Evaluation of high-pressure microwave digestion methods for hydride generation atomic absorption spectrometric determination of total selenium and arsenic in sediment

Five closed-vessel microwave digestion methods were compared for the accurate determination of arsenic and selenium in NIST SRM 1645 River Sediment by flow-injection hydride-generation atomic absorption spectrometric methods. The digestion methods using five different acid mixtures (HNO₃/ H₂SO₄, HNO₃/HCl0₄, HNO₃/HCl, HNO₃/HCl/HF, HNO₃/H₂SO₄/HClO₄) were all found to be reliable for the determination of the analytes. Taking into consideration the safety and suitability for the analysis of other metals, the methods based on the use ofaqua regia are recommended for closed vessel microwave digestion with pressure control. Using the quick digestion program, the presence of up to 10% organic content in soil samples did not adversely affect the closed vessel digestion and did not cause the loss of volatile analytes. After digestion, opening the vessel under an inner pressure of below 345 kPa (50 psi) had no effect on the accuracy of the results. The recommended digestion methods (HNO₃/HCl and HNO₃/ HCl/HF) for the reliable determination of arsenic and selenium in different sediment samples were demonstrated. The calculated detection limits (3 _b ) were less than 0.030 g/g and 0.033 g/g for arsenic and selenium, respectively. All analytical results for arsenic and selenium in SRM 1645 River sediment, NRCC BCSS-1 Marine Sediment and NIES CRM Pond Sediment were within or near the certified and reported ranges, with the exception of selenium in NIES CRM No. 2 Pond Sediment.     

Preservation of arsenic species in water samples using phosphoric acid--limitations and long-term stability

The preservation of arsenic species in water samples is an indispensable method to avoid their changes during storage, if it is not possible to analyse them immediately. The aim of this investigation was to demonstrate the limitations of the suggested method by using phosphoric acid as a preservation agent. The samples remain stable for 3 months, even if they show evidence of high concentrations of iron or manganese. Critical is an increasing pH > 3. Theoretically, a precipitation of strengite (Fe₃(PO₄)₂) could occur, which should be avoided. Phosphoric acid with a final concentration of 10 mM is recommended as a preservation agent, combined with keeping the samples cool (6 °C) and dark. Filtration of samples before preservation may be carried out with respect to the analytical aim to distinguish between the total and soluble fraction (without colloids). It was shown that filtered and non-filtered samples can be preserved by utilising the above mentioned scheme.     

A study on the extraction and quantitation of total arsenic and arsenic species in seafood by HPLC-ICP-MS

Various internal standards and analytical methods were investigated using certified reference materials to evaluate the accuracy of the quantitation of the total As in seafood. Inductively coupled plasma mass spectrometry (ICP-MS) was used to measure the total arsenic. Enhancement of the total arsenic in its concentration caused by the methanol matrix was clearly observed. Selenium (mass 77) was the best internal standard, and the standard addition method combined with the use of Se as an internal standard was the best analytical method. The total arsenic was determined in bluefin tuna, yellowfin tuna, bigeye tuna, and swordfish by ICP-MS. The concentrations of total arsenic in the seafoods ranged from 0.74 to 6.87 mg/kg.Various extraction procedures were also investigated using reference materials to evaluate the extraction efficiency of the different arsenic species in seafood. Inductively coupled plasma mass spectrometry (ICP-MS) was used in conjunction with high performance liquid chromatography (HPLC) to quantitate the arsenic species in seafood. The arsenic species were extracted from tuna fish (BCR 627) with water/methanol mixtures using sonication, a microwave-assisted system, and ultrasonic processor. The major species was arsenobetaine. The total arsenic extraction efficiency ranged from 81 to 87% for water and various methanol concentrations. Chromatograms of the arsenic species extracted from the Korea Research Institute of Standards and Science (KRISS) tuna, bluefin tuna, yellowfin tuna, bigeye tuna, and swordfish were obtained by the optimum extraction methods and the species were quantified.     

