Atomic spectrometry update. Environmental analysis
Owen T.
Butler
*a,
Warren
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.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 6th January 2011
Abstract
This is the twenty-sixth annual review published in JAAS on the application of atomic spectrometry to the chemical analysis of environmental samples. This Update refers to papers published approximately between September 2009 and August 2010 and continues the series of Atomic Spectrometry Updates in Environmental Analysis1 that should be read in conjunction with other related reviews in the series.2–6 In the analysis of air, work is ongoing in developing new and existing air sampler devices with an increasing focus on sampling nanoparticles. Determination of mercury in the atmosphere remains a focus for many research groups and this year has seen a renewed interest in sampling and analysis of respirable silica. There is a growing interest in measuring emissions from transport sources such as aviation and shipping. In the field of water analysis, as in previous years, the main areas of activity are the development of preconcentration and extraction procedures and elemental speciation protocols. In the field of soil, plant and related materials analysis, the past year has seen a marked increase in publications featuring LIBS. The technique now appears well established for screening purposes but has not yet convinced detractors of its suitability for use in quantitative multi-element analysis. There is a growing body of literature examining species stability during sample pre-treatment and extraction. Slurry sampling has experienced renewed interest this year. In the field of geological analysis, as in previous years, considerable effort is being spent not only on the production, characterization and certification of new geological reference materials, but also on enhancing the certification of existing reference materials and the development of reference materials with assigned elemental isotopic ratios. Laser ablation continues to go from strength to strength in being adopted as a solid sampling tool within the geochemical community with a growing interest in coupling laser ablation systems to multicollector ICP-MS systems for in situ isotopic analysis. Feedback on this review is most welcome and the lead author can be contacted using the email address provided.
1 Air analysis
This section highlights noteworthy areas of research and development in the analysis of aerosols, particulates and inorganic gases by atomic spectrometric and complementary analytical techniques of interest to JAAS readership published since the last Update.1 Initially, developments in air sampling and sample preparation are discussed. Advances in analytical instrumentation and methodologies are then reviewed. The Update then looks at specific applications that have caught the eye of this reviewer with respect to particular elemental species and to measurement activities in certain sectors.1.1 Sampling techniques
Monitoring workers' exposure to airborne particles in the workplace through the use of personal air samplers is an ongoing activity in many countries as is the development and evaluation of sampler designs. French researchers have re-evaluated five, widely used sampler designs that conform to the ACGIH-EN-ISO inhalable sampling convention:7 the IOM sampler, widely used in the UK; two versions of the CIP 10 sampler, widely used in France; the 37-mm closed face cassette sampler (CFC), widely used in the USA; a modified version of the 37-mm sampler employing an ACCU-CAP insert; and the Button sampler. The performance of such samplers is strongly dependent on particle size and ambient air velocity. The current focus is on evaluating their performance in near calm air conditions, i.e. low wind speeds of less than 1 ms−1 that predominate in many workplaces. The IOM sampler and CIP 10 (version 2) sampler faired best when benchmarked against the inhalable sampling criterion. The Button sampler and the CIP 10 (version 1) showed lower sampling efficiencies. The 37-mm closed faced cassette sampler produced the poorest performance but this could be improved by using the modified version employing the ACCU-CAP, an insert that prevents particles losses to the internal walls of the sampler. Particles that enter samplers but fail to deposit on the filter can be considered to be transport loss according to Lee and co-workers.8 Many measurement procedures only analyse the filter sample. These workers investigated the size distribution of 0.5–20 μm Pb-containing particles collected from the walls and filters mounted in the CFC and IOM samplers, as part of ongoing work into investigating the extent of such transport losses. Problems can also arise in the analysis of filter samples by direct-on-filter methods such as X-ray techniques. As mentioned in previous Updates, X-ray techniques, because of their small beam size, generally do not interrogate the whole filter and hence the uniformity of particles on a filter becomes somewhat critical for unbiased results. Taiwanese researchers,9 in a very useful publication, studied the effects of uniformity of deposition on filters of respirable particles on the determination of their quartz content using XRD. In a foundry plant, they evaluated the performance of three respirable aerosol samplers: a 25-mm aluminium cyclone, a nylon cyclone and a cyclone developed by the Institute of Occupational Safety and Health in Taiwan. Filter samples were analysed for their crystalline silica content by the direct on-filter XRD approach and subsequently by the NIOSH 7500 method that involves an ashing or dissolution step in which recovered silica is re-filtered onto a silver filter for analysis. The three sampler designs gave fairly equivalent results when measured using the direct-on-filter approach. However, somewhat disconcertingly, this approach gave results that were 1.15–2.89 times higher than the refiltering procedure. Factors at play here can include variable particle deposition uniformity between air sampler designs, particle deposition uniformity effects arising from laboratory filtration procedures and actual sample loss during ashing or dissolution steps. This work compliments ongoing work in this reviewer's laboratory in evaluating sampling and analytical procedures for the determination of crystalline silica in air.Increasing challenges in developing procedures to measure smaller concentrations of species in workplace air have led researchers to evaluate sampler designs operating at higher flow rates than those currently used. Anthony and co-workers10 have developed a new inlet design for the above mentioned CFC sampler to meet the inhalable sample criterion at a revised flow rate of 10 L min−1 compared to the nominal 2 L min−1 currently used. Zhou and Cheng,11 on the other hand, simply investigated the performance of the IOM sampler running at an enhanced flow rate of 10.6 L min−1 and found that it performed fairly comparably when operated at its nominal 2 L min−1, suggesting that, with the advent of lighter more powerful sampling pumps, this widely used sampler could be operated at higher flow rates than previously thought. Lee and co-workers12 have evaluated the performance of three high flow rate samplers for respirable particle collection—the CIP 10R sampler, the GK.2.69 cyclone sampler and the FSP10 sampler—against the ACGIH-EN-ISO respirable sampling criterion.
Sampling nanoparticles, typically particles below 100 nm in size, is another fruitful area of research. Morawska and Australian coworkers13 have reviewed instrumental methods for sampling and analysis of airborne nanoparticles (207 references). Methods to measure physical attributes such as particle number, concentration, size distribution and surface area are initially reviewed. This is followed by a review of methods to characterise the elemental composition of nanoparticles and concludes with a brief summary of sampling considerations related to nanoparticle monitoring. In all, a highly recommended paper for those wishing to learn more about airborne nanoparticle measurements! Wei et al.14 evaluated the particle collection efficiency of a midget impinger sampler for nanoparticles in the range 3–100 nm and found that collection efficiency reached a minimum of 3–10% at a size range of 40–57 nm. Collection efficiency for nominal 10 nm particles was approximately 20%, increasing to 80% for smaller particles. Researchers at NIOSH15 have developed a handheld electrostatic precipitator (ESP) for sampling airborne nanoparticles onto a grid, which can subsequently be examined using electron microscopic techniques. Gross sampler collection efficiencies were 76–94% for particles in the range 30–400 nm. The authors note that not all aerosols may be amenable to sampling with an ESP—explosive atmospheres for one! Semi-volatile and liquid aerosols droplets may undergo transformation during sampling and after collection on the sampling grid. Hence they suggest that prior knowledge of the source aerosol would be advantageous in the effective use of this new sampler. Plans are afoot to license this ESP sampler for commercial manufacture. Li and co-workers16 report the development of a prototype of a miniature disk electrostatic aerosol classifer based on electric mobility and intended for personal exposure monitoring. Satisfactory performance was obtained for particles in the size range 10–120 nm.
Similar research is being carried out by researchers evaluating the performance of ambient air sampler systems. Swiss researchers17 have evaluated the particle deposition uniformity and particle size distribution of ambient aerosol collected with a rotating drum impactor. Aerodynamic particle sizers in conjunction with micro-scanning SRXRF and TEM techniques were used in this evaluation. Phares and Collier18 have characterised the performance of an aerosol inlet that collects particles having a narrow range of electric mobility directly onto a heated filament for subsequent thermal desorption. Semi-volatile particles, such as ammonium nitrate, can be lost from filter samples thus introducing a sampling bias. The extent of such evaporative losses can depend significantly on the velocity of the incoming air arriving at the filter face. McDade and co-workers19 have determined the effective filter face velocities for a number of sampler systems commonly used in ambient air studies in the United States.
Generation of stable atmospheres of particles in a laboratory setting is an ongoing requirement for testing the performance characteristics of new sampler designs and to produce near realistic test samples for proficiency testing schemes. Korean researchers20 have designed and characterised an improved screw-assisted rotary feeding system for the precise aerosolization of ultra-small quantities of particle solids. Researchers at INRS in France have published results of a study into the homogeneity of replicate filters produced from two atmosphere generation systems.21 The variability of material (asbestos fibres or silica-containing dusts) collected on filters, expressed as a coefficient of variation, was in the range 4–10%.
Passive air samplers, i.e., devices that collect gases from air by a diffusion process, are attractive given that they do not require costly pumping systems and hence can be easily deployed. Nash and Leith22 present an overview of their use for sampling of some inorganic gases found in the atmosphere namely CO, H2S and SO2. Ambient NO2 levels are a concern in many urban environments and where diffusive sampling is often deployed. Swedish researchers23 have undertaken a field validation of the Ogawa diffusive sampler design in a cold climate. By using field-calculated diffusion uptake rates, estimated airborne concentrations compared favourably with results obtained with a real-time chemiluminescence monitor. Lin and co-workers24 have field tested a new flow-through directional passive air sampler for NO2. By means of a wind vane, this design is able to directionally discriminate between air pollutant signals. Fairly reasonable quantitative NO2 data were obtained—usually within a factor of two of results obtained from a co-located real-time gas analyser. Garnett and Hartley25 have developed a passive sampling method for the radiocarbon analysis of atmospheric CO2 using a molecular sieve sorbent. The resultant 14C measurements were within the measurement uncertainties obtained on samples collected with a pump-based sampling system. However, fractionation during the passive sampling trapping meant that δ13C values had to be adjusted as all passively collected molecular sieve samples were depleted in 13C compared to pumped samples. American researchers26 report the development of a passive sampler for collection of ambient concentrations of gaseous oxidised mercury. Results from field trials compared favourably with those from a co-located real time Hg analyser (r2 = 0.71, p < 0.01, n = 100 for a one-week sampler deployment and r2 = 0.89, p < 0.01, n = 22 for a two-week sampler deployment). Sampler uptake rate was not significantly affected by changes in temperature, humidity or ozone concentrations, but was somewhat dependent on wind speeds, which is not unusual for passive air samplers. The detection limit for a two-week sampler deployment was determined to be 5 pg m−3.
Papers that consider uncertainties in air sampling, arising from spatial and temporal factors, have not received the same level of attention as that given to laboratory-based analysis. Therefore it is good to see work addressing such issues. Hyslop and White27 examined sampling precision by assessing data from duplicate co-located air samplers. American28 and Spanish29 research groups describe their approaches to locating air samplers in order to optimise spatial distribution.
1.2 Reference materials
In a thought provoking paper, Epstein30 examines two specific cases in the literature where analysts using spectroscopic instrumentation report elemental concentrations that agree with information values reported in reference material certificates that are subsequently found to be incorrect. One example involves the determination of Mn in NIST SRM 1648 Urban particulate matter, a widely used reference material in the air measurement field. Stone and co-workers31 have reported on deliberations from a workshop organised by an EU funded network—NanoImpactNet—on how nanomaterials should be classified in discrete categories, what test materials should be developed as reference materials for use in studies such as ecotoxicological testing, and what physiochemical characterisation information is required on such materials for environmental impact testing. Reseachers at IAEA in Vienna32 describe work undertaken to prepare and characterise a new set of reference air filter samples for use in their proficiency testing scheme. Target values and their standard deviations were established using INAA and PIXE techniques. A Japanese-Chinese consortium33 has characterised the Pu concentration and its isotopic composition in a reference fallout material. It was prepared by the Japanese Meteorological Research Institute from deposition samples collected at 14 air-monitoring stations throughout Japan between the period 1963 and 1979 and characterised by isotope dilution SF-ICP-MS. The Japanese Society for Analytical Chemistry34 has announced the availability of two new coal fly ash CRMs, JSAC 0521 and JSAC 0522, certified for a wide range of elements.1.3 Sample preparation
The risk of sample contamination during air sampling, handling of filters and their subsequent analysis is a recurring theme. Researchers in Sweden35 have published an informative overview on the sources of contamination and remedial strategies for laboratories undertaking multi-element trace analysis. This is particularly useful for researchers analysing ambient air particles where concentrations are often in the pg–ng m−3 range, equating to filter samples containing ng quantities of metallic species.New, improved and quicker sample dissolution procedures are always welcome. American researchers36 have evaluated sample dissolution procedures for the determination of metal impurities in single and multi-walled carbon nanotubes (CNT). They found that microwave-assisted digestion employing a nitric acid-hydrogen peroxide mixture, heated to 185 °C gave recoveries comparable to those obtained by INAA analysis, which was used to determine total metal values. Ultrasonic extraction using water or 1% (v/v) nitric acid on the other hand released little, suggesting that the role of CNT metals in cytotoxicity may be limited owing to this suggested low bioavailability. The authors used both ICP-AES and ICP-MS techniques and, not surprisingly, encountered matrix interferences from carbon. Hungarian researchers37 have developed a rapid sample preparation for the determination by SF-ICP-MS of Pu and U and their isotopes in nuclear forensic swab samples. A microwave digestion procedure was found to be 2–3 times quicker than the commonly used ashing method.
Studies of airborne particles involving sequential extraction procedures continue to attract interest. French researchers38 examined the speciation of Cd and Pb in dusts emitted from sinter plants. They concluded that highly soluble CdCl2 and PbCO3 species were generated. Supporting data were obtained by EXAFS. Workers at NIOSH39 found that welding fumes from stainless steel gas metal arc processes contain multiple Mn species. In a review paper (94 references), French researchers40 present an overview of methods for Cr speciation in solid matrices and their relevance to legislative requirements. The NaOH-NaCO3 leach procedure is widely used in an attempt to selectively leach CrVI from materials that may contain CrIII, as is the case in welding fume and aerosol emissions from chrome-plating workshops. The authors suggest that species interconversion can occur during this leach step and that SIDMS protocols could be suitable for correcting these interconversions. Professor Furuta's group in Japan41 has developed a procedure for the determination of S in size-classified airborne particulate matter. This involved a water leach, followed by an acid leach for the insoluble remainder. Using ion chromatography, they found that 99% of S in particles smaller than 2 μm was soluble ammonium sulfate. Brazilan researchers,42 in a study evaluating the occupational risk due to dust particle inhalation, determined the solubility of Ta, Th and U oxide species contained in pyrochlore (a niobium mineral) dust particles when subjected to a simulant lung fluid (Gamble solution). These experimentally derived dissolution factors were in broad agreement with calculated dissolution factors from worker exposure studies involving air monitoring and subsequent biomonitoring using faecal and urine samples.
Preparatory procedures continue to be developed to preconcentrate analytes, reduce interferences and to automate labour intensive steps. The determination of Pd in airborne particulate matter by ICP-MS remains challenging owing to isobaric interferences. Alsenz and co-workers43 investigated the use of reductive co-precipitation involving either tellurium or mercury, in association with He collision gas and ID techniques, in an attempt to determine ultra-trace levels of Pd. Mercury co-precipitation was deemed to be better than tellurium co-precipitation in reducing interfering matrix constituents. Levels of Pd in air ranged from 0.5 pg m−3 at a rural site to 15 pg m−3 at urban sampling sites in the city of Frankfurt am Main. Canadian researchers44 have developed a novel, rapid and automated procedure for the determination of ultra-trace long-lived actinides—Am, Pu and U—collected on air filters. Analytes were preconcentrated and separated using ion chromatography coupled to ICP-MS. Detection limits of 0.006 (238U), 0.006 (239Pu), 0.041 (240Pu) and 0.062 (241Am) μBq m−3 were obtained on filter samples from a total air volume of 3000 m−3. Spike recoveries were typically better than 90% and the total analysis time was 18 min per sample.
1.4 Instrumental analysis
Mukhtar and Limbeck45 report a method for the determination of Si in airborne particulates by ETAAS. Following a wet ashing step, the remaining in-soluble material is presented for analysis as a homogeneous suspension. A Zr-coated graphite tube was used to minimise the formation of stable SiC species. A cobalt matrix modifier was used in association with a low pyrolysis temperature to minimise the loss of volatile Si species and to minimise charring of residue organic material in the test aliquot. NIST SRM 2907, a soil standard, was used during method development and results obtained on split air filter samples compared favourably with those obtained following a total microwave-assisted digestion.
There seems to be renewed interest in measuring metalloid elements in the atmosphere where hydride generation can be used to increase instrumental sensitivities. Savio and co-workers46 have optimised a method for the determination of As, Bi, Sb and Se in airborne particulate matter by FI-HG-ICP-AES. Initially, filter samples were ultrasonically digested using an HCl/HF acid mixture. As reported in many previous hydride generation applications, a pre-reduction with KI was required to ensure that As and Sb were in optimal oxidation states for hydride formation. It was also noted that boric acid was required to complex free fluoride ions arising from the digestion step and which could form unwanted complexes with As and Sb. Quantitative recoveries were obtained for As, Sb and Se following analysis of NIST SRM 1648 Urban particulate matter. In this SRM, where no data for Bi is included in the certification sheet, these researchers report a value of 34.1 ± 2.2 mg kg−1. Assuming that an air volume of 1440 m3 was used, the limits of detection (3 σ) were calculated to be 0.3 ng m−3 for As, 0.09 ng m−3 for Bi and 0.1 ng m−3 for Sb and Se. In a similar fashion but using a more sensitive ICP-MS detector, an American research group47 used this HG approach thus enabling Se and Te to be determined in cloud droplets and aerosol particles at sub ng L−1 or pg/m3 levels.
