Speciated Isotope Dilution Mass Spectrometry Research Articles

Reconnaissance sampling and determination of hexavalent chromium in potentially-contaminated agricultural soils in Copperbelt Province, Zambia

The distribution of elemental species of chromium (Cr) in potentially-contaminated soil samples warrants investigation due to the differing mobilities and toxicities of trivalent [Cr(III)] and hexavalent chromium [(Cr(VI)]. In addition, the possibility of species interconversions requires the implementation of robust methods that can correct for changes at the point of sampling, extraction and analysis. This work presents the application of speciated isotope dilution mass spectrometry (SIDMS) to accurately quantify Cr(VI) in agricultural soils within close proximity to a mine tailings dam in the Copperbelt Province of Zambia. Interpolated plots of total Cr, produced from data collected through a nested sampling design, were used to optimise the sampling across the spatial domain. Extraction of Cr(VI) was undertaken using a microwave assisted reaction system (80 °C for 5 min) with 50 mM EDTA, to complex Cr(III) and reduce the likelihood of oxidation during the extraction. Isotopically-enriched 53Cr(VI) was added to each sample prior to extraction to account for species interconversions. The accuracy of the method was confirmed using NIST SRM 2700 and 2701. Cr(VI) concentrations in the soil samples ranged between 0.03 and 0.29 mg kg−1, significantly lower than the residential UK screening value for Cr(VI) of 21 mg kg−1. The data indicate that this site poses a low environmental/human health risk with respect to Cr(VI) exposure.

Determination of the Zinc Concentration in Human Fingernails Using Laser-Induced Breakdown Spectroscopy.

The absolute concentration of Zn in human fingernail clippings was determined ex vivo using 1064 nm laser-induced breakdown spectroscopy and confirmed by speciated isotope dilution mass spectrometry. A nail testing protocol that sampled across the nail (perpendicular to the direction of growth) was developed and validated by scanning electron microscopy energy-dispersive X-ray spectrometry. Using this protocol, a partial least squares (PLS) regression model predicted the Zn concentration in the fingernails of five people to within an average of 7 ppm. The variation in the Zn concentration with depth into the nail determined by laser-induced breakdown spectroscopy was studied and showed no systematic variation for up to 15 subsequent laser pulses in one location. The effects of nail hydration (dehydrated and over-hydrated) and nail surface roughness were investigated to explain an anomalously large scatter observed in the measurements. This scatter was attributed to the layered nature and fibrous structure of the fingernails, which resulted in non-uniform ablation as determined by scanning electron microscopy. This work demonstrates that a protocol consisting of low pulse energy (<10 mJ) 1064 nm laser pulses incident on human fingernail clippings in an Ar environment can produce quantifiable Zn emission in the laser-induced plasma and that the measured Zn intensity can be used to accurately predict the Zn concentration in human fingernails.

Reduction of hexavalent chromium by fasted and fed human gastric fluid. I. Chemical reduction and mitigation of mutagenicity

Evaluation of the reducing capacity of human gastric fluid from healthy individuals, under fasted and fed conditions, is critical for assessing the cancer hazard posed by ingested hexavalent chromium [Cr(VI)] and for developing quantitative physiologically-based pharmacokinetic models used in risk assessment. In the present study, the patterns of Cr(VI) reduction were evaluated in 16 paired pre- and post-meal gastric fluid samples collected from 8 healthy volunteers. Human gastric fluid was effective both in reducing Cr(VI), as measured by using the s-diphenylcarbazide colorimetric method, and in attenuating mutagenicity in the Ames test. The mean (±SE) Cr(VI)-reducing ability of post-meal samples (20.4±2.6μgCr(VI)/mL gastric fluid) was significantly higher than that of pre-meal samples (10.2±2.3μgCr(VI)/mL gastric fluid). When using the mutagenicity assay, the decrease of mutagenicity produced by pre-meal and post-meal samples corresponded to reduction of 13.3±1.9 and 25.6±2.8μgCr(VI)/mL gastric fluid, respectively. These data are comparable to parallel results conducted by using speciated isotope dilution mass spectrometry. Cr(VI) reduction was rapid, with >70% of total reduction occurring within 1min and 98% of reduction is achieved within 30min with post-meal gastric fluid at pH2.0. pH dependence was observed with decreasing Cr(VI) reducing capacity at higher pH. Attenuation of the mutagenic response is consistent with the lack of DNA damage observed in the gastrointestinal tract of rodents following administration of ≤180ppm Cr(VI) for up to 90days in drinking water. Quantifying Cr(VI) reduction kinetics in the human gastrointestinal tract is necessary for assessing the potential hazards posed by Cr(VI) in drinking water.

