CZE for the speciation of arsenic in aqueous soil extracts

We developed two separation methods using CZE with UV detection for the determination of the most common inorganic and methylated arsenic species and some phenylarsenic compounds. Based on the separation method for anions using hydrodynamic sample injection the detection limits were 0.52, 0.25, 0.27, 0.12, 0.37, 0.6, 0.6, 1.2 and 1.0 mg As/L for phenylarsine oxide (PAO), p-aminophenylarsonic acid (p-APAA), o-aminophenylarsonic (o-APAA), phenylarsonic acid (PAA), 4-hydroxy-3-nitrobenzenearsonic acid (roxarsone), monomethylarsonic acid (MMA), dimethylarsinic acid (DMA), arsenite or arsenious acid (As(III)) and arsenate (As(V)), respectively. These detection limits were improved by large-volume sample stacking with polarity switching to 32, 28, 14, 42, 22, 27, 26 and 27 microg As/L for p-APAA, o-APAA, PAA, roxarsone, MMA, DMA, As(III) and As(V), respectively. We have applied both methods to the analysis of the arsenic species distribution in aqueous soil extracts. The identification of the arsenic species was validated by means of both standard addition and comparison with standard UV spectra. The comparison of the arsenic species concentrations in the extracts determined by CZE with the total arsenic concentrations measured by inductively coupled plasma-atomic emission spectroscopy (ICP-AES) indicated that CZE is suited for the speciation of arsenic in environmental samples with a high arsenic content. The extraction yield of phenylarsenic compounds from soil was derived from the arsenic concentrations of the aqueous soil extracts and the total arsenic content of the soil determined by ICP-AES after microwave digestion. We found that 6-32% of the total amount of arsenic in the soil was extractable by a one-step extraction with water in dependence on the type of arsenic species.     

Analysis and speciation of arsenic by stripping potentiometry: a review

This paper provides a review that summarizes several examples of the literature from 1980 to 2003, to illustrate the applications of stripping potentiometry for the determination and speciation of arsenic in several samples. A discussion on the main advantages of stripping potentiometry in comparison with other electrochemical methods employed for arsenic determination is presented. Special attention is devoted to stripping modes (constant current or chemical stripping) and to issues related to the choice of working electrodes and supporting electrolyte. This approach has been also applied at arsenic determination in flow systems. A section is dedicated to speciation of arsenic and total arsenic determination and other to analytical characteristic of method and their interferences. An extensive compilation, organize by experimental and analytical parameters and real sample studied is presented.     

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.     

Determination of trace amounts of total arsenic in environmental samples by hydride generation flow injection-AAS using a mixed acid as a pretreatment agent.

A method is described for the determination of total arsenic by hydride generation-atomic absorption spectrophotometry using a mixed acid as a pretreatment. Hydride generation is done by the flow-injection method. The authors investigated in detail the temperature and time of decomposition using inorganic, organic arsenic and environmental standard samples, pretreated with nitric-perchloric-sulfuric mixed acid. By using a mixed acid as a pretreatment agent at 220 degrees C, the decomposition time could be shortened and the blank value of arsenic from the reagents used was reduced. The mixed acid of nitric-perchloric-sulfuric was also found to be effective as a pretreatment agent for organic arsenic compounds in which a dimethylated compound, sodium cacodylate or biological samples, is known to be one of the indecomposables. The present approach was proved to be satisfactory as a pretreatment for the quantitative analysis of trace amounts of total arsenic in liquid or solid environmental samples, such as geothermal water, sediments and biological samples.     

