions. With an elemental ICP-MS detector it was demonstrated that Sr and Zn containing complexes were associated with the first fraction, Mn was bound to the second and Co was detected in the third fraction.
2.4 Instrumental analysis
On account of its sensitivity, ICP-MS is well placed to meet regulatory monitoring requirements for emerging pollutants likely to be imposed in the future. Over 88 and 69% of Gd and Pt, respectively, discharged in hospital waste water was not removed105 by a waste water treatment plant and was consequently discharged into the river Seine. The ETV-ICP-MS LOD106 was 0.12 μg L−1 when Au was used as a modifier in the determination of Hg in production waters arising from crude oil extraction processes. The average spike recovery was 106% for a production water sample which had been spiked with a Biorad Level 1 urine control material to produce a realistic saline matrix simulant. Reduction in argon consumption107 to a mere 1.5 L min−1 as a move to reduce the cost of regulatory measurements was achieved with a novel low-flow ion source/sampling cone geometry. Even at these low flow rates, there was sufficient analytical sensitivity as judged by the good recoveries obtained for NIST SRM 1643e (trace elements in water).
In a HG-ICP-MS procedure for the simultaneous separation, preconcentration and determination of As, Bi and Sn in waters,108 the analytes were retained on a functionalised silica column prior to quantification. Under optimum conditions of a sample loading flow rate of 1.9 mL min−1, a 0.5% (m/v) NaBH4 solution and a carrier gas flow rate of 0.98 L min−1, the enrichment factors ranged from 2.5 (Sn) to 7.0 (Bi) and the LODs from 0.142 (Sn) to 0.002 (Bi) μg L−1. Results for NWRI CRMs TMDA-54.4 and TM-24.3 (both fortified lake waters) were in good agreement with certified values. The volatile generation of Cd109 and Pb110 species was improved 15- and 17-fold, respectively, by the addition of K3Cr(CN)6 and K3Mn(CN)6, to the NaBH4 reductant prior to ICP-MS detection.
The use of isotope dilution analysis in intercomparison exercises can generate data with a high degree of precision. An exact-matching ID-ICP-MS approach111 was used for the determination of Cd, Ni and Pb in a NRCC candidate RM SIM-QM-S2 (trace elements in drinking water). Exact matching was achieved by carrying out an initial analysis of the water using an external calibration approach to ascertain elemental concentrations in the test sample and then adding equal quantities of enriched isotopes to prepare 1:1 spiked solutions. Such ratios are optimal for counting statistics and for achieving the best possible measurement precisions. Results were in good agreement with values generated by other NMIs as part of an intercomparison exercise involving this candidate material.
The use of ICP-MS for isotopic analysis continues to be reported. As part of an interlaboratory analysis112 of CRM IAEA-443 (natural and artificial radionuclides in seawater from the Irish sea), the MC-ICP-MS results for 239Pu, 240Pu, and 241Pu activities and activity ratios were in good agreement with the radiometric data from 12 other laboratories. Such studies demonstrate the usefulness of ICP-MS as an alternative to traditional but slower radiometric techniques. The use of high-efficiency cones and desolvating nebulisers allowed the 143Nd/144Nd ratio to be determined on trace levels (1–5 ng) of Nd.113 Used in conjunction with a high-yield column preconcentration step, the long term measurement precision of the 143Nd/144Nd ratio was (±0.000016, 2 SD), comparable to that of TIMS analysis. An anion-exchange-resin-based clean-up method114 for the measurement of δ34S required only 2 μg of sample and allowed a method precision of 0.24–0.34‰ (2σ) to be achieved. This precision was comparable to that achieved using a previously reported cation-exchange clean-up methodology which, however, required 500 μg of sample.
3 Analysis of soils, plants and related materials
3.1 Review papers
Reviews of analytical methods for the determination of specific analytes included a critical evaluation142 of techniques for the measurement of Se in mushrooms (53 references); a summary143 of methods for the determination of Re in various sample matrices including soils and plants (65 references); and overviews of radiochemical and ICP-MS procedures for measuring Th144 (129 references) and 239Np145 (227 references) in environmental samples.Reviews on specific sample types and applications included the use of chemometrics in authentication of herbal medicines146 by chromatographic, genetic and spectroscopic methods (196 references); the speciation and fractionation of trace elements in tea infusions147 to assess bioaccessibility (60 references); and the capabilities and limitations148 of INAA, XRF spectrometry, ICP-AES, ICP-MS and AAS in plant ionomics research (290 references).
3.2 Reference materials
A brown rice flour CRM (NMIJ CRM 7531-a) was certified149 containing 0.280, 0.308, 4.34, 11.7, 27.6 and 31.8 mg kg−1 As, Cd, Cu, Fe, Mn, and Zn, respectively.3.3 Sample preparation
Microwave-assisted digestion with a 5:4 HNO3–H2O2 mixture gave better precision,153 but similar recoveries, to those obtained by either aqua regia or reverse aqua regia digestion in the determination of 20 analytes in plants by ICP-AES. Disposable, screw-capped polypropylene tubes154 were used as a cheaper alternative to PTFE vessels in the digestion of sediments with 30% HNO3 + 5% HF. Recoveries similar to EPA Method 3052 were obtained for 26 elements, but the approach was unsuitable for Rb, REEs, Th, U or Zr.
Microwave-assisted EDTA extraction 155 was a viable alternative to aqua regia extraction for the estimation of pseudototal Cd, Cu, Mn and Pb concentrations in selected soil and compost samples, but gave poor recoveries for As, Ba, Be, Co, Cr, Ni, V and Zn. Unfortunately, even for the analytes successfully determined, the method may not prove transferable to other types of substrate. Microwave-assisted extraction with EDTA did provide156 the basis for a successful IC-ICP-MS method for the determination of CrVI in solid samples including soil and welding dust. The analyte was extracted quantitatively with 50 mM EDTA at pH 10 in only 10 minutes (5 min at 90 °C + 5 min at 100 °C). Application of a double spike ID procedure showed that species interconversion – the bane of Cr speciation measurement – was inhibited by the formation of a stable CrIII–EDTA complex.
A ultrasound-assisted extraction method157 for the determination of S in mine tailing by ICP-AES – optimised by the use of a two-level, three-factor, full factorial design – involved sonication of 0.1 g samples for 20 minutes at 80 °C in 1 mL HCl + 1 mL HNO3. Sonication158 for 15 min in 0.05 M EDTA solution followed by quantification by ICP-AES was the basis of a fast screening method for estimation of mobile and potentially mobile Cu, Pb and Zn in sediment samples.
Sequential extraction protocols generally specify the use of air-dried samples. However, evidence has emerged that this may underestimate metal availability in flooded field conditions. Researchers in China159 applied the BCR procedure to soils from rice paddies, extracting one portion of each sample at field moisture capacity under N2 and another after air drying. A significant shift of Cd, Cr, Cu and Ni (but not Pb) from acid exchangeable and reducible to residual forms occurred on drying. Huang et al.160 spiked soils with 202Hg and then used ICP-MS to measure the 202Hg/200Hg isotope ratio in the fractions obtained from an eight-step modified Tessier procedure. After two weeks equilibration at room temperature, the tracer was recovered predominantly in the first four fractions: water-soluble, exchangeable, and associated with fulvic and humic acids. Kozak and Niedzielski161 adapted their reactor manifold previously used for water analysis to perform in situ extraction of water-leachable, exchangeable and 2 M HCl-leachable forms of As and Sb in sediment, with quantification by on-line HG-AAS.
Interest has continued in improving physiologically-based extraction tests to assess human bioaccessibility of elements following ingestion. A round-robin study162 compared no fewer than 17 different procedures for assessment of the bioaccessible content of up to 24 elements in NIST SRM 2710 (Montana soil). No preferred method emerged, but several gave results consistent with in vivo testing for As (swine and mouse) and Pb (swine only). The pH of the extractant(s) was found to have a strong influence on results. A modified version163 of the European Standard Toy Safety Protocol EN-71 (sample:extractant ratio 1:2000) extracted similar amounts of As, Cd, Cr, Cu, Ni, Pb and Zn as the well-established SBET method (0.4 M glycine at pH 1.5) from NIST SRMs 2710 and 2711 (both Montana soils) and 2583 and 2584 (both indoor dusts). The modified EN-71 extraction was therefore recommended for use as a simple and rapid soil screening test.
