Continuous flow method for the simultaneous determination of phosphate/arsenate based on their different kinetic characteristics

This paper proposes a new automated spectrophotometric method for the simultaneous determination of phosphate and arsenate without pre-treatment, which is faster, simpler, less expensive and hazardous than other well-known methods used with water samples. Such method is based on the different kinetic characteristics of complex formation of phosphate and arsenate with ammonium molybdate. A flow system was used in order to achieve good mixing and to provide precise time control. All the measurements were performed at the isosbestic point wavelength (885 nm). Chemical variables were optimized by factorial design (ammonium molybdate 0.015 mol L⁻¹, potassium antimony tartrate 1 × 10⁻⁴ mol L⁻¹, and sulphuric acid 0.7 mol L⁻¹). An appropriate linear range for both analytes (0.50-8.00 μmol L⁻¹), good inter-day reproducibility (4.9% [P] and 3.3% [P + As]) and a sample throughput of 6 h⁻¹ were obtained. The detection limits are 0.4 μmol L⁻¹ P and 0.19 μmol L⁻¹ [P + As] (3.3 Sy/x). The method was validated.     

Phosphate desorption kinetics from goethite as induced by arsenate

The kinetics of the arsenate-induced desorption of phosphate from goethite has been studied with a batch reactor system and ATR-FTIR spectroscopy. The effects of arsenate concentration, adsorbed phosphate, pH and temperature between 10 and 45 °C were investigated. Arsenate is able to promote phosphate desorption because both oxoanions compete for the same surface sites of goethite. The desorption occurs in two steps: a fast step that takes place in less than 5 min and a slow step that lasts several hours. In the slow step, arsenate ions exchange adsorbed phosphate ions in a 1:1 stoichiometry. The reaction is first order with respect to arsenate concentration and is independent of adsorbed phosphate under the experimental conditions of this work. The rate law is then r = k_r[As], where r is the desorption rate, k_r is the rate constant and [As] is the arsenate concentration in solution. The values of k_r at pH 7 are 1.87 × 10⁻⁵ L m⁻² min⁻¹ at 25 °C and 7.95 × 10⁻⁵ L m⁻² min⁻¹ at 45 °C. The apparent activation energy of the desorption process is 51 kJ mol⁻¹. Data suggest that the rate-controlling process is intraparticle diffusion of As species, probably As diffusion in pores. ATR-FTIR spectroscopy suggests that adsorbed phosphate species at pH 7 are mainly bidentate inner-sphere surface complexes. The identity of these complexes does not change during desorption, and there is no evidence for the formation of intermediate species during the reaction.     

Arsenite oxidation and arsenate determination by the molybdene blue method

Based on the similarity in properties of arsenate and phosphate, the colorimetric method using the molybdene blue complex was tested in order to determine low As(V) concentration in waters. The influence of complex formation time, daylight, temperature and competitive anions (silicate and sulphate) upon complex formation was determined. Optimal complex formation was reached in 1 h at 20±1 °C and was slightly favoured when developed in daylight. The formation rate declined with decreasing reaction temperature and no influence of any of the competitive anions tested (at concentrations usually found in natural waters of granitic areas) was noted. The detection limit of this method was 20 μg As(V) l⁻¹. This simple, fast and sensitive arsenic determination method is suitable for field analysis, especially for waters containing low levels of phosphate and organic matter. Through arsenate determination, this colorimetric method allowed the arsenite oxidation efficiency of five common industrial oxidants to be compared. H₂O₂ and MnO_2(s) were not considered as effective oxidants as a high excess was necessary to ensure As(III) oxidation. NaOCl and KMnO₄ were promising oxidants as they allowed complete arsenite oxidation with a small excess for NaOCl or even less than the electron stoichiometric ratio in the case of KMnO₄. FeCl₃ was the most effective oxidant among the reagents tested here.     