Comparison of mild extraction procedures for determination of plant-available arsenic compounds in soil

In this work three mild extraction agents for determination of plant-available fractions of elements in soil were evaluated for arsenic speciation in soil samples. Pepper (Capsicum annum, L.) var. California Wonder was cultivated in pots, and aqueous solutions of arsenite, arsenate, methylarsonic acid, and dimethylarsinic acid, at a concentration of 15 mg As kg^–1 soil, were added at the beginning of the experiment. Control pots (untreated) were also included. Deionized water, 0.01 mol L^–1 CaCl₂, and 0.05 mol L^–1 (NH₄)₂SO₄ were used to extract the plant-available fraction of the arsenic compounds in soil samples collected during the vegetation period of the plants. Whereas in control samples the extractable arsenic fraction did not exceed 1% of total arsenic content, soil amendment by arsenic compounds resulted in extraction of larger amounts, which varied between 1.4 and 8.1% of total arsenic content, depending on soil treatment and on the extracting agent applied. Among arsenic compounds determined by HPLC–ICPMS arsenate was predominant, followed by small amounts of arsenite, methylarsonic acid, and dimethylarsinic acid, depending on the individual soil treatment. In all the experiments in which methylarsonic acid was added to the soil methylarsonous acid was detected in the extracts, suggesting that the soil bacteria are capable of reducing methylarsonic acid before a further methylation occurs. No significant differences were observed between analytical data obtained by using different extraction procedures.     

Factors affecting arsenic speciation in environmental samples: sample drying and storage

Arsenic is a ubiquitous element. Its toxicity, mobility, and bioaccumulation depend usually on its chemical form, and therefore, arsenic speciation is indispensable for the assessment of environmental risk and human hazard. Little is known about the effect of sample preparation procedures, such as drying and storage, on the resulting arsenic speciation. In this study, we investigated the influence of different drying methods and storage conditions on the arsenic speciation in mineral soils, organic soils, and plants. Drying soils and plants using different methods may change the concentrations of the total methanol–water (20%,?v/v) extractable arsenic, the proportion of organic arsenic and the ratio of arsenite-to-arsenate. Loss of methanol–water extractable arsenic compounds (up to 63%) was observed particularly in the samples rich in water. Following drying, the speciation of organic arsenic changed less than that of inorganic arsenic. Drying showed little influence on the total arsenic determination. None of the storage methods tested could preserve the arsenic speciation in organic soils and plants, although arsenic speciation after one-month storage varied less in freeze-dried samples than wet samples. Storage of the samples at low temperatures (2 or??20°C) had the largest impact on the samples rich in organic matters, leading to less arsenic being extractable by methanol–water. Both drying and storage of the soil and plant samples changed apparently the arsenic speciation. Therefore, we recommend conducting the arsenic speciation possibly with fresh and wet samples, so that the results of arsenic speciation may be more approaching the original states.     

Determination of arsenic and arsenic compounds in natural gas samples

Organic arsenic compounds (trialkylarsines) present in natural gas were extracted by 10 cm³ of concentrated nitric acid from 1 dm³ of gas kept at ambient pressure and temperature. The flask containing the gas and the acid was shaken for 1 h on a platform shaker set at the highest speed. The resulting solution was mixed with concentrated sulfuric acid and heated to convert all arsenic compounds to arsenate. Total arsenic was determined in the mineralized solutions by hydride generation. The arsenic concentrations in natural gas samples from a number of wells in several gas fields were in the range 0.01–63 μ As dm^?3. Replicate determinations of arsenic in a gas sample with an arsenic concentration of 5.9 μ dm^?3 had a relative standard deviation of 1.7%. Because of the high blank values, the lowest arsenic concentration that could be reliably determined was 5 ng As dm^?3 gas. Analysis of nonmineralized extracts by hydride generation identified trimethylarsine as the major arsenic compound in natural gas. Low-temperature gas chromatography-mass spectrometry showed more directly than the hydride generation technique, that trimethylarsine accounts for 55–80% of the total arsenic in several gas samples. Dimethylethylarsine, methyldiethylarsine, and triethylarsine were also identified, in concentrations decreasing with increasing molecular mass of the arsines.     