Reduction of hexavalent chromium by fasted and fed human gastric fluid. II. Ex vivo gastric reduction modeling

To extend previous models of hexavalent chromium [Cr(VI)] reduction by gastric fluid (GF), ex vivo experiments were conducted to address data gaps and limitations identified with respect to (1) GF dilution in the model; (2) reduction of Cr(VI) in fed human GF samples; (3) the number of Cr(VI) reduction pools present in human GF under fed, fasted, and proton pump inhibitor (PPI)-use conditions; and (4) an appropriate form for the pH-dependence of Cr(VI) reduction rate constants. Rates and capacities of Cr(VI) reduction were characterized in gastric contents from fed and fasted volunteers, and from fasted pre-operative patients treated with PPIs. Reduction capacities were first estimated over a 4-h reduction period. Once reduction capacity was established, a dual-spike approach was used in speciated isotope dilution mass spectrometry analyses to characterize the concentration-dependence of the 2nd order reduction rate constants. These data, when combined with previously collected data, were well described by a three-pool model (pool 1 = fast reaction with low capacity; pool 2 = slow reaction with higher capacity; pool 3 = very slow reaction with higher capacity) using pH-dependent rate constants characterized by a piecewise, log-linear relationship. These data indicate that human gastric samples, like those collected from rats and mice, contain multiple pools of reducing agents, and low concentrations of Cr(VI) (<0.7 mg/L) are reduced more rapidly than high concentrations. The data and revised modeling results herein provide improved characterization of Cr(VI) gastric reduction kinetics, critical for Cr(VI) pharmacokinetic modeling and human health risk assessment.

A novel strategy for Cr(III) and Cr(VI) analysis in dietary supplements by speciated isotope dilution mass spectrometry

In recent years, Cr speciation in dietary supplements has become decisive in the evaluation of their health risks. Despite being an beneficial micronutrient, Cr(III) can be toxic at living organisms at high concentrations, while Cr(VI) is known to be highly toxic and carcinogenic. The main objective of this work was to optimize an analytical methodology for the extraction and accurate quantification of Cr(III) and Cr(VI) in dietary supplements. The extraction of Cr species was carried out with 50mM EDTA solution on a hotplate under optimized conditions. Special attention was paid to bidirectional species transformations. No noticeable oxidation of Cr(III) into Cr(VI) was observed and the reduction to Cr(III) only occurred at very high Cr(VI) concentrations. Cr(III) as Cr(EDTA)- complex was chromatographically separated from Cr(VI), retained as CrO42−, on an anion exchange column coupled to inductively coupled plasma mass spectrometry (LC-ICP-MS). The limit of quantification (0.08µgg−1) was below the limit established for Cr enriched yeasts by the European Union. Eleven dietary supplements were analyzed and Cr(III) and Cr(VI) quantification was carried out by external calibration monitoring 52Cr isotope and by speciated isotope dilution mass spectrometry (SIDMS) adding 50Cr(III) and 53Cr(VI) spikes. Total Cr was also quantified by ICP-MS and mass balance between total Cr and the sum of Cr(III) and Cr(VI) was achieved. In eight of the eleven tested supplements Cr(III) calculated amounts were higher than those indicated by the manufacturer, but only one of them exceeded the 250µgday−1 recommended by World Health Organization (WHO). In contrast, it is worth noting that Cr(VI) amounts beyond the recommendations of the European Union for Cr enriched yeasts were found in five supplements. These results revealed that more accurate and rigorous quality assurance protocols should be applied to the testing of the final products, including the analysis of both Cr(III) and Cr(VI).

Molecular speciated isotope dilution mass spectrometric methods for accurate, reproducible and direct quantification of reduced, oxidized and total glutathione in biological samples.