Researchers based in the UK48 describe the origins of and present supporting validation data for the Dumaery equation which is used in calibration systems for Hg analysers. This equation is used to calculate the concentration of Hg vapour in the headspace above a sealed pool of Hg held at a constant temperature. In an extension to this work, the same researchers49 describe a new automated calibration system for Hg analysers. Here, diluent air, metered using mass flow controllers, is passed over a sealed pool of Hg held at constant temperature, to generate a known Hg in air concentration standard.
Instrumental reviews are welcome publications. In a wide ranging article (147 references), Stockwell and coworkers50 extol the virtues of AFS as a suitable detection technique for the determination of As, Hg, Sb and Se in a range of environmental matrices, including air, biological material and food. They explain and comment on chromatographic and non-chromatographic separation protocols for speciation analysis, as well as considering sample preparation steps. Hahn,51 an exponent of LIBS, has reviewed (73 references) the current status of the technique for aerosol analysis and provides an useful insight into future research directions. Fiddler and coworkers52 present an overview of laser techniques for atmospheric and environmental sensing (407 references).
Naoki Furuta's research group53 in Japan has provided a progress report on the development of their ICP-MS based system for the determination of metals, particularly Pb, in single nanoparticles. Ambient air particles are directly aspirated into the plasma following selective particle sampling using a differential mobility analyser and an aerosol particle mass analyser. A recently developed gas exchange system allows the sampled air to be exchanged for argon. Instrument calibration was achieved by introducing desolvated Pb standards via an ultrasonic nebuliser. To determine the transport efficiency of this USN approach, the introduction of desolvated Cr standards was compared with the introduction of a Cr carbonyl gas standard. The concentration of the Cr gas standard had been previously determined off-line by passing it at a set flow rate through three impinger samplers containing MIBK solutions for a set time period. The Cr content in these solutions was then determined by ICP-MS in order to derive a concentration value for the gas standard. It was then possible to convert Pb concentrations (pg ml−1) in the standard solutions to Pb mass flow rates (ag ms−1). An instrumental detection limit of 2.8 ag ms−1 was calculated, allowing the Pb concentration in single nanoparticles with a nominal diameter of 90 nm and a mass of 0.46 fg to be determined! However, the authors suggest that increased instrumental sensitivities of a factor of 100 or more will be needed to enable elements such as Cd and Sb to determined in similar nanoparticles. Likewise a TOF instrument or a system employing a Mattauch-Herzog detector system would be required to undertake simultaneous multi-elemental analysis.
Brown and co-workers54 used LA-ICP-MS to examine the spatial distribution of metals in airborne particulate matter collected on air filters. Inhomogeneity in the distribution of dust on the filters was noted and thus sub-sampling of filters has to be carefully considered if, as often happens in air monitoring studies, assays are performed on separate test aliquots taken from a filter. A second aim of the study was to assess LA as a means of providing quantitative data from air filter samples. Whilst the absence of sample dissolution may be an attractive attribute of this approach, calibration issues and the lack of automation count against the use of LA for this particular analysis.
Jakob and coworkers55 studied the atmospheric stability of arsines and their oxidative products in PM10 airborne particles. The atmospheric half-life of the arsines ranged from 19 weeks for AsH3 to two days for trimethylarsine for samples stored in the dark at 20 °C. Samples stored in simulated daylight were found to be much less stable. The total As content of airborne particles was below 1 ng m−3. Arsenic species, namely MA, DMA and TMAO were determined using an HPLC-ICP-MS/ES-MS methodology and were detected in 90% of the air samples collected. The predominant species was TMAO, which was found at concentrations of between 4 and 60 pg m−3. Maximum concentrations of DMA and MA determined were 16 and 6 pg m−3 respectively.
Solid-state speciation analysis of airborne particles using XAFS is a powerful tool. British researchers59 describe the use of this technique to study the oxidation state of As in cigarette mainstream smoke, cut tobacco and cigarette ash. Results revealed that the cut tobacco powder and the cigarette ash contained almost exclusively AsV. On the other hand, the smoke particulate samples showed a mixture of both AsIII and AsV species, this latter species however reduced upon aging to AsIII. The study of nanoscale ferrite materials is an emerging field, given that such materials are being used in applications ranging from magnetic media and memory storage cores to catalysis and gas sensors. In an interesting and ‘greening’ article, US researchers60 studied the growth of nanoscale nickel ferrite on a carbonaceous matrix isolated from particle emission byproducts arising from the combustion of residual oil. A selective leaching procedure involving water and dilute HCl was used to isolate the nickel ferrite material and the selectivity and the completeness of this procedure was evaluated using Fe, Ni and S K-edge XAFS.
Data interrogation and processing with X-ray techniques can be challenging. Kellogg and Willis61 have published an optimised protocol for the spectral deconvolution of overlapping elemental spectral lines typically found in the analysis of air filter samples. They state that fitting with many library reference spectra has an unwanted effect of raising the analytical uncertainty. By carefully choosing the order of elemental processing, one can reduce the number of reference spectra required for spectrum fitting, hence reducing this analytical uncertainty. Ceccato62 describes developments with the MAPPIX software, a package for the off-line analysis of data obtained from the analysis of single airborne particles using a μ-PIXE technique. American researchers63 describe a standardised approach to estimating analytical uncertainties for the XRF analysis of PM2.5 filter samples, which has been successful applied to data generated by three laboratories involved in analysing filters samples from US air monitoring programs.
Readers are directed to our companion Update for further information on X-ray developments and applications.5
1.5 Applications
Respirable crystalline silica is classified as a human carcinogen and has been studied in the working environment. However, few measurements have been reported from the wider ambient environment. Richards and co-workers74 report emission factors and ambient air concentrations at three sand and gravel plants. They estimate that the emission factor could be up to 0.00011 lbs of silica per ton of processed stone and that the resultant silica release made up 3–8% of the measured PM10 release. Ambient respirable concentrations of crystalline silica in air, measured upwind and downwind, at such facilities ranged from 0.3–2.8 µg m−3. British researchers questioned whether crystalline silica can be formed and released from sugarcane burning.75 The limited data they collected suggested that no crystalline silica was detected in airborne smoke particles but that the sugarcane trash ash formed after pre-harvest burning contained between 10 and 25% (m/m) SiO2, mostly in an amorphous form, but containing up to 3.5% (m/m) quartz. Likewise, both quartz and cristobalite were identified at levels of 5–15% (m/m) and 1–3% (m/m) respectively in the sugarcane bagasse ash formed in the processing factory. The authors recommend that adequate protection should be given to workers exposed to such dusts and that appropriate disposal routes for the ash material should be sought in order to prevent secondary exposure.
Carbonaceous particles from combustion sources are of continuing interest owing to their impact on health and atmospheric chemistry. Current understanding of their sources, their variability and their subsequent fate in the atmosphere remains incomplete and requires further measurement and data interrogation. A European research consortium has determined the particulate carbon content in precipitation samples collected at five background sites over a west-east European transect, from the Azores to Hungary.76 Tumolva and co-workers77 used TEM/EDS to examine the morphological and elemental properties of soot particles generated in a laboratory setting using a variety of fuel stocks. They subsequently used this information to classify the sources of carbonaceous particles collected at three sampling sites—urban, industrial and coastal—and to apportion their relative contributions.
Emissions from traffic sources continue to attract attention with emerging focus on particle emissions from aviation and shipping. Kinsey and researchers from the US EPA79 report findings from the US Aircraft Particle Emissions Experiment (APEX) in which emissions from the exhaust plumes of nine models of commercial aircraft engine were monitored carefully on a ground test bed. Measurements were made for particle mass and numbers, particle size distributions and total volatile matter using both time-integrated and continuous sampling techniques. Winnes and Fridell80 have studied the particle emissions from ships in relation to fuel type. Irish researchers81 have characterised single particles from in-port ship emissions using ATOFMS, in association with other air quality monitoring equipment, as a means of determining the relative contribution of shipping traffic to air quality in port cities.
Emissions from road transport sources continue to be researched. A US group compared different measurement strategies for emissions from modern diesel engines that emit low levels of particulate matter.82 They evaluated two gravimetric methods, a chemically reconstructed mass measurement method that measured the chemical constituents of particles, and an integrated particle size distribution (IPSD) method where the measured particle number data were converted to mass data using a particle density function. They concluded that, whilst gravimetric methods are traceable to primary standards, the alternative chemical speciation and the IPSD approaches might better characterise ultra low emissions typical of modern engines.
Despite greater efforts to recycle, much waste is still sent to landfill and research continues in evaluating the releases of particles and gases to the atmosphere from such sites. Fowler and fellow UK researchers83 have undertaken a quantitative assessment of dust propagation at a hazardous waste landfill where solid residues from incineration are dumped. In summary, they investigated the use of low cost wind directional passive sampling onto sticky pads to obtain airborne samples. Elemental fingerprinting analysis employing ICP-AES was then carried out in association with chemometric analysis to quantify the extent of these fugitive dust emissions from this waste site.
Studies into emissions from agricultural activities and composting have been reported, in particular where there is a need to derive industry or sector specific emission rates. Spanish researchers have developed a methodology for the determination of ammonia and total VOC in a composting plant. The novel part was to devise a normalisation function relating such emissions to efficiency of the composting process using a biological respiration index.84 It is hoped that, by using such an approach, emissions from differing composting plants can be presented in a more comparative format.
Source apportionment studies attempt to elucidate the origins of airborne pollutants and ascertain the relative contributions from source emitters. Chemical markers are commonly used in such studies, but often these markers are not unique to any one specific emission source. Korean researchers have determined the Nd and Sr isotopic composition of airborne particles and were able to trace dust events back to specific arid locations in China.85 Weinbruch and co-workers,86 using SEM and EDAX microanalysis, characterised the size, morphology and chemical composition of airborne particles collected on moss samples, a substrate often used as a dust deposition sampler. Their study suggests that analysis of individual particles can provide source-specific information that cannot be derived from bulk chemical analysis. In particular, fly ash particles from combustion sources can sometimes be incorrectly adjudged to be derived from geochemical weathering process, given that particles can have similar bulk chemical compositions but where there are differences at an individual particle level.
Numerous atmospheric pollutant surveys continue to be published and, for brevity, it is probably useful just to mention one important paper. A pan-European group87 has distilled data on the physical and chemical characteristics of aerosols obtained over the past decade in research and monitoring programmes conducted at more than 60 locations in Europe, ranging from natural background sampling sites to kerbside sites in urban environments.
Estimating future atmospheric concentrations using projections based upon current data occupies a number of researchers. In an interesting paper, Brown88 compared the estimated UK annual emissions of metals with the measured average ambient air concentrations for the same metals over the period 1980–2007 using a generalised least squares regression technique. Elemental sensitivity factors were calculated, as were ‘predicted’ elemental airborne concentrations in the absence of any future emission sources. For example, it was calculated that ambient airborne levels of Pb could drop to ∼10 ng m−3 in the absence of any future anthropogenic emissions. This value is broadly in agreement with current airborne measurements made at a remote Scottish sampling location deemed to be free from current man-made emission sources.
2 Water analysis
This section highlights new developments and improved analytical methods that use atomic spectrometry for the determination of trace metal(loids) and their associated elemental species in environmental water samples since the last Update.12.1 Sample preparation
For laboratories about to embark on multi-elemental trace and ultra trace analysis for the first time two papers highlight the importance of sample preparation. That by Rodushkin and co-workers35 is full of useful information and sound advice on how to handle samples and reagents to reduce contamination sources. Particularly important in this author's opinion is their advice that, “the targeted purity levels should be fit for purpose to be cost-efficient”, the analyst “should limit the number of steps and general manipulations of the sample” as well as “make sure that available reagents are of sufficient purity”. The determination of Pu isotopes in waters and environmental solids has been reviewed89 (135 references) and includes a summary of related sample preparation procedures such as filtration, acidification, and valence adjustment prior to the preconcentration and analysis of environmental water samples.2.2 Preconcentration, extraction and separation procedures
A driver for the development of analytical methods for water analysis continues to be the preconcentration of the analytes to above the limit of detection of the instrumental techniques to be used. The methodologies are mostly based upon solid phase extraction, liquid-extraction or co-precipitation. Variations on these themes are fertile ground for publications, and the novelty of some of the articles push the definition of analytical novelty to its limit. The aim of this Update is to highlight some of the more interesting papers and new methods for problematic elements.Amongst the truly novel stationary phases for solid phase extraction and specific affinity chromatography that have emerged are imprinted polymers. These are “bio-inspired” materials capable of recognizing a family of compounds. Their production and evaluation for the determination of organotin compounds in seawater has been described by Gallego-Gallegos and co-workers,90 who reported that when used for the SPE of TBT, a breakthrough volume of over 1 L was found for seawater spiked with 50 ng L−1 of TBT resulting in a preconcentration factor of 150, with a spike recovery of 93 ± 12%. Another Spanish research group has reported the use of polymers imprinted with 8-hydroxyquinoline for the on-line SPE of Ni and Pb from seawater with subsequent ICP-AES detection.91 The static adsorption capacities for the metals were found to be 0.023 mmol g−1 for Ni and 0.0015 mmol g−1 for Pb. Standard solutions (50 μg L−1) adjusted to pH 8.5 and passed through a 300 mg bed of molecularly imprinted polymer (MIP), were found to have breakthrough volumes in excess of 200 mL resulting in enrichment factors of 14.8 for Ni and 5 for Pb.
Dispersive liquid-liquid micro-extraction (DLLME) together with homogeneous liquid- liquid extraction in the determination of inorganic elements has been reviewed92 (65 references) covering 1970 to 2009, although the majority of articles are from after 2000. In DLLME a ternary component solvent system is used, which consists of an appropriate mixture of an extractor solvent, such as tetrachloroethene, and a disperser solvent such as acetone that must be highly miscible in the aqueous sample and the extraction solvent. On addition of this mixture, a cloudy solution is produced by the dispersion of the organic solvent in the aqueous phase. The analytes are subsequently extracted into the organic phase after complex formation, and the two phases are separated by centrifugation. The advantages of this method are that it uses micro-volumes of organic solvents, and the extraction surface is extremely large, so that vigorous mixing or shaking is not required. A disadvantage is that the analytes are extracted into an organic solvent making the technique more suitable for FAAS or ETAAS than for ICP-AES or ICP-MS. Another review of DLLME that includes a section on inorganic analysis was published in 200993 (129 references). This will be of interest to researchers who do not just carry out inorganic analysis. Moreover, the widespread use of DLLME in organic analysis suggests that it may well work for organometallic compounds. Ligandless DLLME has also been proposed using 1,2-dichlorobenzene as the extraction solvent and ethanol as the dispersive solvent for the micro-extraction of CuII ions,94 although this has opened the door for a series of “novel” methods where the same authors change the extraction solvent to extract another ion, in this case AgI.95 We are curious to see what other elements they manage to extract next year.
Another growing technique is the use of ionic liquids (IL) to carry out liquid–- extractions. In the extraction of Cd96 the IL is the hydrophobic liquid, 1-hexyl-3-methylimidazolium hexafluorophosphate, that can be used to carry out DLLME without the need for a disperser solvent, as the IL is dispersed using ultrasound. The analyte is complexed with DDTC and extracted into the IL, which is then separated from the aqueous phase by centrifugation. The authors report an enrichment factor of 67 and a LOD of 7.4 ng L−1 with ETAAS determination. These IL lend themselves to the various micro-extraction methods such as SDME97 that already exist in the literature.
Liquid phase micro-extraction for the determination of trace elements has been reviewed98 (104 references), covering the methodologies of SDME, HF-LPME, DLLME and SFODME. The last of these proved popular this year for the determination of Co and Ni,99 Hg,100 Pd101 as well as Pb and Cd.102
The use of cloud point extraction (CPE) with atomic spectrometric methods has been reviewed103 (91 references, in Slovak with an English abstract). An interesting application was the use of CPE followed by ICP-MS detection of radionuclides in aqueous samples.104 The trivalent lanthanides are complexed with APDC and then extracted into Triton X-114®. Extraction efficiencies of up to 90% were achieved with corresponding concentration factors of up to 6.3 for Eu. Meeravali and Jiang105 used CPE for the selective preconcentration of Cr species prior to ICP-MS detection in DRC mode. Hexavalent Cr was selectively extracted from the sample using Aliquat-336® while total Cr (after conversion to CrIII) was extracted using APDC. In both cases the cloud point surfactant was Triton X-114®. The CrIII concentration was estimated by difference. Preconcentration factors of 10 resulted in LODs of 0.01 ng mL−1 for CrVI and 0.025 ng mL−1 for CrIII. A Chinese group has reported the development of an online flow injection CPE method that does not require a chelating agent for the determination of trace metals in sea water by ICP-AES.106
The use of nanoparticles as adsorbents for the solid phase extraction of environmental pollutants including metal ions has been reviewed107 (176 references). Unmodified TiO2 nanoparticles have been employed to extract dissolved Fe from seawater108 and it has been demonstrated that the same material, modified with a calixarene compound, is capable of preconcentrating Cr, Cu, Pb and V from environmental water samples.109
Thanks to a special issue of the Journal of Chromatography A, volume 1217 (16), there have been a large number of review articles recently, including one on carbon nanotubes110(180 references). Their use has been extensively reported over the last few years and this year is no exception. In their oxidized form they are able to extract Cu, Co and Pb111 and Cu, Mn, Pb and Zn.112 They have also been shown to retain complexes of Rh113 and selectively retain CrIII for speciation studies,114 as well as quantitatively adsorb Au and Pd from solutions in the pH range from 2.0 to 5.0.115 The precious metals retained were eluted with a 3% m/v thiourea solution in 0.5 M HCl with a recovery of up to 103%. The LODs were 0.23 ng mL−1 for Au and 0.06 ng mL−1 for Pd, when using ICP-MS.