Speciation analysis of arsenic in groundwater from Inner Mongolia with an emphasis on acid-leachable particulate arsenic

Arsenic in drinking water affects millions of people around the world. While soluble arsenic is commonly measured, the amount of particulate arsenic in drinking water has often been overlooked. We report here determination of the acid-leachable particulate arsenic and soluble arsenicals in well water from an arsenic-poisoning endemic area in Inner Mongolia, China. Water samples (583) were collected from 120 wells in Ba Men, Inner Mongolia, where well water was the primary drinking water source. Two methods were demonstrated for the determination of soluble arsenic species (primarily inorganic arsenate and arsenite) and total particulate arsenic. The first method used solid phase extraction cartridges and membrane filters to separate arsenic species on-site, followed by analysis of the individual arsenic species eluted from the cartridges and filters. The other method uses liquid chromatography separation with hydride generation atomic fluorescence detection to determine soluble arsenic species. Analysis of acidified water samples using inductively coupled plasma mass spectrometry provided the total arsenic concentration. Arsenic concentrations in water samples from the 120 wells ranged from <1 to ∼1000 μg L⁻¹. On average, particulate arsenic accounted for 39 ± 38% (median 36%) of the total arsenic. In some wells, particulate arsenic was six times higher than the soluble arsenic concentration. Particulate arsenic can be effectively removed using membrane filtration. The information on particulate and soluble arsenic in water is useful for optimizing treatment options and for understanding the geochemical behavior of arsenic in groundwater.     

Speciation of arsenic compounds by coupling high-performance liquid chromatography with inductively coupled plasma mass spectrometry

There is considerable evidence that toxicity and physiological behavior of arsenic depends on its chemical forms. Arsenic speciation became therefore the subject of increasing interest in recent years. A sensitive method for the determination of arsenic species has been developed. The proposed procedure involves the use of high-performance liquid chromatography and inductively coupled plasma mass spectrometry (HPLC-ICP-MS). Six arsenic compounds were separated by anion-exchange chromatography with isocratic elution using tartaric acid as mobile phase with an elution order: arsenocholine, arsenobetaine, dimethylarsinic acid, methylarsonic acid, arsenous acid and arsenic acid. The chromatographic parameters affecting the separation of the arsenic species were optimized. Analytical characterization of the method has been realized with standard solutions. The detection limits for six arsenic compounds were from 0.04 to 0.6 g/L as As element. The repeatability (expressed by R.S.D) was better than 7% for all investigated compounds. The HPLC-ICP-MS system was successfully applied to the determination of arsenic compounds in environmental and biological samples in g/L level.     

Determination of total cationic and total anionic arsenic species in oyster tissue using microwave-assisted extraction followed by HPLC-ICP-MS

A microwave-assisted digestion procedure was developed in presence of concentrated nitric acid (2.0 ml) and 30% hydrogen peroxide (0.20 ml) using a closed pressurized microwave digestion system for the determination of total anionic and total cationic arsenic compounds reside in oyster tissue. At 450 W for 15 min digestion, 74% of anionic arsenic, and 31% of cationic arsenic (105% total arsenic) were retrieved. At 300 W microwave power, 68% of anionic and 30.5% of cationic arsenic (98.5% total arsenic), and 100 W, 63% of anionic and 31% of cationic arsenic (94% total arsenic) were extracted out. The methanol water mixture (9:1) was cull out, exclusively 31.6% of anionic and 29% of cationic arsenic compounds (60.6% total). The dimethylarsinoylriboside (phosphate-arsenosugar) was the predominant arsenic species, along with arsenobetaine (AB), dimethylarsinic acid (DMA), inorganic arsenic, methylarsonic acid (MA), arsenocholine (AC), trimethylarsineoxide (TMAO) and tetramethylarsonium ion (TMI). Some other arsenic compounds, those were not matched with the retention time of the available standards, were also detected. Arsenosugar was fragile and adequately transmuted to DMA (100%), AB and AC to TMAO (100%) when 450 W microwave power was applied for 15 min. The separation and quantification of arsenic compounds in the microwave digests and extracts, were carried out in anion (PRP-X100) and cation (LC-SCX) exchange columns using ICP-MS as arsenic specific detector. The procedure was also validated by determining the total cationic and total anionic arsenic compounds present in DORM 1.     