| Analyte(s) | Matrix | Method | Reagent(s) | Detector | Attributes LOD (μg L−1) unless otherwise stated | CRMs or other validation approaches | Reference |
|---|---|---|---|---|---|---|---|
| Au, Pd, Pt | Soil | CPE | Tergitol TMN-6 and ammoniumpyrolysine dithiocarbamate | FAAS | 1.2, 1.4, 2.8 for Au, Pd, Pt, respectively | Spike recovery | 221 |
| Bi | Ash, coal, metals, ointment, soil, urine, water | CPE | Triton X-114® and iodide + thiourea | HG-ICP-AES | 0.01 | Spike recovery | 222 |
| Cd, Co, Ni, Pb | Water | DLLME | 2-(2′-Benzothiazolylazo)-p-cresol in trichloroethylene and ethanol | ICP-AES | 0.2–0.7 | NIST SRM 1944 (waterway sediment) | 223 |
| Hg species | Sediment | VALLME | L-Cysteine and carbontetrachloride | HPLC-cold vapour-AFS | 0.028 ng g−1 for MeHg+, 0.057 ng g−1 for EtHg+, and 0.029 ng g−1 for Hg2+. | IRMM ERM-CC580 and IAEA-405 (both estuarine sediments) for MeHg, spike recovery for other analytes | 224 |
| Mn | Fish, mollusc, water | SDME | 1-(2-Pyridylazo)-2-naphthol in chloroform | ETAAS | 0.03 | NIST SRMs 1515 (apple leaves), 1570a (spinach leaves), 1577 (bovine liver), IRMM ERM CE 278k (mussel tissue) | 225 |
| Pb, Pd | Catalytic converter, radiology waste, soil, street dust, urban aerosol | CPE | Triton X-114® and dimethylglyoxime | FAAS | 1.4 for Pb, 1.0 for Pd | NIST SRM 2711 (Montana soil), 2704 (Buffalo river sediment), and IRMM BCR CRM 142R (light sandy soil) | 226 |
| Pd | Anode slime, catalytic converter, road sediment, water | DLLME | 2,2′-Furyldioxime in chloroform and methanol | FAAS | 0.04 | CDN-PGMS-10 (platinum group ore) | 227 |
| Zn | Hair, rice, soil, tea, waters | LLE | Ethyl-2-(4-methoxybenzoyl)-3-(4-methoxyphenyl)-3-oxopropanoylcarbamate in MIBK | FAAS | 0.2 | LGC 6019 (river water), NIST SRM 1547 (peach leaves) | 228 |
| Analyte(s) | Matrix | Carrier | Detector | Attributes LOD (μg L−1) unless otherwise stated | CRMs or other validation approaches | Reference |
|---|---|---|---|---|---|---|
| a Coprecipition of CrIII, then reduction with sulfuric acid and ethanol, determination of total Cr, and estimation of CrVI by difference. | ||||||
| Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb | Food, soil | Holmium hydroxide | AAS | IRMM BCR CRM 141R (calcareous loam soil), CRM 025-050 (soil) | 229 | |
| Cd, Co, Pb, Zn | Baby food, dried eggplant, water | Mo(VI)-diethyldithiocarbamate | FAAS | 1.1–2.2 | NWRI TMDW-500 (drinking water) | 230 |
| Cr speciesa | Soil, water | Copper-violuric acid | FAAS | 1.2 | Spike recovery | 231 |
| Analyte(s) | Matrix | Substrate | Substrate coating or modifying agents | Detector | Attributes LOD (μg L−1) unless otherwise stated | CRMs or other validation approaches | Reference |
|---|---|---|---|---|---|---|---|
| Au | Water | Alumina nanoparticles | 9-Acridinylamine | FAAS | 13 | 232 | |
| Cd, Co, Mn, Ni, Pb | Street sediment, tea, water | Poly(2-thiozylmethacrylamide-co-divinylbenzene-co-2-acrylamido-2-methyl-1-propanesulfonic acid) | FAAS | 0.23–1.1 | NWRI TMDA-70 (fortified lake water), SPS-WW1 batch 111 (waste water), NIST SRM 8704 (Buffalo River sediment), NRCCRM GBW 07605 (tea) | 233 | |
| Co, Cu, Fe, Pb | Table salt | MWCNT | Violuric acid | FAAS | 0.15–0.43 μg g−1 | NWRI TMDA 54.4 (fortified lake water), HR-1 (Humber River sediment) NIST SRM 1515 (apple leaves) | 234 |
| Cu, Fe, Ni, Pb | Animal feed, water | Single walled carbon nanotube discs | 2-(5-Bromo-2-pyridylazo)-5-diethylaminophenol | FAAS | 0.3–4.6 | NWRI TMDA 54.4 (fortified lake water), NWRI HR-1 (Humber River sediment) | 235 |
| Cu, Zn | Vegetables, water | Silica-coated Fe3O4 nanoparticles | 2,6-Diaminopyridine | AAS | 0.14 for Cu, 0.22 for Zn | NRCCRM GBW 08301 (river sediment), 08607 (water solution) | 236 |
| Fe, Pb | Environmental samples | MWCNT | 2,9-Dimethyl-4,7-diphenyl-1,10-phenanthroline | FAAS | 1.3 for Fe, 2.9 for Pb | NWRI TMDA 54.4 (fortified lake water), HR-1 (Humber River sediment) | 237 |
| Hg | Liver, lotus leaf, water | Silica gel | Diethylenetriamine and thiourea | ICP-AES | 0.023 | NRCCRM GBW 08301 (river sediment), 09101 (hair powder) | 238 |
| Ir, Pd, Pt | Sediment, water | Aminopropyl-controlled pore glass | 1,5-bis(2-pyridyl)-3-sulfophenyl methylene thiocarbonohydrazide | ICP-MS | 0.0001 for Ir, 0.056 for Pd, 0.079 for Pt | NIST SRM 2557 (autocatalyst) | 139 |
| La, Nd, Pr, Sm | Soil | Carbon ferrite magnetic nanocomposite | 4-(2-Pyridylazo) resorcinol | ICP-AES | 0.5–10 | IAEA Soil-7 | 239 |
| Mo | Vegetables, water | Amino-functionalised silica | 2,6-Diacetylpyridine-monosalicyloylhydrazone | FAAS | 0.5 | Mo wire | 240 |
| Ni | Fish, plants, soil, water | Pyridine-functionalised MCM-41 and MCM-48 | FAAS | 3.5 | Spike recovery | 241 | |
| Ni | Fish, sediment, vegetables, water | Nanostructured ion imprinted polymer | FAAS | 0.15 | NACIS DC 73323 (soil), OREAS BG 326 (ore polymetallic gold) | 242 | |
| Pb | Water | MWCNT | 3-Mercaptopropyl trimethoxysilane | FAAS | 1.7 | NRCC PACS-2 marine sediment | 243 |
| Pb | Fish, sediment, soil, water | Magnetic metal–organic framework | FAAS | 0.37 | NRCCRM GBW 07293 (platinum ore), inter-laboratory comparison | 244 | |
| Pd | Soil, water | Pyridine-functionalised magnetic nanoparticles | FAAS | 0.15 | Spike recovery, inter-laboratory comparison | 245 | |
| Pd, Pt, Rh | Soil | Activated alumina | ICP-AES | 0.3–2.1 | Spike recovery | 246 |
A combination of preconcentration approaches83 was used successfully for Cr speciation in soil extracts and water by FAAS. The CrVI concentration was determined following DLLME using APDC, carbon tetrachloride and ethanol as chelating agent, extraction solvent and disperser solvent, respectively. The total Cr concentration was determined by coprecipition following oxidation of the sample with 0.02% Ce(SO4)2 in 0.07 M H2SO4. The CrIII concentration was estimated by difference. A four-step procedure164 reduced interference from Mo and Sn in the determination of Cd in rice and sediments by ICP-MS. The steps were: thiourea SPE for removal of the majority of matrix elements; coprecipitation with Mg(OH)2 to eliminate Mo; acid dissolution of the precipitate; and strong anion exchange SPE to remove Sn.
3.4 Instrumental analysis
Developments in AAS included a method for the determination of Ag that introduced a new design95 of CVG integrated-atom-trap. A 2 min in situ preconcentration gave a 143-fold improvement in sensitivity over conventional FAAS and an LOD of 0.7 μg L−1. A method166 for the determination of Cd in soil extracts combined high-speed self-reversal background correction with HG-AAS. The LOD was 1 ng mL−1 and the RSD 5% (n = 10). Results were statistically similar to those obtained by ETAAS using a Pd/MgNO3 matrix modifier. Li et al.167 more than doubled both sensitivity and linear range for the determination of Cu and Zn in plants by coating the inner wall of a slotted quartz tube atomiser with SiO2 nanoparticles. Recoveries of analytes from NIST SRMs 1515 (apple leaves) and 1573 (tomato leaves) were in the range 95–102%.
Developments in ETAAS included a new chemical modifier168 of 6 μg Pb + 1600 μg citric acid that overcame chloride interference in the determination of Ag in aqua regia digests of soils, sediments and sludges, and a procedure169 for the simultaneous determination of As, Cd, Cu, Cr, Ni, Pb and Tl in sediment digests and extracts, optimised via a 33 Box–Behnken factorial design. Interest in slurry sample introduction continued. One reported method170 used Ir as a permanent modifier in the measurement of Cd and Pb in medicinal plants. Another171 used dimethylglyoxime-impregnated activated carbon to preconcentrate Co and Ni from plant CRMs prior to introduction to the graphite tube as a carbon slurry. Although good recoveries were obtained (105–114% for Co and 95–100% for Ni), before the sorbent could be added the samples had to be digested in HNO3 and the digests taken to near dryness, reconstituted in de-ionised water, and adjusted to pH 7 with sodium hydroxide. This is a disadvantage relative to conventional slurry sample introduction that can be applied directly to solid samples.