2.3 Speciation
Feldmann and co-workers116 critically reviewed (91 references) elemental speciation analysis, covering three main analytical methodologies: electrochemical, X-ray absorption spectroscopy and chromatography coupled to mass spectrometric detection. Researchers from the University of Vienna provided a more specific review (141 references) of the environmental applications of HPLC or GC coupled to ICP-MS.117Non-chromatographic speciation methods continue to dominate the literature. Reviews by Vieira et al.118 (176 references) and Gonzalvez et al.119 (43 references) both note that, although atomic spectroscopy coupled to chromatographic separation gives the most complete information, non-chromatographic techniques are competitive as they are less time consuming and more cost effective.
The fractionation of trace elements in water samples is now being studied more closely. Puls and Limbeck120 have described a flow injection system for the identification of the neutral, anionic and cationic fractions of Al, Ca, Cd, Co, Cr, Cu, Mg, Mn, Pb and Zn in urban snow samples. Three different SPE cartridges were employed in sequence, followed by ICP-AES detection. Turkish researchers121 determined the distribution of P in seawater between Al-bound, Fe-bound, Ca-bound and loosely bound P forms with ICP-AES detection. Studies of this kind are important for understanding the bioavailability of this marine nutrient.
Chromatographic and non-chromatographic methods for arsenic speciation continue to be investigated. The use of the SFODME preconcentration technique to obtain a 1000-fold enhancement factor, followed by ETAAS detection of the As species (AsIII and total As after reduction of AsV to AsIII) has been reported by Iranian researchers.122 They achieved a LOD of 9.2 pg mL−1 with an RSD of < 8.6%. A method for the speciation analysis of arsenic and antimony using DLLME and ETAAS has been developed in a Spanish laboratory.123 Samples were acidified to pH 1, the AsIII and SbIII complexes with APDC were extracted into carbon tetrachloride and determined by ETAAS. Total As and Sb were obtained after the reduction of the sample with sodium thiosulfate and the AsV and SbV concentrations obtained by difference. A narrow bore HPLC SF-ICP-MS method for the determination of AsIII, AsV, MMA, DMA and AsB has been developed.124 A NH4NO3 buffer allowed high salt gradients to be run. Separation was achieved in less than 9 min using the Dionex AS11 and AS7 columns. Column deposits that could cause reversible adsorption of AsV were eliminated by having the samples in an excess of phosphate buffer that prevented artefacts in the AsIII to AsV ratios. Retention times were stable and LODs were in the low ng L−1 range.
A multi-elemental speciation method for the determination of AsIII, AsV, MMA, CrIII, CrVI, SeIV and SeVI has been reported.125 Chromatographic separation was achieved within 11 min using a Hamilton PRP-X100 column and a binary mobile phase mixture of 0.5 mM NH4H2PO4/10 mM NH4NO3 at pH 6 and 30 mM NH4NO3 at pH 6. An ICP-MS instrument equipped with a dynamic reaction cell was used to detect the analytes, resulting in LODs of between 0.12 to 0.44 μg L−1 for the individual species. A method for the analysis of waste waters for AsIII, MMA, DMA, AsV, SeIV, SeVI and SeCN− has been developed by a Brazilian research team126 using a binary gradient elution of H2O and 100 mM NH4NO3 at pH 8.5 and a Metrosep A Supp10 (250 × 4.0 mm) analytical separation column. Using ICP-MS detection, LODs of between 56 and 81 ng L−1 were reported for the Se species and 16 to 25 ng L−1 for the As species.
The speciation of chromium continues to be of interest. Brazilian researchers127 have developed a non-chromatographic method based on CPE and preconcentration to determine CrIII and CrVI species, by difference, in river waters contaminated with leather effluents. Trivalent Cr is complexed and trapped in a TritonX-114® surfactant solution after being brought to the cloud point temperature by microwave-assisted heating. Under optimized conditions, an enrichment factor of 48 was achieved for a 50 mL sample. Detection of Cr was by ETAAS calibrated over a linear range of 2.5 to 80 μg L−1, giving a LOD of 0.7 μg L−1. Method repeatability was 2.0% for 10 replicates of a 50 μg L−1 CrIII solution. Micro-extraction techniques seem to be much in vogue in Iran, so it was no surprise that Iranian workers had developed a method for the speciation analysis of chromium based on DLLME followed by FAAS detection.128 Enrichment factors of up to 275 were achieved resulting in a LOD of 0.07 μg L−1 for CrVI, a linear working range of 0.3 to 20 μg L−1 and precision of 2.0% (n = 7)]. These figures of merit are comparable to those achieved by ETAAS.127 Xing and Beauchemin129 have described an HPLC-ICP-MS method for chromium speciation at trace levels in potable water using cell-based ICP-MS for interference elimination. Chromium species were separated on an IonPac AG-7 column using a gradient elution of 0.1 M NH4NO3 and 0.8 M HNO3. Accuracy was verified against the NRCC river water CRM SLRS-2 and detection limits of 0.02 μg L−1 for CrVI and 0.04 μg L−1 for CrIII were attained.
Methods for the determination and speciation of mercury in natural waters have been reviewed130 (120 references). The ability of sodium diethyldithiocarbamate (NaDDC) to form complexes with mercury species has been exploited, either by immobilizing it on polyurethane foam131 or by extracting the complexes using CPE.132 Alternatively, the use of a dithiazone functionalized C18 column for SPE of Hg2+, MeHg+ and EtHg+ has been reported.133
Inorganic selenium speciation in environmental samples has been carried out using selective electrodeposition coupled to ETAAS.134 In this technique, SeIV and selenocystine are selectively electrodeposited onto a mercury electrode before removal and detection by ETAAS. Spike recoveries of between 91 and 99% are achieved with a selenium LOD of 1.0 μg L−1 and an RSD of 3.5% (n = 6) at a level of 100 μg L−1.
A method for the speciation analysis of tellurium by electrodeposition followed by ETAAS detection has been described.135 Trace amounts of TeIV were preconcentrated using DLLME prior to detection. The TeVI concentration was found by difference after reduction to TeIV.136 The enrichment factor of 125 meant that a LOD of 0.004 ng mL−1 and a precision of 3.6% RSD (n = 6) for a 0.5 ng mL−1 standard were achieved.
A Dowex Monosphere 550A (OH) anion exchange resin has been used to successfully retain and separate thallium species in waters.137 The species TlI and TlIII were eluted using thiourea followed by ICP-MS detection. The use of the column resulted in a 100-fold enrichment factor for the analytes.
Given their toxicity, even at low concentrations, the analysis of water for organotin compounds continues to attract research interest. A liquid–- extraction followed by HPLC separation and ICP-MS detection was used by Chinese researchers138 to quantify organotin compounds in seawater; LODs were better than 3 ng L−1. They found that the only compound present was TPhT at a concentration of 53 ng L−1. Although an atomic spectrometric detector has not been used in the following methods, it will of interest that two research groups from Spain have demonstrated that HS-SPME after in situ ethylation improved method sensitivity with GC-FID139 with LODs (n = 9) of 0.17 μg L−1 for MBT, 0.012 μg L−1 for DBT and 0.009 μg L−1 for TBT. Using a more sensitive GC-MS technique,140 the same researchers obtained LODs between 0.025 and 1 ng L−1 for the target tin species. A Japanese research group has succeeded in quantifying TPhT and TBT using HILIC-ESI-MS.141 Following preconcentration of a 500 mL water sample using mixed mode reversed phase and weak anion-exchange SPE cartridges, they determined a method LOD of 3 ng L−1 for TBT and 6 ng L−1 for TPhT.
The speciation of vanadium in water samples has been studied by Chinese researchers142 using on-line separation and preconcentration. A silica column modified with CTAB retains VV in a pH range between 2.0 and 7.0, while VIV is retained between pH 5.0 and 7.0. Hence, VV was retained at pH 2.5 and total V at pH 6.0 and the VIV concentration was obtained by difference. With ICP-AES detection, the LOD of 0.03 μg L−1 for Vv was obtained after preconcentration of a 3 mL sample with an enrichment factor of 27.9; the RSD for a 5 μg L−1 standard (n = 9) was 4.3%.
2.4 Instrumental analysis
Chinese researchers148 have instead used tungsten coil electrothermal vaporization atomic fluorescence spectrometry to detect trace Cd in water after CPE. They report an enrichment factor of 152 and a LOD of 0.01 μg L−1.
An ICP-AES method149 for the indirect determination of F− in aqueous samples following precipitation of CeF3 has been reported. The excess CeIII remaining after precipitation is used indirectly to determine the F− concentration. The method has been shown to work well for F− in the range 0–20 mg L−1 in solution giving a LOD of 1.4 mg L−1.
A review of atomic fluorescence spectrometry as a suitable detection technique in speciation studies for As, Hg, Sb, Se50 (147 references) has been published.
Mercury in river water 151 was detected using a FAPES low power atmospheric pressure He plasma source directly coupled to either a conventional vapour generation system, using SnIICl2 as a reductant, or a UV photochemical system for the generation of elemental mercury vapour. The concentration of mercury was monitored using the characteristic 253.7 nm resonance line, and the 70 W plasma was found to be tolerant of the water vapour generated by the gas-liquid separator. With the furnace at 400 °C, a measurement precision of 2.4% RSD at the 1 ng mL−1 concentration level was achieved. With chemical reduction a LOD of 240 pg mL−1 was obtained, and with photochemical reduction the LOD was 250 pg mL−1. The addition of a gold amalgamation system improved the LODs 5-fold. Chemical vapour generation using NaBH4 cannot be coupled to this emission source since the large quantity of H2 generated extinguishes the He FAPES source!
The determination of Bi by tungsten trap hydride generation AAS has been extensively investigated.152 Bismuth hydride is generated and trapped on a tungsten coil held at 289 °C. After preconcentration, the coil is heated to 1348 °C to release the trapped Bi species, which are subsequently transported to a flame heated quartz atom cell for detection. Interferences from As, Au, Ca, Cd, Cu, Fe, Mg, Mn, Na, Ni, Pb, Se and Zn, as well as Cl−, SO42−, and PO42− were examined. The calibration was found to be linear between 0.1 and 10.0 μg L−1 for an 8 mL sample giving a LOD of 25 ng L−1. The tungsten trap gave an enhancement factor of 21.
Work on the development of vapour generation methods for non-traditional elements continues. Zheng and co-workers153 have developed a UV photochemical vapour generation method for Ni with ETAAS detection. This is based on the photochemical generation of highly toxic Ni(CO)4 and its trapping on the surface of a graphite furnace. With a NiII solution containing formic acid, the vapour generation efficiency was 35%. A 2 mL sample gave a LOD of 8 pg mL−1 and the measurement precision was better than 3% RSD at the 1 ng mL−1 level. The method was validated using the NRCC CRM SLRS-4 River water. The same researchers154 have developed a high yield photochemical vapour generation method for Fe with ICP-AES detection. In this approach, Fe2+ and Fe3+ in solutions containing low molecular weight acids such as formic, acetic or propionic acids are exposed for 250 s to a UV source, in this case a 17 W low pressure mercury lamp. Optimum vapour generation efficiency (60 ± 2%) was obtained when the sample solution contained 50% v/v formic acid at pH 2.5. Compared to conventional solution nebulisation, the sensitivity was improved 80-fold and the LOD 100-fold when monitoring the 238.204 nm Fe line. An analytical precision of 0.75% RSD was obtained at the 100 ng mL−1 level and the method was validated against the NRCC CRM SLRS-5 River water. This research team155 also developed a thin film HG method with ETAAS detection for the determination of trace Cu. Using tetrahydroborate as the reductant, the overall efficiency was 8–12%. A solution LOD of 100 pg mL−1 was achieved with a measurement precision of 4% RSD for a 1 mL sample spiked at 1 ng mL−1. A Czech research group156 has developed a method for the determination of sulfide in water samples by vapour generation ICP-AES. Hydrogen sulfide and acid volatile sulfides are transformed into a vapour by acidification. The method gave results comparable to those from iodometric titration over a linear range of 0.06 to 22.0 mg L−1 and a LOD of 0.03 mg L−1 for H2S.
Tolokonnikov et al.161 considered the application of EDXRF to the analysis of potable water. They discuss the use of thin layer samples and X-ray optics with total internal reflection. They report that, when compared to thick layer optic geometries, LODs can be lowered 1000-fold, making the instrument suitable for the analysis of potable waters.
Preconcentration methods for the analysis of liquid samples by XRF techniques have been reviewed162 (118 references) with emphasis on new efficient variations that extend the possibilities of XRF for liquid sample analysis. To the same end, powdered silica163 has been used to preconcentrate trace elements before WDXRF detection, and the ion-exchange resin Amberlite IRC748 to preconcentrate Cu, Fe, Ni, Pb and Zn from surface waters before detection with transportable EDXRF.164
Although cell-based instruments eliminate many of the inherent interference problems in ICP-MS, chromatographic separations for interference removal are sometimes necessary. An in-line chromatographic separation of Ba from Cs for the measurement of 135Cs and 137Cs at trace levels in ground waters has been reported.167 A Dionex CG3 IonPac guard column is used to reduce the blank Ba levels and preconcentrate Cs. To improve instrument sensitivity, the in-line chromatographic system was used in conjunction with a high efficiency desolvating nebulizer. Detection limits were 2 fg mL−1 for 135Cs and 0.9 fg mL−1 for 137Cs.
Sector field high resolution ICP-MS is sometimes the only alternative to matrix separation/removal techniques. When measuring trace element profiles in high altitude ice cores,168 sample manipulation can be reduced to a minimum, thus reducing any contamination risk. A Lithuanian research group169 determined As in seawater by SF-ICP-MS at medium resolution where 75As is resolved from 40Ar35Cl. In contrast, where interferences are less of an issue, a low-resolution mode is often used to maximize ion transmission and take advantage of the low background signals. To further boost sensitivities, microconcentric nebulizers and high efficiency nebulization/desolvation units can be used. Two such sample introduction systems have been compared for the determination of REE in surface and subsurface waters.170 The authors report that, for spiked (1 μg L−1) NRCC water CRMs, REE recoveries were about 100% and that, overall, LODs were of the order of 0.05–0.2 ng L−1 and not significantly different between either system.
Isotope dilution as a calibration strategy has been investigated for the determination of total Cr in seawater.171 All the Cr present was reduced to CrIII with hydroxylamine hydrochloride, before the sample pH was adjusted to 9.3. The CrIII was retained on silica immobilized diphenylcarbazone and subsequently eluted with 1.5 M HNO3. A throughput of ten 7.5 mL samples per hour and a LOD of 0.005 ng g−1 were obtained. Results were in agreement with the certified values for the NRCC seawater CRMs NASS-5 and CASS-4. The validated method provided results of 0.1132 ± 0.0047 ng g−1 for the candidate CRM NASS-6 and 0.0942 ± 0.0040 ng g−1 for the candidate CRM CASS-5. The same research group reported the use of ID-ICP-MS for the determination of Mo in seawater172 after matrix separation on an immobilized 8-hydroxyquinoline microcolumn. A concentration of 9.42 ± 0.22 ng g−1 was obtained for the NRCC NASS-5 CRM, which was in good agreement with the certified value of 9.4 ± 1.0 ng g−1. Milne et al.173 determined Co, Cd, Cu, Fe, Mn, Ni, Pb and Zn in seawater using ID-SF-ICP-MS and applying standard additions after a preconcentration step on a microcolumn containing Toyopearl AF-Chelate-650M resin. The samples were irradiated with a UV lamp to destroy any organic ligands before preconcentration and extraction. The results were in good agreement with those of the NASS-5 CRM and consensus values from inter-comparison exercises. The method was used to produce vertical profiles of the target elements at the Bermuda Atlantic Time Series station. Tagami and Uchida174 have used ID-ICP-MS to survey Re concentrations in 45 Japanese rivers. After a 20-fold preconcentration, ID-ICP-MS gave a recovery of almost 100%. A geometric mean Re value of 0.81 ng L−1 was found and the values had a high correlation with SO42− concentrations. Isotope dilution ICP-MS has also been used to determine the vertical profile of dissolved Pb in Lake Baikal.175 The results were validated against the river water reference material JSAC 0302. Concentrations in the profile ranged from 6.39 to 14.5 pg mL−1 with the exception of surface water, which had a value of 194 pg mL−1. This was attributed to a surface input source. The mean concentration was approximately 9.6 pg mL−1, which was much lower than the results found by other researchers.