Speciation of arsenic in natural waters by HPLC-NAA

Neutron activation analysis (NAA) in combination with mainly high-performance liquid chromatography (HPLC) has been developed for the determination of low levels of five arsenic species, namely As(III), As(V), monomethylarsonic acid (MMA), dimethylarsinic acid (DMA), and arsenobetaine (AsB) in water samples. Organically bound arsenic (OBAs) and total arsenic have also been determined. In addition to anion-exchange HPLC, solid phase extraction and open-column cation-exchange chromatographic methods have also been used. The detection limits of the method have been found to be 0.005 ng·cm⁻³ for OBAs, 0.02 ng·cm⁻³ for AsB, DMA, MMA, As(III), and As(V) and 0.12 ng·cm⁻³ for total arsenic. This revised version was published online in July 2006 with corrections to the Cover Date.     

Extraction and speciation of arsenic in plants grown on arsenic contaminated soils

A sequential arsenic extraction method was developed that yielded extraction efficiencies (EE) that were approximately double those using current methods for terrestrial plants. The method was applied to plants from two arsenic contaminated sites and showed potential for risk assessment studies. In the method, plants were extracted first by 1:1 water-methanol followed by 0.1 M hydrochloric (HCl) acid. Total arsenic in plant and soil samples collected from contaminated sites was mineralized by acid digestion and detected by inductively coupled plasma-atomic emission spectrometry (ICP-AES) and hydride generation-atomic absorption spectrometry (HG-AAS). Arsenic speciation was done by high performance liquid chromatography coupled with HG-AAS (HPLC-HGAAS) and by HPLC coupled with ICP-mass spectrometry (HPLC-ICP-MS). Spike recovery experiments with arsenite (As(III)), arsenate (As(V)), methylarsonic acid (MA) and dimethylarsinic acid (DMA) showed stability of the species in the extraction processes. Speciation analysis by X-ray absorption near edge spectroscopy (XANES) demonstrated that no transformation of As(III) and As(V) occurred due to sample handling. Dilute HCl was efficient in extracting arsenic from plants; however, extraction and determination of organic species were difficult in this medium. Sequential extraction with 1:1 water-methanol followed by 0.1 M-HCl was most useful in extracting and speciating both organic and inorganic arsenic from plants. Trace amounts of MA and DMA in plants could be detected by HPLC-HGAAS aided by the process of separation and preconcentration of the sequential extraction method. Both organic and inorganic arsenic compounds could be detected simultaneously in synthetic gastric fluid extracts (GFE) but EEs by this method were lower than those of the sequential method. The developed sequential method was shown to be reliable and applicable to various terrestrial plants for arsenic extraction and speciation.     

Preliminary studies on arsenic species in some environmental samples

Arsenic compounds have been determined in some environmental samples from the German Environmental Specimen Bank (ESB) (marine mussels, freshwater mussel and fish, sea-gull eggs) and certified reference materials (DORM-1, DOLT-1, NBS Oyster Tissue) after separation by open column cation and anion exchange chromatography by two different methods of total arsenic determination in separated fractions (instrumental neutron activation analysis or hydride generation atomic absorption spectrometry). Arsenobetaine has been identified as the major species in all the different materials.     

An evaluation of extraction techniques for arsenic species from freeze-dried apple samples

The extraction of arsenic from freeze-dried apples and subsequent determination of individual arsenic species by HPLC-ICP-MS is described. Solvent extraction with sonication using various aqueous and aqueous/solvent mixtures was initially evaluated by measuring total arsenic extracted by ICP-MS. A two step procedure using overnight treatment with alpha-amylase enzyme followed by sonication for 6 h with 40:60 acetonitrile-water was found to provide good extraction efficiency. The concentration of arsenic extracted was compared with the concentration of total arsenic in the samples determined using ICP-MS after microwave digestion in order to calculate extraction efficiency. Individual arsenic species in the extracts were measured using HPLC-ICP-MS. The three most abundant arsenic species found were arsenite, arsenate and dimethylarsinic acid. Total arsenic concentrations in the freeze-dried apple samples ranged from 8.2 to 80.9 micrograms kg-1 As, dry mass. By HPLC-ICP-MS, the relative amount of inorganic arsenic in the samples ranged from 73 to 90% of the sum of the arsenic species detected in each sample.     