High-resolution continuum-source AAS has seen use with both flame and electrothermal atomisers. Researchers in Romania172 developed a fast sequential method for determination of Ag, Cd, Co, Cr, Cu, Ni, Pb and Zn in soil digests by HR-CS-FAAS. This was shown by the Bland and Altman statistical method to have accuracy, precision and LODs statistically similar to those obtained using ICP-AES. Researchers in Brazil, who have long championed CS-AAS, reported methods for the determination of Cr173 and Pb174 in medicinal plants by HR-CS-ETAAS with direct solid sampling. Both procedures yielded accurate results, at the 95% confidence level, for analysis of RMs including NIST SRM 1515 (apple leaves), 1547 (peach leaves) and 1575a (pine needles). The use of wavelength-integrated absorbance and/or summation of absorbance at different lines enhanced175 sensitivity in the determination of B, P and S in plant digests by HR-CS-FAAS.
In contrast, lower plasma loading and sample consumption was achieved177 in the development of a high-temperature, single-pass spray chamber (also called a torch-integrated sample introduction system, see Section 3.4.4). Combined with segmented sample injection into an air carrier gas and single point standard addition, it was used in the ICP-AES analysis of marine samples including CRMs GBW 07313 and ISS-A1 (both marine sediment). Matrix interferences from Al, Ca, K and Na on the analytes Ag, Al, Cd, Mn and Zn were reduced.
Mulitvariate optimisation using two-level factorial design178 allowed development of a method for the determination of mercury by FI-cold vapour-ICP-AES with a LOD of 0.11 μg L−1. The recovery was 99% for NIST SRM 2704 (Buffalo River sediment) and 98% for CRM IAEA 085 (human hair).
A simplified method180 for determination of mercury species in sediments by HPLC-cold vapour-AFS used 0.1% mercaptoethanol microwave-assisted extraction. Results similar to certified values were obtained for MeHg+ in CRMs ERM-C580 and IAEA 405 (both estuarine sediments). The LODs were 0.5, 1.1 and 0.5 ng g−1 for MeHg+, EtHg+ and Hg2+, respectively.
The accuracy of the determination of lanthanides in plants by ICP-MS was improved182 by the removal of silica and fluoride by volatilisation as SiF4 and BF3 after microwave digestion of the sample. The low recoveries obtained if this was omitted were attributed to the formation of tiny silica particles that could act as analyte sorbents and interfere with ion production.
Claverie et al.183,184 developed a LA-ICP-MS method with a fast spinning platform on which samples and standards, prepared as lithium metaborate fused beads, were ablated alternately. The standard was spiked with Gd isotopically enriched in 157Gd, whilst the sample was spiked with Gd of natural isotopic abundance. Differences in ablation rates between the two materials could therefore be calculated from analysis of the mixed aerosol. In one set of experiments183 the two glasses were mounted symmetrically about the spin axis; in another184 they were positioned on the platform such that it was possible to ablate either only the sample or, by moving the laser farther off-axis, an increasing proportion of standard, thus performing in-cell standard additions. The approach was used successfully to determine Ba, Cu, Ni, Sr, Pb and Zn in glass, rock, sediment and soil SRMs.
The excellent review of hyphenated techniques involving ICP-MS (89 references) presented by Maher et al.185 predominantly concerned HPLC-ICP-MS but also covered HPLC-HG-ICP-MS, cryogenic trapping ICP-MS, and GC-ICP-MS. Example applications were discussed, challenges highlighted, and recommendations for future work provided. Articles featuring HPLC-ICP-MS published since 1978 were reviewed by Shoaib and Al-Swaidan77 (79 references).
Various procedures involving chromatographic separation coupled to ICP-MS have been reported for specific analytes. A gradient elution HPLC-ICP-MS method186 successfully separated organoarsenic feed additives and other As species present in the 0.5 M H3PO4 extracts of soil and sediment from the vicinity of pig farms in China. Complementary SEC-ICP-MS and CZE-ICP-MS methods187 were developed and applied to the characterisation of low-, medium- and high-molecular weight complexes of cadmium with glutathione and phytochelatins in model solutions and extracts of thale cress (Arabidsopsis thaliana). Gadolinium-based contrasting agents89 used in MRI were detected in river water and aquatic plants near Berlin by hydrophilic interaction chromatography ICP-MS. Application of a variety of techniques188 including HPLC-ICP-MS gave a comprehensive characterisation of low molecular weight (<5 kDa) selenometabolites in rice, wheat and maize grown on soils naturally rich in Se. A speciation method189 involving on-line preconcentration IC-ICP-MS gave LODs for both selenite and selenate in Se deficient soils of <10 pg.
The first application of LA-LIBS to soil analysis has been published.197 In this technique, the processes of solid sample ablation and generation of the LIBS plasma for analyte measurement are performed separately, allowing each to be optimised independently, although both can be performed with the same laser following beam-splitting. The LA-LIBS and conventional LIBS procedures were compared for the measurement of K in soils spiked with N–P–K fertiliser. Superior linearity was obtained with LA-LIBS (r2 = 0.861 cf. r2 = 0.665). Although acceptable results were obtained for only Ca, Fe, K and Mg in the analysis of 65 soil samples with known concentrations of Ca, Fe, K, Mg, Mn and Na, there is considerable scope for further development of the approach.
Another interesting innovation was the application of microwave-assisted LIBS to soil analysis.198 The LIBS signal intensity could be increased markedly by application of microwave radiation to extend plasma emission lifetime. The sensitivity for measurement of Cu was thereby increased 23-fold and concentrations as low as 30 mg kg−1 could be measured. Additional analytes such as Ag, undetectable using conventional LIBS analysis, could be detected.
Applications of LIBS to plant analysis included the development of a method199 for measurement of Cu in algae by standard additions, and a detailed investigation200 of how laser fluence and focusing affected the determination of Al, B, Ca, Cu, Fe, K, Mg, Mn, P and Zn in pellets of plant material. Best results were obtained when the Q-switched Nd:YAG laser (1064 nm) was operated at 50 J cm−1 with a spot size of 750 μm.
Research has continued to improve the performance of LIBS for analysis of soils. Methods have been reported for the determination of Pb,201 Zn,202 and Ca, Cu and Zn,203 all with LODs <50 mg kg−1. Repeatedly firing the laser at the same spot on a soil sample enhanced204 both signal intensity and signal-to-background ratio for Fe, K, Mn and Ti, potentially improving LODs. Frydenvang et al.205 developed a novel approach for determining elemental ratios that combined the fundamental mathematical framework of calibration-free LIBS with empirical calibration using 22 CRMs, including NIST SRMs 2709 (San Joaquin soil), 2710 (Montana soil – highly elevated trace elements) and 2711 (Montana soil – moderately elevated trace elements). The usefulness of ANNs206 in overcoming matrix effects and non-linear response in LIBS was clearly illustrated when a transportable instrument was deployed at a former mining site in France. A separate ANN model was built for each analyte, if necessary incorporating lines for other elements to improve the prediction. Results obtained for Al, Ca, Cu and Fe agreed well with those obtained using ICP-AES.
Plessow208 highlighted some major problems in the analysis of soil by XRF spectrometry for B, Cl and F. Fluorescence intensities changed dramatically over time, usually increasing over replicate analyses, but occasionally decreasing, depending on the nature of the sample. He attributed this to radiolysis, suggesting that irradiation could mobilise the analytes, which could then either move into the analytical zone, or through it and be lost to the vacuum. Difficulties in the application of PIXE to soil and other thick insulating samples209 due to charge build-up, which contributes to the Bremsstrahlung background, were overcome by use of an electron flood gun and beam profile monitor. The utility of the approach was demonstrated by analysis of CRM IAEA Soil 7. Wovkulich et al.210 ran soil column experiments in a synchrotron beam line. Both μ-XRF and μ-XANES spectrometries were used to measure changes in As concentration and speciation following treatment of the soil with oxalic acid, a potential reagent for use in the remediation of As-contaminated soil. Malherbe and Claverie211 presented the intriguing possibility of using a commercially available high-resolution WDXRF spectrometer for direct determination of Cr speciation in solid samples, including soils, based on differences in Kβ transition profiles between Cr0, CrIII and CrVI.
In the analysis of plants, Paltridge et al. confirmed the suitability of EDXRF spectrometry for measuring Fe and Zn in rice and pearl millet212 and Fe, Se and Zn in wheat213 and recommended wider use of the technique to assess nutrient status in biofortification programmes. Other workers used μ-SR-XRF to image and quantify Fe in tomato roots214 and to investigate the distribution of macronutrients and micronutrients in Dioscorea balcanica.215 A rather unusual study216 combined ultrasound-assisted extraction with TXRF spectrometry for the measurement of Ca, Cr, Cu, Fe, K, Mn, Ni, P, Pb and Zn in medicinal plants, spices and herbs. The optimised procedure involved 15 min sonication of 10 mg finely ground sample in 1 mL of 10% v/v HNO3 + 10% v/v HCl + 10% v/v HF, followed by centrifugation to separate residual solid and drying of 10 μL of the supernatant onto a quartz sample carrier for analysis.