Isotope ratio measurements are becoming more important for identifying sources of pollutants. Lead concentrations and isotope ratios have been measured on rainfall collected in Central Tokyo.176 A sequential rain sampler was used to collect 1 mm precipitation intervals. Lead concentrations decreased rapidly during a rainfall event and the isotope ratios clearly changed. This was explained with an atmospheric mixing model that involved four possible sources: diesel vehicle emissions, natural soil components, airborne particulate matter and fly ash from a local incinerator. Lithium isotope ratios in seawater and natural carbonates have been determined by quadrupole ICP-MS.177 A single step ion chromatographic method was used to recover and separate Li from matrix elements, involving a 2 mL column packed with AG 50W-X8 resin. High column yields (> 99.98%) were obtained with low procedural blanks (< 500 fg mL−1). The precision was < ± 0.8‰ (2 σ) for L-SVEC Li standards and better than ± 1.5‰ (2 σ) for natural samples. Seawater δ7Li values of 30.75 ± 0.41‰ (2 σ) were similar to those reported by other workers (31 ± 0.5‰). Lithium isotope ratios have also been determined using a MC-ICP-MS coupled with a low memory APEX IR high efficiency nebuliser.178 A simple one-step column separation was used to isolate Li from the matrix elements. The long-term precision of δ7Li values was ± 0.12‰ (2 σ, n = 46). Using this method, δ7Li values for the following seawater RMs were determined: IAPSO, 30.84‰, NASS-5, 30.72‰, CASS-4, 30.69‰ and SLEW-3, 30.45‰. A value of 9.36‰ was found for NIST SRM 1640 River water. Results are also reported for two silicate rock standards.
Continuous flow hydride generation laser induced fluorescence spectrometry has been investigated for the determination of Pb in water and sediment samples. The Pb hydride generated was excited with a laser at 283.306 nm, and fluorescence was detected at 365 and 405.8 nm. Method repeatability was 3.5% RSD at the 10 ng mL−1 level.182
Laser induced thermal lens spectrometry has been used in combination with CPE for the detection of Pd in water samples.183 After extraction, the sample was transferred into a quartz microcell and probed with a single laser thermal lens spectrometry instrument. Under optimum conditions the calibration was found to be linear between 0.3 to 60 ng mL−1, with a LOD of 0.04 ng mL−1.
2.5 Data quality and method intercomparison
Kumar and Riyazuddin184 made a detailed study of the effect of Fe in ground waters on the determination of total Cr. They found that digestion with HNO3 or hydroxylamine was not enough to release organically bound Cr or Cr fixed to FeIII hydroxide precipitates, and that microwave digestion of the samples before the measurement of total Cr gave the best results.Plutonium isotopes have been determined in seawater by semiconductor alpha spectrometry, ICP-MS and Accelerator Mass Spectrometry.185 All the techniques were able to produce depth profiles and the results were comparable between all three analytical methods.
A review of Bi determination in environmental matrices has been published186 (148 references), with the objective of establishing a range of typical concentrations for this element in natural water. The authors report that this objective was impossible to achieve due to the wide variability in the published data. The detection limits of most methods published after 2000 are well above those required and are therefore of limited usefulness. Analysis of existing information on total Bi concentrations and its chemical speciation in seawater reveals that the uncritical reproduction of old data is widespread. The gauntlet has been thrown down! It should be picked up by the readers of this update as there is obviously a need for sensitive, accurate and precise measurements of Bi and its species in natural waters.
A blind intercomparison exercise has been carried out to validate the standard portable flow injection chemiluminescence method for the determination of Fe in seawater187 against ID-ICP-MS. To investigate the effect of the seawater matrix, samples were collected at various depths (0–200 m) from different water masses in the Atlantic Ocean. These were filtered through 0.02 μm and 0.2 μm pore size filters. The results showed good agreement over the concentration range of 0.15–2.1 nM of Fe. Where disagreement occurred the authors put this down to random error caused by contamination during handling and matrix effects, rather than systematic errors.
Sector field ICP-MS and TIMS have been compared for B isotope ratio measurements in Water Framework Directive monitoring programs.188 As expected, the precision of δ11B measurements by sector field ICP-MS (2 σ = ± 2.6‰) was poorer than that obtained using a TIMS instrument (2 σ = ± 0.3‰). However, ICP-MS was found to be a suitable screening method although the superior precision of TIMS allowed the tracing of nitrate pollution sources.
3 Analysis of soils, plants and related materials
3.1 Sample preparation
Assessing the bioavailable metal fraction in soil is important but there is still little consensus on the best reagent(s) to use. Bortolon and Gianello193 recommended the Mehlich-1 extractant (0.05 mol L−1 HCl + 0.0125 mol L−1 H2SO4, pH 1.2) over 1 mol L−1 KCl or 0.1 mol L−1 HCl in their study of 441 Brazilian soils. However, Schroder et al.,194 following a major inter-comparison involving 23 laboratories, concluded that the Mehlich-3 recipe (0.2 mol L−1 CH3COOH + 0.25 mol L−1 NH4NO3 + 0.013 mol L−1 HNO3 + 0.015 mol L−1 NH4F + 0.001 mol L−1 EDTA) constituted a suitable extractant for assessing plant-available Cu, K, Mg, Mn and Zn, but not Ca or Fe. A simple water leaching procedure195 was applied to assess element bioaccessibility in 161 soils from two transects across the United States as part of the North American Soil Geochemical Landscapes Project. Sixty-three elements were measured in the extracts by ICP-AES or ICP-MS.
An attempt has been made196 to harmonise dynamic fractionation methods for metals in soils by comparing results obtained by sequential injection micro-column extraction and the rotating coiled column extraction methods on NIST SRM 2711 Montana soil and GBW 07511 Sediment. A 5-step procedure was used: water soluble/exchangeable; acid soluble; easily reducible; easily oxidisable and moderately reducible. The residual fraction was also analysed after microwave digestion in aqua regia. Similar trends in metal distribution were generally found with the two methods.
The stability of arsenic species during sample pre-treatment and extraction is of considerable current interest, as discussed in a review by Rubio et al.197 with 70 references. A comparison198 of four extraction procedures applied to two CRMs and five soils, using HPLC-ICP-MS detection, found that microwave-assisted extraction in 1 mol L−1 H3PO4 + 0.5 mol L−1 ascorbic acid gave good As recovery and minimised conversion of AsIII to AsV. Similar findings were reported by other workers199 using IC-ICP-MS. They recommended 1 mol L−1 H3PO4 for the extraction of inorganic As species from soils but noted that, even when extracts were stored at 4 °C in darkness, the AsIII peak area decreased over time. The same research group200 recommended the use of protein-extracting solution at pH 5.6 for isolation of AsIII, AsV, DMA and MMA from spinach, claiming that no interspecies conversion occurred in extracts over 45 days. Meanwhile, a study on the uptake and transformation of As by Sinapis alba (White Mustard)201 found highest extraction efficiency, reproducibility and stability of arsenic species in extracts when samples of dry, homogenised plant tissue were extracted in water, without use of liquid nitrogen.
Species stability during extraction is also discussed in a review40 (94 references) focusing on recent developments in EU regulation and analytical methods relevant to Cr. A study202 has been carried out to assess whether naturally-occurring substances co-extracted from plant tissue can affect Se speciation. Using HPLC-ICP-MS it was discovered that the presence of two phenolic antioxidants, rutin and tannin, at a ratio of 1![[thin space (1/6-em)]](https://www.rsc.org/images/entities/char_2009.gif)
:100 w/w, decreased the signal for SeIV. In addition, the SeVI was found to be unstable during 4 days at 4 °C.
A high-throughput microwave-assisted digestion method suitable for plant samples weighing as little as 1–20 mg has been developed.203 The procedure, which typically involved treatment of ≤ 5 mg of sample with 125 μL of H2O2 and 250 μL of HNO3, was carried out using a 64-position rotor with 5 mL disposable glass vials. Results for NIST SRM 1515 Apple leaves were similar to certified values for Al, Ca, Cu, K, Mg, Mn, P and Zn for a 1 mg sample, but 5 mg was required for accurate quantification of Mo and S. The method was applied to single rice grains and small batches of Arabidopsis seeds, yielding results similar to conventional digestion of larger samples. Microwave digestion was found superior to open vessel digestion for the measurement of rare earth elements in tomato plants.204 Procedures without HF addition, e.g. treatment of 0.5 g of sample with 6 mL HNO3 + 2 mL H2O2, gave lower blanks than when HF was added. Limits of detection from around 0.02 to 2 μg kg−1 were obtained by both quadrupole ICP-MS and SF-ICP-MS.
| Analyte(s) | Matrix | Digestion | Surfactant | Chelator | Detector | Comments | CRMs | Reference |
|---|---|---|---|---|---|---|---|---|
| Cd, Pb | Tobacco | HNO3 | Triton X-114® | 2-(bromo-2-pyridylazo)-5-diethyl-amino-phenol | TSFFAAS | LOD 4.0 μg kg−1 for Cd, 13 μg kg−1 for Pb | GBW 08505 Tea leaves, 08501, Peach leaves | 278 |
| Co, Cu, Pb | Canned fish, honey, tea, tomato sauce, water | Microwave, HNO3/H2O2 | Triton X-114® | 1-phenylthiosemicarbazide | FAAS | LOD 1.0, 0.7, 3.4 μg L−1 for Co, Cu and Pb, respectively; RSD 1.7–4.8% (n = 7) | NIST SRM 1515 Apple leaves, GBW 07605 Tea | 279 |
| Cu, Fe | Plant materials | Microwave, HNO3/H2O2 | Triton X-114® | 1,2-thiazolylazo-2-naphthol | FAAS | Preconcentration factor 30; LOD 1.0 μg L−1 for Cu, 10 μg L−1 for Fe | NIST SRMs 1515 Apple leaves, 1547 Peach leaves, 1572 Citrus leaves, 1573a Tomato leaves | 280 |
| Mn | Flaxseed flower, oat, shrimp powder, soy flour, wheat flour | HNO3 | Triton X-114® | 2-[2′-(6-methyl-benzothiazolylazo)]-4-bromophenol | FAAS | Preconcentration factor 17; LOD 0.7 μg L−1 | NIST SRMs 1515 Apple leaves, SRM 1570a Spinach leaves | 281 |
| Ni | Plant materials | Microwave | Triton X-114® | 1,2-thiazolylazo-2-naphthol | FAAS | Preconcentration factor 30; LOD 5 μg L−1 | CRMs | 282 |
| Analyte(s) | Matrix | Carrier | Detector | Comments | CRMs | Reference |
|---|---|---|---|---|---|---|
| Cd, Co, Cr, Cu, Fe, Mn, Pb | Soil, water | N-cetyl N,N,N trimethylammonium bromide | FAAS | Preconcentration factor 10; LOD 0.2 –17 μg L−1 | BCR 141R Calcareous loam, CRM 025–050 Soil | 283 |
| Cr | Cheese, fish, meat, tea, water, wheat | Ni2+/2-nitroso-1-naphthol-4-sulfonic acid | FAAS | Preconcentration factor 50; LOD 1.3 μg L−1 | GBW 07605 Tea, 0703 Bush branches and leaves | 284 |
| Cr | Water | 3-ethyl-4-(p-chlorobenzylidenamino-4,5-dihydro-1H-1,2,4-triazol-5-one | FAAS | Preconcentration factor 50; LOD 1.0 μg L−1 | NIST SRM 1573a Tomato leaves, GBW 0703 Bush branches and leaves | 285 |
| Mn | Food, water | Zirconium (IV) hydroxide | FAAS | Preconcentration factor 50; LOD 0.75 μg L−1 | NIST SRMs 1515 Apple leaves, 1568a Rice flour | 286 |
| Analyte(s) | Matrix | Substrate coating | Column substrate | Detector | Notes | CRMs (or other validation) | Reference |
|---|---|---|---|---|---|---|---|
| Au | Soil, water | Benzoylthiourea | Silica gel | FAAS | Preconcentration factor 267; LOD 1.4 μg L−1 | NIST SRMs 2709 San Joaquin soil, 2710 Montana soil | 287 |
| Au, Pd | Soil, wastewater | Aminopropyl- | MCM-41 and MCM-48 | FAAS | Preconcentration factors 800 for Au, 400 for Pd; LOD 0.06 μg L−1 for Au and 0.1 μg L−1 for Pd | Spike recovery | 288 |
| Cd | Tea leaves, water | None | B2O3/TiO2 composite | FAAS | Preconcentration factor 50; LOD 1.4 μg L−1 | GBW 07605 Tea leaves | 289 |
| Cd | Soil, tobacco, water | 2-(2-quinolinylazo)-4-methyl-1,3-dihydroxydibenzene | Graphitised carbon black | FAAS | Preconcentration factor 500; LOD 0.05 μg L−1 | Comparison with ICP-MS | 290 |
| Cd, Co, Cu, Ni, Pb | Pea, potato | Carboxylic acid | Silica gel | ICP-AES | Analytes as 1,10-phenanthroline complexes; LOD 2.1–4.4 μg L−1 except Pb (18 μg L−1) | CTA-OTL-1 Oriental tobacco leaves | 291 |
| Cd, Co, Cu, Ni, Pb, Zn | Potato | 1,10-phenanthroline | Silica gel | ICP-AES | LOD 6–30 μg L−1 | CTA-OTL-1 Oriental tobacco leaves | 292 |
| Cd, Pb | Plants, water | Sodium dodecyl sulfate-1-(2-pyridylazo)-2-naphthol | Nanometer sized γ alumina | FAAS | Preconcentration factor 250; LOD 0.15 μg L−1 for Cd, 0.17 μg L−1 for Pb | NIST SRM 1640 Natural water | 293 |
| Cr, Fe | Food, plants | None | Single-walled carbon nanotubes | FAAS | LOD 4.1 μg L−1 for Cr, 2.1 μg L−1 for Fe | IAEA 336 Lichen, CRM 025–050 Metals on soil, BCR 032 Moroccan phosphate rock | 294 |
| Cr, Fe, Pb, Th, Ti | Water | 2-nitroso-1-naphthol | MCI GEL CHP20P | ICP-MS | Preconcentration factor 20; LOD 0.03–0.6 μg L−1 | BCR 114R Sewage sludge, BCR 141R Calcareous loam, SRMs 1568a Rice flour, 1577b Bovine liver | 295 |
| Cu | Tea leaves | None | Surface ion imprinted silica gel | AAS | GBW 07301a Sediment, 07401 Soil | 296 | |
| Cu | Water | Dimethylglyoxime | Silica gel | LOD 6.0 μg L−1 | NIST SRMs 1515 Apple leaves, 1643e Simulated freshwater | 297 | |
| Hg | Plants, waters | 1,5-diphenylcarbazide | Magnetic nanoparticles | CVAAS | LOD 0.16 μg L−1 | GBW 08603 Water | 298 |
| Mn | Cassava, corn, rice | 4-(5′-bromo-2′-thiazolylazo)orcinol | Polyurethane foam | FAAS | Preconcentration factor 17; LOD 0.7 μg L−1 | NIST SRMs 1570a Spinach leaves, 1573a Tomato leaves, 1575a Pine needles | 299 |
3.2 Instrumental analysis
Slurry sampling with conventional (line source) FAAS has been used to determine Mn in leaves of cassava (Manihot esculenta crantz).212 Samples were dried, milled, sieved to < 100 μm particle size, and sonicated for 20 min in 2 mol L−1 HNO3 before aspiration. A method for measuring Mn and Zn in tea leaves213 reported optimal performance when 200 mg of ground sample was sonicated for 10 min in 2 mol L−1 HNO3 + 2 mol L−1 HCl + 2.5% Triton X-100®, in the proportions 50:12:38. Results were statistically similar to those obtained following acid digestion of the same samples (paired t-test at 95% confidence interval).
Slurry sampling ETAAS has also experienced renewed interest. Methods have been reported for the determination of As and Sb214 and for Cd, Hg, Pb and Se215 in soil (the latter in Japanese with English abstract), Sn in sediment,216 and V217 and Cd, Cu, Pb and Ni218 in plant material.
Applications of AAS in speciation analysis include a method coupling FAAS with HPLC,219 specifically an IonPac CS5A bifunctional ion-exchange column, which allowed a Polish group to separate three classes of Al species in water extracts of soil from the vicinity of an aluminium smelter. The first peak eluted corresponded to AlF2+ and/or AlF4−, the second to AlF2− and/or AlF3, and the third to Al3+. Researchers in Brazil220 used a Box-Behnken design to optimise a method for determination of AsIII (directly) and total As (following reduction) in slurries of phosphate fertilisers by HG-AAS. Speciation of inorganic Se in soil digests by ETAAS134 was achieved by workers in Iran. Their approach involved measurement of total Se, electrodeposition of SeIV on a mercury-coated electrode, and then measurement of SeVI in the spent electrolyte.
A recently available commercial background correction method for AAS–high speed self-reversal–has been evaluated221 for its ability to remove spectral interference from As in the determination of Cd. This occurs in As-rich soils because the weak As line at 228.812 nm is only 0.01 nm from a sensitive Cd line. The method not only overcame the As problem but also appeared to eliminate the need to add chemical modifier (Pd) in the determination of Cd in 0.01 mol L−1 CaCl2, a reagent often used to assess phytoavailability.
There has been interest in non-ICP excitation methods for atomic emission spectrometry. Work on tungsten coil atomic emission spectrometry has included the first demonstration of the measurement of In and V by this technique.146 Results obtained for Cr, Ga, In and V in nitric acid digests of NIST SRM 2711 Montana soil were statistically similar to certified values, although RSDs were at least 15% for all analytes and 36% for V. Limits of detection, based on summation of results at multiple wavelengths for each element, were 4, 80, 20 and 300 μg L−1 for Cr, Ga, In and V, respectively. An article in Japanese with English abstract224 described a simple, cheap approach for the measurement of Pb in soil based on liquid electrode plasma atomic emission spectrometry. The sample was extracted with water, then Pb was separated from other elements in the extract by SPE. A small amount of sample extract was placed in a narrow channel and a high voltage applied from both ends, generating a micro-plasma. A LOD of 1.3 mg L−1 was achieved and calibration was linear up to 800 mg L−1. A two-jet argon arc plasmatron225 has been used to study the effect of major elements K, Na, Ca, Mg and Fe on the determination of trace elements in natural samples including rock, soil and ashed plant material. Matrix interferences could be controlled by careful selection of operating conditions and use of reference samples.
