Complementary chromatography separation combined with hydride generation-inductively coupled plasma mass spectrometry for arsenic speciation in human urine

This study aimed to establish complementary high performance liquid chromatography (HPLC) methods including three modes of separation: ion pairing, cation exchange, and anion exchange chromatography, with detection by inductively coupled plasma mass spectrometry (ICPMS). The ion pairing mode enabled the separation of inorganic arsenate (As(V)), monomethylarsonic acid (MMA(V)), and dimethylarsinic acid (DMA(V)). However, the ion pair mode was unable to differentiate inorganic arsenite (As(III)) from arsenobetaine (AsB); instead, cation exchange chromatography was used to isolate and quantify AsB. Anion exchange chromatography was able to speciate all of the aforementioned arsenic species. Potential inaccurate quantification problem with urine sample containing elevated concentration of AsB, which eluted immediately after As(III) in anion exchange or ion pairing mode, was overcame by introducing a post-column hydride generation (HG) derivatization step. Incorporating HG between HPLC and ICPMS improved sensitivity and specificity by differentiating AsB from hydride-forming arsenic species. This paper emphasizes the usefulness of complementary chromatographic separations in combination with HG-ICPMS to quantitatively determine concentrations of As(III), DMA(V), MMA(V), As(V), and AsB in the sub-microgram per liter range in human urine.     

On-line coupling of an ultraviolet titanium dioxide film reactor with a liquid chromatography/hydride generation/inductively coupled plasma mass spectrometry system for continuous determination of dynamic variation of hydride- and nonhydride-forming arsenic species in very small microdialysate samples

We describe a method for continuously monitoring both hydride- and nonhydride-forming arsenic species in 10-microL microdialysate samples by coupling together on-line high-performance liquid chromatography (HPLC), a post-column UV/TiO2 film reactor, and hydride generation (HG) inductively coupled plasma mass spectrometry (ICP-MS). To maximize the signal intensities of the desired arsenic species, we optimized the photocatalytic oxidation efficiency of the analyte species and used a rapid on-line pre-reduction process to convert the oxidized species into As(III) prior to HG-ICP-MS determination. The UV/nano-TiO2 film reactor was manufactured by coating nano-TiO2 onto the interior of a glass tube. Impregnation and sol-gel methods were employed to deposit the TiO2 films, and their effectiveness for the oxidation of organic arsenicals was compared. To enhance the decomposition efficiency of organic arsenicals, we investigated the effects of the acidity and the composition of the column effluent. Because of the improved HG efficiency toward the tested arsenicals and the adoption of a segmented flow technique to retain the peak resolution in our on-line LC-UV/nano-TiO2 film reactor-HG-ICP-MS instrument, the detection limits for arseneous acid [As(III)], monomethylarsonic acid (MMA), dimethylarsinic acid (DMA), arsenic acid [As(V)], and arsenobetaine (AsB) were all in the submicrogram-per-liter range (based on 3 sigma) for 10-microL injections. A series of validation experiments--analyses of certified reference urine and rabbit serum samples--indicated that these methods can be applied satisfactorily to the continuous determination of As(III), MMA, DMA, As(V), and AsB in blood and in the extracellular space of target organs.     

Arsenic Compounds Accumulated in Sedimentary Microorganisms Cultivated in Media Containing Several Arsenicals

Sediments, as sources of microorganisms, were added to two kinds of media, 1/5 ZoBell 2216E and a solution of inorganic salts, which contained inorganic arsenic(III), inorganic arsenic(V), methanearsonic acid, dimethyl- arsinic acid, trimethylarsine oxide, tetramethylarsonium salt or arsenocholine. After 17 days of incubation at 20 °C, the arsenicals that had accumulated in the microorganisms were analysed by high-performance liquid chromatography (HPLC). While the more toxic arsenicals [inorganic arsenic(III), inorganic arsenic(V), methanearsonic acid, dimethylarsinic acid] were not converted in the microorganisms, trimethylarsine oxide and tetramethylarsonium salt were considerably degraded to inorganic arsenic(V), and arsenocholine to arsenobetaine. Arsenobetaine that had accumulated in the microorganisms was extracted and confirmed by thin-layer chromatography (TLC) and fast atom bombardment (FAB) mass spectrometry.     

