Arsenic speciation analysis

Nearly two dozen arsenic species are present in the environmental and biological systems. Differences in their toxicity, biochemical and environmental behaviors require the determination of these individual arsenic species. Considerable analytical progresses have been made toward arsenic speciation analysis over the last decade. Hyphenated techniques involving a highly efficient separation and a highly sensitive detection have become the techniques of choice. Methods based on high-performance liquid chromatography separation with inductively coupled plasma mass spectrometry, hydride generation atomic spectrometry, and electrospray mass spectrometry detection have been shown most useful for arsenic speciation in environmental and biological matrices. These hyphenated techniques have resulted in the determination of new arsenic species, contributing to a better understanding of arsenic metabolism and biogeochemical cycling. Methods for extracting arsenic species from solid samples and for stabilizing arsenic species in solutions are required for obtaining reliable arsenic speciation information.     

Arsenic speciation in contaminated soils

A method for arsenic speciation in soils is developed, based on extraction with a mixture of 1 mol l(-1) of phosphoric acid and 0.1 mol l(-1) of ascorbic acid, and further measurement with the coupling liquid chromatography (LC)-ultraviolet (UV) irradiation-hydride generation (HG)-inductively coupled plasma mass spectrometry (ICP/MS). The stability of the arsenic species in the extracts is also studied. The speciation method applied to several Spanish agricultural contaminated soils from the Aznalcollar zone shows that arsenate is the main species in all the soils analysed and that in some samples arsenite and methylated species could also be detected. The determination of the "pseudototal" arsenic in these soils, obtained by applying extraction with aqua regia (ISO Standard 11466), is also carried out. Both the speciation method and the aqua regia method are applied to several certified reference materials (CRMs) in which total arsenic content is certified. Finally, the same LC-UV-HG coupling with atomic fluorescence spectrometry (AFS) detection reveals to be a valid coupling system to perform arsenic speciation in the soils according to its fair quality parameters and easy utilisation.     

Arsenic speciation in hair extracts

Ingested arsenic is known to be not only excreted by urine, but to be stored in sulphydryl-rich tissue like hair, nail or skin. We developed an extraction method for arsenic species from these tissues and studied the stability of different arsenic species during the extraction process. Inorganic and pentavalent methylated arsenic was found to be stable under the extraction conditions, whereas trivalent methylated arsenicals and the thio-analogue of DMA^V (DMAS) showed reduced stability. The absorption ability of hair for these different species was studied as well. Inorganic arsenic is better absorbed by hair than monomethyl- or dimethyl-arsenicals, whereby the trivalent forms are taken up better than the pentavalent forms. Independent of which methylated arsenical was used for the incubation, the pentavalent form was always the dominant form after extraction. Hair and nail samples from humans suffering from chronic arsenic intoxication contained dominantly inorganic arsenic with small and strongly varying amounts of DMA^V and MA^V present. DMAS was only found in some nail sample extracts containing unusually high amounts of DMA^V and is believed to be formed during the extraction process.     

Arsenic speciation patterns in freshwater fish

Muscle of 16 freshwater fish (9 different species belonging to 4 different families) was analysed for arsenic species using HPLC separation (anion and cation exchange) followed by on-line UV-decomposition, hydride generation and AFS detection. The main arsenic compounds found in the extracts were arsenobetaine (AsB), which accounted for 92–100% of extractable arsenic in species of salmonids (Salmo marmoratus, Oncorhynchus mykiss, Salmo trutta m. fario), and dimethylarsinic acid (DMAA), which accounted for 75% of extractable arsenic in burbot (Lota lota). AsB was also found in lower concentrations in almost all other fish species analysed (Silurus glanis, L. lota, Barbus barbus, Rutilus pigus virgo, Chondrostoma nasus). Arsenite (As(III)) and trimethylarsine oxide (TMAO) were detected in low concentrations in some representatives of Cyprinidae only (R. pigus virgo, C. nasus). Except in salmonids, an unknown cationic compound was present in most of the samples in relatively low concentrations. Cluster analysis of the generated data seems to indicate that there is a correlation between fish family and the arsenic speciation pattern. This is especially clear for the salmonids which show a completely separate cluster and thus a very distinct arsenic speciation pattern.     

Arsenic speciation in iron hydroxide precipitates

In this study a special sequential extraction method is proposed to discriminate between arsenic adsorbed and co-precipitated in precipitates arising mainly from iron hydroxides or bound in low solubility mineral phases. Synthetic iron hydroxide precipitates were prepared to investigate the influence of the amount of arsenate, of the manganese additionally added and of the valence state of arsenic on the remobilisation of arsenic. After preparing the precipitates with arsenate no arsenic could be detected in the supernatant solution. About 82% (w/w) of the arsenate is adsorbed to the precipitate and the remaining part can be dissolved by shaking with an oxalate buffer. A significant difference between the amount of arsenic added and the amount analysed in the two steps was not found. Consequently, compounds with a low solubility, such as scorodite, were not formed in the synthesized precipitates. The valence of the arsenic and addition of manganese influence significantly the uptake of arsenic by iron hydroxides. Natural precipitate samples from a percolate water of tin mill tailings were investigated using this method.     