As noted in our sister review,4developments and applications of PXRF spectrometry continue to be reported. Bosco217 summarised presentations delivered at the James L Waters Symposium at Pittcon in 2012 and provided a fascinating insight into the commercial evolution of the technique. Researchers in Brazil218 proposed a new method to correct for the effects of moisture in the analysis of soil based on the low energy background under the Ti Kα peak. More significantly, two groups—one in Australia and one in the UK—almost simultaneously reported the ‘first’ application of PXRF spectrometry to the analysis of plants. McLaren et al.219 determined Ca, Co, Cr, Fe, K, Mn, Ni, P, S, Si and Zn in agricultural plants from New South Wales whereas Reidinger et al.220 focussed on the measurement of P and Si.
4 Analysis of geological materials
4.1 Reference materials
Current interest in developing suitable RMs for microanalytical and isotopic analysis was highlighted in a review247 of papers published in 2011 containing analytical data for geochemical and environmental RMs and CRMs. The on-line version, containing all the references and supporting information in appendices, is particularly valuable because it is fully integrated with the GeoReM database (http://georem.mpch-mainz.gwdg.de), thereby providing the geoanalyst with access to information on approximately 2600 RMs. The IGGE has reported248 certified and indicative values for 18 oxides and elements in four new chromium ore CRMs with Cr2O3 contents between 17.6 and 57.8% m/m.The increased use of microanalytical techniques in recent years has fuelled the demand for suitable reference glasses. Two soda lime reference glasses (BAM-S005-A and BAM-S005-B), initially prepared as standards for bulk analysis by techniques like XRF spectrometry, were assessed for their suitability as glass standards for microanalysis using EPMA, LA-ICP-MS and SIMS.249 All major and 33 trace elements were homogeneously distributed at the μm scale in both glasses but Cl, Cr, Cs, Mo and Ni were not. Information values for major elements on the original certificate were confirmed by EPMA, while LA-ICP-MS data for 18 trace elements were within the uncertainties of the certified concentrations; no explanation for the lack of agreement with the certified values for Ba, Ce, Pb and Sr could be given. New LA-ICP-MS data for a further 22 trace elements were presented and will enhance the utility of these materials. The paucity of commercially available RMs for the determination of Au and the PGEs in sulfides by LA-ICP-MS prompted a comparative study250 of some PGE-bearing sulfide materials employed as in-house RMs in various laboratories around the world. A new nickel sulfide RM (NiS-3) was developed and used to calibrate LA-ICP-MS measurements of the five in-house sulfide RMs. Gold and PGE concentrations were not homogeneously distributed in all the RMs tested. Calibration against different RMs did not produce consistent results for all elements, highlighting the importance of the widespread availability of well-characterised sulfide RMs.
More RMs for isotope studies are required to facilitate easy comparison between data sets. Although NIST SRM 3108 (Cd solution), certified for its Cd concentration, is not an isotopic RM per se, a round-robin intercalibration251 of various in-house Cd reference solutions involving seven laboratories demonstrated that it fulfilled all reasonable criteria for its adoption as a zero-delta RM, pending the certification of another material as a proper isotopic RM for Cd. Similarly, there is no universally accepted zero-delta RM for the Mo stable isotope system, making it difficult to compare results generated in different laboratories using different analytical approaches. In an inter-laboratory calibration252 of various in-house Mo standard solutions, the δ98Mo values differed by up to 0.37‰, significantly exceeding the typical reproducibility of the determinations. Data from this study facilitated a rigorous assessment of previously published Mo isotope data. Mutually consistent δ98Mo data for NIST SRM 3134 (Mo solution) supported the proposal by Greber et al.253 that the Mo-isotope community should adopt this material as the zero-delta RM. Greber et al. also refined the working values for Mo concentrations in NIST glass RMs SRM 610 and 612, using an ID double-spike technique, and advocated their use as solid standards, as they have identical and homogeneous 98Mo/95Mo ratios at a test portion of 0.02 g. Despite its significantly different ablation behaviour, Darling et al.254 found that NIST SRM 610 was effective in correcting for instrumental mass fractionation during in situ Pb isotope analysis of a range of Fe–Ni–Cu–Zn sulfides from the Sudbury impact structure using 193 nm LA-MC-ICP-MS. Although matrix-dependent melting and condensate accumulation around the sulfide ablation pits may have reduced the sensitivity of the measurements, they did not result in any detectable isotopic fractionation.
Twelve natural monazites were analysed to find a suitable RM with homogeneous Sm–Nd composition and concordant U–Th–Pb age to act as a standard for in situ microanalysis.255 From determinations of trace element concentrations by quadrupole ICP-MS and of isotope ratios by LA-MC-ICP-MS, with some check data for Nd–Sm isotopes by ID-TIMS, two monazites were found to be sufficiently homogeneous to act as potential standards.
4.2 Solid sample introduction
Several new investigations have provided insights into causes of elemental fractionation in LA-ICP-MS. When silicate glasses were ablated with a 193 nm excimer laser in a single-volume ablation chamber,261 differential responses of volatile and refractory elements (relative to 43Ca) at different ablation sites were found to be correlated with the local helium gas velocity at the position of analysis. This fractionation was thought to be related to the different behaviour of these elements during condensation after ablation, as well as interaction between condensate and the carrier gas. In contrast, ablation within a two-volume sample chamber, designed to produce a uniform helium flow regime throughout the ablation cell, showed little evidence of this type of elemental fractionation. Flamigni et al.262 used 2D optical emission spectrometry and quadrupole ICP-MS to investigate the processes involved in the vaporisation of laser-produced aerosols and particles within the ICP. They demonstrated a shift of a few mm along the axial channel in the onset and maximum positions of atomic emission, depending on whether the aerosols were produced from the ablation of silicate glasses or of metals. These shifts arose from the aerosol penetrating to different depths along the ICP axial channel, depending on its composition, resulting in varying ion extraction efficiencies through the vacuum interface. Although further work is required, these findings may influence the design of future LA-ICP-MS systems.
Hardware modifications for improving the analytical performance of LA-ICP-MS included the development of a signal-smoothing device,263 which consisted of a copper cylinder (internal volume ca. 94 cm3) filled with steel wire and placed between the ablation cell and the ICP torch. This device produced smooth signals at laser repetition rates as low as 1–2 Hz without any detrimental effect on the signal decay time and was successfully used for U–Pb dating of zircons. The development of a spinning ablation cell platform enabled ID183 and standard addition184 techniques to be implemented with LA-ICP-MS. Two samples were placed close together in a sample holder on a rotating platform in the ablation cell and ablated using a stationary laser beam, allowing quasi-simultaneous ablation of the materials. Isotope dilution183 was performed on four NIST SRMs, two soils and two sediments, prepared as lithium borate fusions spiked with a natural Gd solution. By ablating an isotopically spiked glass, containing known amounts of 135Ba, 157Ga, 206Pb, 86Sr, and 67Zn, alongside each sample on the spinning platform, a homogeneous aerosol was produced after a period of equilibration. The main advantage of this method was the possibility of analysing a large number of samples with the same isotopically spiked glass RM, which could be reused after polishing to remove previous ablations. The spinning platform was employed in a similar manner to determine Ba, Cu, Ni, Pb, Sr and Zn concentrations in geological materials by standard addition.184 A glass RM, prepared by spiking lithium borate flux with a multielement solution containing the elements of interest and a solution of 157Gd, was mounted alongside the fused sample in an off-axis alignment so that it was possible to increase the proportion of standard in the aerosol mix by performing the ablation further from the axis of rotation. The validity of the proposed method for the direct and fast analysis of solid samples of different matrices by standard additions using a single standard sample was demonstrated using NIST SRMs. A new design264 of skimmer and sample cones with addition of nitrogen to the central gas flow enhanced signal intensities for LA-MC-ICP-MS measurements. This modification was utilised for the in situ determination of Hf isotope ratios in zircons using Faraday detectors, for which large ions beams are necessary for precise and accurate results.
Laser ablation ICP-MS is now considered an essential tool for analysing individual fluid inclusions. The main limitation in determining Br and Cl by LA-ICP-MS is their high first ionisation energies, which result in low signal intensities. Using well-characterised natural scapolite standards, Hammerli et al.265 routinely achieved LODs of 8 μg g−1 for Br and 500 μg g−1 for Cl during multi-element analysis of individual fluid inclusions by quadrupole ICP-MS coupled to a 193 nm ArF excimer laser. Corrections for polyatomic interferences on the Br signal were required, particularly when analysing K-bearing minerals, with measurements at mass 81Br recommended in preference to 79Br. In most fluid inclusion analysis by LA-ICP-MS, microthermometric measurements are used to provide an absolute concentration for one major element, usually Na, which is then applied as an internal standard to the LA-ICP-MS data. Recently, concerns have been voiced about the thermodynamic and numerical models used to estimate the absolute concentration of the internal standard from the microthermometric data, particularly for complex fluids containing salts other than NaCl. Microthermometric measurements of two phase transitions allowed Schlegel et al.266 to quantify absolute concentrations of CaCl2 and NaCl in Ca–Na dominated brine inclusions. Using this approach, accurate Na concentrations for internal standardisation of LA-ICP-MS data could be obtained irrespective of the host mineral, e.g., quartz or calcium-rich minerals like fluorite or carbonate. Leisen et al.267 presented an improved method for calculating cation concentrations in quartz-hosted fluid inclusions of low to moderate salinity. When combined with a signal processing protocol, this enabled them to estimate analytical uncertainties for each major and trace element determined in a single inclusion. The method was validated against concentrations in materials previously analysed by LIBS or crush-leach methods, and was applied as part of a wider strategy268 involving microthermometry and LA-ICP-MS for the determination of Br, Cl and Na concentrations in individual fluid inclusions of variable size and bulk salinity. The calculated uncertainty on Cl/Br and Na/Br ratios was found to be between 30 and 38%, regardless of the salinity of the inclusion.
