Determination of arsenic species: A critical review of methods and applications, 2000–2003

Simultaneous Determination of Arsenic, Selenium and Mercury in Foodstuffs by Chemical Vapour Generation Inductively Coupled Plasma Optical Emission Spectroscopy

A procedure for the simultaneous determination of arsenic, selenium and mercury in foodstuffs has been developed. After a two-step microwave-assisted wet digestion in closed vessels, using concentrated nitric acid and hydrogen peroxide, the solution was analysed by inductively coupled plasma multichannel-based emission spectrometry using chemical vapour generation as the sample introduction system. All steps of the procedure, such as solid sample dissolution, pre-reduction to the suitable oxidation state, vapor generation, transport and atomization have been designed and optimised taking into account the concomitant presence of all the analytes considered. Temporal variation of analytical signals as well as interfering effects due to transition elements were also studied. Under the optimised operating conditions, the achieved detection limits for the simultaneous determination of arsenic, selenium and mercury in foodstuffs were 0.006, 0.023 and 0.018 microg g(-1), respectively, allowing their determination in real samples. Precision of the analytical procedure was 6.8% for arsenic, 5.2% for selenium and 7.7% for mercury (n=7). The accuracy and reliability of the method was verified by the analysis of both standard reference materials (rice flour and spinach leaves) and real samples (natural and Se-enriched rice).

Biomarkers of exposure: a case study with inorganic arsenic

The environmental contaminant inorganic arsenic (iAs) is a human toxicant and carcinogen. Most mammals metabolize iAs by reducing it to trivalency, followed by oxidative methylation to pentavalency. iAs and its methylated metabolites are primarily excreted in urine within 4-5 days by most species and have a relatively low rate of bioaccumulation. Intra- and interindividual differences in the methylation of iAs may affect the adverse health effects of arsenic. Both inorganic and organic trivalent arsenicals are more potent toxicants than pentavalent forms. Several mechanisms of action have been proposed for arsenic-induced toxicity, but a scientific consensus has not been achieved. Biomarkers of exposure may be used to quantify exposure to iAs. The most common biomarker of exposure for iAs is the measurement of total urinary arsenic. However, consumption of seafood containing high concentrations of organic arsenic can confound estimation of iAs exposure. Because these organic species are thought to be relatively nontoxic, their presence in urine may not represent increased risk. Speciation of urinary arsenic into inorganic and organic forms, and even oxidation state, gives a more definitive indication of the exposure to iAs. Questions still remain, however, as to how reliably the measurement of urinary arsenic, either total or speciated, may predict arsenic concentrations at target tissues as well as how this measurement could be used to assess chronic exposures to iAs.

Improved chromatographic separation of thio-arsenic compounds by reversed-phase high performance liquid chromatography-inductively coupled plasma mass spectrometry

A new group of arsenic species, thio-arsenicals, have recently been reported in several natural samples such as molluscs, algae, and urine. These compounds are the sulfur analogues of oxo-arsenicals, a large group of naturally-occurring compounds, whereby the arsinoyl (As=O) group is substituted by an arsinothioyl group (As=S). The most common separation technique for oxo-arsenicals is anion-exchange HPLC with polymer-based columns, but under these conditions the thio-arsenicals show strong retention, resulting in unacceptably long analysis times and broad peaks. We report the development of a reversed-phase HPLC method, with ICPMS detection, which allows separation of the known thio-arsenicals within 15 min with significantly improved peak shapes. The detection limit is about 0.6 microg As/L based on 10 microL injection volume. Further, we have applied the method to the identification and quantification of thio-arsenic species in two standard reference materials, BCR 710 oyster tissue and NIES 18 human urine.