Arsenic speciation in clams of British Columbia

The water-soluble arsenic compounds in five species of clams – Butter clam (Saxidomus giganteus), Horse clam (Schizothoerus nuttalli), Soft-shelled clam (Mya arenaria), Native littleneck clam (Protothaca staminea), and Manila clam (Venerupis japonica) – are described. Varying amounts of arsenobetaine and tetramethylarsonium ion are the major arsenicals found in all species. Butter clams show the presence of a third compound which appears to be trimethylarsine oxide. Small amounts of as-yet-unidentified arsenicals can also be isolated.     

Arsenic speciation by capillary gas-liquid chromatography

Specific environmentally significant arsenic compounds are determined by capillary gas-liquid chromatography. Inorganic (arsenite, arsenate) and organic (monomethylarsonate, dimethylarsinate) arsenicals are measured as the corresponding methylthioglycolate derivatives, which are simultaneously separated on wide-bore borosilicate glass and fused-silica columns under conditions of temperature programming. Inorganic arsenate and arsenite cannot be differentiated by the derivatization technique. Flame-ionization and electron-capture detection are evaluated. A simple and rapid sample preparation procedure is used for water, urine, blood, and tissue.     

Arsenic speciation in chinese seaweeds using HPLC-ICP-MS and HPLC-ES-MS

Three common Chinese edible seaweeds, one brown (Laminaria japonica) and two red (Porphyra crispata and Eucheuma denticulatum), were examined for their total arsenic content. The As species were extracted with yields of 76.4, 69.8 and 25.0%, respectively. Anion-exchange and cation-exchange high-performance liquid chromatography (HPLC) in combination with inductively coupled plasma mass spectrometry (ICP-MS) were used for the separation of the different arsenic species in two of the three seaweed extracts (Laminaria and Porphyra). The main arsenic species in the algal extracts are arseno sugars, although it has been shown that the Laminaria seaweed contains significant amounts of dimethylarsinic acid (DMA). HPLC was coupled with electrospray mass spectrometry (ES-MS) for structural confirmation of the arsenic species. The mass spectrometer settings for the arseno sugars were optimised using standards. The conclusions drawn on the basis of HPLC-ICP-MS were confirmed by the HPLC-ES-MS data. The HPLC-ES-MS method is capable of determining both arseno sugars and DMA in the seaweeds. The unknown compounds seen in the HPLC-ICP-MS chromatogram of Laminaria could not be ascribed to trimethylarsenic oxide or tetramethylarsonium ion.     

Arsenic speciation using HPLC-HG-ICP-AES with gas-liquid separator

Summary The determination of different chemical species of arsenic in aquatic media has been carried out using several separation and detection techniques. As separation techniques GC, HPLC, HG and cold trap-selective thermal desorption are the most frequently used. As detection techniques FAAS, ETAAS or, to a lesser extent, ICP-AES are applied. In this work the optimization conditions are presented for As(III), As(V), monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA) determination using (IC)HPLC coupled to HG with gas-liquid separator, and using ICP-AES as detector is presented. The gas-liquid separator impedes the entrance of mobile phase into the nebulizer and consequently into the plasma torch. The experimental conditions for HG (kind of acids and concentration, borohydride concentration, reagent flows) as well as plasma conditions have been optimized.The quality parameters: LOD, precision and accuracy are reported for all the species studied.     

Arsenic speciation in natural waters by cathodic stripping voltammetry

Contamination of groundwater with arsenic (As) is a major health risk through contamination of drinking and irrigation water supplies. In geochemically reducing conditions As is mostly present as As(III), its most toxic species. Various methods exist to determine As in water but these are not suitable for monitoring arsenic speciation at its original pH and without preparation. We present a method that uses cathodic stripping voltammetry (CSV) to determine reactive As(III) at a vibrating, gold, microwire electrode. The As(III) is detected after adsorptive deposition of As(OH)₃⁰, followed by a potential scan to measure the reduction current from As(III) to As(0). The method is suitable for waters of pH 7-12, has an analytical range of 1 nM to 100 μM As (0.07-7500 ppb) and a limit of detection of 0.5 nM with a 60 s deposition time. The As speciation protocol involves measuring reactive As(III) by CSV at the original pH and acidification to pH 1 to determine inorganic As(III) + As(V) by anodic stripping voltammetry (ASV) using the same electrode. Total dissolved As is determined by ASV after UV-digestion at pH 1. The method was successfully tested on various raw groundwater samples from boreholes in the UK and West Bengal.     