Analysis of gaseous arsenic species and stability studies of arsine and trimethylarsine by gas chromatography-mass spectrometry

A rapid method based on gas chromatography-mass spectrometry was developed for analysis of four volatile arsenic species: arsine, monomethylarsine (MMA), dimethylarsine (DMA) and trimethylarsine (TMA). With the proposed method gaseous arsenic species could be determined in less than 2 min and no pre-treatment for gas phase samples was needed, which minimized the risks of species conversion before analysis. The detection limits for different species were 24–174 pg. The standards for arsine, MMA and DMA were prepared by reaction between arsenic acid, monomethylarsonic acid or dimethylarsinic acid with tetrahydroborate(III) and nitric acid. The effect of pH on recovery of different arsine species was examined and is discussed. The TMA was obtained commercially as liquid. Also stability of inorganic (arsine) and organic (TMA) gaseous arsenic species in air was studied as a function of time.     

Determination of metals in marine species by microwave digestion and inductively coupled plasma mass spectrometry analysis

A microwave digestion method suitable for determination of multiple elements in marine species was developed, with the use of cold vapor atomic spectrometry for the detection of Hg, and inductively coupled plasma mass spectrometry for all of the other elements. An optimized reagent mixture composed of 2 ml of HNO₃, 2 ml of H₂O₂ and 0.3 ml of HF used in microwave digestion of about 0.15 g (dry weight) of sample was found to give the best overall recoveries of metals in two standard reference materials. In the oyster tissue standard reference material (SRM 1566b), recoveries of Na, Al, K, V, Co, Zn, Se, Sr, Ag, Cd, Ni, and Pb were between 90% and 110%; Mg, Mn, Fe, Cu, As, and Ba recoveries were between 85% and 90%; Hg recovery was 81%; and Ca recovery was 64%. In a dogfish certified reference material (DORM-2), the recoveries of Al, Cr, Mn, Se, and Hg were between 90% and 110%; Ni, Cu, Zn, and As recoveries were about 85%; and Fe recovery was 112%. Method detection limits of the elements were established. Metal concentrations in flounder, scup, and blue crab samples collected from coastal locations around Long Island and in the Hudson River estuary were determined.     

One step derivatization with British Anti-Lewsite in combination with gas chromatography coupled to triple-quadrupole tandem mass spectrometry for the fast and selective analysis of inorganic arsenic in rice

We developed a new fast and selective analytical method for the determination of inorganic arsenic (iAs) in rice by a gas chromatography – tandem mass spectrometry (GC-MS/MS) in combination with one step derivatization of inorganic arsenic (iAs) with British Anti-Lewsite (BAL). Two step derivatization of iAs with BAL has been previously performed for the GC-MS analysis. In this paper, the quantitative one step derivatization condition was successfully established. The GC-MS/MS was carried out with a short nonpolar capillary column (0.25 mm × 10 m) under the conditions of fast oven temperature ramp rate (4 °C/s) and high linear velocity (108.8 cm/s) of the carrier gas. The established GC-MS/MS method showed an excellent linearity (r² > 0.999) in a tested range (0.2–100.0 μg L⁻¹), ultra-low limit of detection (LOD, 0.08 pg), and high precision and accuracy. The GC-MS/MS technique showed far greater selectivity (22.5 fold higher signal to noise ratio in rice sample) on iAs than GC-MS method. The gas chromatographic running time was only 2.5 min with the iAs retention time of 1.98 min. The established method was successfully applied to quantify the iAs contents in polished rice. The mean iAs content in the Korean polished rice (n = 27) was 66.1 μg kg⁻¹ with the range of 37.5–125.0 μg kg⁻¹. This represents the first report on the GC-tandem mass spectrometry in combination with the one step derivatization with BAL for the iAs speciation in rice. This GC-MS/MS method would be a simple, useful and reliable measure for the iAs analysis in rice in the laboratories in which the expensive and element specific HPLC-ICP-MS is not available.     