Determination of urinary arsenic species using an on-line nano-TiO2 photooxidation device coupled with microbore LC and hydride generation-ICP-MS system

We have developed an on-line digestion device-based on the nano-TiO₂-catalyzed photooxidation of arsenic species—for coupling between microbore anion-exchange chromatography (μ-LC) and hydride generation (HG)-inductively coupled plasma mass spectrometry (ICP-MS) systems that can be used for the determination of urinary arsenic species. To maximize the signal intensities of the desired arsenic species, we optimized the photocatalytic oxidation efficiency of the analyte species and developed a rapid on-line pre-reduction process for converting the oxidized species into As(III) prior to HG-ICP-MS determination. Under the optimized conditions for the nano-TiO₂-catalyzed photooxidation-i.e., using 1 g of nano-TiO₂ per-liter, at pH 5.2, and illuminating for 3 min- As(III), monomethylarsenoic acid (MMA), and dimethylarseinic acid (DMA) can be converted quantitatively into As(V). To attain maximal hydride generation efficiency, 0.5% Na₂S₂O₄ solution, which can reduce As(V) to As(III) virtually instantaneously upon on-line mixing, was added as a pre-reductant prior to performing the HG step. In light of all the HG efficiency of tested arsenicals were improved and a segmented-flow technique was employed to avoid the loss of peak resolution when using our proposed on-line μ-LC-UV/nano-TiO₂/HG-ICP-MS, the detection limits for As(III), MMA, DMA, and As(V) were all in the range of sub-microgram-per-liter (based on 3 sigma). A series of validation experiments-analysis of neat and spiked urine samples-indicated that our proposed methods can be applied satisfactorily to the determination of As(III), MMA, DMA, and As(V) in urine samples.     

Simultaneous co-extraction of organometallic species of different elements by accelerated solvent extraction and analysis by inductively coupled plasma mass spectrometry coupled to liquid and gas chromatography

This paper describes the development of an accelerated solvent extraction methodology that is capable of simultaneously extracting organometallic species of As, Sn and Hg in a semi-automated manner. Accelerated solvent extraction (ASE) methods based on previous research on the separate extraction of organotin and -arsenic species in our laboratory were adapted for the co-extraction of six different species from an oyster tissue certified reference material (BCR 710). For the first time, the extraction of MeHg by this technique is also investigated. The proposed ASE conditions employed 50% acetic acid in methanol at a temperature of 100 degrees C with up to five consecutive extraction cycles of 3 minutes. Extraction efficiencies for organoarsenic species ranged from 80% (dimethylarsinic acid, DMA) to 99% for arsenobetaine (AsB). Species of toxicological interest, such as dibutyltin (DBT), tributyltin (TBT) and methylmercury (MeHg), were extracted with mean recoveries of 81, 84 and 76%, respectively. The extracted species were analysed by gas chromatography/inductively coupled plasma mass spectrometry (GC/ICPMS; DBT, TBT and MeHg) and liquid chromatography/inductively coupled plasma mass spectrometry (LC/ICPMS; MMA, DMA, AsB) after ethylation with sodium tetraethylborate or dilution with water, respectively. In addition to those species for which the extraction efficiency was assessed during this study, a further five arsenic species (arsenite, arsenate and three unidentified species), as well as monobutyltin (MBT) and mono-, di- and triphenyltin, could also be extracted from other matrices. The developed ASE method provides a significant improvement over many currently available routine monitoring methods for trace element speciation due to the fact that it is capable of extracting several species of toxicological interest simultaneously and quantitatively.     

Lipid-soluble and water-soluble arsenic compounds in blubber of ringed seal (Pusa hispida)

High concentrations of arsenic were observed in the blubber of ringed seals (Pusa hispida) in our previous study. To better understand the arsenic accumulation in blubber of marine mammals, arsenicals in the blubber of ringed seal were characterized using high performance liquid chromatography–inductively coupled plasma mass spectrometry (HPLC–ICPMS). The most predominant water-soluble arsenical in the blubber was dimethylarsinic acid (DMA), in spite of the predominance of arsenobetaine in other tissues. Lipid-soluble fraction was hydrolyzed under mild (tetraethylammonium hydroxide (TEAH) hydrolysis) and strong (NaOH hydrolysis) conditions, and then an aliquot of hydrolysate was injected onto HPLC–ICPMS. Both TEAH-labile and TEAH-stable/NaOH-labile lipid-soluble fractions contained precursors of DMA. These results suggest that the blubber might be the pool of DMA and DMA-containing precursors in ringed seals.     