The application of LA-ICP-MS to U–Pb geochronology continues to be a major growth area. Nemchin et al.269 summarised the state-of-the-art capability of LA-ICP-MS and SIMS for resolving age variations in U and Th-bearing minerals at the μm-scale. A review270 of the development of LA-ICP-MS for U–Pb dating of accessory minerals illustrated how recent advances have increased the precision and accuracy of such analysis. Included was a typical workflow for the in situ analysis of assessory minerals in standard 30 μm polished thin sections by 193 nm ArF excimer LA-ICP-MS. Data reduction was performed using Iolite™ and VizualAge™. The latter is a new software tool271 developed for reducing and visualising U–Pb geochronology data obtained by LA-ICP-MS. In addition, it provides computational tools to correct for the presence of common lead. In a recent zircon dating study,272 systematic differences observed in 206Pb/238U data obtained by LA-ICP-MS and TIMS were related to differences in radiation damage in the RM and samples. Annealing unknown and reference zircons to the same state prior to analysis appeared to overcome this problem. A modified protocol273 based on single shot LA-ICP-MS, with semi-automated data reduction and robust error propagation, offered rapid sample throughput for analysing detrital zircon and monazite grains without compromising data quality. The time taken to analyse 120 sample grains and associated RMs was reduced from ca. 2 h using conventional LA protocols with continuous laser pumping to 20 min, with only a modest increase in uncertainity from 2% to 4%. Significant matrix effects were encountered during in situ U–Pb dating of titanite274 by 193 nm LA coupled to quadrupole ICP-MS. Consistent ages were obtained when titanite RMs rather than zircons were used for external calibration.
The suitability of various RMs for LA-ICP-MS multi-element analysis of Roman and late Iron Age glasses275 was evaluated. Standards from the Corning Museum of Glass were considered to be more appropriate for this purpose than the NIST SRM 600 series of glass RMs in terms of their matrix composition and concentration range of trace elements, but the former were not as well characterised as the NIST ones in terms of element concentrations and their homogeneity. Quantification was achieved by adopting a strategy developed by previous workers, in which an average response factor for each analyte was calculated from a set of glass RMs. Comparison of individual response factors obtained from different standards provided an indication of any standard that suffered from specific issues, so that any deemed unsuitable could be discarded. Calibration using sets of response factors produced more accurate results than single-point calibrations and enabled the quantification of trace elements in glasses with very different compositions. As an alternative to the glass RMs currently available, Ulrich and Kamber276 proposed the use of natural obsidian glass as an inexpensive and abundant QC material for LA-ICP-MS analysis of silicates and oxides. The obsidian samples were homogeneous at the 40 μm scale for many trace elements including REEs. Silicon was recommended as the internal standard for these matrices.
The suitability of carbon as the internal standard for biological and medical applications has long been a matter of debate. A fundamental study277 on the ablation behaviour of carbon revealed a matrix-dependent partitioning of carbon into gaseous species and particles, whereas trace elements were exclusively transported in the particulate phase. Calcium carbonate powder and diamonds used as cutting tool tips were included in the 12 common carbon matrices examined.
4.3 Sample preparation
In a systematic reassessment of the optimum conditions for high pressure digestion of felsic rocks in HF–HNO3 acids, Zhang et al.285 concluded that the use of HF alone was more effective in dissolving refractory phases, such as zircon, than the use of a mixture of HF with other minerals acids. It was recommended that a test portion of ≤100 mg of ultrafine powder (<30 μm nominal particle size) be taken to prevent the formation of insoluble fluorides even though this might result in unrepresentative subsampling.
For samples with low REE content or matrix components that cause severe polyatomic interferences, separation and preconcentration of REEs prior to analysis is desirable; the determination of the REEs and Th at ng kg−1 levels in ultramafic rocks being one such case. Preconcentration methods are often based on ion-exchange resins, as in that developed by Sun et al.288 in which anion exchange and inorganic co-precipitation were used to separate and preconcentrate REEs from a HF–HNO3 digest treated with boric acid to prevent the formation of insoluble fluorides. In a novel approach, Fedyunina et al.289 used a chelating sorbent, Pol-DETATA (cross-linked polystyrene grafted with diethylene-triaminetetraacetate groups), to extract REEs from lithium metaborate fusions of rock CRMs. Because Pol-DETATA sorbs metals ions other than the REEs, 5-sulfosalicylic acid was added as a masking agent to minimise the retention of Fe, which would otherwise reduce REE sorption. Flow injection of 50 μL of eluate in 1 M HNO3 into the ICP-MS instrument was deemed a reasonable compromise between maximising analytical sensitivity and minimising the risk of corrosion of the interface. Data obtained were within 15% of the reference value for most REEs in the four CRMs analysed. The authors claimed that the method was faster, required less reagent and provided lower LODs than more conventional preconcentration methods based on ion-exchange or extraction chromatography.
4.4 Instrumental analysis
While quadrupole ICP-MS is firmly established as a routine workhorse for elemental determinations, improvements in isotope ratio measurements by MC-ICP-MS continue to support advances in geochemical research. The emphasis is often on novel methods for chemical separation and purification to improve the accuracy of isotope ratio measurements. A one-step separation293 of Mg from matrix elements using AG50W-X8 cation-exchange resin in a purification process taking about 5 h gave recoveries of ≥97% for Mg whereas ≤0.2% of the matrix components were recovered. The total procedural blank of <3 ng Mg was considered to be negligible. Magnesium isotope measurements were made by MC-ICP-MS under cool plasma conditions using a high-sensitivity skimmer cone (X-cone) to maximise signal intensity and to reduce background interferences. Although the samples contained significant Ca concentrations, Ca:Mg ratios of up to 100:1 had little impact on the measured Mg isotope data. The accuracy of the procedure was confirmed through the analysis of seven rock RMs. The analytical precision was 0.03–0.09‰ (2σ). In a method for the isolation of Sb from stibnite ores,294 care was taken not to change the oxidation state which could influence the Sb isotope ratio. After aqua regia digestion, separation was achieved using a combination of cation-exchange chromatography on Dowex AG50-X8 resin and AEC on Amberlite IRA 743 resin. Indium was used to correct for instrument-induced mass bias during measurements of 123Sb/121Sb by MC-ICP-MS at medium resolution (R = 4000). Variations in Sb isotope ratios in stibnite ores of 0.1%, compared to a measurement reproducibility of 0.01%, demonstrated the potential of this method for provenance studies of Roman glasses. Lithium isotope ratios295 in three RMs were measured by MC-ICP-MS after a three-step separation technique employing cation-exchange chromatography on AG50W-X8, with 2.8 M HCl, 0.15 M HCl and 0.5 M HCl in 30% ethanol as eluants at each of the three stages. The precision of the technique (±0.72–1.04‰, 2σ) was similar to that obtained by other MC-ICP-MS laboratories.
A method296 for the simple and rapid determination of Os and Re isotope ratios by MC-ICP-MS was based on Carius tube digestion and sparging introduction of Os. Measurement times were reduced significantly by employing four channeltron multi-ion counters for simultaneous measurement of the Os isotopes. Because the amplification efficiencies of the multi-ion counters differed from one another and varied with time, Os isotope ratios were corrected using the sample-standard bracketing method. Rhenium isotopes were measured by means of eight Faraday cups with solution nebulisation via a USN. The method was sufficiently sensitive to determine Re–Os isotope systematics in samples such as chert with low concentrations. Precisions for 187Os/188Os ratios for RMs JMS-2, JCh-1 (GSJ sedimentary rock RMs) and DROsS (Durham Romil Os solution) were 0.35–0.71, 1.56–3.31 and 0.99–1.28% RSD, respectively, depending on the ion intensities. These precisions were considered to be adequate for reconstructing changes in marine Os isotope compositions. Lawley and Selby297 proposed a room-temperature chemical separation technique using HF to isolate ultrafine (<50 μm) molybdenite grains for Re–Os geochronology. Exposure to HF had no effect on the Re–Os molybdenite systematics. The procedure allowed the determination of reproducible ages for ultrafine samples that were unsuitable for Re–Os determinations using traditional physical mineral separation techniques.