Novel protocols were developed to accurately quantify reduced (GSH), oxidized (GSSG) and total (tGSH) glutathione in biological samples using molecular speciated isotope dilution mass spectrometry (SIDMS). For GSH and GSSG measurement, the sample was spiked with isotopically enriched analogues of the analytes ((310)GSH and (616)GSSG), along with N-ethylmaleimide (NEM), and treated with acetonitrile to solubilize the endogenous analytes via protein precipitation and equilibrate them with the spikes. The supernatant was analyzed by liquid chromatography-tandem mass spectrometry (LC-MS/MS), and the analytes were quantified with simultaneous tracking and correction for auto-oxidation of GSH to GSSG. For tGSH assay, a (310)GSH-spiked sample was treated with dithiothreitol (DTT) to convert disulfide-bonded glutathione to GSH. After removing the protein, the supernatant was analyzed by LC-MS/MS and the analyte was quantified by single-spiking isotope dilution mass spectrometry (IDMS). The mathematical relationships in IDMS and SIDMS quantifications are based on isotopic ratios and do not involve calibration curves. The protocols were validated using spike recovery tests and by analyzing synthetic standard solutions. Red blood cell (RBC) and saliva samples obtained from healthy subjects, and whole blood samples collected and shipped from a remote location were analyzed. The concentrations of tGSH in the RBC and whole blood samples were 2 orders of magnitude higher than those found in saliva. The fractions of GSSG were 0.2-2.2% (RBC and blood) and 15-47% (saliva) of the free glutathione (GSH + 2xGSSG) in the corresponding samples. Up to 3% GSH was auto-oxidized to GSSG during sample workup; the highest oxidations (>1%) were in the saliva samples.

Optimization and validation of strategies for quantifying chromium species in soil based on speciated isotope dilution mass spectrometry with mass balance

Strategies were designed to quantify hexavalent (Cr(VI)), soluble trivalent (Cr(III)) and insoluble chromium (Cr) species in soil by integrating existing methods of Cr(VI) and total Cr determination with speciated isotope dilution mass spectrometry (SIDMS, EPA Method 6800). Two different extraction methods that utilize a NaOH–Na2CO3 solution (EPA Method 3060A) and alkaline solution of ethylenediaminetetraacetic acid (EDTA) were used to extract Cr(VI) (along with soluble Cr(III) in the latter case). The extracted Cr was speciated by ion chromatography-inductively coupled plasma mass spectrometry (IC-ICP-MS), and the separated species were quantified using the mathematical relationships in SIDMS with simultaneous correction for their method-induced transformations. The Cr species that fell out as insoluble solid during extraction were determined by isotope dilution mass spectrometry (IDMS) after decomposing the extraction residues in a mixture of mineral acids according to EPA Method 3052. Several certified reference materials and a soil sample were analyzed using the proposed strategies. The measured mass fractions of Cr(VI) and total Cr in the reference materials statistically agreed with the certified values at 95% CL. The insoluble fraction of Cr accounted for 64–107% of the total Cr in the samples. The validity of the strategies was proved using mass balance by comparing the sum of the mass fractions of Cr(VI), soluble Cr(III) and insoluble Cr with the total Cr measured in the corresponding samples; the latter was determined by IDMS after decomposing the soils using EPA Method 3052 with single spiking.

Measurement of mercury species in whole blood using speciated isotope dilution methodology integrated with microwave-enhanced solubilization and spike equilibration, headspace-solid-phase microextraction, and GC-ICP-MS analysis.

A biomonitoring method was developed for the determination of inorganic-, methyl-, and ethylmercury (Hg(2+), CH3Hg(+), and C2H5Hg(+), respectively) in whole blood by triple-spiking speciated isotope dilution mass spectrometry (SIDMS) using headspace (HS) solid-phase microextraction (SPME) in combination with gas chromatographic (GC) separation and inductively coupled plasma mass spectrometric (ICP-MS) detection. After spiking the blood sample with isotopically enriched analogues of the analytes ((199)Hg(2+), CH3(200)Hg(+) and C2H5(201)Hg(+)), the endogenous Hg species were solubilized in 2.0 mol L(-1) HNO3 and equilibrated with the spikes using a microwave-enhanced protocol. The microwaved sample was treated with a 1% (w/v) aqueous solution of sodium tetrapropylborate (buffered to pH 5.2), and the propylated Hg species were sampled in the HS using a Carboxen/polydimethylsiloxane-coated SPME fiber. The extracted species were thermally desorbed from the fiber in the GC injection port and determined by GC-ICP-MS. The analytes were quantified, with simultaneous correction for their method-induced transformation, on the basis of the mathematical relationship in triple-spiking SIDMS. The method was validated using a bovine blood standard reference material (SRM 966, Level 2). Analysis of human blood samples demonstrated the accuracy and reproducibility of the method, which can detect the Hg species down to 30 pg g(-1) in blood. The validity of the analytical results found for the blood samples was demonstrated using mass balance by comparing the sum of the concentrations of the individual Hg species with the total Hg in the corresponding samples; the latter was determined by isotope dilution mass spectrometry (IDMS) after decomposing the blood using EPA Method 3052 with single-spiking.