Use of perchloric acid as a reaction medium for speciation of arsenic by hydride generation-atomic absorption spectrometry

Simple and inexpensive methods for the speciation of arsenite, arsenate, monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA) in environmental water samples were developed. In these methods a hydride generation-atomic absorption spectrometry (HG-AAS) technique was employed and perchloric acid (as a reaction medium), L-cysteine (as a pre-reducing agent for a certain contact time between its addition and analysis) and sodium tetrahydroborate(III) (NaBH4, as a reducing agent) were used. The use of L-cysteine greatly enhances the absorption signals of all four arsenic species at low acid concentration (0.001-0.04 M). The methods developed for the determination of total arsenic and total inorganic arsenic and speciation of the four arsenic species in environmental water samples are as follows. (i) DMA: 0.005 M acid and 0.04% NaBH4 in the absence of L-cysteine. DMA can also be speciated in the presence of L-cysteine as follows: 2 M acid, 2.5% L-cysteine after a contact time of approximately 5 min and 0.6% NaBH4. (ii) As(III): 5 M acid and 0.08% NaBH4 in the absence of L-cysteine. (iii) Total inorganic arsenic (As(III) + As(V)]: 8 M acid and 0.6% NaBH4 in the absence of L-cysteine. (iv) Total arsenic: 0.01 M acid, 5% L-cysteine after a contact time of 5 min and 2% NaBH4. (v) MMA: 8 M acid, 3% L-cysteine after a contact time of 50 min and 0.6% NaBH4. (vi) As(V): by difference. Detection limits and recoveries of added spikes for all analyses were found to be 0.5-1.7 ppb and 90-112% respectively.     

Determination of total arsenic and arsenic (III) in phosphate fertilizers and phosphate rocks by HG-AAS after multivariate optimization based on Box-Behnken design

In the present paper, a procedure for the determination of total arsenic and arsenic (III) in phosphate fertilizers and phosphate rocks by slurry sampling (SS) with hydride generation atomic absorption spectrometry (HG-AAS) is proposed. Arsenic (III) is determinated directly and total arsenic is determinated after reduction reaction. The procedure was optimized for the flow rate of NaBH₄, NaBH₄ and hydrochloric acid concentrations using a full two-level factorial and also a Box-Behnken design. Slurry preparation with hydrochloric acid in an ultrasonic bath allowed the determination of arsenic (III) with limits of detection and quantification of 0.1 and 0.3 μg L⁻¹, respectively. The precision of results, expressed as relative standard deviation (RSD), was always lower than 3%. The accuracy of this method was confirmed by analysis of certified sediment reference materials, while the procedure also allows for calibration using aqueous external standards. This method (SS/HG-AAS) was used to determine total arsenic and arsenic (III) in two phosphate rock samples and two phosphate fertilizer samples. In these samples, total arsenic concentrations varied from 5.2 to 20.0 mg kg⁻¹, while As (III) concentrations varied from 2.1 to 5.5 mg kg⁻¹, in agreement with published values. All samples were also analyzed using acid digestion/HG-AAS. Both, a paired t-test and a linear regression model demonstrated no significant difference (95% CL) between the results obtained using these two sample preparation procedures.     