Kramchaninov et al.298 sought to refine the procedure for Sr isotope measurements by MC-ICP-MS to achieve performance data to rival those obtainable by TIMS. By correcting for mass bias effects using a combination of normalisation to zirconium isotope ratios and standard-sample bracketing, they obtained precisions of ±0.015–±0.05‰ for 88Sr/86Sr, depending on sample composition. These precisions were better than those reported previously by similar protocols based on MC-ICP-MS but worse than those achievable by double spike TIMS (±0.02‰). The advantages cited were greater productivity (5–6 times higher) and the ability to measure 88Sr/86Sr and 87Sr/86Sr simultaneously with high precision. However, at least 95% Sr recovery from the chromatographic column was essential for accurate results. Building on earlier work using a heated torch integrated spray chamber, Paredes et al.299 demonstrated that it was possible to measure Sr isotope ratios at a continuous liquid flow rate of <10 μL min−1 with a repeatability of ca. 40 ppm on the 87Sr/86Sr ratio measured in NIST SRM 987 (strontium carbonate). This figure of merit was similar to that (30–50 ppm) previously reported for liquid flows rates 5 to 10 times higher. The new instrumental setup allowed a sheathing gas to be introduced and included a semi-automated FI sample-delivery system to minimise manipulation of small sample volumes.
Several new procedures for the measurement of Nd isotopes by MC-ICP-MS have been published. Huang et al.113 employed a combination of high-sensitivity cones and a new generation desolvator (Aridus II or APEX-IR), together with column chemistry based on Eichron LN resin, to achieve optimal conditions for Nd isotope determination in a variety of sample matrices with low Nd content, including carbonates and seawater. The long-term precision of ±0.000016 (2SD) for 143Nd/144Nd in a sample size of ca. 1.25 ng Nd, was a factor of 5 better than that for existing MC-ICP-MS methods using <2 ng Nd. Their column separation chemistry provided yields (>92%) similar to those of more traditional multi-step separations but was simpler and more rapid, with negligible procedural blanks. As a practical alternative to ID-TIMS, Sanchez-Lorda et al.300 proposed an MC-ICP-MS procedure for determining 143Nd/144Nd and 147Sm/144Nd ratios on solutions of silicate rocks without resorting to ID or separating Nd and Sm from each other. The LREEs were separated from matrix elements with yields of ≥97% by cation-exchange on Dowex AG50W followed by extraction chromatography on Eichrom TRU resin. Oxide formation, a major cause of mutual fractionation of Nd and Sm in the ICP, was minimised by introducing the sample into the ICP via a commercially available two-step desolvation system. After correction for instrumental mass bias effects and isobaric interferences, the overall reproducibility for 147Sm/144Nd was 0.2–0.7% RSD. Although this performance cannot compete with that achievable by TIMS (typically 0.1% RSD), the precision and accuracy were sufficient for many geological applications. Interestingly, the within-run uncertainty of 0.000014 for 143Nd/144Nd in a depleted basalt (GSJ JB-2), was similar to the long-term precision reported by Huang et al.113 In a study of isotopic fractionation of Ce and Nd in geological samples by MC-ICP-MS, Ohno and Hirata301 employed a similar two-step separation scheme to isolate the LREEs before further purification on an Ln Spec column to separate Ce and Nd. Mass discrimination effects were corrected by means of an external correction technique based on an exponential law, using 149Sm/147Sm and 153Eu/151Eu ratios to correct the isotope data for Ce and Nd, respectively. The reproducibilities of the measurements for 142Ce/140Ce, 146Nd/144Nd and 148Nd/144Nd were 0.08‰ (2SD, n = 25), 0.06‰ (2SD, n = 39) and 0.12‰ (2SD, n = 39), respectively. With this level of precision it was possible to demonstrate that the Ce and Nd isotope ratios in two igneous and two sedimentary rocks were the same, within analytical uncertainty, whereas the ratios for both elements were significantly higher in a carbonate RM.
The 100th anniversary of the publication of “Age of the Earth” in 1913 by Arthur Holmes, the first person to use radioactivity to date rocks, has been marked by various conferences and review articles on isotope geochronology and its future. An issue of Elements (volume 9, number 1) dedicated to this theme contained perspectives on various aspects of modern geochronology by some of the leading practitioners in the field. In his review of the evolution of U–Pb geochronology as a robust dating technique, Corfu302 focussed on the causes and implications of discordance between the 206Pb/238U and 207Pb/235U decay systems. Although targeted at the nuclear industry, a guide303 to the fundamental concepts and recent applications of U chronometry may also be of interest to some geochemists.
An article on high precision geochronology304 highlighted the continuing efforts of the EARTHTIME community (http://www.earth-time.org) to identify and mitigate inter-laboratory biases through group inter-calibration experiments. Important developments in the application of U–Pb dating to speleothems were discussed by Woodhead and Pickering.305 Although the mechanisms for the transport and incorporation of Pb and U in speleothem calcite remain a matter of debate, LA-ICP-MS and autoradiography were considered ideal screening methods for reconnaissance studies.306 On the basis of exploratory studies already undertaken, this is likely to be a major growth area in the next decade as the potential palaeoclimatic information contained within selected speleothems offers insights in many fields of research.
4.4.3.1 Thermal ionisation mass spectrometry. The analytical capabilities of the current generation of TIMS instruments have renewed the debate256 over the best strategies for achieving the lowest uncertainties and best accuracy for geological applications, as well as choice of the most appropriate RM.
As noted in Section 4.3.2, double spike TIMS is the method of choice when the best precision for Nd isotope determinations is required. Wakaki and Tanaka307 refined this procedure by including internal normalisation and an iterative calculation procedure to improve precision and to minimise possible systematic bias during data analysis. The average 143Nd/144Nd ratio for GSJ JNdi-1 (neodymium oxide) was 0.512106 ± 0.000010 (2SD, n = 44). Significant mass-dependent isotope fractionation occurred during the analysis of commercial high-purity Nd oxide reagents and the La Jolla RM, relative to JNdi-1, suggesting that these data should always be normalised to JNdi-1 rather than the La Jolla RM, which is often used for this purpose. By replacing the 1011 Ω resistors in the feedback loop with 1012 Ω resistors to reduce the noise in the Faraday cup current amplifiers, Koornneef et al.308 achieved a two-fold improvement in the analytical precision on 143Nd/144Nd and 87Sr/86Sr ratio measurements. Their data suggested that it should be possible to resolve the variability in the fourth decimal place for Nd and Sr isotope ratios in geological samples as small as 100 pg. The 1012 Ω resistors were expected to provide a similar improvement in S/Ns when used in MC-ICP-MS detection systems, although any limitations imposed by the slower response time of the 1012 Ω resistors would need to be investigated. Normally, the determination of 143Nd/144Nd and 147Sm/144Nd ratios by TIMS requires purified Nd and Sm fractions to be loaded onto different filaments because of severe isobaric interferences, making the procedure lengthy and laborious. Li et al.309 proposed a new method, using a mixed 152Sm–148Nd spike rather than the traditional 149Sm–150Nd spike, in which both ratios and concentrations of Nd and Sm were measured simultaneously on the same filament loading. Measurements of ten silicate rock CRMs confirmed that neither precision nor accuracy of the results had been compromised.
One of the major problems in the measurement by TIMS of boron isotope ratios in organic-rich natural samples, such as fossil carbonates, is interference from isobaric ions from the organic matter. Characterisation of natural biocarbonates310 confirmed these isobaric interferences originated from organic compounds with an acylamino group (–CO2–NH2). However, total elimination of organic matter by pretreating the samples is still a considerable challenge. A method311 for the determination of B isotope composition of tourmalines employed K2CO3 fusion and a three-column ion-exchange procedure to achieve full recovery of B prior to measurement by positive ionisation TIMS and MC-ICP-MS. Although the δ11B values obtained by the two techniques were generally in good agreement, some discrepancies observed for a couple of samples were thought to be due to interferences from isobaric ions formed from trace organic matter during ion-exchange. Thus, it would appear that MC-ICP-MS may be preferred over TIMS for organic-rich samples.
Experiments312 were performed to assess whether an approach to U–Pb dating of monazite involving multi-step chemical abrasion plus TIMS could reveal complexities in either age or composition that might be masked by single-step analysis. It was recommended that high temperature annealing should not be used for monazite because the annealing induced recrystallisation that affected the style of dissolution and produced age and composition data that were more variable than those for unannealed samples. Instead, slow partial dissolution of monazite in weak acid proved suitable for obtaining consistent information on complex samples.
An investigation313 into the suitability of various commercial silica gels as emission activators for the ionisation of Pb in TIMS concluded that the emitter produced by Sigma-Aldrich was an excellent alternative to Merck silica gel, which is no longer manufactured.
4.4.3.2 Secondary ion mass spectrometry. An authoritative overview of the current state of SIMS technology formed part of a more wide-ranging review256 of recent developments in geochemical analysis. Applications of SIMS to geochronology, palaeoclimate studies and quantification of volatile elements were highlighted, together with the increased popularity of NanoSIMS due to its capability for high-spatial-resolution imaging. The main applications of NanoSIMS within the Earth sciences continue to be imaging and isotope ratio determinations on presolar grains. For a detailed account of the technical aspects and application of NanoSIMS in cosmochemistry and biogeochemistry, a critical review (170 references) by Hoppe et al.314 is recommended.