Arsenic speciation by ion-exchange separation and graphite-furnace atomic-absorption spectrophotometry

As(III), As(V), monomethylarsenic acid (MMA) and dimethylarsenic acid (DMA) were determined by graphite-furnace atomic-absorption spectrophotometry after separation of the species by ion-exchange chromatography. The detection limits (ng/ml) were DMA 0.02, MMA 2.0, As(V) 0.4 and total arsenic 4.0. As(III) was determined by difference. This system gave better detection limits and/or shorter analysis times than previously reported systems.     

Selenium speciation in foods: Preliminary results on potatoes

The present paper deals with the speciation of selenium in potatoes (enriched or not in selenium). The study was carried out by using differential pulse cathodic stripping voltammetry (DPCSV) for quantifying selenium. Results obtained provide evidence that the selenium content in the protein fraction is rather independent from the selenium added to the plants during their growth. On the contrary, the amount of Se in the non-protein fraction (water and starch) in Se-enriched sample is significantly higher than in non-enriched one, suggesting that it is the main selenium-storing site. In this fraction the Se(VI)/Se(IV) ratio seems independent from selenium application but it may be related to the redox conditions. The accumulation of selenium in the non-protein fraction is tentatively ascribed to the “Se–starch interaction” that should be able to modulate both the Se absorption into proteins and, possibly, its toxic effect for the plant itself.     

Arsenic speciation in solid biological specimens by temperature-controlled pyrolysis and NAA

A pyrolysis-neutron activation analysis (NAA) procedure has been developed and applied to the speciation of arsenic in solid biological samples. The method involves the retention of the inorganic arsenic in the pyrolysis boat by the addition of NaOH, the volatilization and trapping of the organic arsenic on a cation exchange resin and the subsequent NAA of the resin for the determination of the trapped arsenic. The method, developed with the aid of radiochemically labelled arsenic compounds, has been applied to the determination of the ratio of inorganic to organic arsenic species in commercical shrimps as well as in NBS standard reference materials such as oysters and orchard leaves. The results show different relative amounts of inorganic arsenic content in the samples analysed. In the shrings the fraction of inorganic arsenic was of the order of 20%, in the oysters the inorganic arsenic consfituted 60% of the total arsenic concentration while in the samples of vegetable origin more than 98% of the arsenic was of inorganic nature.     

Arsenic speciation in humans and food products: a review.

Although acute intoxication has become rare, arsenic (As) is still a dangerous pollution agent for industrial workers and people living in the vicinity of emission sources. In humans, only inorganic As is toxic; organic forms present in large amounts in the environment are nontoxic. It is therefore important to be able to differentiate one group from the other using appropriate speciation methods. The authors review the present knowledge of the distribution of As in humans and food products. The three steps of the speciation methods (sample preparation, species separation, and detection) are described. For liquid samples, a clean-up step (C18 cartridge extraction, dilution, or freezing) is necessary to eliminate proteins and salts from the matrix. For solid organic samples, the first step consists of the digestion of tissues followed by solvent extraction sometimes coupled with a C18 extraction. The separation of As species is accomplished by different high-performance liquid chromatography (HPLC) methods (ion-exchange, ion-pairing, and micellar liquid chromatography). The detection methods are compatible with HPLC and are able to detect As species in the microgram-per-liter range. Inductively coupled plasma (ICP) atomic emission spectrometry is more frequently used, but suffers from interference by organic solvents in the mobile phases. Atomic absorption spectrometry methods give sensitivities of the same order. ICP-mass spectrometry has the advantage of specificity and can be 100- to 1000-fold more sensitive than previous methods.     

Tritium concentration in some European foods

High specific activity ratios of tissue-bound³H to free water³H were reported for foods in Europe und USA during 1970s and 1980s. To examine these high specific activity ratios, small number of some recent food samples imported from European countries were previously analyzed in our laboratory. The specific activity ratios obtained were around unity, and not so high as those reported by other investigators. To confirm our results, additional food samples were collected during 1988–90 and analyzed for³H. The specific activity ratios in the food samples were found to be around unity, and the previous results was confirmed.     

High-performance liquid chromatographic determination of tocopherols in infant formulas

A method for the simultaneous determination of alpha-tocopherol acetate and alpha-, delta-, and gamma-tocopherols by normal-phase high-performance liquid chromatography (HPLC) with a fluorescent detector in infant formula is proposed. The values obtained in the determination of the analytical parameters: linearity, precision, limit of detection and accuracy (analysis of a standard reference material, SRM 1846), confirm the quality of the method. The proposed method is useful for the determination of alpha-, delta-, and gamma-tocopherols and alpha-tocopherol acetate in infant formulas at a low cost and in a total time of 2 h.     