The single-ring electrode radio-frequency capacitively coupled plasma torch previously used to measure Cd and reported in the previous Update 1 has now been applied successfully to the determination of Zn230 in soil, water and biological reference materials. The LOD was 8.2 μg L−1 and RSD values were < 10% (n = 10).
Ezer182 described the first application of continuous flow hydride generation-laser induced fluorescence (HG-LIF) for trace measurement of Pb. After optimisation of hydride generation conditions, the LOD was 0.3 μg L−1 using the 405.8 nm line and the dynamic range was 0.5–50 μg L−1. Results similar to those from ICP-AES were obtained for a multi-element solution, but not when the techniques were applied to an acid digest of a contaminated river sediment (316 mg kg−1 by HG-LIF vs. 501 mg kg−1 by ICP-AES at 220.4 nm and 735 mg kg−1 at 217.0 nm). Ezer attributed these discrepancies to spectral interference in ICP-AES caused by the unusually high levels of iron present in the sample, up to 245,000 mg kg−1.
The feasibility of using sample combustion and gold amalgamation in the determination of Hg by cold vapour-AFS was demonstrated for the first time by Cizdziel et al.231 They coupled together a cold vapour atomic fluorescence spectrometer and a combustion-based cold vapour-AAS mercury analyser in a configuration that allowed analyte from a single combustion event to be measured sequentially by both techniques. Results obtained by AAS and AFS for a range of environmental and biological samples were highly correlated (R2 = 0.998). However, the sensitivity was higher with AFS and the LOD lower (0.002 ng for AFS vs. 0.016 ng for AAS).
There is growing use of ICP-MS for measuring ratios of stable and long-lived isotopes. However, although the addition of oxygen gas to the collision/reaction interface166 allowed 129I to be measured in environmental samples from around a nuclear reprocessing plant, the method was not sensitive enough to measure 129I/127I ratios in relatively uncontaminated environmental samples. Sahoo et al.233 avoided this problem in their study of soils from Chernobyl by measuring 127I by ICP-MS but 129I by AMS. Current concentrations of 129I, together with knowledge of the 129I/131I ratio discharged by the damaged reactor, were used to calculate the amounts of the short-lived radionuclide 131I (t1/2 = 8 days) deposited in the vicinity of the plant in the period April-May 1986. This allowed the probable radiation dose to the local population arising from 131I to be estimated. Meanwhile, a Canadian research group234 has developed a method of measuring 107Ag/109Ag using MC-ICP-MS and performed the first study of isotopic fractionation of silver in a range of environmental CRMs. Silver was isolated from samples by a two-stage column procedure involving anion and cation-exchange resins. Mass discrimination and instrument drift were corrected using a combination of Pd internal standard and standard-sample-standard bracketing. Precision better than 0.015‰ (2 σ) was obtained which is adequate for measuring δ107Ag/109Ag fractionation in natural samples. There is also continuing interest in isotope ratio measurements by MC-ICP-MS as a tool to improve understanding of the biogeochemical cycling of Hg.235
Two detailed studies focus on the optimisation of collision/dynamic reaction cell conditions in the measurement of Se. Pick et al.236 used 1.1 mL min−1 CH4 as a reaction gas to reduce polyatomic interference from 40As40As on 80Se in the analysis of HNO3/H2O2 digests of plants and animal feedstuffs. Results were comparable to those obtained by both collision cell ICP-QMS and SF-ICP-MS; the latter measured 77Se since it could not resolve 80Se from the interferent. Floor et al.237 applied a collision cell in the analysis of volcanic soils. Best results were obtained by the addition of ca. 2% methanol to the sample digests and quantifying 78Se using a collision/reaction cell pressurised with hydrogen.
Brazilian workers238 optimised a method for measuring Dy, Eu, Sm, Te and Yb in soil and sediment by slurry sampling ETV-ICP-MS. Powdered sample (50 mg) ground to < 50 μm particle size was sonicated in a mixture of 0.8 mL HNO3, 0.4 mL HF and 0.4 mL H2O2, then introduced to the instrument along with NaCl carrier. Results for four CRMs were within the certified ranges.
The use of chromatographic methods with ICP-MS remains of major interest, with applications becoming more widespread and routine. Interesting examples from the past year include the measurement of organophosphates in sediment239 by gas chromatography ICP-MS, and a rapid automated method for the separation of Pu240 from potential interferent Hg, Pb, Th and U by sequential injection extraction chromatography using TEVA® sorbent followed by ICP-MS. The procedure, which was validated by the analysis of CRMs IAEA 375 Soil and IAEA 135 Sediment, proved capable of handling the large samples–up to 200 g of soil, 20 g of seaweed, or 200 L of water–typically required in environmental radiochemical analysis. Size exclusion chromatography ICP-MS was used to investigate the speciation of Fe and Zn in barley241 as part of a study focused on the growing worldwide problem of human trace element deficiency. Phosphorus and S were also measured, as 47PO+ and 48SO+ ions after addition of oxygen to the octapole reaction cell, because they are major constituents of phytic acid and protein, respectively. Most of the Fe co-eluted with P and most of the Zn with S suggesting that, in cereal grains, Fe is mainly associated with phytic acid and Zn with peptides. Lombi et al.242 also found evidence of Fe phytate in their analysis of rice grains by HPLC-ICP-MS and X-ray techniques, and of an association between Zn and S consistent with metal binding to phytochelatins or metallothionein. A HPLC-ICP-MS method,243 employing a mixed mode (anion-exchange with reversed phase) column, provided baseline separation of selenocystine, Se-methyl-selenocysteine, selenomethionine, methylseleninic acid, selenite, γ-glutamyl-methyl-selenocysteine and selenate in standard solutions. Selenium-methyl-selenocystiene, selenomethionine and selenate were identified in extracts of watercress grown in Se-enriched soil. The identities of the two organic species were confirmed by on-line HPLC-ESI-MS-MS.
Several developments focusing on the measurement and application of isotopes of plutonium were presented at the 11th International Conference on AMS (Rome, September 2008). Chamizo et al.245 reported that 240Pu/239Pu ratios can now be measured on the compact AMS system at the Centro Nacional de Acceleradores in Seville and presented sediment core data that confirmed weapon-grade Pu, released during an accident on land, had been transported into the marine environment. Bisinger et al.246 applied a combination of AMS and alpha particle spectrometry to samples of soil, moss and sediment. The AMS technique was used to measure the β− emitter 241Pu, and the α-emitters 239Pu and 240Pu, which cannot be distinguished by alpha spectrometry because of their similar decay energies. The 238Pu, which suffers interference from 238U in AMS, was measured by alpha spectrometry. This approach allowed Pu released from Chernobyl to be distinguished from Pu released by other sources, Tims et al.247,248 proposed that Pu isotopes derived from atmospheric weapons testing could prove a valuable alternative to the more commonly used 137Cs as a tracer for soil erosion and sediment accumulation in major world rivers.
Numerous element specific LIBS methods for soil analysis have been reported. Double-pulse LIBS and normalisation of results to the Fe emission line at 248.814 nm successfully increased signal-to-noise ratio in the measurement of As252 in mine tailing soils. A screening method for Cd253 used soil and sediment CRMs for calibration and achieved a LOD of 1.3 mg kg−1. Similar LOD values were reported for Cd and Zn254 by Indian researchers using spiked soil calibrants and Si as internal standard. The potential of LIBS to measure U in soils and on surfaces as an aid in the detection of nuclear material associated with weapons of mass destruction has been assessed.255 The LOD in soil measured at a stand-off distance of around 75 cm was 0.5% w/w. Articles in Chinese with English abstracts describe further LIBS studies focusing on Cu,256 Cr and Sr257 and Pb.258
The determination of total carbon content in soil by LIBS has attracted interest. Two different ablation/excitation modes have been compared:259 a double laser pulse vs. the superposition of a single laser pulse and a spark discharge. Measurements were made at 247.86 nm in the first case, but at 833.52 nm in the second to avoid spectral overlap with W lines from the electrodes. Both methods produced curved calibration graphs. However, results for topsoils with C contents ranging from approximately 1 to 8% were broadly similar to those from a commercial CHNOS analyser. Martin et al.260 obtained linear correlations between emission at 247.85 nm and soil C content, but these were dependent on soil type. A multivariate approach–construction of a robust calibration model based on several C emission lines–helped overcome this problem, but study of a broader range of soils was recommended.
Several workers have sought to optimise experimental or calibration procedures for application of LIBS to plants. A novel chemometric approach261 has been used to obtain the best compromise working conditions for the simultaneous measurement of macronutrients and micronutrients in plants, with NIST SRM 1570a Spinach leaves as the test sample. Some of the same researchers compared univariate and multivariate calibration262 in the measurement of B, C, Fe, Mn and Zn in plant CRM and samples. They found that the development of a specific calibration model for each analyte, based on partial least squares regression and only the spectral regions containing analyte emission lines, was preferred. Meanwhile, workers interested in the development of LIBS methods that do not require external calibration studied the fundamental characteristics of the ablation plasma generated from the skin of potato.263 The temporal evolution in electron density and temperature showed that local thermal equilibrium (LTE)—a pre-requisite for calibration-free LIBS—occured only if high ablation energy (20 mJ) was used and sufficient time (600 ns) allowed after impact of the laser pulse for LTE to be established.
An attempt to develop a high-throughput LIBS method for the routine analysis of crop plants264 was partially successful. The method generally produced results within 10% of values obtained by AAS or ICP-AES for Ca, K, Mg and P when applied to samples of rape and wheat, which are chemically similar to the barley and poppy leaves used for calibration. But poorer performance was obtained when the method was applied to a range of CRM representing widely different plant types and tissues, such as GBW 07603 Bush branches and NIST SRM 1575 Pine needles.
Portable X-ray fluorescence (PXRF) spectrometry and laboratory based systems have been compared. Radu and Diamond268 studied the highly-contaminated Silvermines site in Ireland and obtained satisfactory correlations between data from the portable instrument and results from AAS analysis of aqua regia digests of the same 17 soils (R2 values of 0.991, 0.959, 0.995 and 0.843 for As, Cu, Pb and Zn, respectively). Spectra of a variety of CRMs and cultural heritage objects269 obtained by PXRF were compared with spectra from a laboratory instrument. Results suggested that, despite poorer limits of detection, PXRF could be useful for analysing artefacts that cannot be moved because of their size, weight or value. The ability to carry out multi-element analysis, on both solid and liquid samples, at remote locations, using a single portable instrument system, was demonstrated by an Australian research group.164 They used Amberlite IRC748 cation-exchange resin to develop an in-field preconcentration technique that allowed PXRF to quantify Cu, Ni, Pb and Zn in water samples at concentrations as low as 20 μg L−1. Analytes were collected from 1 L of water buffered to pH 5.5 with 2 mol L−1 ammonium acetate, then eluted in 15 mL of 3 mol L−1 HNO3. Results obtained were generally within ± 20% of ICP-MS data except where concentrations approached the PXRF limits of detection.
High resolution micro-PIXE images of leaves from the Ni hyperaccumulator Hybanthus floribundus (Violaceae)270 were obtained following optimisation of the ion beam of the microprobe at the Australian Nuclear Science Technology Organisation. Using an ion source current of almost 1 nA and a 3 μm spot, sub-cellular resolution was achieved. The study showed that both Ca and Ni are accumulated in the cell walls of the plant epidermis.
A rapid X-ray absorption near edge spectrometry (XANES) method has been developed and applied successfully to map Pu species271 in a ‘hot’ particle from a nuclear test site in the Pacific Ocean. Only PuIV species were found. Haverkamp and Marshall272 used XANES to measure Ag in Brassica juncea (Indian Mustard) grown hydroponically, and to follow the reduction of AgI to Ag0. They discovered that there is a limit to the amount of Ag that the plant can store as metal nanoparticles (ca. 0.35% dry weight) with the remainder accumulated as salt. In other work, difficulties in distinguishing between AsIII-S and AsIII-O species in plants273 based on XANES spectra alone, were overcome by parallel application of EXAFS to the samples. The combination of information from X-ray absorption spectrometry and chromatography coupled to mass spectrometry, for speciation analysis in environmental samples, has been reviewed116 (91 references).
3.3 Reference materials
Several studies have reported new information on isotopic fractionation or speciation of analytes in CRMs. The Hg isotopic composition274 of two new reference materials and nine established CRMs has been measured. The CRMs include soils, sediment, jasperoid, ore deposit, fly ashes and lichen and have δ202Hg values from −1.75 to +0.11‰. Scientists from the European Commission's Institution for Reference Materials and Measurements275 have reported not only 240Pu/239Pu but also, for the first time, 241Pu/239Pu and 242Pu/239Pu isotope ratios in IAEA 135 Irish Sea sediment, IAEA 368 Pacific Ocean sediment, IAEA Soil 6 and SRM 4354 Freshwater lake sediment. Measurements were made by TIMS with multiple ion counting. The 87Sr/86Sr isotope ratios276 in soil CRMs BCR 141 Calcareous loam soil and BCR 142 Light sandy soil have been determined by MC-ICP-MS as part of wider assessment of the suitability of a commercial resin for isolation of Sr from Ca-rich samples. Mandiwana and Panichev277 determined VV in 0.1 mol L−1 Na2CO3 extracts of five soil CRMs certified for total V content by ETAAS, finding concentrations ranging from 3.6 to 86 μg g−1 equating to 4 to 88% of the certified values.4 Analysis of geological materials
4.1 Reference materials
Developments in analytical techniques have fuelled a demand for reference materials that are characterised as comprehensively as possible. Values for Br, Cl, F and I in six igneous rock RMs (including BHVO-2 from the USGS and JB-2, JR-1 JG-1 and JG-1b from the GSJ) were obtained by combining extraction by pyrohydrolysis with detection by IC and ICP-MS.300 Scrutiny of the data and associated errors indicated that the nature of the geological material had a greater influence on the data distribution than the absolute halogen concentration. It was concluded that basalts are more suitable as RMs for halogens, even though their halogen contents are lower than those of granites and rhyolites, where halogens can be heterogeneously distributed in 0.5 g sub-samples. Selenium is difficult to determine in silicate rocks because the aggressive reagents employed to dissolve the rock can lead to loss of Se by volatilisation, even at relatively low temperatures. For this reason, Savard et al.301 employed INAA after preconcentration on thiol-cotton fibre to estimate the Se content of 26 geological RMs. They proposed consensus values for seven of the RMs and gave suggested or information Se values for the rest, supported by a literature compilation of Se values for all these RMs. In an alternative approach, Forrest and colleagues302 used HG-ICP-MS and ID to obtain Se and Te data for basalt RMs. Method LODs were 0.01 ng g−1 for Se and 0.003 ng g−1 for Te, well below the concentrations expected in basalts, but more work is required to confirm the accuracy of their data. As part of a study of the geochemical behaviour of W, Babechuk and co-workers303 have published their long-term data for a number of USGS and GSJ RMs. They concluded that some of the materials are heterogeneous and contaminated with respect to W. Although strictly outside the remit of this Update, the reader's attention is drawn to the simultaneous determination of C, H, N and S in 27 geochemical RMs using an automated elemental analyser.304The preparation and characterisation of five new sediment RMs from the China Sea and continental shelf has been reported.305 These are in the form of ultra-fine powders with an average particle size of <4 μm. Twelve laboratories participated in the co-operative study and recommended values for over 50 analytes were assigned in compliance with Chinese national norms.
With an increasing requirement for RMs with certifications that comply, as far as possible, with metrological standard ISO Guide 35, two CRMs initially prepared and certified by the Central Geological Laboratory of Mongolia have been recertified.306 Serpentinite GAS and alkaline granite OShBO were recertified under the direction of the International Association of Geoanalysts. While it was possible to increase the number of elements certified for OShBO from 21 to 30, the greater analytical difficulties posed by the serpentinite matrix in meeting the more stringent metrological requirements resulted in 12 rather than 15 elements being certified for GAS. Uncertainties for these values were established in accordance with international protocols and traceability of the certified values was demonstrated by concurrent analyses of existing matrix-matched CRMs by all participating laboratories.
New analytical data on reference glasses and other RMs suitable for the calibration of LA-ICP-MS are available. Hu et al.307 reported values for Ag, As, Au, B, Be, Bi, Cd, Ge, In, Ir, Mo, Pd, Pt, Re, Rh, Sb, Sn, Tl and W in MPI-DING, USGS silicate glasses and the NIST 610 series of glasses by 193 nm ArF excimer LA-ICP-MS. Small amounts of nitrogen were added to the central channel of the plasma to improve the LODs and reduce oxide interferences, resulting in LODs between 0.1 and 10 ng g−1. In general, for most elements, any chemical heterogeneities were smaller than the analytical uncertainty. A combination of EPMA, LA-ICP-MS and SHRIMP was used to determine Se in NIST SRMs 610, 612, 614 and a range of glass geological RMs.308 Initially, the Se content of NIST SRM 610 was verified by EPMA and SHRIMP, and used to constrain the Se concentration in NIST SRM 612. The latter was then employed as an external calibrant in the determination of Se in the other geological materials by LA-ICP-MS. After a detailed examination of potential interferences from geological matrices, low uncertainties for in situ analysis were obtained by measuring 77Se and correcting for 154Sm2+ and 154Gd2+. Unfortunately, there is virtually no overlap in the materials analysed in this study and those measured by INAA for Se.301 Two new USGS carbonate RMs, MACS-1 and MACS-2, doped with trace elements at various concentrations, were analysed by ICP-MS after dissolution and by LA-ICP-MS.309 Differences of <10% relative between the solution and laser data were obtained for all elements except Cu and Zn in MACS-1 and Co, Cr and Zn in MACS-2, which has a lower trace element content. Good agreement was observed between LA-ICP-MS calibrations based on the MACS RMs and NIST SRM 612 glass when analysing landfill calcites. Although these carbonate RMs were promising, the NIST glass was considered to be easier to use and had the advantage of containing all the elements of interest. However, the production of other carbonate RMs was thought to be highly desirable. Two candidate RMs, spinel lherzolite LSHC-1 and amphibole Amf-1, are being developed at the Institute of Geochemistry in Irkutsk.310 Indicative values for REE, Hf, Nb, Ta, Th, U, Y and Zr obtained by solution and LA SF-ICP-MS, together with INAA and XRF data, have been published. High resolution mode was required to overcome interferences for some elements in the solution work, whereas all elements could be measured at low mass resolution during LA-ICP-MS because of low oxide formation in this mode.