The increased capabilities of SIMS have resulted in more research projects using the technique to resolve variations in the growth of biocarbonate structures at the μm scale. Vetter et al.315 conducted experiments in which planktonic foraminifera were grown at a constant temperature and transferred every 12 h between unmodified seawater and seawater with modified Ba/Ca and δ18O chemistry to produce time-resolved bands of shell calcite with predictable geochemistries. Elemental (Ba/Ca) data were obtained by LA-ICP-MS and cross-correlated with δ18O measurements by SIMS with a 3 μm spot and average precision of 0.6‰ (2 SD). The finding that Ba and δ18O tracers were incorporated into the shell calcite synchronously demonstrated the power of SIMS, in combination with high resolution LA-ICP-MS, to measure chemical variations in daily growth increments in the fossil record. NanoSIMS316 was used to study the skeletal structures of a coral exposed to 86Sr-enriched seawater at intervals of 48 h interspersed with 5 days of growth under normal seawater conditions. Images of the 86Sr/44Ca ratio in the skeleton formed during the experiment allowed the growth rate of components of the coral skeleton to be calculated.
New approaches to U–Pb geochronology by SIMS have taken advantage of the high spatial resolution that can be achieved. Schmitt and Zack317 used an O2+ primary ion beam coupled with surficial O2 gas deposition (so-called O2 flooding) for U–Pb dating of rutile. Natural and synthetic rutiles were conductive under O2+ bombardment so higher sputter rates were possible than by conventional SIMS with a O2− beam, without incurring detrimental effects due to sample charging. Both Pb–Pb and U–Pb ages generated were accurate to within <1% for Early Paleozoic to Archean rutile, without evidence of any significant bias due to crystal orientation. The coupling of two new approaches enabled Ault et al.318 to analyse zircons too small to be dated by standard SIMS and TIMS methods. Automated mineralogy was used instead of mineral separation to identify rapidly sub-20 μm zircons in a single thin Section. A modified SIMS protocol then preferentially collected secondary ions emitted from a domain a few μm in size within the ca. 20 μm diameter analysis pit. In this manner, in situ U–Pb data were acquired for zircons grains with dimensions of <10 μm.
Other novel geological applications of SIMS included high precision in situ measurements of Mg isotopes in meteoritic materials319 using 30–40 μm analytical spots. The data reduction procedure was critical for achieving the level of precision required, typically ±5 ppm. By modelling factors such as Faraday cup background drift and instrumental fractionation induced by the sample matrix, the fully corrected Mg isotopic data attained the levels of precision and accuracy needed to establish the chronology of events in the early solar system based on the decay of 26Al to 26Mg. Following an evaluation of methods for correcting the significant instrumental mass fractionation observed during in situ determination of oxygen isotopes in magmatic glasses,320 a simple correction scheme based on the SiO2 content of appropriate standards spanning a compositional range from basalt to rhyolite was recommended. Using this method, the δ18O values of a range of glass RMs were reproduced to within ±0.4‰ of their nominal value. An unusual approach321 to the quantification of abundant volatile elements (Cl, F and H) in apatites by SIMS was based on the stoichiometric constraint that the sum of the volatile elements must closely approach 100% occupancy of their collective structure sites. The main advantage of this procedure is that it does not require any independently known homogeneous RMs. Whitehouse322 demonstrated the potential of SIMS for high spatial resolution measurements of all four S isotopes in sulfides. In addition, two sulfides from SW Greenland and Minas Gerais, Brazil were shown to be sufficiently homogenous to be used as secondary RMs to monitor the accuracy of Δ33S measurements by SIMS.
The use of TOF-SIMS for quantitative analysis of geological materials is still in its infancy. Marques et al.323 studied a single melt inclusion in a single phenocryst to evaluate the advantages and limitations of this technique for elemental and isotopic determinations. Two basalt glass RMs (USGS BCR-1 and GSJ JB-2) with similar matrix composition and physical properties to the melt inclusion were used for quantification. The composition of an ore-metal sublimate on the wall of an exposed vapour bubble inside the melt inclusion could be determined as well as the composition of its host clinopyroxene. Results for the melt inclusion and the clinopyroxene were close to those obtained previously by EPMA and LA-ICP-MS. Values for B, Cl, Li, O and S isotope ratios in the two glass RMs differed significantly from literature values so the authors concluded that more research was necessary to establish whether these discrepancies related to sample heterogeneity at the nanoscale or to analytical factors such as matrix effects in SIMS analysis.
4.4.3.3 Accelerator mass spectrometry. A review of advances in AMS256 noted that the focus of current geochemical applications has shifted from extraterrestrial materials to tectonics and climate research, and hot topics now include archaeometry and anthropology. The precision and accuracy of cosmogenic 3He measurements by AMS are dependent on the passive He blank from the extraction apparatus. A new high temperature single vacuum furnace324 was capable of extracting He from minerals such as apatite, pyroxene and olivine with virtually 100% recovery. After heating at 1450 °C for 20 min, the blanks of 3.7 × 10−21 mole for 3He and 1.1 × 10−15 mole for 4He were an order of magnitude better than those of a conventional double vacuum furnace. Large accelerators are normally necessary to separate 36Cl from its stable isobar 36S but these measurements can now be performed with modern 5 MeV AMS instruments using gas stripping to produce the highest quality beams. A protocol325 for the accurate determination of 36Cl included ID for stable Cl measurement and separation from 36S on a relatively small automated spectrometer using 30 MeV ions. Improvements in AMS measurement precisions and low Pt machine backgrounds contributed to the first reported observation that the 198Pt/195Pt ratio in two presolar nanodiamonds326 from the Allende meteorite may be enhanced by 6–7%. Although they require verification, these results may shed light on nucleosynthesis processes taking place in the parent star.
Ten laboratories participated in the first international interlaboratory comparison327 for the measurement of the long-lived radionuclide10Be. The results for three samples (10Be/11Be = 10−12–10−14) were made traceable to NIST SRM 4325 (beryllium chloride solution) to avoid discrepancies from the use of different calibration materials. Multi-variate statistical analysis of the data indicated that participating AMS facilities fell into two distinct groups, with maximum discrepancies of 6–31% depending on the absolute 10Be/11Be value. Various issues were discussed openly in the paper because the laboratories had waived their right to anonymity. Unfortunately, NIST SRM 4325 is no longer available, so another material of the same metrological quality is required in the very near future. Instead of following the common practice of adding stable 9Be as a carrier during sample preparation prior to 10Be AMS measurements, researchers at ETH Zurich advocated328 a two-step leaching procedure for the measurement of 10Be/11Be in marine sediments without the addition of 9Be carrier. An elaborate procedure was necessary to separate authigenic Be from detrital Be while keeping contamination from 9Be to a minimum. In a procedure329 for measuring 10Be in small amounts of sediments, 1–10 mg of sediment were spiked with several hundred μg of 9Be carrier. The 10Be values obtained by this method agreed within 3–5% with values previously determined on much larger 200 mg samples at the same AMS facility at the University of Tokyo. Efficient extraction and purification of quartz from rocks, soils and sediments are essential for in situ AMS measurement of cosmogenic nuclides such as 26Al and 10Be in the determination of surface exposure ages, erosion rates and burial ages. The use of hot phosphoric acid,330 which preferentially dissolved silicates but not quartz, was considered an alternative to the usual procedure involving repetitive etching in dilute HF. This method was particularly effective at recovering quartz from samples with very low quartz abundance or with cryptocrystalline silica, e.g., greywacke and chert.
Polarised-beam EDXRF instruments are routinely used for the determination of major and trace elements in a wide range of materials including soils and sediments332 and chert artefacts.333 Redus and Huber334 suggested that a suitable figure of merit for comparison of EDXRF instruments would be the time required to achieve a given statistical uncertainty and they demonstrated how this could be used to select the optimum detector and spectrometer configuration for a specific application. A comparison335 of EDXRF detectors concluded that CdTe detectors generally gave better precision and accuracy for K lines greater than 20 or 25 keV but that high resolution silicon drift detectors were superior at lower energies. For applications requiring accurate measurement of high Z elements in complex matrices, instruments with a combined or dual detector system provided both high resolution and high efficiency.
The popularity of XRF core scanning for obtaining elemental data from sediment cores has increased, particularly for applications such as paleoclimate studies in which relative rather than absolute element concentrations are required. The capabilities of a new core scanner,336 with a resolution of 0.8 mm, were compared with ICP-AES and ICP-MS analysis after acid digestion of discrete samples collected at 1 mm intervals on a 5 m marine sediment core. Scanning was suitable for obtaining an overview on the chemical patterns along the core and so identifying the most appropriate sampling locations to take discrete samples for ICP analysis to address specific paleoclimatic questions. Errors calculated by the core scanner software were underestimated, especially at high count rate, so it was recommended that the XRF data should always be compared with quantitative data from discrete samples for calibration purposes. Hennekam and de Lange337 noted the measurement variability caused by different thicknesses of the water film underneath the plastic foil during XRF core scanning and suggested ways of correcting for this.