Interference from arsenate when determining phosphate by the malachite green spectrophotometric method

The effect of arsenate on phosphate determination by the malachite green spectrophotometric method was investigated. The molar absorptivities of the molybdophosphate and malachite green–molybdoarsenate species at 625 nm and a final acidity of 0.38 M were calculated as 10.4±0.13×10⁴ and 7.2±0.17×10⁴ l mol⁻¹ cm⁻¹ respectively, indicating that arsenate could interfere in phosphate measurement. Arsenate concentrations as low as 23 μg l⁻¹ caused increase in colour development in phosphate solutions. However, the extent of colour development for both anions depended on the final acid concentration of the solution. An acidified sodium sulphite solution (0.83 M NaSO₃, 0.83 M H₂SO₄) quantitatively prevented arsenate colour development up to 300 μg l⁻¹ As(V). It was also demonstrated that the method removed As(V) interferences in mixed As/P solutions and therefore can be used to treat natural water samples with elevated arsenate concentrations before phosphate measurement.     

Determination of arsenite, arsenate and monomethylarsonic acid in aqueous samples by gas chromatography of their 2,3-dimercaptopropanol (bal) complexes

Procedures are described for the determination of arsenite, arsenate and monomethylarsonic acid in aqueous samples. The arsenicals (after reduction of arsenic to the tervalent state) readily react with 2,3-dimercaptopropanol (BAL) to yield their BAL complexes. The products are extracted with benzene and introduced into a gas Chromatograph equipped with a flame-photometric detector for sulphur. One aliquot of sample is treated with stannous chloride solution and potassium iodide solution to reduce arsenate and monomethylarsonic acid, then BAL is added and the complexes are extracted with benzene. The extract is analysed for total inorganic As plus monomethylarsonic acid. Magnesia mixture and phosphate solution are added to another aliquot to remove arsenate by co-precipitation with magnesium ammonium phosphate. The precipitate is filtered off and arsenite determined in the filtrate. The detection limits are 0.02 ng of As for arsenate and arsenite and 0.04 ng of As for monomethylarsonic acid.     

Kinetic spectrophotometric determination of submicromolar orthophosphate by molybdate reduction

A kinetic spectrophotometric procedure was developed for determination of submicromolar orthophosphate based on the reaction in which orthophosphate serves as a catalyst in the reduction of molybdenum, and the initial rate of molybdenum-blue formation (λ_max = 780 nm) is proportional to the concentration of orthophosphate in the samples. The detection limit (3 × standard deviation of blank, n = 8) was 6 nM and the linear calibration ranged from 10 to 100 nM (r² = 0.997). The precisions of this method were 3.3% at 10 nM and 5.4% at 50 nM (n = 8), respectively. Similar to other molybdate based methods, silica and arsenate in the samples can interfere with phosphate determination. The responses of silicate and arsenate were about 25% and 7% of that of orthophosphate, respectively, and their interferences were enhanced in the presence of phosphate in the samples due to the synergistic effect of phosphate with arsenate or silicate on the molybdate reagent.     

Retention of arsenate using genetically modified coryneform bacteria and determination of arsenic in solid samples by ICP-MS

A novel method for the retention of arsenate [As(V)] combining time-controlled solid-phase extraction with living bacterial biomass is presented. As(V) retention was carried out by exposing the extractant, consisting of a living double-mutant of Corynebacterium glutamicum strain ArsC1-C2, to the sample for a retention time of 1-7 min, before the arsenic distribution equilibrium between the sample solution and the extractant was established. The amount of As(V) retained in the biomass was measured by inductively coupled plasma-mass spectrometry (ICP-MS) after the sample had been treated with nitric acid. A theoretical model of the retention process was developed to describe the experimental retention-time profiles obtained with the bacterial cells. This relationship provided a feasible quantification of the retention process before steady-state was reached, providing that the agitation conditions and the retention time had been controlled. An analytical procedure for the retention/quantification of As(V) was then developed; the detection limit was 0.1 ng As(V) mL⁻¹ and the relative standard deviation 2.4-3.0%. The maximum effective retention capacity for As(V) was about 12.5 mg As (g biomass)⁻¹. The developed procedure was applied to the determination of total arsenic in coal fly ash, using a sample that had undergone oxidative pre-treatment.     