Inhibition by methylated organoarsenicals of the respiratory 2-oxo-acid dehydrogenases

Inorganic arsenic that is ingested through drinking water or inhalation is metabolized by biological methylation pathways into organoarsenical metabolites. It is now becoming understood that this metabolism that was formerly considered to be detoxification may contribute as much or more to increasing the toxicity of arsenic. One proposed mode of the toxic action of arsenic and its organoarsenic metabolites is through its binding to proteins and inactivating their enzymatic activity. The classic case has been considered the affinity of the proximal 1,3 sulfhydryl groups of the lipoic acid cofactor of the pyruvate dehydrogenase complex for arsenic. A 2:1 stoichiometry of sulfhydryl to arsenic groups has been measured in proteins and arsenical complexes can be synthesized using free d,l-lipoic acid. The relative importance of this site for arsenic binding has come in to question through the use of methylating bifunctional arsenic complexes, and the suggestion that arsenic inhibits the pyruvate dehydrogenase complex indirectly by elevating mitochondrial hydrogen peroxide generation. In order to separate the effects of direct trivalent arsenite toxicity from that of hydrogen peroxide and activated oxygen, we studied the inhibition of the PDH complex under conditions that did not generate hydrogen peroxide but did expose the lipoic acid group in its reduced state to arsenicals. We also studied the effects of arsenicals in the inhibition of the α-ketoglutarate dehydrogenase complex. We found that only trivalent arsenical compounds inhibited the activity of both dehydrogenase complexes and only when the lipoic acid was in its reduced form. Arsenite inhibited both enzyme complexes approximately equivalently while monomethylarsenite inhibited the PDH complex to a greater extent than the KGDH complex - although both complexes were very sensitive to inhibition by this complex. Dimethylarsenite inhibition of both complexes was only observed with longer pre-incubation periods. Cumulative inhibition by the reduced arsenical was observed for all complexes indicating a binding mode of inhibition that is dependent upon lipoic acid being in its reduced state.     

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.     

The effect of arsenicals on alkaloid production by cell suspension cultures of Catharanthus roseus

The effect of arsenic compounds on indole alkaloid production by cell suspension cultures of Catharanthus roseus was investigated. The analysis of indole alkaloids was achieved by using thermospray liquid chromatography-mass spectrometry (LC MS) which facilitated the rapid screening of alkaloid composition in cultures treated with different arsenicals at different times in their growth cycle. Treatment with dimethylarsinate (DMA), a non-selective herbicide, has a drastic inhibitory effect on alkaloid production although it is the least toxic arsenical to growth. Tryptamine, an early precursor in the biosynthesis of indole alkaloids, accumulates in cells treated with DMA, indicating that the initial step of condensation of tryptamine with secologanin is inhibited. Treatment with DMA during the early stationary phase of culture growth enhances the accumulation of some alkaloids, although some, such as catharanthine, are suppressed. The arsenicals arsenate and methylarsonate (MMA) have an inhibitory effect on alkaloid production when applied during the early growth stages. In contrast to MMA and DMA, arsenate has a stimulatory effect on catharanthine production when introduced to the culture during its early stationary phase. Thus the changes in the pattern of alkaloid accumulation on addition of arsenicals are dependent on the arsenic species and its concentration, as well as the time of application. This variable response indicates that each arsenical has a distinct mode of action on the secondary metabolic pathways of C. roseus.     

A systematic study on extraction of total arsenic from down-scaled sample sizes of plant tissues and implications for arsenic species analysis

An easily feasible, species-conserving and inexpensive protocol for the extraction of total arsenic and arsenic species from terrestrial plants was designed and applied to the investigation of accumulation and metabolization of arsenite (As(III)), arsenate (As(V)), monomethylarsonate (MMA(V)), and dimethylarsinate (DMA(V)) by the model plant Tropaeolum majus. In contrast to existing extraction methods hazardous additives and elaborate procedures to enhance the extraction yields were omitted. The proposed protocol is suited to down-scale the sample sizes used for the extractions and to promote a compartmentally resolved analysis of the arsenic distribution within individual leaves, leaf stalks, and stems instead of the conventional extraction of pooled samples. In a two-step extraction, the high extraction efficiencies (85-92%) for arsenic achieved by phosphate buffer from larger amounts (200mg) of homogenized leaf material in a one-step extraction, could be enhanced to 94-100% in a second extraction step. A strong dependence of the arsenic extractability on the type of arsenic species accumulated in the tissue as well as on the type of the tissue (leaf, leaf stalk, stem) was found. For the extraction of 5mm long segments cut from individual leaves without previous homogenization of the plant parts yields between 75 and 93% depending on arsenic species prevailing in the cells were obtained using 1 or 10mM phosphate buffer. The total extraction and analysis protocol was validated using a standard reference material as well as by spiking experiments. The arsenic species analysis by IC/ICPMS revealed a number of nine unidentified metabolites in the plant extracts in addition to the species MMA(V), DMA(V), As(III), and As(V) that were provided to the plants during their growth phase.     