Disposition of arsenobetaine in two marine fish species following administration of a single oral dose of [14C]arsenobetaine

The distribution and excretion of arsenobetaine in fish were investigated using whole body autoradiography and liquid scintillation counting. A single dose of synthesised [(14)C]arsenobetaine was orally administered to Atlantic salmon, Salmo salar L., and Atlantic cod, Gadus morhua L. Arsenobetaine was distributed to most organs within both species. Nevertheless, there were species differences in tissue distribution and excretory pattern. The highest level of arsenobetaine in Atlantic salmon was present in muscle tissue, while high levels of arsenobetaine were found in both muscle and liver (including gall bladder) from Atlantic cod. The results suggest that the major route of excretion was via urine, which seemed to be more important in Atlantic cod than in Atlantic salmon. Elimination of arsenobetaine via bile appeared to be negligible in both species.

Sonically assisted electroanalytical detection of ultratrace arsenic

A simple portable handheld electrochemical sensor with an integrated sound source for the detection of ultratrace quantities of arsenic using square wave anodic stripping voltammetry is described. The sensor uses low-frequency sound (250 Hz) during the arsenic deposition step to enhance the sensitivity of the arsenic stripping response. It is found that under quiescent (silent) conditions a detection limit of 2.1 x 10(-7) M with a sensitivity of 0.51 M(-1) A is achievable using a 120-s accumulation period, while applying low-frequency sound using a "sonotrode" reduced this detection limit to 3.7 x 10(-9) M with an increased sensitivity of 27.2 M(-1) A. Thus, the low-frequency sonotrode is shown to increase the sensitivity by ca. 50 times while reducing the limit of detection by 2 orders of magnitude. A study of the effect of copper contamination is carried out as well as analysis in real samples; it is found that although as expected copper detrimentally effects the arsenic limit of detection, it does not rise significantly above 10(-8) M levels.

Arsenosugars and other arsenic compounds in littoral zone algae from the Adriatic Sea

In 10 different marine algae from the littoral zone (found between the highest and lowest tide marks on the seashore) arsenic compounds were determined by means of a high-performance liquid chromatography (anion and cation exchange)-UV photochemical digestion-hydride generation-atomic fluorescence spectrometry (HPLC-UV-HGAFS) system. Samples (Ceramium sp., Cystoseira barbata, Enteromorpha sp., Fucus virsoides, two different species of Gelidium, Padina pavonica, Polisyphonia sp. and Ulva rigida) were collected along the Adriatic Sea coast of Slovenia. The total arsenic content of the algal samples, as determined by ICP-MS, ranged from 1.35 to 28.1 microg g(-1) (fresh weight). In all algae but two, the most abundant arsenic species found were arsenosugars with minor amounts of other arsenic compounds. Cystoseira barbata and Ceramium sp. contained high amounts of mainly inorganic arsenic. A small quantity of arsenobetaine was detected in most of the investigated Adriatic algae, which probably originates from mesofauna attached to the algae in their natural habitat.

Speciation of alkylated metals and metalloids in the environment

The analytical methodology for speciation of metals and metalloids associated with alkyl groups and biomacromolecules is critically reviewed. Alkylated metals and metalloids are not only known to be produced by microbial methylation within most anaerobic compartments in the environment, but also in the course of enzymatic transformations during human metabolism. Because of the toxicological relevance of these compounds present in trace to ultratrace concentrations, firm species identification and exact quantification are essential. While many instrumental techniques coupling chromatography (GC, HPLC, CE, GE) with plasma mass spectrometry (ICP-MS) are available for quantification, methods used for structural identification often suffer from inadequate sensitivity (EI-MS, ESI-MS, MALDI-MS, FT-ICRMS). Other problems encountered are sample derivatisation artefacts, lack of suitable standards for quantification, lack of equilibrium between spikes and sample, and the integrity of metal-protein association during separation, in particular during SDS-PAGE. Selected application examples with respect to mercury and arsenic speciation will be discussed critically.

Mass spectrometry in bioinorganic analytical chemistry

A considerable momentum has recently been gained by in vitro and in vivo studies of interactions of trace elements in biomolecules due to advances in inductively coupled plasma mass spectrometry (ICP MS) used as a detector in chromatography and capillary and planar electrophoresis. The multi-isotopic (including non-metals such as S, P, or Se) detection capability, high sensitivity, tolerance to matrix, and large linearity range regardless of the chemical environment of an analyte make ICP MS a valuable complementary technique to electrospray MS and MALDI MS. This review covers different facets of the recent progress in metal speciation in biochemistry, including probing in vitro interactions between metals and biomolecules, detection, determination, and structural characterization of heteroatom-containing molecules in biological tissues, and protein monitoring and quantification via a heteroelement (S, Se, or P) signal. The application areas include environmental chemistry, plant and animal biochemistry, nutrition, and medicine.