Simple and selective determination of arsenite and arsenate by electrospray ionization mass spectrometry

Arsenic pollution of public water supplies has been reported in various regions of the world. Recently, some cancer patients are treated with arsenite (As^III); most Japanese people consume seafoods containing large amounts of negligibly toxic arsenic compounds. Some of these arsenic species are metabolized, but some remain intact. For the determination of toxic As^III, a simple, rapid and sensitive method has been developed using electrospray ionization mass spectrometry (ESI-MS). As^III was reacted with a chelating agent, pyrrolidinedithiocarbamate (PDC, C₄H₈NCSS^-) and tripyrrolidinedithiocarbamate-arsine, As(PDC)₃, extracted with methyl isobutyl ketone (MIBK). A 1 μL aliquot of MIBK layer was directly injected into ESI-MS instrument without chromatographic separation, and was detected within 1 min. Arsenate (As^V) was reduced to As^III with thiosulfate, and then the total inorganic As was quantified as As^III. This method was validated for the analysis of urine samples. The limit of detection of As was 0.22 μg L⁻¹ using 10 μL of sample solution, and it is far below the permissible limit of As in drinking water, 10 μg L⁻¹, recommended by the WHO. Results were obtained in < 10 min with a linear calibration range of 1-100 μg L⁻¹. Several organic arsenic compounds in urine did not interfere with As^III detection, and the inorganic As in the reference materials SRM 2670a and 1643e were quantified after the reduction of As^V to As^III.     

Determination of As(III) and As(V) in soils using sequential extraction combined with flow injection hydride generation atomic fluorescence detection

An analytical procedure for determination of As(III) and As(V) in soils using sequential extraction combined with flow injection (FI) hydride generation atomic fluorescence spectrometry (HG-AFS) was presented. The soils were sequentially extracted by water, 0.6 mol l⁻¹ KH₂PO₄ solution, 1% (v/v) HCl solution and 1% (w/v) NaOH solution. The arsenite (As(III)) in extract was analyzed by HG-AFS in the medium of 0.1 mol l⁻¹ citric acid solution, then the total arsenic in extract was determined by HG-AFS using on-line reduction of arsenate with l-cysteine. The concentration of arsenate (As(V)) was calculated by the difference. The optimum conditions of extraction and determination were studied in detail. The detection limit (3σ) for As(III) and As(V) were 0.11 and 0.07 μg l⁻¹, respectively. The relative standard deviation (R.S.D.) was 1.43% (n=11) at the 10 μg l⁻¹ As level. The method was applied in the determination of As(III) and As(V) of real soils and the recoveries of As(III) and As(V) were in the range of 89.3-118 and 80.4-111%, respectively.     

Determination of arsenite and arsenate by differential pulse polarography

Summary Arsenate was determined by differential pulse polarography in acidic solutions in the presence of polyhydroxy compounds. The best medium was found to be 2.0 M aqueous HClO₄ containing 4.5 g d-mannitol in 50 ml solution. The peak heights measured at –0.55 V gave linear calibration curves in the concentration range 20 g/l to 160 mg/l As. Arsenite was similarly determined with mannitol at –0.34 V or without mannitol at –0.42 V. When arsenite and arsenate were present in solution, the simultaneous determination of these compounds in the presence of mannitol was generally not possible because the peak heights at –0.34 V and –0.55 V were influenced by arsenite as well as arsenate. In these cases arsenite was determined at –0.42V in the absence of mannitol. After oxidation of arsenite to arsenate by chlorine water and addition of mannitol, total inorganic arsenic was determined as arsenate at –0.55 V. The arsenate concentration in the sample was found as the difference between the concentrations of total inorganic arsenic and arsenite. The detection limit for arsenite and arsenate was found to be approximately 10 g/l As. This method was successfully used to determine arsenite and arsenate in a synthetic river water sample and some arsenic-containing drinking water samples.