Dissolution of the metal sensitizers Ni, Be, Cr in artificial sweat to improve estimates of dermal bioaccessibility

Dermal exposure to sensitizing metals is a serious occupational and public health problem. The usual approach to dermal exposure assessment is to process samples by chemical methods that use reactants to digest the metal particles and quantify the mass. In the case of dermal exposure assessment, these reactants are not representative of the skin surface film liquids and hence, may overestimate bioaccessibility. We hypothesize that the amount and form of sensitizer on a sample that leaches in a biological fluid, as can be estimated using artificial sweat, may be a more relevant metric for assessing health risks. Beryllium metal (Be), nickel metal (Ni), and chromium carbide (Cr3C2) particles were characterized and masses of sensitizing ions were measured using established reactant-assisted digestion procedures and extraction in artificial sweat under physiologically relevant conditions. Chromium ions released into artificial sweat were speciated to understand valence states. The ratios of the fraction of metal dissolved in artificial sweat relative to that dissolved by chemical-specific reactants were 1/2 (Be), 1/108 (Ni), and 1/2500 (Cr). The divalent Be and Ni cations were stable in artificial sweat over time (did not precipitate) whereas hexavalent chromium [Cr(VI)] ions decayed over time. Further analysis using speciated isotope dilution mass spectrometry revealed that the decay of Cr(VI) was accompanied by the formation of Cr(III) in the sweat model. Use of reactant-assisted analytical chemistry to quantify amounts of metal sensitizers on samples could overestimate biologically relevant exposure. In addition to mass, the valence state also influences penetration through the outer stratum corneum of the skin and is an important consideration when assessing exposure to complex sensitizers such as Cr which have multiple valence states with differing penetration efficiencies.

Determination of Chromium Species in Dietary Supplements Using Speciated Isotope Dilution Mass Spectrometry with Mass Balance

In order to determine the health impact of chromium in dietary supplements, the Cr(III) and Cr(VI) must be independently measured and verified with mass balance (sum of both species equaling independent measurements of total chromium), as both may be present in finished products. Because Cr(III) is stable in acidic conditions and Cr(VI) in alkaline conditions, interconversions between species may occur in complex matrices and during analytical extraction, increasing the difficulty of quantification. A study was conducted to determine Cr(VI) and Cr(III) in dietary supplements. EPA Method 3060A extraction protocol was performed to extract Cr(VI), and EPA Method 3052 was performed on the extracted residue to digest the remaining Cr(III). Speciated isotope dilution mass spectrometry (SIDMS), as described in the EPA Method 6800 (update V), was implemented with ion-exchange chromatography inductively coupled plasma mass spectrometry (IC-ICP-MS). Method 6800 uniquely enables tracking and correcting for the bidirectional chromium interspecies conversions that occur during extraction and sample handling prior to instrumental analysis. Mass balance results indicated that the off-the-shelf dietary supplements analyzed during this study contained hexavalent chromium ranging from <DL (detection limit) to 122.4 ± 13.0 μg/g, which corresponds to concentrations from below detection levels to up to 16% of the total chromium content. This type of variation in the final products raises public health issues and points to a need to use a robust method that can accurately and reliably make species measurements including correcting for species conversions.