Precursor ion scanning for the non-targeted detection of individual arsenosugars in extracts of marine organisms

Arsenosugars are a group of arsenic compounds reported to be present in a wide variety of marine organisms. Numerous such compounds have been identified and characterized in marine organisms; however, unknown arsenosugar species may also be present. This indicates the need for an analytical technique suitable for their non-targeted detection. One such technique is tandem mass spectrometry operated in the precursor ion scanning mode. This technique is based on scanning for precursor ions that give specific product ions, characteristic of the compounds under investigation. In the present study two subgroups of arsenosugar species were examined, the oxo- and the thioarsenosugars, the CID behavior of which is well known from previous studies. In the case of the oxoarsenosugars characteristic product ions were observed at m/z 237 and 97, and for the thioarsenosugars at m/z 253 and 97. Validation of this approach was carried out by analyzing extracts of two commercial kelp powders with known contents of arsenosugar species. All arsenosugars reported to exist in these materials were detected successfully using the precursor ion scanning approach. The limits of detection for the oxo- and the thioarsenosugar species, and the selectivity and sensitivity of the method, strongly indicate the suitability of this approach for the non-targeted detection of arsenosugars in extracts of marine origin.

Development of a liquid chromatography/electrospray selected reaction monitoring method for the determination of organoarsenic species in marine and freshwater samples

Cation- and anion-exchange high-performance liquid chromatography/electrospray selected reaction monitoring (HPLC/ES-SRM) methods were developed for the determination of 15 organoarsenic compounds in marine and freshwater samples. The results demonstrate that the developed HPLC/ES-SRM methods are powerful approaches for the identification of organoarsenic species in crude sample extracts. The detection limits, linearity as well as reproducibility for most of the species are comparable or even better than those measured by the HPLC/inductively coupled plasma mass spectrometry (ICPMS) technique. The qualitative analysis of the extracts shows that the developed methods allow for the identification of arsenicals which were not detectable by ICPMS. It was also demonstrated that the signal suppression caused by matrix effects means a significant limitation in the quantification of arsenicals by ES-SRM detection. This drawback is manifested especially in the case of the slightly retained species. The three sample-cleanup chromatographic methods including off-line size-exclusion, on-line reversed-phase and on-line oppositely charged ion-exchange approaches proved to be ineffective for separation of the signal-suppressive matrix from the analytes. The standard addition calibration seems to be a suitable solution for such problems.

Speciation analysis of inorganic arsenic by microchip capillary electrophoresis coupled with hydride generation atomic fluorescence spectrometry

A novel method for speciation analysis of inorganic arsenic was developed by on-line hyphenating microchip capillary electrophoresis (chip-CE) with hydride generation atomic fluorescence spectrometry (HG-AFS). Baseline separation of As(III) and As(V) was achieved within 54 s by the chip-CE in a 90 mm long channel at 2500 V using a mixture of 25 mmol l(-1) H3BO3 and 0.4 mmol l(-1) CTAB (pH 8.9) as electrolyte buffer. The precisions (RSD, n=5) ranged from 1.9 to 1.4% for migration time, 2.1 to 2.7% for peak area, and 1.8 to 2.3% for peak height for the two arsenic species at 3.0 mg l(-1) (as As) level. The detection limits (3sigma) for As(III) and As(V) based on peak height measurement were 76 and 112 microg l(-1) (as As), respectively. The recoveries of the spikes (1 mg l(-1) (as As) of As(III) and As(V)) in four locally collected water samples ranged from 93.7 to 106%.