There is a continuing requirement for well characterised isotopic standards. Amini and co-workers311 reported δ44Ca/40Ca values (relative to NIST SRM 915a) for various igneous and metamorphic rocks, including several international RMs. The MPI-DING glasses were shown to be isotopically homogeneous at the millimetre scale and it was concluded that they were suitable RMs for Ca isotope measurements by LA-ICP-MS or ion microprobe. Molybdenum isotope ratios are typically reported relative to an in-house laboratory RM as there is no internationally accepted standard RM for Mo isotopes. Consequently, Wen et al.312 used MC-ICP-MS to investigate the Mo isotopic composition of six synthetic reference solutions. These were found to have identical Mo isotopic values, despite their different chemical compositions. The authors proposed that NIST SRM 3134 Mo solution should be adopted as the delta zero RM when reporting the Mo isotopic composition of natural samples. A stream sediment, RM JSd-1 issued by the GSJ, has been characterised for 50 major, minor and trace elements, plus Hf, Nd, Pb and Sr isotope ratios.313 The original JSd-1 powder displayed heterogeneity for Group II elements (Hf, Mo, Nb, Sb, Sn, Ta and Zr); this problem was eliminated on regrinding the material. The emergence of new analytical methods for measuring Hg isotopic compositions has accentuated the need for a wide range of RMs to validate and assure the accuracy of such data. Estrade and co-workers274 have reported the Hg isotopic compositions of nine RMs, including soils, ores and sediments using cold vapour MC-ICP-MS. In addition, two solutions with δ202Hg values at the extremities of the range of natural Hg variations were proposed as secondary RMs and are available from the CRPG.
4.2 Solid sample introduction
The influence of sample matrix on laser ablation characteristics is still the subject of much research. Gaboardi and Humayun317 examined elemental fractionation during laser ablation of a range of silicate glass standards at 213 nm. They observed that the transparency of the material was a significant factor within the NIST SRM 61X series. In contrast, all the non-transparent glasses analysed (MPI-DING and USGS glasses BHVO-2G and BCR-2G) showed no matrix-dependent fractionation and could be employed interchangeably for calibration. For many elements in non-transparent materials, the greatest obstacle in the accuracy of measurement by LA-ICP-MS was the uncertainty in the reported RM concentrations. They concluded that calibration with transparent standards should be avoided unless an element with a similar temperature of condensation to the analytes was available for normalisation.
In a study of the fractionation of alkali elements during laser ablation at 213 nm, LA-ICP-MS and SIMS were used to determine differences in sample composition before and after laser interaction with various silicate reference glasses.318 Trends in fractionation of alkali elements were found to be different from those of other lithophile elements such as Ca and REEs. The rate of fractionation varied between different sample matrices and between different alkali elements in the same matrix. A combination of thermally-driven diffusion and size-dependent particle fractionation was thought to be responsible for these observations. The significance of this research is that elements such as Si and Ca, which are often used to correct for ablation yield in quantitative analysis of silicate rocks by LA-ICP-MS, are strongly influenced by thermal effects adjacent to the LA craters. Because these effects are very different for Ca and Si, for example, the choice of internal standard will affect the precision and accuracy of analysis.
Fundamental studies of femtosecond lasers have demonstrated reduced elemental fractionation compared to ns laser ablation, mainly because of the much smaller amount of heat transfer into material around the ablation crater. Research into the structural changes resulting from infrared (800 nm) fs LA in monazite by Seydoux-Guillaume and co-workers319 highlighted the intense damage induced by the high-pressure shock wave associated with a fs laser pulse. They showed that these mechanical effects largely dominated the thermally-induced ones and were limited to a thin layer (200 nm) of resolidified nanocrystalline monazite. Although in the fs LA regime most of the energy goes into producing mechanical defects, it is still necessary to understand the cause of the remaining chemical fractionation observed. Hence investigations by Claverie et al.320 on the influence of high repetition rates on elemental fractionation in silicate glass NIST SRM 610 in IR fs LA-ICP-MS are relevant. Working with wet plasma conditions to minimise fractionation within the ICP, a special arrangement of pulses was used to produce craters 100 μm in diameter. Two scanning speeds, five laser repetition rates and three fluence values were evaluated. Fractionation related to particle size was observed by monitoring 238U/232Th ratios. This was minimised by using high repetition rates and low scanning speeds, both of which had the effect of diluting the large particles ejected from the sample surface with a larger number of smaller particles originating from deeper levels in the sample. High repetition rates had little influence on elemental fractionation whereas the fluence value was a major factor, with high fluence values resulting in lower fractionation. However, SEM measurements revealed no significant differences in the particle size and structure of the laser-generated particles whatever the ablation conditions. Short particle washout times from ablation cells are particularly desirable for applications such as 2-D imaging or depth profiling. With this in mind, Lindner et al.321 describe their optimised cell geometry, based on computational fluid dynamics, for use with a fs laser operating at 795 nm at high repetition rates. Washout times of 140 ms were achieved, allowing single-pulse analysis with a 7 Hz laser repetition rate.
Recent reports of the use of fs LA in geochemical applications have been dominated by the measurement of Fe isotopes. Steinhoefel and co-workers322 verified the accuracy and reproducibility of Fe isotope data in a range of iron siliceous matrices obtained by UV fs LA-MC-ICP-MS at high spatial resolution using non-matrix-matched calibration. Careful consideration of corrections for Cr isobaric interferences was necessary for samples with high Cr contents. Related studies included the measurement of Fe and Si isotope signatures in banded iron formations323 and the use of the Fe isotopic composition of individual pyrite grains in Precambrian shallow marine carbonates as a proxy for redox conditions in ancient seawater.324
Several developments in the measurement of Pb isotopes by LA-ICP-MS have been reported. Analytical problems unique to the determination of Pb isotopes in pyrite have been addressed by Woodhead et al.413 Technical issues included the low melting point of sulfide, highly variable and often high Pb contents, and the potential presence of relatively radiogenic inclusions. They concluded that controlled ablation of pyrite can be achieved only at low laser fluence and that, under these conditions, calibration with natural pyrite standards is preferred. Where pyrites contain hundreds to thousands of ppm of Pb, MC-ICP-MS can provide Pb isotope data of a comparable quality to that of TIMS. However, quadrupole ICP-MS was utilised where Pb concentrations were inappropriate for Faraday cup detection or where age corrections for radiogenic ingrowth required simultaneous measurement of Pb, Th and U. In such cases, pooling of data from multiple spot analyses could improve the analytical precision markedly but needed to be undertaken with care. At the other end of the scale, Souders and Sylvester325 evaluated the extent to which matrix matching is necessary for Pb isotope measurements of feldspar and sulfide minerals with low Pb contents (<70 μg g−1 Pb). The analytical methodology involved a 193 nm excimer laser coupled to MC-ICP-MS and standard-sample-standard bracketing to correct for mass discrimination. Somewhat to their surprise, acceptable results were obtained when feldspars containing <40 μg g−1 Pb were analysed against silicate glass standards, in spite of the large differences in the physical and chemical responses of the feldspar minerals and silicate glasses to the LA conditions used in the study. Similarly, for sulfides that contained little or no mercury, the mean Pb isotope ratios obtained were within 0.40% of the average TIMS measurements made on the same grains, with only subtle differences in the results between matrix-matched (sulfide) and non-matrix-matched (glass) standards. However, for samples containing significant amounts of mercury, external calibration standards that are matrix-matched in terms of their Hg/Pb ratios are required to correct properly for the isobaric interference of 204Hg on 204Pb.
A novel approach to laser ablation U-Pb geochronology is described by Cottle and co-workers.326 Their method aims to acquire accurate isotopic data from single laser pulses while minimising sample destruction and maximising the spatial resolution of the analyses. Data were obtained using an in-house low volume ablation cell that facilitates the production of a high density particle stream with short sample washout of ca. 0.5 s. Isotope ratios from an individual laser pulse were calculated by integrating the total number of counts for the entire pulse after subtracting the baseline and assigning an uncertainty based on counting statistics. This method eliminates the effects of differing detector response times in MC-ICP-MS and provides an alternative way of quantifying transient signals. Results indicated that sample consumption could be reduced by as much as 90% without significant loss of precision, with a depth resolution of about 0.1 μm per pulse. Hence this technique has the potential to generate accurate age information from zoned and complex accessory minerals.
Patterns of fractionation with crater depth can vary markedly between laboratories and with changes in operating conditions. Thus an improved method of correcting for downhole fractionation effects observed when acquiring data for U-Pb geochronology by LA-ICP-MS has been proposed by Paton et al.327 They suggest a general approach in which users develop an appropriate model of downhole fractionation based upon their own data acquired during each analytical session, rather than attempting to fit the data to a preconceived fractionation model. This methodology is capable of producing high quality ages that are accurate to within 1% of accepted values and allows analytical uncertainties to be estimated using a single reference standard in a manner that best reflects the actual uncertainties of individual spot analyses. A data reduction module that implements this protocol is available as part of a software package designed for the rapid reduction of large quantities of data with maximum feedback to the user at each stage.
The feasibility of determining Li isotope ratios in natural and synthetic glasses by LA-MC-ICP-MS has been demonstrated.328 Because of the large relative difference in abundance between the two Li isotopes, asymmetric tuning of the quadrupole lenses was required for simultaneous measurement of 7Li and 6Li on a Faraday–ion counter configuration. The NIST series of standard glasses proved to be unsuitable for external calibration. After evaluation of USGS and MPI-DING standard glasses, USGS BCR-2G was selected as the bracketing standard by reason of its moderate Li content and isotopic composition. This protocol yielded accurate Li isotope data with an external δ7Li precision (2σ) of <1‰ at Li concentrations of 3–35 ppm.
Although in situ measurements of Sr isotopes are routinely made by LA-ICP-MS, most materials studied have high Sr contents and low Rb/Sr ratios. Samples with low Rb/Sr ratios and young ages are still analysed by TIMS after separation because of the requirement for high precision data. Jochum and co-workers329 have demonstrated the potential of analysing silicates with low Sr concentrations of 30 to 400 μg g−1 using single collector SF-ICP-MS combined with a 193 nm Nd:YAG LA system. Corrections made in the determination of 87Sr/86Sr included dead time of the ion counting system, blanks, isobaric interferences of Kr and Rb, and mass discrimination for Sr and Rb. An external precision (1 SD) of about 0.0002–0.0004 was achieved for Sr contents of 100–400 μg g−1 and spot sizes of 50 μm. A comparison of LA-ICP-MS data obtained for glass RMs agreed with high precision TIMS data within the limits of uncertainty. Mainly because of isobaric interferences from 87Rb, the technique is restricted to samples with Rb/Sr ratios of ca. <0.1.
The ongoing development of portable LIBS instruments for geochemical fingerprinting by the US military has been described.333–335 The focus of these studies is the utilisation of full broadband LIBS spectra between 200 and 965 nm for rapid material discrimination and identification in the field. Harmon et al.333 discuss the particular problems associated with using LIBS to identify minerals and to differentiate between minerals with similar structures with a high degree of accuracy. By comparing a single LIBS spectrum against a library of LIBS spectra a high degree of correct classification was achieved for carbonates, feldspars and pyroxenes. However, it proved more difficult to correctly identify different varieties of beryl, which is representative of a group of minerals with a fixed major element composition and a high degree of trace element variability. These experiments were performed with a bench-top instrument, as were the ones reported by Alvey and co-workers335 who were able to discriminate between garnets of different composition using similar methodology. In another study, the performances of a commercial single-pulse LIBS system, a laboratory bench-top double pulse LIBS system and a prototype stand-off LIBS system designed for analysis at a distance of 25 m were compared.334 The chemometric techniques of partial least squares discriminant analysis (PLS-DA) and PCA were applied to the spectra to classify the materials and to identify the distinguishing characteristics of a large suite of carbonate, fluorite and silicate geological samples. The laboratory double-pulse system did not provide any advantage for sample classification over the single-pulse system, except for soil samples. The stand-off LIBS system provided comparable results to the laboratory systems, demonstrating its potential for its intended application of geochemical fingerprinting. The work also showed how PCA can be used to identify spectral differences between similar sample types based on minor impurities.
4.3 Sample treatment
New digestion protocols suitable for geological materials continue to be published, although it is often difficult to detect much of any novelty. Makishima and colleagues338 lament the lack of appreciation by geochemistry students of the behaviour of different elements during sample decomposition using HF. They provide an overview of the problems involved and precautions required. A microwave-assisted digestion procedure with HNO3–H2O2–HF (2 + 1 + 1) in high-pressure vessels for the determination of As in mining residues from gold beneficiation plants by FAAS was reported to be fast and accurate.339 However, its accuracy could only be evaluated by standard addition, there being no suitable CRM.
The use of ultrasound energy to assist the extraction of analytes from solid samples has been advocated as a safe and relatively cheap alternative to microwave-assisted procedures.191,340 De Vallejuelo et al.191 describe the use of an ultrasonic probe in a procedure involving various HNO3–HCl mixtures for leaching trace elements from sediments. The method was capable of extracting 13 elements in 6 mins with similar recoveries to those obtained using the microwave-assisted EPA 3051 method, which is based on extraction with HNO3 only. One drawback of the use of the probe was that, although the procedure is rapid, the samples had to be treated individually. In contrast, Ilander and Vaisanen340 employed an ultrasonic water bath, which allowed them to process 30 fly ash samples simultaneously in 18 mins prior to the determination of Cr, Cu, Ni Pb, V and Zn by ICP-AES.340 Variations included a one-step protocol employing aqua regia and HF (1 + 1) and a two-step procedure, in which samples are first leached with 0.1 M HNO3 and then a HNO3–HF mixture (1 + 1). Data from the analysis of NIST SRM 1633b (coal fly ash) enabled the authors to claim that this was the first time a digestion method using ultrasound had achieved a greater extraction efficiency for Cr and Ni than an equivalent EPA microwave-based method.
All sample pre-treatment must be appropriate for the specific geochemical application to which it relates. Revillon and Hureau-Maxaudier341 revisited methods for the complete digestion of sediments, including refractory phases such as zircons. Alkali fusion with NaOH–Na2O2 followed by preconcentration with Fe(OH)3–Ti(OH)4 was ruled out because not all the elements of interest were fully recovered and the blanks were too high for measurements of Sr and Pb isotope ratios. Of the four acid digestion procedures they tested, their preferred option was a combination of HF, HNO3 and HClO4 acids in high pressure digestion vessels for 7 days at 160 °C. Complete dissolution and very low blanks were achieved, making it suitable for a broad range of analysis, including isotopic characterisation. In contrast, Townsend and co-workers342 recommended partial extraction with 1 M HCl in a study of Pb isotopic signatures in Antarctic marine sediment cores, in preference to total dissolution with HF. This partial extraction offered the greatest discrimination between contaminated and natural samples as over 90% of the easily extractable Pb was anthropogenic.
Simple methods for the determination of gold and other precious metals continue to attract interest. One of the more novel approaches involved the use of a microcolumn packed with powdered leaves from the Azadirachta Indica tree for the separation and preconcentration of Au and Pd in geological materials prior to their determination by ICP-AES.346 Disappointingly, data for only one RM were reported. El-Naggar and co-workers347 employed brilliant green as the complexing agent in cloud point extraction for selective preconcentration of trace amounts of Au in silicate rocks. Brilliant green is a basic water soluble dye, which has the advantage of being stable in acidic solution. Problems with the introduction of the highly viscous surfactant-rich phase into a FAAS instrument were circumvented by dilution with methanol. Acceptable results were obtained for three CRMs with Au contents between 500 and 5000 ng g−1.
4.4 Instrumental analysis
Dittert et al.210 determined Ag in geological samples using direct solid sampling and high resolution continuum source ETAAS. For soils and sediments, good accuracy was achieved with calibration against aqueous standards. However, the analysis of rocks and ores required calibration against a CRM of similar composition, as the sulfur-rich matrices reduced the atomisation efficiency. An optimised atomisation temperature of 2300 °C and the application of a least squares background correction algorithm resulted in a background-free spectrum. A LOD of 2 ng g−1 was obtained with RSDs of typically <15%.
In a novel application of AAS for speciation analysis, Campos and co-workers352 interfaced a commercially available Hg analyser based on CV-AAS with HPLC separation for the determination of total inorganic Hg and MeHg in sediments. The detector contained a high intensity source, a multipath absorption cell and a system for Zeeman-effect background correction. Post-column CVG was performed with a UV lamp to decompose MeHg and reduction with sodium borohydride to generate Hg0. For a 200 μL injection volume, the LOD was 50 ng L−1 for both species.