Hexavalent chromium reduction kinetics in rodent stomach contents

Reduction of hexavalent chromium (Cr(VI)) to trivalent chromium (Cr(III)) in the stomach prior to absorption is a well-recognized detoxification process thought to limit the toxicity of ingested Cr(VI). However, administration of high concentrations of Cr(VI) in drinking water cause mouse small intestinal tumors, and quantitative measures of Cr(VI) reduction rate and capacity for rodent stomach contents are needed for interspecies extrapolation using physiologically-based toxicokinetic (PBTK) models. Ex vivo studies using stomach contents of rats and mice were conducted to quantify Cr(VI) reduction rate and capacity for loading rates (1–400mg Cr(VI)L−1 stomach contents) in the range of recent bioassays. Cr(VI) reduction was measured with speciated isotope dilution mass spectrometry to quantify dynamic Cr(VI) and Cr(III) concentrations in stomach contents at select time points over 1h. Cr(VI) reduction followed mixed second-order kinetics, dependent upon concentrations of both Cr(VI) and the native reducing agents. Approximately 16mg Cr(VI)-equivalents of reducing capacity per L of fed stomach contents (containing gastric secretions, saliva, water and food) was found for both species. The second-order rate constants were 0.2 and 0.3Lmg−1h−1 for mice and rats, respectively. These findings support that, at the doses that caused cancer in the mouse small intestine (⩾20mg Cr(VI)L−1 in drinking water), the reducing capacity of stomach contents was likely exceeded. Thus, for extrapolation of target tissue dose in risk assessment, PBTK models are necessary to account for competing kinetic rates including second order capacity-limited reduction of Cr(VI) to Cr(III).

Chromium speciation in solid matrices and regulation: a review

In recent years, the extensive use of chromium in industrial processes has led to the promotion of several directives and recommendations by the European Union, that try to limit and regulate the presence of Cr(VI) in the environment and to protect industrial workers using chromium and end-users of manufactured products. As a consequence, new standard methods and analytical procedures have been published at the EU level for Cr(VI) determination in soil, sludge, sediment, and similar waste materials, workplace atmospheres, cement, packaging materials, industrially produced samples, and corrosion-protection layers on some components of vehicles and electrical and electronic equipment. The objective of this article is to summarize the different directives and recommendations and to critically review the currently existing standard methods and the methods published in the literature for chromium speciation in the above mentioned solid matrices, putting the emphasis on the different extraction procedures which have been developed for each matrix. Particular attention has been paid to Cr(III) and Cr(VI) inter-conversions that can occur during extraction and efforts to minimize these unwanted reactions. Although the use of NaOH-Na(2)CO(3) solutions with hot plate extraction seems to be the more widespread procedure, species transformation can still occur and several studies suggest that speciated isotope-dilution mass spectrometry (SIDMS) could be a suitable tool for correction of these interconversions. Besides, recent studies have proved the role of Cr(III) in chromium toxicology. As a consequence, the authors suggest an update of standard methods in the near future.

Sensitivity analysis for speciated isotope dilution mass spectrometry

Speciated isotope dilution mass spectrometry is a technique that uses mass spectrometer readings to determine the species-specific isotopic profile of a chemical element in solution. The problem admits several approaches, each with its own computational quirks. In our view, matrix methods are attractive because they are conceptually clear, easy to implement, and amenable to formal analysis. The purpose of this note it to illustrate these virtues by using a matrix method to analyze the numerical sensitivity of a particular published example under both observation noise and changes in parameter space. The analysis blends numerical and theoretical techniques, and has implications both for experimental design and for practical uncertainty assessment.

Application of speciated isotope dilution mass spectrometry to evaluate methods for efficiencies, recoveries, and quantification of mercury species transformations in human hair

Nine different analytical extraction methods commonly used for mercury speciation in human hair samples were evaluated by analyzing hair certified reference material (IAEA-085) certified for the content of monomethyl mercury and total mercury. Extraction efficiencies, recoveries, as well as their potential of species transformations during sample processing, were evaluated by applying speciated isotope dilution mass spectrometry (SIDMS) as both a quantitative and diagnostic tool. The extraction methods evaluated in this study were based on acid leaching with HCl and HNO3. Detection of total mercury and mercury species from all extraction methods were carried out by inductivley coupled plasma mass spectrometry (ICP-MS) and high performance liquid chromatography-ICP-MS, respectively. Microwave-assisted extraciton (4 mol l−1 HNO3, at 100 °C for 10 min) and mechanical shaking (2 mol l−1HCl, at room temperature for 24 h) were found to be the most efficient protocols that demonstrated approximately 100% mecury recovery with less species transformations (6% or less). Ultrasound-assisted extraction (2% HCl + 10% ethanol, at 60 °C for 7 min) or extraction with 2 mol l−lHCl in a water bath (at 100 °C for 5 min) resulted in the second highest extraction efficiency (∼90%) with little or no mercury species transformations. Demethylation as high as 22% using 5 mol l−1 HNO3 in a water-bath heating (at 100 °C for 2 h) and 90% with cold acid digestion using concentrated HNO3 and H2O2 (room temperature for 24 h) were observed, however, all the extraction methods tested produced accurate quantitation when corrected in accordance with the SIDMS method standardized in the US Environmental Protection Agency's method 6800.