微波消解–电感耦合等离子体质谱法测定酱卤肉中铬、砷、镉、铊、铅

建立微波消解–电感耦合等离子体质谱法测定酱卤肉中铬、砷、镉、铊、铅5种元素含量的检测方法。采用微波消解法处理酱卤肉样品,对预处理条件和仪器工作参数进行了优化,以内标校正法降低基体影响。结果表明,铬、砷、镉、铊、铅5种元素的质量浓度在0.1~50.0 μg/L范围内与质谱响应值具有良好的线性关系,相关系数均大于0.999,方法检出限为0.2~6.2 μg/kg。在3种加标浓度水平下平均回收率为72.4%~113.5%,测定结果的相对标准偏差为3.1%~7.7%(n=6)。该方法简便,快速,高效,准确,可为酱卤肉中铬、砷、镉、铊、铅的测定提供技术支持。     

Non-chromatographic speciation analysis of arsenic and antimony in milk hydride generation atomic fluorescence spectrometry

A rapid, high sensitivity method has been developed for the determination of As(III), As(V), Sb(III) and Sb(V) in milk samples by using hydride generation atomic fluorescence spectrometry. The method is based on the leaching of As and Sb from milk through the sonication of samples with aqua regia followed by direct determination of the corresponding hydrides both before and after reduction with KI. It was confirmed by recovery experiments on spiked commercially available samples that neither the reduced nor the oxidized forms of the elements under study or mixtures of the two oxidation states were modified by the room temperature sample treatment with aqua regia. The methodologies developed provided 3σ limit of detection values of 8.1, 10.3, 5.4 and 7.7 ng l⁻¹ for As(III), As(V), Sb(III) and Sb(V) in the diluted samples. Average relative standard deviation values of 5.7, 5.5, 8.2 and 4.7% were found for determination of As(III), As(V), Sb(III) and Sb(V) in commercially available samples of different composition and origin containing from 3.5 to 13.6 ng g⁻¹ total As and from 4.9 to 11.8 ng g⁻¹ total Sb, it being confirmed that As(V) and Sb(V) are the main species present in the samples analyzed (62±5 and 73±5%, respectively). The time required to determine As and Sb species in milk involves 10 min sonication and 30 min prereduction but these steps can be carried out for several sample simultaneously. Additionally the fluorescence measurement step involves less than 20 min for three replicates of all the four measurements required. So, in less than 2 h it is possible to determine the content of As(III), As(V), Sb(III) and Sb(V) in four samples.     

Separation of arsenic species by ion-pair and ion exchange high performance liquid chromatography

Arsenite, arsenate, monomethylarsonate, dimethylarsinate, arsenobetaine, arsenocholine and the tetramethylarsonium ion were subjected to ion-exchange and ion-pair reversed phase HPLC. The ion exchange method was superior in selectivity and time of analysis for the arsenic anions. The ammonium ions used for the ion-pair method only resulted in separation of some of the anionic arsenic compounds. Flame atomic absorption spectrometry was used for on-line arsenic-specific detection.     

微波消解-电感耦合等离子体质谱法同时测定化妆品中的汞、铅、砷、锑

采用HNO_3-H_2O_2微波消解样品,电感耦合等离子体质谱法(ICP-MS)同时测定化妆品中的汞、铅、砷、锑.实验中探讨了最佳的消解程序,选择适当的同位素,并用铋、铟、锗做内标元素,有效地抑制了分析信号的漂移.结果表明,各元素的检出限在0.089～0.37μg/kg,相对标准偏差(RSD)在0.709%～2.10%,回收率为80.10%～102.67%.该方法具有简便、快速、准确、灵敏度高的优点,适用于化妆品中的汞、铅、砷、锑元素的同时测定.     