人肝细胞中甲基硫代砷酸的表征及其代谢过程的推测

利用高效液相色谱-电感耦合等离子体质谱(HPLC-ICP-MS)联用技术对染砷后Chang肝细胞中的砷代谢产物进行分析.借助合成标准,采用标准加入法,表征出肝细胞中一种未知砷代谢产物甲基硫代砷酸(MMTA).定量分析显示,MMTA含量随染砷浓度升高呈上升趋势.采用质量平衡分析法对双氧水处理前后的染砷细胞样品进行研究.结果显示,除小分子砷代谢产物外,细胞中部分砷代谢产物以大分子形态存在,推测其为砷蛋白类物质.由于MMTA系首次在人体肝细胞中发现,本研究结合实验现象和相关文献推测该物质系细胞代谢产物,并且其代谢途径与二甲基硫代砷酸相似,是以细胞内与其对应的一甲基亚砷酸为代谢起点.     

Investigation of the interaction between arsenic species and thiols via electrospray ionization tandem mass spectrometry

Arsenic species have been known to participate in a number of chemical and biological reactions, including oxidation-reduction reactions, acid-base reactions, covalent interactions, and methylation-demethylation reactions because of the element's multiple and interconvertible oxidation states. Little is known about the structure or bonding behavior between arsenic species and thiolcontaining biomolecules. Therefore, a better understanding of the bonding behavior and detailed information on the molecular structure for arsenic-thiol complexes is needed. As a result, we have investigated the interaction between arsenic species (arsenate (As^V), arsenite (As^III), monomethylarsonic acid (MMA^V), and dimethylarsinic acid (DMA^V)) with biomolecules containing thiol groups (glutathione and cysteine) by electrospray ionization mass spectrometry (ESI-MS). These compounds were dissolved in methanol/water solution and introduced into the MS instrument in order to elucidate the direct bonding behavior of thiol group of biomolecules with arsenic species. In addition, further detailed structural information on this complex was obtained by collision-induced dissociation (CID) measurements.In each mass spectrum for mixture solutions between arsenic species and thiol compounds, various peaks such as protonated arsenic-thiol complexes, protonated noncomplexed thiol compounds, sodium bound cluster ions, and proton bound cluster ions were observed. In these mass spectra, the arsenic complexes were formed by interaction with thiol groups on the cysteine residues. These arsenic-thiol complexes produced a variety of fragment ions by cleavage of chemical bonds, and by interaction of other binding site on thiol compounds in tandem mass spectrometry experiments.     

Methods for the separation and quantification of arsenic species in SRM 2669: arsenic species in frozen human urine

Two independent liquid chromatography inductively coupled plasma-mass spectrometry (LC/ICP-MS) methods for the separation of arsenic species in urine have been developed with quantification by standard additions. Seven arsenic species have been quantified in a new NIST frozen human urine Standard Reference Material (SRM) 2669 Arsenic Species in Frozen Human Urine, Levels 1 and 2. The species measured were: arsenite (As(III)), arsenate (As(V)), monomethylarsonate (MMA), dimethylarsinate (DMA), arsenobetaine (AB), arsenocholine (AC), and trimethylarsine oxide (TMAO). The purity of each arsenic standard used for quantification was measured as well as the arsenic species impurities determined in each standard. Analytical method limits of detection (L _D) for the various species in both methods ranged from 0.2 to 0.8 μg L⁻¹ as arsenic. The results demonstrate that LC/ICP-MS is a sensitive, reproducible, and accurate technique for the determination of low-level arsenic species in urine. Measurements of the arsenic species 3 years after initial production of the SRM demonstrate the stability of the arsenic species in the urine reference material.     