Determination of arsenic in organic compounds rapid micro and semimicro methods

Rapid micro and semimicro methods for determination of arsenic in organic compounds have been developed using 28% chloric acid as the digesting agent for the organic arsenical. This wet acid digestion procedure does not lead to loss of arsenic even in the presence of 4 M chloride.The semimicro method completes the assay by reduction of the pentavalent arsenic with iodide, destruction of the iodine liberated with thiosulfate and re-oxidation to pentavalent arsenic with iodine in a buffered solution. The micro method completes the assay by development of a heteropoly molybdenum blue using a single solution reagent at room temperature. The color is reproducible so that only a single calibration curve for the instrument used is necessary.     

Analytical methods for inorganic arsenic in water: a review

Inorganic arsenic, a term which encompasses both As (III) and As (V) species, constitutes the highest toxicological risk associated with arsenic in water in contrast to the organic arsenic species. Different determination methods of inorganic arsenic have been developed over 40 years providing timely and efficient risk assessments of inorganic arsenic contamination world wide. The current report gives an overview of more than 100 papers, regarding existing methods for analysis of As (III) and As (V) in water, including various spectroscopic, ICP and electrochemical techniques. Recent field portable analytical applications are also reviewed.     

Assessment of total arsenic and arsenic species stability in alga samples and their aqueous extracts

In order to achieve reliable information on speciation analysis, it is necessary to assess previously the species stability in the sample to analyse. Furthermore, in those cases where the sample treatment for species extraction is time-consuming, an assessment of the species integrity in the extracts is of paramount importance. Thus, the present paper reports total arsenic and arsenic species stability in alga samples (Sargassum fulvellum and Hizikia fusiformis), as well as in their aqueous extracts, which were stored in amber glass and polystyrene containers at different temperatures. Total arsenic determination was carried out by inductively coupled plasma atomic emission spectroscopy (ICP-AES), after sample acid digestion in a microwave oven, while arsenic speciation was conducted by anion exchange high performance liquid chromatography on-line coupled to ICP-AES, with and without sample introduction by hydride generation (HPLC-ICP-AES and HPLC-HG-ICP-AES), after aqueous microwave-assisted extraction. The results obtained for solid alga samples showed that total arsenic (for Hijiki alga) and arsenic species present (As(V) for Hijiki and NIES No. 9 Sargasso) are stable for at least 12 months when samples are stored in polystyrene containers at +20 degrees C. On the other hand, a different behaviour was observed in the stability of total arsenic and As(V) species in aqueous extracts for both samples, being the best storage conditions for Sargasso extracts a temperature of -18 degrees C and polystyrene containers, under which they are stable for at least 15 days, while Hijiki extracts must be stored in polystyrene containers at +4 degrees C in order to ensure the stability for 10 days.     

Non-chromatographic screening procedure for arsenic speciation analysis in fish-based baby foods by using electrothermal atomic absorption spectrometry

A procedure for the speciation analysis of arsenic in fish-based baby foods is presented. Inorganic arsenic, methylarsonic acid (MA), dimethylarsinic acid (DMA) and arsenobetaine (AB) were determined by electrothermal atomic absorption spectrometry (ETAAS) using suspensions prepared in a 0.01 mol L⁻¹ tetramethylammonium hydroxide (TMAH) solution. Speciation is based on the use of three different chemically modified ETAAS atomizers to obtain the analytical signals. Using a palladium salt as the chemical modifier, the signal corresponding to the total arsenic concentration is obtained. When palladium is replaced by Ce(IV), the signal is solely due to inorganic arsenic (III and V) + MA. If no signal is obtained in this latter case, it is possible to distinguish between DMA and AB using a zirconium coated atomizer. The signal obtained in this way is due solely to DMA, and the concentration of AB can be obtained by the difference with the total arsenic content. Determinations by ETAAS require the use of the standard additions method. The limits of detection for the determination of AB, DMA and inorganic arsenic (+MA) are 15, 25 and 50 ng g⁻¹ expressed as arsenic, respectively. These detection limits are good enough for the procedure to be appropriate for the rapid determination of these compounds, avoiding extraction processes and/or chromatographic separations. Data for commercial samples, as well as for four standard reference materials, are given.