Accelerated tryptic digestion for the analysis of biopharmaceutical monoclonal antibodies in plasma by liquid chromatography with tandem mass spectrometric detection

Accelerated tryptic digestion of a therapeutic protein including microwave irradiation and thermal transfer by convection at 60 °C and 37 °C was investigated. An analytical setup was devised to follow the protein digestion rate using 1D gel electrophoresis and liquid chromatography coupled a triple quadrupole linear ion trap mass spectrometer. The formation kinetic of its tryptic peptides was monitored in the selected monitoring mode (LC-SRM/MS). Different digestion end points (e.g. 2, 5, 10, 15, 30 and 60 min) as well as an overnight digestion were tested using a therapeutic human monoclonal antibody (mAb) with the goal of its LC-SRM/MS quantification in human plasma. The peptides from the human mAb were generated at different rates and were classified into three categories: (1) the fast forming peptides, (2) the slow forming peptides and (3) the peptides degrading over time. For many monitored peptides, a heating temperature of 37 °C with a 750 rpm mixing applied for at least 30 min provided equivalent results to microwave-assisted digestion and generally allowed the achievement of an equivalent peptide concentration as an overnight digestion carried out at 37 °C. The disappearance of the protein of the heavy and light chains can be monitored by 1D gel electrophoresis but was found not to be representative of the final tryptic peptide concentrations. For quantitative purposes a stable isotope labeled version (¹³C₄, ¹⁵N₁) of the therapeutic protein was used. The labeled protein as internal standard was found to be very efficient to compensate for incomplete digestion or losses during sample preparation.     

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.     

The separation of arsenic species in soils and plant tissues by anion-exchange chromatography with inductively coupled mass spectrometry using various mobile phases

Anion-exchange chromatography (Hamilton, PRP-X100) with inductively coupled plasma mass spectrometry (ICP-MS) is commonly used for the speciation of arsenic in environmental and biological samples. However, retentions for As species are frequently different because of the use of widely different mobile phases. In addition, chloride in matrices interferes with arsenic determination. In this study, we systematically investigated various mobile phases based on ammonium salts affecting arsenic retention to eliminate chloride interference chromatographically. Hence, various mobile phases based on ammonium salts, including NH₄H₂PO₄, NH₄HPO₄, NH₄Ac, NH₄HCO₃ and NH₄NO₃, were examined for reasonable resolution and to separate chloride from arsenic species. The best result was obtained with a mobile phase containing 30 mM NH₄H₂PO₄ at pH 5.6, where the separation of arsenic species, including arsenite [As(III)], arsenate [As(V)], dimethylarsinic acid (DMA) and monomethylarsonic acid (MMA)], was achieved within 9 minutes with reasonable resolution and free of chloride interference at its high level (500 mg L^− 1). The detection limits for the arsenic species were in the range of 0.1-0.3 μg L^− 1 with a direct injection of sample without removing matrix. Finally, the proposed method was used for the determination of arsenic species in contaminated soil and plant tissues.     

Rapid sample digestion by fusion and chemical separation of Hf for isotopic analysis by MC-ICPMS

A new method for rapid sample digestion and efficient chemical separation of Hf and REE from rock samples for precise isotopic analysis is presented. Samples are digested by fusion in the presence of a lithium borate flux at 1100 °C and dissolved whilst molten in dilute nitric or hydrochloric acid. Prior to chemical separation using ion exchange techniques, Li and B from the flux material and Si from the sample are separated from the remaining major elements, REE and high field strength elements (HFSE) in the sample by Fe-hydroxide co-precipitation. The chemical separation of Hf is a two-stage procedure designed to first remove the remaining matrix elements (e.g. Fe, Ba) in the sample using standard cation exchange techniques, followed by separation of Hf from the REE and HFSE on TEVA extraction chromatographic resin. Hf yields are >90% and total procedural blanks are ca. 50 pg. Hf isotope ratios of a synthetic standard solution and replicate digestions of international rock standards BHVO-1 and BCR-1 measured on multi-collector inductively coupled plasma mass spectrometer (MC-ICPMS) reproduce similarly to ≤50 ppm (2 S.D.). The following elemental ratios are routinely obtained for elements, which interfere isobarically or may affect the ionisation and/or fractionation behaviour of Hf during analysis: ¹⁷⁶Yb/¹⁷⁶Hf<0.0001; ¹⁷⁶Lu/¹⁷⁶Hf<0.00001; Ti/Hf<0.05. This technique also provides a means of separating Nd from the REE fraction for isotopic analysis and, potentially, may be adapted for measurement of Lu/Hf ratios by isotope dilution techniques.     