Advances in analytical methodology for bioinorganic speciation analysis: metallomics, metalloproteomics and heteroatom-tagged proteomics and metabolomics

The recent developments in analytical techniques capable of providing information on the identity and quantity of heteroatom-containing biomolecules are critically discussed. Particular attention is paid to the emerging areas of bioinorganic analysis including: (i) a comprehensive analysis of the entirety of metal and metalloid species within a cell or tissue type (metallomics), (ii) the study of the part of the metallome involving the protein ligands (metalloproteomics), and (iii) the use of a heteroelement, naturally present in a protein or introduced in a tag added by means of derivatisation, for the spotting and quantification of proteins (heteroatom-tagged proteomics). Inductively coupled plasma mass spectrometry (ICP MS), used as detector in chromatography and electrophoresis, and supported by electrospray and MALDI MS, appears as the linchpin analytical technique for these emerging areas. This review focuses on the recent advances in ICP MS in biological speciation analysis including sensitive detection of non-metals, especially of sulfur and phosphorus, couplings to capillary and nanoflow HPLC and capillary electrophoresis, laser ablation ICP MS detection of proteins in gel electrophoresis, and isotope dilution quantification of biomolecules. The paper can be considered as a followup of a previous review by the author on a similar topic (J. Szpunar, Analyst, 2000, 125, 963).

Comparison of atomic absorption and fluorescence spectroscopic methods for the routine determination of urinary arsenic

OBJECTIVE

Interpretation of urinary arsenic measurements is sometimes difficult because of the absorption of seafood that contains trimethylated arsenic forms (arsenobetaine and arsenocholine). The objective of this study was to develop a rapid and robust technique for the measurement of the sum of inorganic arsenic metabolites.

METHODS

Measurement of arsenic was performed in urine after hydride generation in acid medium. Using atomic fluorescence spectrometry (AFS) as the detection system, we developed a rapid (one determination in less than 2 min) technique using 50 microl urine without pre-treatment. Standardisation was done externally with a mixed standard solution containing inorganic trivalent arsenic (As(i) (III)), inorganic pentavalent arsenic (As(i) (V)), monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA) (15/15/12.5/57.5).

RESULTS

Samples distributed in the frame of an international comparison programme were used to assess accuracy of the AFS procedure, which gives a linear response up to 50 microg/l and proves more precise [coefficient of variation (CV)< 4% at 5 microg/l] and sensitive than the atomic absorption spectroscopy (AAS) technique using a quartz cell. An additional adaptation that allows the detection of non-directly reducible arsenic forms has also been validated for samples with high arsenic concentrations.

CONCLUSIONS

The present study demonstrates the superiority of AFS over atomic absorption spectrometry (AAS) in arsenic determination and the interest of online mineralisation prior AFS detection for the determination of arsenic concentration in urine.

Pretreatment procedures for characterization of arsenic and selenium species in complex samples utilizing coupled techniques with mass spectrometric detection

Research interest in analyzing arsenic and selenium is dictated by their species-dependent behavior in the environment and in living organisms. Different analytical methodologies for known species in relatively simple chemical systems are well established, yet the analysis of complex samples is still a challenge. Owing to the complex matrix and low concentrations of target species that may be chemically labile, suitable pretreatment of the sample becomes a critical step in any speciation procedure. In this paper, the pretreatment procedures used for arsenic and selenium speciation are reviewed with the emphasis on the link between the analytical protocol applied and the biologically-significant information provided by the results obtained. In the first approach, the aim of pretreatment is to convert the original sample into a form that can be analyzed by a coupled (hyphenated) technique, preventing possible losses and/or species interconversion. Common techniques include different leaching and extraction modes, enzymatic hydrolysis, species volatilization, and so on, with or without species preconcentration. On the other hand, if the speciation analysis is performed for elucidation of elemental pathways and specific functions in a living system, more conscious pretreatment and/or fractionation is needed. The macroscopic separation of organs and tissues, isolation of certain types of cells, cell disruption and separation of sub-cellular fractions, as well as isolation of a specific biomolecules become important. Furthermore, to understand molecular mechanisms, the identification of intermediate-often highly instable--metabolites is necessary. Real life applications are reviewed in this work for aquatic samples, soils and sediments, plants, yeast, and urine.