Arsenic speciation in marine biological materials by LC-UV-HG-ICP/OES

Arsenic compounds have been extracted with methanol/water (1:1) from three Certified Reference Materials (CRMs): CRM 278 (mussel tissue), CRM 422 (cod muscle) and DORM-1 (dogfish muscle). The extracts obtained were analyzed by an LC-UV-HG-ICP/OES coupled system, which permits the one-line determination of arsenocholine, arsenobetaine, dimethylarsinate, monomethylarsonate, As(III) and AS(V) at concentration in the range of gl^–1. The main species found in all CRMs extracts was arsenobetaine (97.3% of total As in CRM 422, 89.5% of total As in DORM-1 and 22.2% of total As in CRM 278).     

Arsenic speciation in freshwater organisms from the river Danube in Hungary

Total arsenic and arsenic species were determined in a range of freshwater samples (sediment, water, algae, plants, sponge, mussels, frog and fish species), collected in June 2004 from the river Danube in Hungary. Total arsenic concentrations were measured by ICPMS and arsenic species were measured in aqueous extracts of the samples by ion-exchange HPLC-ICPMS. In order to separately determine the efficiency of the extraction method and the column recovery, total arsenic concentrations in the extracts were obtained in three ways: (i) ICPMS determination after acid digestion; (ii) flow injection analysis performed directly on the extract; (iii) the sum of arsenic species eluting from the HPLC column. Extraction efficiencies were low (range 10-64%, mean 36%), but column recovery was acceptable (generally >80%) except for the fish samples, where substantial, currently unexplained, losses were observed. The dominating arsenic species in the extracts of freshwater algae were arsenosugars, whereas arsenate [As(V)] was present only as a minor constituent. On the other hand, plant extracts contained only inorganic arsenic, except for two samples which contained trace amounts of dimethylarsinate (DMA) and the tetramethylarsonium cation (TETRA). The oxo-arsenosugar-phosphate (ca. 35% of extractable arsenic) and the oxo-arsenosugar-glycerol (ca. 20%) as well as their thio-analogues (1-10%) were found in the mussel extracts, while arsenobetaine (AB) was present as a minor species only. In general, fish extracts contained only traces of arsenobetaine, and the oxo-arsenosugar-phosphate was the major arsenic compound. In addition, samples of white bream contained thio-arsenosugar-phosphate; this is the first report of a thio-arsenical in a fish sample. The frog presented an interesting arsenic speciation pattern because in addition to the major species, arsenite [As(III)] (30%) and the tetramethylarsonium cation (35%), all three intermediate methylation products, methylarsonate (MA), dimethylarsinate and trimethylarsine oxide (TMAO), and arsenate were also present. Collectively, the data indicate that arsenobetaine, the major arsenical in marine animals, is virtually absent in the freshwater animals investigated, and this represents the major difference in arsenic speciation between the two groups of organisms.     

Arsenic accumulation and speciation analysis in wool from sheep exposed to arsenosugars

Wool or hair fibre is a metabolically dead material after it has left the epidermis. During growth the fibre in the root is a metabolically very active organ, which is highly influenced by the health status of the living being. Arsenic is one of the elements that is easily taken up by the cells of the root and stored in the fibre afterwards. Here we show that arsenic can quantitatively be extracted by boiling the wool fibre or hair in water. The high intake of arsenic species by the sheep of North Ronaldsay (the seaweed-eating sheep) leads to a high arsenic concentration in wool (mean 5.2+/-2.3 mug g(-1)). The wool of lambs of these sheep, which are not exposed to seaweed, contains about 10 times less arsenic, which is still elevated compared to uncontaminated wool. The arsenic species identified in wool extract are arsenite (As(III)), arsenate (As(V)), monomethylarsonic acid (MMA(V)) and monomethylarsonious acid (MMA(III)) as minor species. The major species is dimethylated arsenic DMA in its tri- and pentavalent form (dimethylarsinous acid (DMA(III)) and dimethylarsinic acid (DMA(V))) accounting for 85% of the specified arsenic in the wool which reflects the amount of dimethylated species (i.e. the arsenoribofuranosides) taken up by seaweed being the main food source of the sheep. However, there are unknown arsenic species in the extract, which are not eluting from a strong anion exchange column. In vitro incubation experiments with this kind of wool showed that it has reducing properties but no demethylation was recorded. The absorption ability of the wool for methylated arsenic species is negligible, while inorganic arsenic is easier to be absorbed in the fibre (11-17%). This means that the species integrity is only guaranteed in terms of the degree of methylation but not in terms of their redox status.