Stability of arsenic peptides in plant extracts: off-line versus on-line parallel elemental and molecular mass spectrometric detection for liquid chromatographic separation

The instability of metal and metalloid complexes during analytical processes has always been an issue of an uncertainty regarding their speciation in plant extracts. Two different speciation protocols were compared regarding the analysis of arsenic phytochelatin (As^IIIPC) complexes in fresh plant material. As the final step for separation/detection both methods used RP-HPLC simultaneously coupled to ICP-MS and ES-MS. However, one method was the often used off-line approach using two-dimensional separation, i.e. a pre-cleaning step using size-exclusion chromatography with subsequent fraction collection and freeze-drying prior to the analysis using RP-HPLC–ICP-MS and/or ES-MS. This approach revealed that less than 2% of the total arsenic was bound to peptides such as phytochelatins in the root extract of an arsenate exposed Thunbergia alata, whereas the direct on-line method showed that 83% of arsenic was bound to peptides, mainly as As^IIIPC₃ and (GS)As^IIIPC₂. Key analytical factors were identified which destabilise the As^IIIPCs. The low pH of the mobile phase (0.1% formic acid) using RP-HPLC–ICP-MS/ES-MS stabilises the arsenic peptide complexes in the plant extract as well as the free peptide concentration, as shown by the kinetic disintegration study of the model compound As^III(GS)₃ at pH 2.2 and 3.8. But only short half-lives of only a few hours were determined for the arsenic glutathione complex. Although As^IIIPC₃ showed a ten times higher half-life (23 h) in a plant extract, the pre-cleaning step with subsequent fractionation in a mobile phase of pH 5.6 contributes to the destabilisation of the arsenic peptides in the off-line method. Furthermore, it was found that during a freeze-drying process more than 90% of an As^IIIPC₃ complex and smaller free peptides such as PC₂ and PC₃ can be lost. Although the two-dimensional off-line method has been used successfully for other metal complexes, it is concluded here that the fractionation and the subsequent freeze-drying were responsible for the loss of arsenic phytochelatin complexes during the analysis. Hence, the on-line HPLC–ICP-MS/ES-MS is the preferred method for such unstable peptide complexes. Since freeze-drying has been found to be undesirable for sample storage other methods for sample handling needed to be investigated. Hence, the storage of the fresh plant at low temperature was tested. We can report for the first time a storage method which successfully conserves the integrity of the labile arsenic phytochelatin complexes: quantitative recovery of As^IIIPC₃ in a formic acid extract of a Thunbergia alata exposed for 24 h to 1 mg As^v L⁻¹ was found when the fresh plant was stored for 21 days at 193 K. Figure On-line HPLC–ICP-MS/ES-MS (bottom) is the preferred method for MS determination of unstable arsenic peptide complexes in plant extracts, since this avoids fractionation and subsequent freeze-drying that are responsible for loss of arsenic phytochelatin complexes in the 2D off-line method (top) Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

AE-HG-AFS测定长期汞暴露人群补硒后尿中硒的形态

建立了一种利用阴离子交换高效液相色谱与氢化物发生原子荧光光谱联用同时测定四、六价硒及硒代半胱氨酸(SeCys)形态的方法。优化了六价硒的还原条件及仪器检测参数,以不同浓度的柠檬酸铵作为流动相,在10 min内同时分离了四、六价硒及硒代半胱氨酸(SeCys)。采用加标法定量,加标回收率在90%～119%之间,相对标准偏差为1.6%～3.1%(100μg/L),四、六价硒及硒代半胱氨酸(SeCys)的检出限分别为0.32μg/L、0.47μg/L和0.44μg/L(进样量为100μL)。应用该法对长期汞暴露人群补硒后尿中的小分子硒的形态进行了分析,仅检测到硒代半胱氨酸(SeCys)。     

Determination of toxic and non-toxic arsenic species in urine by microwave assisted mineralization and hydride generation atomic absorption spectrometry