Recent applications of PXRF devices, in use for over a decade, included mudrock chemostratigraphy in drill cores,338 characterisation of stoneware ceramics339 and the provenacing of glass beads.340 The user-friendly nature of the technique means that handheld and portable bench-top instruments are regularly employed in activities such as mineral prospecting, ore-grade evaluation and contaminated land surveys by operators with very limited knowledge of the underlying spectrometry. A cautionary tale in relation to the use of PXRF devices in mineral exploration341 was highlighted in a study commissioned by the Canadian Mining Industry Research Organisation (CAMIRO), in which the performance of three handheld and two benchtop models were evaluated. No reliance could be placed on preset factory calibrations so recalibration of portable instruments with well-charactersied RMs was recommended. The CAMIRO Phase I report, which is available at http://www.appliedgeochemists.org, gives a detailed picture of the performance of this type of instrumentation and should be essential reading for anyone interested in using PXRF instrumentation.
5 Glossary of terms
| 2D | Two-dimensional |
| A4F | Asymmetrical flow field flow fractionation |
| AAS | Atomic absorption spectrometry |
| AEC | Anion exchange chromatography |
| AES | Atomic emission spectrometry |
| AFS | Atomic fluorescence spectrometry |
| ALF | Artificial lysosomal fluid |
| ANN | Artifical neural network |
| AMS | Accelerator mass spectrometry |
| APDC | Ammonium pyrrolidine dithiocarbamate |
| ASU | Atomic spectrometry update |
| aTOF-MS | Aerosol time of flight mass spectrometry |
| BAM | Federal Institute for Materials Research and Testing (Germany) |
| BCR | Community Bureau of Reference (of the European Community) now IRMM |
| CCD | Charge coupled detector |
| CDN | CDN Resource Laboratories Ltd (Canada) |
| CI | Confidence interval |
| CNT | Carbon nanotube |
| COD | Chemical oxygen demand |
| CPC | Condensation paricle counter |
| CPE | Cloud point extraction |
| CRM | Certified reference material |
| CRS | Cavity ringdown spectroscopy |
| CS | Continuum source |
| CVG | Chemical vapour generation |
| CZE | Capillary zone electrophoresis |
| DDTC | Diethyldithiocarbamate |
| DEEE | Diesel engine exhaust emission |
| DGT | Diffusion gradient in thin films |
| DLLME | Dispersive liquid liquid microextraction |
| DPM | Diesel particulate matter |
| DRC | Dynamic reaction cell |
| EC | Elemental carbon |
| ED | Energy dispersive |
| EDS | Energy dispersive spectrometry |
| EDTA | Ethyldiaminetetraacetic acid |
| EDXRF | Energy dispersive X-ray fluorescence |
| EN | European standard |
| EPA | Environmental Protection Agency (USA) |
| EPMA | Electron probe microanalyser |
| ERM | European Reference Material |
| ES | Electrospray |
| ESI-MS | Electrospray ionisation mass spectrometry |
| ETAAS | Electrothermal atomic absorption spectrometry |
| ETV | Electrothermal vaporisation |
| EUSAAR | European Supersites for Atmospheric Aerosol Research |
| FAAS | Flame atomic absorption spectrometry |
| FFF | Field flow fractionation |
| FI | Flow injection |
| fs | Femto second |
| FTIR | Fourier transform infrared |
| GC | Gas chromatography |
| GSJ | Geological Survey of Japan |
| HDC | Hydrodynamic chromatography |
| HEN | High efficiency nebuliser |
| HG | Hydride generation |
| HILIC | Hydrophilic liquid interaction chromatography |
| HPLC | High performance liquid chromatography |
| HPS | High Purity Standards (USA) |
| HR | High resolution |
| IAEA | International Atomic Energy Agency |
| IC | Ion chromatography |
| ICP | Inductively coupled plasma |
| ICP-AES | Inductively coupled plasma atomic emission spectrometry |
| ICP-MS | Inductively coupled plasma mass spectrometry |
| ICP-SF-MS | Inductively coupled plasma sector field mass spectrometry |
| ID | Isotope dilution |
| IERM | Institute for Environmental Reference Materials (of Ministry of Environmental Protection, China) |
| IGGE | Institute of Geophysical and Geochemical Exploration (China) |
| IMPROVE | Interagency Monitoring for Protected Visual Environments |
| INAA | Instrumental neutron activiation analysis |
| INCT | Institute of Nuclear Chemistry and Technology (Poland) |
| IR | Infrared |
| IRMM | Institute for Reference Materials and Measurements |
| IRMS | Isotope ratio mass spectrometry |
| ISO | International Organisation for Standardization |
| ISS | Instituto Superiore di Sanita (Italy) |
| ITU | Institute for Transuranium Elements |
| LA | Laser ablation |
| LC | Liquid chromatography |
| LGC | Laboratory of the Government Chemist (UK) |
| LIBS | Laser induced breakdown spectroscopy |
| LLE | Liquid liquid extraction |
| LLME | Liquid liquid microextraction |
| LOD | Limit of detection |
| LREE | Light rare earth element |
| MAAP | Multiangle absorption photometer |
| MC | Multicollector |
| MIBK | Methyl isobutyl ketone |
| MOUDI | Micro orifice uniform deposition impactor |
| MRI | Magnetic resonance imaging |
| MS | Mass spectrometry |
| MWCNT | Multiwalled carbon nanotube |
| NACIS | National Analysis Centre for Iron and Steel (China) |
| NCS | China National Analysis Centre for Iron and Steel |
| NDIRS | Non-dispersive infrared spectroscopy |
| Nd:YAG | Neodymium doped:yttrium aluminum garnet |
| NIOSH | National Institute of Occupational Safety and Health (USA) |
| NIR | Near infrared |
| NIST | National Institute of Standards and Technology (USA) |
| NMI | National Measurement Institute |
| NMIJ | National Metrology Institute of Japan |
| NP | Nanoparticle |
| NRCC | National Research Council of Canada |
| NRCCRM | National Research Centre for Certified Reference Materials (China) |
| ns | Nano-second |
| NTIMS | Negative thermal ionisation mass spectrometry |
| NWRI | National Water Research Institute (Canada) |
| OC | Organic carbon |
| OES | Optical emission spectrometry |
| OREAS | Ore Research and Exploration Pty Ltd Assay Standards (Australia) |
| PFA | Perfluoroalkyl |
| PGE | Platinum group element |
| PIXE | Particle induced X-ray emission |
| PM0.07–0.34 | Particulate matter (with an aerodynamic diameter of between 0.07 and 0.34 μm) |
| PM0.34–1.15 | Particulate matter (with an aerodynamic diameter of between 0.34 and 1.15 μm) |
| PM1.15–2.5 | Particulate matter (with an aerodynamic diameter of between 1.15 and 2.5 μm) |
| PM2.5 | Particulate matter (with an aerodynamic diameter of up to 2.5 μm) |
| ppbv | Part per billion volume |
| ppm | Part per million |
| PTFE | Poly(tetrafluoroethylene) |
| PXRF | Portable X-ray fluorescence |
| QA | Quality assurance |
| QC | Quality control |
| REE | Rare earth element |
| rf | Radio frequency |
| RM | Reference material |
| RP | Reversed phase |
| RSD | Relative standard deviation |
| RTC | Resource Techology Corporation (USA) |
| SBET | Simplified bioaccessibility extraction test |
| SD | Standard deviation |
| SDME | Single drop microextraction |
| SEC | Size exclusion chromatography |
| SEM | Scanning electron microscopy |
| SF | Sector field |
| SFE | Supercritical fluid extraction |
| SFODME | Solidification of floating organic drop microextraction |
| SIMS | Secondary ion mass spectrometry |
| SMPS | Scanning mobility particle sizer |
| S/N | Signal-to-noise ratio |
| SPE | Solid phase extraction |
| SPME | Solid phase microextraction |
| SR | Synchrotron radiation |
| SRM | Standard reference material |
| TC | Total carbon |
| TDS | Total dissolved solid |
| TEM | Transmission electron microscopy |
| TEOM | Tapered element oscillating microbalance |
| TGA | Thermal gravimetric analysis |
| TIMS | Thermal ionisation mass spectrometry |
| TOF | Time of flight |
| TOT | Thermal optical transmission |
| TSP | Total suspended particles |
| TXRF | Total reflection X-ray fluorescence |
| USGS | United States Geological Survey |
| USN | Ultrasonic nebuliser |
| UV | Ultra violet |
| VALLME | Vortex-assisted liquid liquid microextraction |
| VG | Vapour generation |
| VOC | Volatile organic compound |
| WDXRF | Wavelength dispersive X-ray fluorescence |
| WHO | World Health Organisation |
| XANES | X-ray absorption near edge structure |
| XAS | X-ray absorption spectrometry |
| XRD | X-ray diffraction |
| XRF | X-ray fluorescence |
| z | Atomic number |
| σ | Population standard deviation |
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Footnote |
| † Review coordinator. |
| This journal is © The Royal Society of Chemistry 2014 |