Robust microwave-assisted extraction protocol for determination of total mercury and methylmercury in fish tissues

A rapid and efficient closed vessel microwave-assisted extraction (MAE) method based on acidic leaching was developed and optimized for the extraction of total mercury (Hg), inorganic mercury (Hg 2+) and methylmercury (CH 3Hg +) from fish tissues. The quantitative extraction of total Hg and mercury species from biological samples was achieved by using 5 mol L −1 HCl and 0.25 mol L −1 NaCl during 10 min at 60 °C. Total Hg content was determined using inductively coupled plasma mass spectrometry (ICP–MS). Mercury species were measured by liquid chromatography hyphenated with inductively coupled plasma mass spectrometry (LC–ICP–MS). The method was validated using biological certified reference materials ERM-CE464, DOLT-3, and NIST SRM-1946. The analytical results were in good agreement with the certified reference values of total Hg and CH 3Hg + at a 95% confidence level. Further, accuracy validation using speciated isotope-dilution mass spectrometry (SIDMS, as described in the EPA Method 6800) was carried out. SIDMS was also applied to study and correct for unwanted species transformation reactions during and/or after sample preparation steps. For the studied reference materials, no statistically significant transformation between mercury species was observed during the extraction and determination procedures. The proposed method was successfully applied to fish tissues with good agreement between SIDMS results and external calibration (EC) results. Interspecies transformations in fish tissues were slightly higher than certified reference materials due to differences in matrix composition. Depending on the type of fish tissue, up to 10.24% of Hg 2+ was methylated and up to 1.75% of CH 3Hg + was demethylated to Hg 2+.

Comparison of methods with respect to efficiencies, recoveries, and quantitation of mercury species interconversions in food demonstrated using tuna fish

Eight different analytical extraction procedures commonly used to extract mercury species from biological samples were evaluated by analyzing Tuna Fish Tissue Certified Reference Material (ERM-CE464) certified for the content of total mercury and methylmercury. Speciated isotope dilution mass spectrometry (SIDMS; US Environmental Protection Agency's method 6800) was utilized to evaluate and effectively compensate for potential errors during measurement and accurately quantify mercury species using all the extraction methods. SIDMS was used to accurately evaluate species transformations during sample pretreatment, preparation and analysis protocols. The extraction methods tested in this paper were based on alkaline extraction with KOH or tetramethylammonium hydroxide; acid leaching with HCl, HNO(3) or CH(3)COOH; extraction with L: -cysteine hydrochloride; and enzymatic digestion with protease XIV. Detection of total mercury and mercury species from all extraction methods was carried out by inductively coupled plasma mass spectrometry (ICP-MS) and high-performance liquid chromatography-ICP-MS, respectively. Microwave-assisted extraction and ultrasound-assisted extraction were found to be the most efficient alkaline digestion protocols that caused the lowest levels of transformation of mercury species (6% or less). Extraction with 5 M HCl or enzymatic digestion with protease resulted in the second-highest extraction efficiency, with relatively lower transformation of methylmercury to inorganic mercury (3 and 1.4%, respectively). Despite frequent use of acid leaching for the extraction of mercury species from tuna fish samples, the lowest extraction efficiencies and the highest mercury species transformation were obtained when microwave-assisted extraction with 4 M HNO(3) or CH(3)COOH was used. Transformations as high as 30% were found using some literature protocols; however, all the extractions tested produced accurate quantitation when corrected in accordance with the SIDMS method standardized in the US Environmental Protection Agency's method 6800. [figure: see text]

Development of a standard reference material for Cr(vi) in contaminated soil

Over the last several decades, considerable contamination by hexavalent chromium has resulted from the land disposal of Chromite Ore Processing Residue (COPR). COPR contains a number of hexavalent chromium-bearing compounds that were produced in high temperature industrial processes. Concern over the carcinogenic potential of this chromium species, and its environmental mobility, has resulted in efforts to remediate these waste sites. To provide support to analytical measurements of hexavalent chromium, a candidate National Institute of Standards and Technology (NIST) Standard Reference Material® (SRM 2701), having a hexavalent chromium content of approximately 500 mg kg−1, has been developed using material collected from a waste site in Hudson County, New Jersey, USA. The collection, processing, preparation and preliminary physico–chemical characterization of the material are discussed. A two-phase multi-laboratory testing study was carried out to provide data on material homogeneity and to assess the stability of the material over the duration of the study. The study was designed to incorporate several United States Environmental Protection Agency (USEPA) determinative methods for hexavalent chromium, including Method 6800 which is based on speciated isotope dilution mass spectrometry (SIDMS), an approach which can account for chromium species inter-conversion during the extraction and measurement sequence.