A relatively inexpensive method for the determination of REE in uraninite samples by ICP-AES, based on solid phase extraction on activated carbon, has been developed.355 After digestion with a mixture of HNO3, HF and HClO4, the solution was neutralised with NH4OH and then excess NH4OH added to precipitate U, Th and REEs as hydroxides. After washing and filtration, the precipitate was dissolved in HF and activated carbon added. The slurry was then filtered and the filter paper ignited to remove the activated carbon before dissolution in 10% HNO3 and dilution prior to analysis. Uranium was almost entirely eliminated in this procedure (<10 μg mL−1 in the final solution) but judicious selection of the REE emission lines was essential to minimise interference from the relatively large concentrations of Th.
A novel method of determining bioavailable concentrations of As and Se in sediments by HG-ICP-AES employed a commercial ‘concomitant metals analyser’ as a hydride generator.356 This device is a modified cyclonic chamber developed to measure elements that form hydrides alongside those that do not. Multivariate techniques were employed in the optimisation of the procedure. It was found that interferences from transition metals such as cobalt, copper, iron and nickel could be minimised by using highly concentrated HCl solutions (6 mol L−1). The bioavailable content of estuarine sediments was assessed using a microwave-assisted extraction with concentrated HNO3 at 175 °C and LODs of 0.025 μg g−1 for As and 0.030 μg g−1 for Se were obtained.
4.4.3.1 Quadrupole ICP-MS. Many people involved with the operation and application of modern ICP-MS instruments are not necessarily specialists in this field, as was usually the case in the early days of the technique. A recent review (124 refs) of quadrupole ICP-MS,357 written with the novice user in mind, is valuable in reminding all practitioners of the strengths and limitations of the technique, particularly now it is being applied to a vast range of sample types. Because ICP-MS has the sensitivity to determine trace elements at concentrations of ng L−1 and below, one of the constraints in realising the best possible detection limits is contamination in the laboratory. Rodushkin et al.35 reviewed (43 refs) potential sources of contamination during sample preparation and instrumental analysis and proposed remedial strategies for ameliorating their impact on the results of multi-elemental analysis by ICP-MS. Papers on ETV-ICP-MS published in the last 10 years (186 refs) have been reviewed by Aramendía and co-workers.358 From this evidence, it would appear that one of the main reasons why ETV has not been widely adopted as a sample introduction technique for ICP-MS is the lack of interest shown by the main manufacturers of analytical instrumentation in providing suitable instrumental systems and the associated methodological support. Wider acceptance is also hindered by the perception that it is a difficult technique to work with, together with the lack of well-defined standard conditions for analysis. This review seeks to provide the reader with a better understanding of ETV-ICP-MS and proposes a guide for method development for challenging analytical applications based on existing literature. Advantages of coupling an ETV unit to newer types of ICP-MS instrumentation equipped with collision/reaction cells, TOF or SF spectrometers are also discussed. The contribution of quadrupole ICP-MS to the advancement of geochemical fingerprinting and its application in the earth and environmental sciences over several decades has also been reviewed (81 refs).359
The widespread availability of reaction cells in ICP-MS instrumentation has facilitated the determination of Se and other elements that suffer from spectral interferences from argon-based polyatomic ions. In a study of the Se content of volcanic soils, Floor et al.237 recommended the addition of 2% methanol to the nitric acid extractions and the use of hydrogen as the cell gas while making measurements at 78Se. Makishima and Nakamura360 developed an ID-ICP-MS method for the determination of As, Ge, Se and Te in silicate samples, based on their finding that the use of helium as a reaction gas suppressed the formation of iron oxides, and Rb and Sr argide ions. A Ge-Se-Te spike was added during digestion with a HF-HNO3-HBr mixture, before the sample was dried and re-dissolved with HF. The supernatant was directly aspirated into the ICP-MS instrument, using helium as the reaction gas for As, Ge and Te, and hydrogen for Se. Interferences were assessed and corrections applied where necessary. Limits of detection were 4, 2, 1 and 0.1 ng g−1 for As, Ge, Se and Te respectively. Advantages of this method included the capability to analyse very small amounts of material, as only 0.13 mg was required for a test portion, and the same solution could be used to determine Hf, Mo, Nb, S, Sb, Sn, Ta, Ti and Zr.
Quadrupole ICP-MS is capable of providing sufficiently precise isotopic data for a wide range of applications, at comparatively low cost. The challenge of measuring Li isotope ratios in natural carbonates was tackled by separating Li from the matrix elements on AG 50W-X8 cation-exchange resin and eluting with 6 ml of 0.5 N HCl.177 Measurements of 6Li/7Li were made by ICP-QMS under cool plasma conditions (600 W) to minimise interferences from 12C2+ and 14N2+, using soft extraction, peak jumping and pulse detection mode combined with sample-standard bracketing. The method was optimised for natural carbonates containing 1–2 ppm of Li and a precision (2σ) of better than ±1.5‰ was achieved for this type of sample. Marelli et al.361 proposed an alternative protocol for measuring 63Cu/65Cu ratios by ICP-QMS. Their modified bracketing approach incorporated regular monitoring of a blank solution, which was used to compensate for long-term drift in signal sensitivity. Accurate ratios with RSDs as low as 0.025% (1s) were reported, with a faster throughput than normally achieved with a typical standard bracketing protocol. This precision was sufficient to discriminate between copper ores. It is worth noting here a parallel paper on the application of Cu isotope fractionation in mineral exploration of porphyry copper deposits based on MC-ICP-MS.362 Although magnetic sector mass spectrometers dominate the field of 40Ar/39Ar geochronology, Schneider and colleagues363 have demonstrated that a triple filter quadrupole mass spectrometer can be an economic alternative for 40Ar/39Ar dating when small sample sizes and very high precision measurements are not the first priority. The instrument was connected to a furnace extraction system designed for the incremental heating of large (500 mg) samples of young volcanic rocks. The system produced stable flat-topped peaks with good resolution and was capable of measuring Ar ratios with precisions in the 1‰ range. Rapid and accurate U-Th dating of ancient carbonates by ICP-QMS has been explored by Douville et al.364 The analytical procedure included a simplified chemical separation using Eichrom UTEVA® resin, which resulted in rapid extraction of Th and U from 100–1000 mg of carbonate on a micro-column. During the simultaneous measurement of Th and U isotopes, the count rate was maintained below 2 Mcps for each isotope in order to maintain data acquisition in pulse counting mode. Up to 50 U-Th dates could be measured per day by ICP-QMS with reproducibilities (2σ) of 3–4‰ for δ234U and 1‰ for δ230Th. Sensitivities of greater than 3 × 105 cps/ppb combined with low backgrounds (<0.5 cps) allowed U-Th dating of ancient deep-water corals (15–260 kyr) and stalagmites (30–85 kyr) at precisions of <2%. Kamber and Gladu365 compared standard methods of Pb purification by anion-exchange for the measurement of Pb isotope ratios by ICP-QMS. They found that matrix removal was much more effective using a procedure based on HBr-HCl rather than HBr-HNO3. A careful single-pass separation using HCl removed more than 99.9% of the silicate rock matrix except for very zinc-rich matrices. Elemental concentrations were determined on the sample digest first to allow accurate prediction of the expected ion signal and permit optimal spiking with Tl, if desired, for mass bias correction. Long-term reproducibility of data obtained for geological CRMs was better than 1% for Th/U and 1.5% for U/Pb, which was more than adequate for most geological applications of Pb isotopes. Godoy and co-workers366 demonstrated that ICP-QMS is capable of determining the concentrations and isotopic compositions of Pu and U in environmental samples with sufficient precision and accuracy to satisfy IAEA requirements. The separation procedure applied to soils and sediments involved acid leaching, followed by ion-exchange on Dowex 1X8 and extraction chromatography using 2 mL TEVA® and UTEVA® columns to produce separate Pu and U fractions. The samples were introduced into the ICP-QMS instrument via an ultrasonic nebuliser coupled to a membrane desolvation system. Several CRMs and working RMs were incorporated into the analytical scheme for mass bias corrections and ID measurements; the uncertainty calculations are given in detail in several appendices. The reader's attention is drawn to a similar separation procedure for the determination of Pu and U in coral soils by MC-ICP-MS.367
4.4.3.2 Sector field ICP-MS. Considerable improvements in instrumentation, combined with improved access to the technique worldwide, have resulted in a dramatic increase in the number of isotopic measurements made by MC-ICP-MS over the past few years. A recent review368 on the determination of isotope ratios by single collector and multicollector ICP-MS (182 refs) is an excellent tutorial for those new to the field. The capabilities of various types of ICP-MS instrumentation are addressed and issues surrounding mass discrimination and detectors are discussed. In covering a wide range of applications, including geochronological dating and provenance studies, this review provides an insight into some of the more important aspects of the published literature. In another review (170 refs) Yang369 highlights the inconsistencies and errors evident in many MC-ICP-MS publications, including errors in mass bias correction models. She suggests some general rules to achieve precise and accurate measurements by MC-ICP-MS. The advantages and disadvantages of three mass bias correction procedures, depending on the elements and sample types measured, the chemical separation employed, plus the instrument used and its degree of stability, are discussed. She emphasizes the importance of reporting isotope ratio data with their associated combined uncertainties to facilitate comparison of results from different laboratories.
A comparatively simple and rapid method for the determination of7Li/6Li by MC-ICP-MS has been applied to the analysis of silicate rocks and marine biogenic carbonates, as well as waters of varying salinities.178 Sample digests were loaded onto columns containing AG 50W-X8 cation-exchange resin and eluted with 1 M HNO3 in 80% methanol (cf.ref. 177 in section 4.4.3.1). To reduce the Li background signal, a new desolvating sample introduction system with high sensitivity and low memory effects was employed, which reduced the amount of Li required for isotope measurements. A residual stable background of ∼10 mV was achieved with a 120 s wash time; measurement of various CRMs demonstrated that this did not affect the accuracy of the Li ratios significantly.
Huang and colleagues370 addressed several issues to improve the accuracy and precision of Mg isotope measurements by MC-ICP-MS and applied them to the analysis of 11 rock standards. Factors examined included the storage of the Mg standard solution, the effect of matrix elements (e.g. Al, Na, Mn and Ni) on the instrumental mass bias of Mg isotopes, the total Mg concentration of the nebulised solutions, interference from the 12C14N+ isobar, and the effect of Mg blank and organics leached from the resin during chemical purification. After dissolution, the samples were purified using a column filled with 0.5 mL BioRad AG50-X12 cation-exchange resin and Mg isotopes measured by sample-standard bracketing using HR-MC-ICP-MS. Prior to sample introduction, samples and standards were diluted to produce ∼0.2 Mg ppm solutions in 0.3 N HNO3, keeping the variation in 24Mg intensity to within 5%. Even with the use of a desolvating nebuliser, the size of the 12C14N+ interference could be significant relative to the 26Mg signal. Hence, it was important to ensure that the total Mg concentrations were high enough so that the contribution from 12C14N+ was minimal and that the concentrations of samples and standards were closely matched. The long-term reproducibility of δ26Mg for samples with relatively high MgO content was 0.11‰ (2 SD) and ∼0.2‰ for granites with lower MgO content. Wombacher and co-workers371 reported a method for the chemical separation of Ca, Fe and Mg on a 1 mL column of BioRad AG50W-X8 cation-exchange resin. This resin was preferred because of the somewhat faster fluid flow and less extensive cleaning required compared to X12 resin. For many geological matrices, Mg was separated by a single pass through the column. To separate Mg from Ca-dominated samples, such as carbonate, or effect a separation of Ca, Fe and Mg from the same sample aliquot, Ca and Fe were first separated in an additional step, using the same ion-exchange columns but higher acid molarities (10 M HCl). Following purification, Mg and Fe isotopes were determined by MC-ICP-MS, while Ca isotopes were measured by double-spike TIMS. Average external repeatabilities (2 σ) were ±0.16‰ for 26Mg/24Mg, ±0.26‰ for 44Ca/40Ca and ±0.05‰ for 56Fe/54Fe.
Dauphas et al.372 developed a protocol for the determination of Fe isotope ratios in natural materials and evaluated the variables that can affect the accuracy of δ56Fe measurements using HR-MC-ICP-MS. Chemical separation and purification involved two-step chromatography using columns containing 1 mL of AG1-X8 anion-exchange resin. The analyte concentrations of the sample and RM were matched within 5%; most critical was ensuring the acid molarity of the samples and standards were matched exactly (0.3 M HNO3). The measurements were not sensitive to the presence of other transition elements in solution as long as their concentrations remained below ∼10 ppm. Accurate δ56Fe measurements in a range of natural materials with precisions of better than about 0.03‰ (2 SD) were routinely achieved. A similar procedure was adopted in a study of the Fe isotope composition of ordinary chondrites and their constituent components.373
A method for the measurement of Co, Cr, Fe, Mn and Ni concentrations in silicate samples by HR-MC-ICP-MS without chemical separation or matrix matching has been described.374 Chromium was determined by ID after the addition of a Cr spike at a judicious point after sample digestion to avoid its loss as Cr2O2Cl2. The concentrations of the other elements were calculated by an ID-IS method developed previously by researchers at the same Japanese institute. Matrix effects were shown to be negligible down to a dilution factor of ∼3000 for basalt and ∼104 for peridotite, within an uncertainty of ±1%. A repeatability of about 1% was achieved, which is >10 times better than HR-ICP-MS and comparable to ID-TIMS.
Precautions need to be taken in the measurement of Si isotope ratios by MC-ICP-MS to avoid the introduction of mass dependent fractionation375 and sulfur-induced offsets.376 Samples are normally fused with NaOH and purified using cation-exchange. In a study of meteorite and terrestrial samples, Fitoussi and colleagues375 found that the pH of the solutions loaded onto the cation-exchange column containing BioRad 50W-X12 resin was a critical parameter. They concluded that the solution should be adjusted to pH ≥ 2.1 to avoid analytical artefacts. Because concentrations of anionic species in rocks are generally low, it is often assumed that they do not compromise Si isotope determinations. However, van den Boorn et al.376 have demonstrated significant errors in the accuracy of such measurements by MC-ICP-MS in sulfur-rich materials. They recommend heating the sample powder at 1350 °C in a stream of oxygen to remove sulfur prior to sample dissolution; this was shown to have no effect on the Si isotope ratios. Mechanisms to account for the observed offset in δ26Si values were explored. Isobaric interferences could be excluded because sulfur isotopes have higher masses than Si ones; their hypothesis was that sulfur in the matrix brought about changes in the instrumental mass bias.
The presence of high concentrations of sulfur can be particularly problematical in the measurement of Cu isotopes in some geological samples such as sulfides because of the formation of polyatomic species such as 32S14N16O1H that overlap with the 63Cu isotope.377 Although routine separation normally incorporates anion-exchange chromatography, variable column breakthrough can occur if the initial sulfur content is high. In these circumstances, a second separation of the Cu fraction was recommended, although it may be worth considering the simpler approach taken in the study of Si isotopes above.376
A new procedure for the quantitative separation of Mo and Re from geological samples has been developed by Pearce and co-workers.378 The chromatography employs 2 ml of BioRad AG 1-X8 anion-exchange resin in 10 mL columns, and involves the sequential use of three solutions: 1 mol L−1 HF/0.5 mol L−1 HCl, 4 mol L−1 HCl and 3 mol L−1 HNO3 for loading, washing and collecting the Mo and Re fractions. In conjunction with a 100Mo-97Mo double spike and 185Re single spike, the Mo isotope composition and Mo and Re abundances were determined by MC-ICP-MS. Quantitative recovery of Mo and Re from the column was confirmed and a long-term precision of <0.12‰ (2 σ) for 98Mo/95Mo established. This procedure offers some analytical advantages over others in current use and is suited to palaeo-redox studies requiring knowledge of Mo isotopic composition and the Re/Mo ratio from the same sample.
Poirier and Doucelance379 demonstrated that the measurement of 187Re/185Re ratios in geological materials by MC-ICP-MS suffered from matrix effects that could not be corrected by the standard–solution bracketing technique. By spiking samples and calibrators with W, and measuring Re and W isotopes simultaneously, they were able to compensate for changes in mass bias induced by the matrix.
Yang and co-workers380 present a three-column procedure for separating Hf, Lu, Nd, Rb, Sm and Sr from a single rock digest prior to measurements of the Lu-Hf, Rb-Sr and Sm-Nd isotope systems using MC-ICP-MS and TIMS. The simplified chemical separation employs mainly HCl as the eluting reagent, resulting in rapid purification in three working days. The same Chinese research group381 demonstrated precise and accurate measurements of 143Nd/144Nd without separating Nd from Sm. Instrumental mass discrimination was corrected using 146Nd/144Nd as an internal standard, after correction of the isobaric interference of 144Sm on 144Nd using interference-free 147Sm/149Sm for mass fractionation. Their data indicated that the mass discrimination of Sm and Nd was not the same and varied during an analytical session, hence the need to calculate mass biases for each element on individual samples. Martin et al.382 showed that Nd extracted from the >63μm decarbonated fraction of deep sea sediments using a solution of 0.02 M hydroxylamine hydrochloride can provide high resolution records of Nd isotopes for interpretation in terms of deep water circulation. An excellent correlation with Nd isotope ratios obtained from cleaned fossil fish teeth from the same samples was noted. Yuan and co-authors383 proposed a method based on a sectional power-law correction to provide precise and accurate measurements of 176Lu/175Lu by MC-ICP-MS, after spiking with 176Lu and 178Hf, and correcting for the interference of 176Yb on 176Lu. The success of this correction method meant that complete chemical separation of Yb from Lu was not a critical requirement and that simple one-step chemical chromatography could be employed. A systematic evaluation of the performance of Sr spec® resin in Sr isotope ratio measurements of samples with a complex or calcium-rich matrix has been carried out by a research group in Belgium.276 Resin volumes between 250 and 2000 μL, depending on the absolute Sr mass loaded on the resin, were preferred over the smaller quantities often deployed (<300μL), to ensure complete recovery and matrix separation. Soil and bone CRMs were analysed but the method is likely to be applicable to a broad range of Sr isotope applications. They also demonstrated that it was possible to regenerate the resin without detectable loss of performance. The same laboratory developed a simplified procedure for the isolation of Sr and Pb from volcanic rock powders using this resin.384 Savings in cost and time were made by reducing the steps for pre-cleaning the Sr spec® resin and cutting out the use of HBr without compromising the quality of the isotope ratio data obtained by MC-ICP-MS.