Flow injection — microwave oven — hydride generation — atomic absorption spectroscopy (FI-MO-HG-AAS) has been optimized for the determination of the total and toxic arsenic in urine with and without persulfate, respectively. With microwave oven assisted digestion of urine with 5% (w/v) K₂S₂O₈ and 5% (w/v) NaOH all arsenicals completely can be converted to arsenate, which is determined by HG-AAS to give the total concentration of the six species present in urine. The detection limits of 4–6 g l^–1, the relative standard deviation of 3–7% and the high sample throughput make the methods suitable for rapid routine on-line determination. Application of the proposed procedures to the analysis of urine from people on a diet rich in seafood revealed a significant increase in total urinary arsenic due to the rapid excretion of organoarsenicals. Efficient decomposition and quantitative recovery of all arsenic species in spiked urine is achieved by using 5% K₂S₂O₈ in 5% NaOH at 4.6 ml min^–1, microwave power of 700 W and a 1.5 m coil.     

Speciation analysis of inorganic arsenic in natural water by carbon nanofibers separation and inductively coupled plasma mass spectrometry determination

In this paper, carbon nanofibers (CNFs) as a novel solid phase extraction sorbent were developed for speciation preconcentration and separation of inorganic arsenic species As(III) and As(V) prior to determination by inductively coupled plasma mass spectrometry (ICP-MS). It was found that during all the steps of the separation, As(III) was selectively sorbed on the microcolumn packed with CNFs within a pH range of 1.0-3.0 in the presence of ammonium pyrroinedithiocarbamate (APDC), while As(V) was passed through the microcolumn without the retention. Various experimental parameters affecting the separation and determination of As(III) and As(V) have been investigated in detail. Under the optimized conditions, the detection limits of this method for As(III) were 0.0045 ng mL⁻¹ with an enrichment factor of 33 and 0.24 ng mL⁻¹ for As(V), and the relative standard deviations for As(III) and As(V) were 2.6% and 1.9% (n = 9, c = 1.0 ng mL⁻¹), respectively. In order to verify the accuracy of the method, a certified reference of water sample was analyzed, and the results obtained were in good agreement with the certified values. The proposed method was applied for the analysis of inorganic arsenic species in groundwater and lake water with the recovery of 92-106%.     

高效液相色谱-电感耦合等离子体质谱测定蟾酥中铜、砷、镉、汞、铅含量及砷化学形态

建立了微波消解-电感耦合等离子体质谱(ICP-MS)测定蟾酥中Cu、As、Cd、Hg、Pb等元素含量的分析方法,检出限为0.28～5.7 μg/L,元素加标回收率均在91.5%～115%.用1.2 mol/L HCl浸提蟾酥中的As形态,利用高效液相色谱(HPLC)与电感耦合等离子体质谱(ICP-MS)联用技术对蟾酥样品中的As化学形态进行了初步探讨,发现其中As主要以有毒的无机As(Ⅴ)形态存在,并讨论了形态分析的方法及结果.本方法适用于蟾酥中药样品质量控制和安全评估的要求.     

A simple method using on-line continuous leaching and ion exchange chromatography coupled to inductively coupled plasma mass spectrometry for the speciation analysis of bio-accessible arsenic in rice

A simple method for the speciation analysis of bio-accessible arsenic (As) in rice was developed using a continuous on-line leaching method to release the bio-accessible fraction. The continuous on-line leaching method has several advantages over commonly used batch methods including quicker and easier sample preparation, reduced risk of contamination and access to real time leaching data. The bio-accessibility of As in the samples was monitored using inductively coupled plasma mass spectrometry (ICP-MS). Results from a certified reference material as well as cooked and uncooked white rice showed that the majority of As was leached by saliva. Results obtained using the continuous on-line leaching method were comparable to those obtained using a batch method. Speciation analysis of the saliva leachate was performed using ion exchange chromatography coupled to ICP-MS. The four most toxic forms of As (As(III), monomethylarsonic acid (MMA), dimethylarsinic acid (DMA) and As(V)) were clearly separated within 5 min in a single chromatographic run. Over 92% of bio-accessible As in the certified reference material and uncooked white rice sample was in the form of DMA and As(V), whereas it was present as DMA and As(III) in the cooked white rice.