High yield synthesis and characterization of isotopically enriched monoethylmercury chloride (C2H5201HgCl)

AbstractAn important but commercially unavailable compound isotopically enriched monoethylmercury chloride (C2H5201HgCl), has been synthesized from commercially available 201HgO (98.11% enriched isotopic purity) and tetraethyltin. The required synthesis time is 1 h at 90 °C, and the product is the single product of monoethylmercury chloride, yielding more than 95% as 201Hg in C2H5201Hg+ (98.19 ± 0.22% enriched isotopic purity). The synthesized product was analyzed with high‐performance liquid chromatography coupled with inductively coupled plasma mass spectrometry (HPLC‐ICP‐MS) to determine its concentration, isotopic composition and purity. The synthetic isotopically enriched monoethylmercury synthesized can be used in speciated isotope dilution mass spectrometry (SIDMS) and isotope dilution mass spectrometry (IDMS) analyses as a standard. Copyright © 2005 John Wiley &amp; Sons, Ltd.

Determination of hexavalent chromium by using speciated isotope-dilution mass spectrometry after microwave speciated extraction of environmental and other solid materials

Precise and accurate determination of hexavalent chromium in different types of solid environmental sample is regarded as a technical challenge with significant potential error if historically accepted methods are used. Microwave-assisted alkaline extraction (0.5 mol L(-1) NaOH + 0.28 mol L(-1) Na2CO3) followed by anion-exchange chromatographic separation and inductively coupled plasma mass spectrophotometric detection has been shown to provide accurate and precise results. To obtain a better understanding of potential species conversion during and/or after extraction steps, speciated isotope-dilution mass spectrometry (SIDMS) (EPA Method 6800) metrology has been successfully applied as a diagnostic tool with the modified accompanying extraction version of EPA Method 3060A. In our study, aggregate materials distributed over a large area of a major western US state were found to contain a high concentration of total chromium (195 +/- 13 to 709 +/- 19 microg g(-1)) and significant amounts of Cr6+ (141 +/- 6 to 341 +/- 29 microg g(-1)) which are at least three orders of magnitude higher than the US EPA threshold limit (0.5 microg g(-1)). Sediment samples from a major western US state, studied independently, were found to contain less (1.77 +/- 0.34 microg g(-1)) or no Cr6+ in the presence of significant total chromium.

Mercury in environmental samples: Speciation, artifacts and validation

Speciation of mercury compounds in environmental samples requires rigorous analytical procedures at each stage of sample collection, treatment and measurement. Sampling, the first step, is generally only critical for water samples. The best materials for water-sample storage and processing are Pyrex and Teflon (PTFE or FEP) after a severe cleaning procedure. Extraction of the Hg species from its matrix, the second step, requires an aggressive treatment, such as acid extraction (mostly combined with solvent extraction), distillation or alkaline extraction. Extraction is one of the most critical steps, and, for biota and sediments, almost certainly the most critical, because two conflicting issues need to be addressed – obtaining adequate recovery, and preventing losses. Extraction efficiency and validation are discussed as well as methylation artifacts, especially when the distillation method is applied to sediments. Separation of the Hg species, the third step, includes derivatization followed by gas chromatography (GC), new GC improvements and liquid chromatography (LC), which has, since the development of more sensitive detectors, resulted in wider applications in environmental studies. Detection of the Hg species is the fourth and last step in the analytical procedure. The development of commercial, relatively inexpensive, extremely sensitive, selective cold vapor atomic fluorescence spectrometry (CV-AFS) instrumentation in the late 1980s and 1990s made this the most popular detector for the laboratories working on the biogeochemical cycling of Hg. In recent years, the use of inductively coupled plasma mass spectrometry (ICP-MS) in speciation analysis has increased tremendously. Besides its high sensitivity and selectivity, ICP-MS offers the opportunity to perform speciated isotope dilution mass spectrometry (SID-MS). Finally, as other measurement fields, speciation analysis requires suitable reference materials to be available for the verification of accuracy and to meet quality-assurance needs. We also address these important aspects.