Direct measurement of lipid-soluble arsenic species in biological samples with HPLC-ICPMS

Lipid-soluble arsenicals (arsenolipids) occur in a wide range of biological samples where they may play a key role in the biosynthesis of organoarsenic compounds from inorganic arsenic. The study of these compounds has been hindered, however, by the lack of a suitable analytical technique able to separate and measure the various lipid species. As a source of arsenolipids, we used 10 crude fish oils from various regions of the world. Total arsenic analyses on the fish oils, performed with ICPMS following acid digestion with microwave-assisted heating, gave concentrations from 4.3 to 10.5 mg As kg(-1). All of the arsenic was soluble in non-polar solvents such as hexane. Analysis of the fish oils for arsenolipids was performed by normal phase HPLC-ICPMS with various mixtures of organic solvents as mobile phases. Inherent problems of instability associated with the introduction of organic solvents to the plasma were overcome by the use of reduced column flow, a chilled spray chamber, and the addition of oxygen directly to the plasma. All ten fish oils appeared to contain the same 4-6 major arsenolipids, but in varying amounts depending on the origin of the fish. Further chromatography with both normal phase and reversed-phase conditions on some of the oils indicated the presence of many more minor arsenolipids. Quantification was achieved by external calibration against triphenylarsine oxide or triphenylarsine sulfide, and the sum of species following HPLC of the oils matched well the total arsenic results (92-107%). The method was applied to samples of food supplements (fish oil capsules) and a packaged food product (cod liver) whereby arsenolipids were measured and found to be significant arsenic constituents. This study represents the first attempt to directly measure intact arsenolipids and, with appropriate sample preparation, may be suitable for quantitative measurement of these arsenicals in a range of biological samples, including foodstuffs.

Report on three aliphatic dimethylarsinoyl compounds as common minor constituents in marine samples. An investigation using high-performance liquid chromatography/inductively coupled plasma mass spectrometry and electrospray ionisation tandem mass spectrometry

Three water-soluble aliphatic arsenicals, dimethylarsinoyl acetate (DMAA), dimethylarsinoyl ethanol (DMAE), and dimethylarsinoyl propionate (DMAP), were identified in marine biological samples. Sample extracts in methanol/water (1 + 1) were analysed by cation-exchange high-performance liquid chromatography/inductively coupled plasma mass spectrometry (HPLC/ICPMS). Eluate fractions from the HPLC/ICPMS analyses containing the compounds in question were collected and subjected to analysis by electrospray ionisation tandem mass spectrometry (ESI-MS/MS), which provided supportive evidence for the structures of the three compounds. The concentrations of the three arsenicals were determined in 37 marine organisms comprising algae, crustaceans, bivalves, fish and mammals by HPLC/ICPMS. The three arsenicals DMAA, DMAE and DMAP, which occurred at microg kg(-1) concentrations, were detected in 25, 23 and 17 of the 37 samples analysed, respectively. The limits of detection were 2-3 microg kg(-1) dry mass. The data illustrate that the three compounds are common minor constituents in marine samples. This is the first report on DMAE and DMAP as naturally occurring species in marine samples. The presence of DMAA and DMAE supports a proposed biosynthesis of arsenobetaine (AB) from dimethylarsinoylribosides. Alternative proposals, which explain the presence of the compounds in marine samples, are addressed briefly in the paper.

A comparison of different types of gold?carbon composite electrode for detection of arsenic(III)

A study has been conducted using abrasively modified basal and edge-plane graphite, carbon-paste, and carbon-epoxy electrodes to create gold-carbon composite electrodes. Using either nano or micro-sized gold particles their suitability for use in detecting arsenic(III) is assessed. It was found that gold arrays prepared from micron-sized particles gave the best performance for arsenic detection. In particular micron arrays produced in carbon-paste electrodes with an easily renewable surface work well for detection of arsenic, producing a detection limit of 5(+/-2)x10(-9) mol L(-1), with a high sensitivity of 10(+/-0.1) A mol(-1) L.