Speciation of arsenic metabolites in the free-living mouse Mus spretus from Do?ana National Park used as a bio-indicator for environmental pollution monitoring

A speciation approach based on orthogonal chromatographic systems coupled to inductively coupled plasma mass spectrometry (ICP-MS) was used to characterise the biological response of free-living mice Mus spretus to environmental pollution caused by arsenic in different areas of the Do?ana National Park (south-west Spain). The relative presence of inorganic and organic forms of arsenic was studied in cytosolic extracts from high metabolic activity organs of Mus spretus mice: kidneys, liver, and brain. An instrumental coupling of size-exclusion chromatography with UV and collision/reaction cell-ICP-MS detectors (SEC-UV-ICP-ORC-MS) both in analytical and preparative scale was used for this purpose. The results showed the presence of low molecular mass (LMM) molecules linked to arsenic in these tissues especially in the kidneys, where the presence of these arsenic metabolites was higher. On the other hand, the presence of these arsenicals varied from one area to the other, which can be related to a different occurrence of contaminants. These low molecular mass fractions were collected by preparative SEC chromatography for later study with ion exchange chromatography and detection by ICP-ORC-MS, using both anionic and cationic columns. The results showed the higher presence of MMA and DMA in kidneys of mice caught in contaminated areas and the existence of small amounts of unidentified arsenicals when cation-exchange chromatography was used, which could be related to the presence of dimethylarsinoylethanol (DMAE), thioarsenic species, or arsenocholine (AsC).     

Determination of urinary arsenic and impact of dietary arsenic intake

An analytical method based on microwave decomposition and flow injection analysis (FIA) coupled to hydride generation atomic absorption spectrometry (HGAAS) is described. This is used to differentiate arsenite [As(III)], arsenate [As(V)], monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA) from organoarsenic compounds usually present in seafood. Without microwave digestion, direct analysis of urine by HGAAS gives the total concentration of As(III), As(V), MMA and DMA because organoarsenic compounds such as arsenobetaine, usually found in most seafood, are not reducible upon treatment with borohydride and therefore cannot be determined by using the hydride generation technique. The microwave oven digestion procedure with potassium persulfate and sodium hydroxide as decomposition reagents completely decomposes all arsenicals to arsenate and this can be measured by HGASS. Microwave decomposition parameters were studied to achieve efficient decomposition and quantitative recovery of arsenobetaine spiked into urine samples. The method is applied to the determination of urinary arsenic and is useful for the assessment of occupational exposure to arsenic without intereference from excess organoarsenicals due to the consumption of seafood. Analysis of urine samples collected from an individual who ingested some seafood revealed that organoarsenicals were rapidly excreted in urine. After the ingestion of a 500-g crab, a 10-fold increase of total urinary arsenic was observed, due to the excretion of organoarsenicals. The maximum arsenic concentration was found in the urine samples collected approximately between 4 to 17 hr after eating seafood. However, the ingestion of organoarsenic-containing seafoods such as crab, shrimp and salmon showed no effect on the urinary excretion of inorganic arsenic, MMA and DMA.     

Can we trust mass spectrometry for determination of arsenic peptides in plants: comparison of LC–ICP–MS and LC–ES-MS/ICP–MS with XANES/EXAFS in analysis of <Emphasis Type="Italic">Thunbergia alata</Emphasis>

The weakest step in the analytical procedure for speciation analysis is extraction from a biological material into an aqueous solution which undergoes HPLC separation and then simultaneous online detection by elemental and molecular mass spectrometry (ICP-MS/ES-MS). This paper describes a study to determine the speciation of arsenic and, in particular, the arsenite phytochelatin complexes in the root from an ornamental garden plant Thunbergia alata exposed to 1 mg As L(-1) as arsenate. The approach of formic acid extraction followed by HPLC-ES-MS/ICP-MS identified different As(III)-PC complexes in the extract of this plant and made their quantification via sulfur (m/z 32) and arsenic (m/z 75) possible. Although sulfur sensitivity could be significantly increased when xenon was used as collision gas in ICP-qMS, or when HR-ICP-MS was used in medium resolution, the As:S ratio gave misleading results in the identification of As(III)-PC complexes due to the relatively low resolution of the chromatography system in relation to the variety of As-peptides in plants. Hence only the parallel use of ES-MS/ICP-MS was able to prove the occurrence of such arsenite phytochelatin complexes. Between 55 and 64% of the arsenic was bound to the sulfur of peptides mainly as As(III)(PC(2))(2), As(III)(PC(3)) and As(III)(PC(4)). XANES (X-ray absorption near-edge spectroscopy) measurement, using the freshly exposed plant root directly, confirmed that most of the arsenic is trivalent and binds to S of peptides (53% As-S) while 38% occurred as arsenite and only 9% unchanged as arsenate. EXAFS data confirmed that As-S and As-O bonds occur in the plants. This study confirms, for the first time, that As-peptides can be extracted by formic acid and chromatographically separated on a reversed-phase column without significant decomposition or de-novo synthesis during the extraction step.     