Two new procedures for the measurement of Ca and Cd isotopes in geological and biological materials by TIMS have been published recently by the same lead author.386,387 The method for Cd386 involves spiking with a mixed 106Cd/108Cd tracer prior to anion-exchange on AG1-X8 resin. It was found necessary to remove iron by solvent extraction into 4-methylpentan-2-one as traces of iron inhibit the thermal ionisation of Cd. The long-term reproducibility on the 112Cd/110Cd ratio was ±14 ppm (2 σ), based on 100 ng loads of standards treated as unknowns. The natural isotope fractionation of Ca is very limited, so excellent external precision and sensitivity are required in the measurement of Ca isotope ratios. Automated high pressure ion chromatography was shown to effect complete separation of Ca from K, Mg and Sr, thereby avoiding isobaric interferences, which is critical in TIMS.387 Recoveries of close to 100% were achieved, leading to the absence of any fractionation of Ca isotopes during purification. This procedure will be of particular interest for applications that are not amenable to the use of the double spike technique.
Chen and co-workers388 developed an improved protocol for the determination of Ru isotopes in chondrites and meteorites by NTIMS. Samples were dissolved with reverse aqua regia in Carius tubes and Ru separated by ion-exchange and further purified by distillation. This improved chemistry, plus the use of zone-refined, outgased Pt filaments, reduced the isobaric interferences from Mo to a level where no correction was necessary. Data were normalised to 99Ru/101Ru to correct for mass fractionation.
Algorithms commonly used to correct for mass dependent isotope fractionation in TIMS have been assessed using a large set of Nd isotope data.389 The authors concluded that the exponential law is fully adequate to correct for mass fractionation at the current level of precision obtainable. However, this law assumes that sample evaporation takes place from a single, homogenous domain on the filament, which is probably not justified as mass fractionation is temperature dependent. They estimated that up to 50% of the external reproducibility could be explained by ion emission from multiple domains of somewhat different isotopic composition on the filament. Chu et al.390 have developed a sensitive method for measurements of Nd isotopes as NdO+ by TIMS using a single tungsten filament with TaF5 as an ion emitter. Advantages of this procedure included higher sensitivity, a more stable ion beam and no need for oxygen gas to be bled into the ion source chamber compared to previous methods based on NdO+ determinations. Combined with a highly efficient and low-blank column chemistry to separate Nd from Sm, Ce and Pr, this method has the potential to determine Nd and Sm concentrations plus Nd isotopic compositions of geological materials with very low Nd and Sm contents.
Lasers are often employed for the extraction of noble gases from geological samples prior to the measurement of their isotopic composition by mass spectrometry. In K-Ar geochronology, the use of lasers to release argon from the sample can result in lower blanks and complete extraction of the argon in tens of seconds. Sole391 reported the practicalities of using an infrared CO2 laser for this purpose. In an alternative approach, Ignatiev et al.392 extracted argon from geological samples with a continuous Nd:YAG laser. After preconcentration, the argon was separated from possible contaminants by passage through a chromatographic capillary column in a flow of helium. They demonstrated that argon isotope measurement by continuous flow IRMS could be used for the analysis of radiogenic argon in picogram quantities. The sensitivity and accuracy of the measurements were shown to be comparable to those provided by the classical static method, as well as being simpler and more reliable. A combination of three techniques to extract argon for Ar-Ar dating has been reported.393 This involved stepwise heating of single grains and small separates with a laser, spot fusions with a UV laser on polished sections, and an in vacuo crushing technique for liberating radiogenic argon from fluid inclusions. An internally consistent thermal history was derived from the Ar-Ar ages obtained from these three sampling strategies. Zimmerman and co-workers394 selected a 193 nm excimer laser to extract nitrogen captured in targets exposed to ions emitted by the Sun during a space mission. A low volume nitrogen purification line was employed, with a cryogenic trap to avoid loss of nitrogen by dilution in the line. After purification, the nitrogen was analysed by static mode mass spectrometry using electron multipliers and Faraday cup detectors. The blanks obtained by this procedure were an order of magnitude lower than those achieved by other workers.
Applications of ion microprobe measurements in the study of zircons have been widely reported in this review period. Some of the first determinations of oxygen isotope ratios in zircon from oceanic crust have been made by ion microprobe on zircon in polished rock chips of gabbro and veins in serpentinised peridotite drilled from the Mid-Atlantic Ridge.395 Gordon et al.396,397 used SIMS to acquire high resolution depth profiles of the rims of natural zircons from high-grade metamorphic rocks. They demonstrated that the evolution of highly metamorphosed terrains, including the fluid history, could be revealed in unprecedented detail through data acquired at the sub-micrometre scale for U-Pb geochronology, Ti thermometry and oxygen isotope ratios using this technique. A rapid and accurate protocol398 has been developed for the determination of U-Pb ages in ancient zircons using a fully automated multi-collector ion microprobe. Sample locations were determined off-line before the samples were loaded into the instrument, reference points calibrated and target positions visited sequentially. The zircons were screened initially, with a 5 s acquisition, and suitable candidates then analysed in a longer routine to obtain better measurement statistics, U/Pb and concentration data. In multi-collector mode the analytical time taken for a single mount containing 400 zircons is approximately 6 h; in single collector mode, the analysis takes about 17 h. These routines have been used to analyse over 100,000 zircons.
In many cases, ion microprobe is the preferred method for U-Pb and Th-Pb dating of minerals because it combines good analytical precision with μm-scale spatial resolution. Like all techniques, it is important to quantify any matrix effects in SIMS. Fletcher and co-workers399 have made a comprehensive assessment of the effect of different matrix compositions on the ionisation efficiencies of different secondary ionic species during the dating of monazites. They concluded that matrix corrections could be applied without measuring all the LREEs, using ion probe data alone. In addition, other complications such as Pb-Pb fractionation and an isobar at 204Pb, could be addressed without iterative data reduction. Observed variations in instrumental mass fractionation (IMF) during the measurement of radiogenic Pb isotopes in zircon by SIMS has been investigated.400 The specific causes of variability in IMF were unclear, but generally reflected subtle differences in analytical conditions between instruments and between analytical sessions. It was recommended that an igneous zircon RM such as OD1, characterised for U-Pb isotopes by ID-TIMS, should be incorporated into the analytical routine to improve the accuracy and reproducibility, particularly when dating Precambrian events. Problems encountered in the determination of Pb and U isotopes in highly altered zircons from sediments overlying the Bangombé natural fission reactor in the Gabon using a SHRIMP have been described.401 These zircons have very variable U contents from <100 to 59 000 ppm. The conventional U-Pb calibration technique for in situ analysis was found to be unreliable for U contents greater than 2500 ppm. Accurate ages for these samples required U/Pb elemental ratios determined by EPMA prior analysis by ion probe.
A new method to measure the natural 10Be/9Be ratio of a sample directly with low energy AMS has been developed.402 This AMS technique is faster and more precise compared to conventional analysis, which relies on the determination of the total Be concentration by another method, such as ICP-MS. Possible applications of this new method are 10Be/9Be dating or the reconstruction of the geomagnetic field strength from marine sediments. Chmeleff and co-workers403 used a combination of MC-ICP-MS and scintillation counting to measure the half-life of 10Be; a value of 1.387 ± 0.012 My is recommended as a result of their investigations. They also prepared a 10Be-rich master solution that is available as an absolute Be ratio standard for AMS measurements. The current debate over the half-life of 10Be is relevant to the determination of cosmogenic 10Be/21Ne and 26Al/21Ne production rates in quartz by AMS.404
In contrast to siliceous environments, there are few cosmogenic nuclides that can be used for in situ dating of calcareous materials. Thus, Merchel et al.405 investigated nuclides such as 10Be, 26Al and 41Ca as possible dating tools through cross-calibration with 36Cl. They found that cosmogenic 10Be in calcite is highly contaminated with atmospheric 10Be, which is extremely difficult to remove quantitatively. Very low 41Ca/Ca ratios (<5 × 10−15) hinders the measurement of 41Ca, whereas 26Al can be easily determined in calcite, even though its production rate is only ∼4.6%, and would merit further research.
The accurate determination of Cl concentrations in terrestrial rocks is important for the interpretation of in situ cosmogenic 36Cl. The results of an inter-laboratory comparison406 of Cl measurements by prompt gamma activation analysis and ID-AMS concluded there was no significant difference between the Cl concentrations obtained by either methods when analysing Icelandic basalts with Cl contents between 40 and 99 μg g−1, with comparable uncertainties.
Developments in geological applications of XRF have included an EDXRF method for the rapid determination of Ce, La, Nd, Pr and total REE content of rare earth ores from Mongolia.408 Synthetic calibration standards were prepared from pure oxides of Ce, La, Nd and Pr in a matrix of SiO2 and Al2O3. Artificial samples made by successive dilution of the calibration standards with oxides of known interfering elements, such as Ba, Nd and Sr, were used to calculate corrections for spectral overlaps. The measurement of the FeO content of rocks by WDXRF has been revisited by Finkelshtein and Chubarov.409 From their experiments, they selected the Kβ2,5/Kβ1,3 ratio as the analytical signal. For igneous samples with a FeO/Fe2O3total of >0.25, they claimed that the accuracy of the XRF determination of FeO content was comparable to that of wet chemical analysis.
A variety of micro X-ray techniques can be employed to provide chemical information at high spatial resolution. Recent examples include the use of XANES to investigate whether REEs are incorporated in the carbonate phase or in other minor mineral phases, such as Fe-Mn oxides and phosphate, within a limestone.410 Impact materials from the Barringer Meteor Crater have been examined by a combination of μ-XANES and μ-PIXE techniques,411 with a focus on understanding the complex character of their Fe-rich inclusions. Spatially resolved X-ray absorption and fluorescence techniques were employed to study the movement and speciation of a Np radiotracer in fractured granite core from the Swedish Aspo Hard Rock Laboratory.412 Elemental distributions were obtained from μ-XRF measurements and the Np oxidation state was determined by μ-XAFS. It was observed that the Np, originally introduced as NpV, was reduced to NpIV in the fractured granite and that the distribution of the NpIV was correlated with that of zinc. From this, it was concluded that ZnS may have played a role in the reduction and subsequent immobilisation of the Np tracer.
5 Glossary of terms
| AAS | atomic absorption spectrometry |
| ACGIH | American Conference of Governmental Industrial Hygienists |
| AES | atomic emission spectrometry |
| AFS | atomic fluorescence spectrometry |
| AMS | accelerator mass spectrometry |
| APDC | ammonium pyrrolidinedithiocarbamate |
| AsB | arsenobetaine |
| ASTM | American Society for Testing And Materials |
| ATOFMS | atomic time of flight mass spectrometry |
| BCR | Community Bureau of Reference (of the European Community) |
| CCRMP | Canadian Certified Reference Material Project |
| CEN | European Committee for Standardisation |
| CPE | cloud point extraction |
| CRM | certified reference material |
| CRPG | Centre de Recherches Petrographiques et Geochimiques |
| CV | cold vapour |
| CV-AAS | cold vapour atomic absorption spectrometry |
| CV-AFS | cold vapour atomic fluorescence spectrometry |
| CVG | chemical vapour generation |
| DBT | dibutyltin |
| DDTC | diethyldithiocarbamate |
| DLLME | dispersive liquid liquid microextraction |
| DMA | dimethyl arsenic acid |
| DRC | dynamic reaction cell |
| EDAX | energy dispersive X-ray spectrometry |
| EDXRF | energy dispersive X-ray fluorescence |
| EN | European Standard |
| EPMA | electron probe microanalysis |
| ESI-MS | electrospray ionisation mass spectrometry |
| ETAAS | electrothermal atomic absorption spectrometry |
| ETV | electrothermal vaporisation |
| ETV-ICP-AES | electrothermal vaporisation inductively coupled plasma emission spectrometry |
| ETV-ICP-MS | electrothermal vaporisation inductively coupled plasma mass spectrometry |
| EU | European Union |
| EXAFS | extended X-ray absorption fine structure |
| FAAS | flame atomic absorption spectrometry |
| FAPES | furnace atomization plasma excitation spectrometry |
| FI-CVG-AFS | flow injection cold vapour atomic fluorescence spectrometry |
| FI-HG-ICP-AES | flow injection hydride generation inductively coupled plasma atomic emission spectrometry |
| fs | femto second |
| FTIR | fourier transform infrared |
| GBW | reference materials produced by the National Research Center for Certified Reference Materials in China |
| GC | gas chromatography |
| GC-FID | gas chromatographic flame ionization detector |
| GC-MS | gas chromatography mass spectrometry |
| GSJ | Geological Survey of Japan |
| HF-LPME | hollow fiber liquid phase microextraction |
| HG | hydride generation |
| HG-AAS | hydride generation atomic absorption spectrometry |
| HG-AFS | hydride generation atomic fluorescence spectrometry |
| HG-ICP-MS | hydride generation inductively coupled plasma mass spectrometry |
| HPLC | high performance liquid chromatography |
| HPLC-ICP-MS | high performance liquid chromatography inductively coupled plasma mass spectrometry |
| HR-CS-FAAS | high resolution continuum source flame atomic absorption spectrometry |
| HR-CS-ETAAS | high resolution continuum source electrothermal atomic absorption spectrometry |
| HR-ICP-MS | high resolution inductively coupled plasma mass spectrometry |
| HR-MC-ICP-MS | high resolution multi collector inductively coupled mass spectrometry |
| IAEA | International Atomic Energy Agency |
| IC | ion chromatography |
| IC-ICP-MS | ion chromatography inductively coupled plasma mass spectrometry |
| ICP-AES | inductively coupled plasma atomic emission spectrometry |
| ICP-MS | inductively coupled plasma mass spectrometry |
| ICP-QMS | inductively coupled plasma quadrupole mass spectrometry |
| ID | isotope dilution |
| ID-ICP-MS | isotope dilution inductively coupled plasma mass spectrometry |
| INAA | instrumental neutron activation analysis |
| INRS | Institut National de Recherche et de Sécurité |
| IOM | Institute of Occupational Medicine |
| IR | infrared |
| IRMS | isotope ratio mass spectrometry |
| IS | internal standard |
| ISO | International Standards Organisation |
| JSAC | Japanese Society for Analytical Chemistry |
| LA | laser ablation |
| LA-ICP-MS | laser ablation inductively coupled plasma mass spectrometry |
| LA-MC-ICP-MS | laser ablation multi collector inductively coupled plasma mass spectrometry |
| LIBS | laser induced breakdown spectroscopy |
| LOD | limit of detection |
| LOQ | limit of quantification |
| LPME | liquid drop microextraction |
| MA | methylarsonic acid |
| MBT | monobutyltin |
| MC-ICP-MS | multi collector inductively coupled plasma mass spectrometry |
| MIBK | methyl isobutyl ketone |
| MMA | monomethyl arsenic |
| Nd:YAG | neodymium doped:yttrium aluminium garnet |
| NIOSH | National Institute of Occupational Safety and Health |
| NIST | National Institute of Standards and Technology |
| NRCC | National Research Council of Canada |
| ns | nano second |
| NTIMS | negative thermal ionisation mass spectrometry |
| PCA | principal component analysis |
| PGE | platinum group elements |
| PIXE | particle induced X-ray emission |
| PXRF | portable X-ray fluorescence |
| REE | rare earth element |
| RM | reference material |
| RSD | relative standard deviation |
| sd | standard deviation |
| SDME | single drop microextraction |
| SEM | scanning electron microscopy |
| SF | sector field |
| SF-ICP-MS | sector field inductively coupled plasma mass spectrometry |
| SFODME | solidified floating organic drop microextraction |
| SHRIMP | sensitive high mass resolution ion microprobe |
| SIDMS | speciated isotope dilution mass spectrometry |
| SIMS | secondary ion mass spectrometry |
| SPE | solid phase extraction |
| SPME | solid phase microextraction |
| SRM | standard reference material |
| SRXRF | synchrotron radiation X-ray fluorescence |
| TBT | tributyltin |
| TEM | transmission electron microscopy |
| TIMS | thermal ionisation mass spectrometry |
| TMAO | trimethylarsine oxide |
| TOF | time of flight |
| TPhT | triphenyltin |
| TXRF | total reflection X-ray fluorescence |
| US EPA | United States Environmental Protection Agency |
| USGS | United States Geological Survey |
| USN | ultrasonic nebulisation |
| UV | ultra violet |
| VOC | volatile organic compound |
| WDXRF | wavelength dispersive X-ray fluorescence |
| XAFS | X-ray absorption fine structure spectrometry |
| XANES | X-ray absorption near edge structure |
| XRD | X-ray diffraction |
| XRF | X-ray fluorescence |
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