Stability study of As(III), As(V), MMA and DMA by anion exchange chromatography and HG-AFS in wastewater samples

The stability of arsenic species (arsenate [As(V)], monomethylarsonate [MMA], dimethylarsinate [DMA] and arsenite [As(III)]) in two types of urban wastewater samples (raw and treated) was evaluated. Water samples containing a mixture of the different arsenic species were stored in the absence of light at three different temperatures: +4 degrees C, +20 degrees C and +40 degrees C. At regular time intervals, arsenic species were determined by high performance liquid chromatography (HPLC)-hydride generation (HG)-atomic fluorescence spectrometry (AFS). The experimental conditions for the separation of arsenic species by HPLC and their determination by AFS were directly optimised from wastewater samples. As(III), As(V), MMA and DMA were separated on an anion exchange column using phosphate buffer (pH 6.0) as the mobile phase. Under these conditions the four arsenic species were separated in less than 10 min. The detection limits were 0.6, 0.9, 0.9 and 1.8 micro g L(-1) for As(III), DMA, MMA and As(V), respectively. As(V), MMA and DMA were found stable in the two types of urban wastewater samples over the 4-month period at the three different temperatures tested, while the concentration of As(III) in raw wastewater sample decreased after 2 weeks of storage. A greater stability of As(III) was found in the treated urban wastewater sample. As(III) remained unaltered in this matrix at pH 7.27 over the period studied, while at lower pH (1.6) losses of As(III) were detected after 1 month of storage. The results show that the decrease in As(III) concentration with time was accompanied by an increase in As(V) concentration.     

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.     

Seasonal Changes in Methylarsenic Distribution in Tosa Bay and Uranouchi Inlet

The distribution of arsenic species, including trivalent methylarsenicals, was observed in coastal seawater of Tosa Bay and Uranouchi Inlet Japan. In Tosa Bay, most arsenic was dissolved in the inorganic form throughout the year and the concentration of total dissolved arsenic was higher than that in Uranouchi Inlet. The sum of methylarsenicals found in surface waters comprised 2–25% and 10–82% of the total dissolved arsenic in Tosa Bay and Uranouchi Inlet, respectively. In Uranouchi Inlet, seasonal variations in the concentrations of arsenicals were observed both in the water column and in surface sediments. The maximum concentrations of methylarsenicals appeared during summer, and became comparable to those of inorganic arsenicals in surface water. The concentration of trivalent methylarsenicals was usually low, and their seasonal changes seemed to be independent of those of the pentavalent species. The variations in methylarsenic(V) concentration did not coincide with those of chlorophyll a in either Tosa Bay or Uranouchi Inlet. These results suggested that methylarsenic(V) in natural waters was produced not directly by the activity of phytoplankton but through decomposition of organic matter by bacteria.     

Thermodynamics of the As(III)-thiol interaction: arsenite and monomethylarsenite complexes with glutathione, dihydrolipoic acid, and other thiol ligands

Colorimetric (near-UV absorption spectroscopy) and calorimetric (isothermal titration calorimetry) methods have been used to quantify the equilibrium and thermodynamics of arsenite and monomethylarsenite (MMA) coordinating to glutathione (GSH) and the dithiols dimercaptosuccinic acid (DMSA), dihydrolipoic acid (DHLA), and dithiothreitol (DTT). We found that both arsenite and MMA form moderately stable complexes (beta = 10(6)-10(7)) with GSH; that arsenite forms a particularly stable 2:3 complex (beta approximately 10(18)) with the biological cofactor DHLA; that MMA has a somewhat higher affinity than arsenite for thiol ligands; and that entropic factors modulate the overall stability of As(III) complexes with thiols, which are favored by the exothermic formation of As(III)-thiolate bonds. The implications of these results for arsenic toxicity are discussed.