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Bestimmung von Arsenit und Arsenat durch Differential-Pulspolarographie

Zusammenfassung Arsenat wurde durch Differential-Pulspolarographie in saurer Lösung in Gegenwart von Polyhydroxyverbindungen bestimmt. Das günstigste Medium war 2,0 M wäßrige HClO₄ mit 4,5 g d-Mannit in 50 ml. Die bei –0,55V gemessenen Peakhöhen ergaben eine lineare Eichkurve für den Bereich von 20 g/l bis 160 mg/l As. Arsenit wurde auf ähnliche Weise mit Mannit bei –0,34 V oder ohne Mannit bei –0,42 V bestimmt. Bei Anwesenheit von Arsenit + Arsenat in Lösung war eine Simultanbestimmung in Gegenwart von Mannit im allgemeinen nicht möglich, weil die Peakhöhen bei –0,34 V und –0,55 V sowohl von Arsenit als auch von Arsenat beeinflußt werden. In diesen Fällen wurde Arsenit ohne Mannit bei –0,42 V bestimmt. Nach Oxidation zu Arsenat mit Chlorwasser und Zugabe von Mannit wurde dann das Gesamtarsen als Arsenat bei –0,55 V bestimmt; der Arsenatgehalt in der Probe ergab sich aus der Differenz. Die Nachweisgrenze für Arsenit und Arsenat lag bei etwa 10 g/l As. Das Verfahren wurde mit gutem Erfolg für eine synthetische Flußwasserprobe sowie einige Trinkwasserproben angewendet.

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On leave from Jadavpur University, Calcutta, India     

Adsorption kinetics of phosphate and arsenate on goethite. A comparative study

The adsorption kinetics of phosphate and arsenate on goethite is studied and compared. Batch adsorption experiments were performed at different adsorbate concentrations, pH, temperatures and stirring rates. For both oxoanions the adsorption rate increases by increasing adsorbate concentration, decreasing pH and increasing temperature. It does not change by changing stirring rate. The adsorption takes place in two processes: a fast one that takes place in less than 5 min and a slow one that takes place in several hours or more. The rate of the slow process does not depend directly on the concentration of phosphate or arsenate in solution, but depends linearly on the amount of phosphate or arsenate that was adsorbed during the fast process. Apparent activation energies and absence of stirring rate effects suggest that the slow process is controlled by diffusion into pores, although the evidence is not conclusive. The similarities in the adsorption kinetics of phosphate and arsenate are quantitatively shown by using a three-parameters equation that takes into account both the fast and the slow processes. These similarities are in line with the similar reactivity that phosphate and arsenate have in general and may be important for theoretical and experimental studies of the fate of these oxoanions in the environment.     

Determination of melamine and related triazine by-products ammeline, ammelide, and cyanuric acid by micellar electrokinetic chromatography

In this study, micellar electrokinetic chromatographic (MEKC) methods were developed for the detection of traces of melamine and its related by-products (ammeline, ammelide, and cyanuric acid). Two on-line sample concentration steps namely reversed electrode polarity stacking mode (REPSM) and cation-selective injection (CSI) were used for improving the detection sensitivity. For REPSM, a borate-NaOH buffer (pH 10, 35 mM) composed of 60 mM SDS and 10% (v/v) methanol, was used as carrier electrolyte, and samples were prepared in an aqueous solution of 10 mM NaOH. In CSI, a phosphate buffer (pH 2, 50 mM) containing 41 mM SDS was used as the carrier electrolyte, and samples were prepared with an aqueous solution of 10 mM NaOH and a phosphate buffer (pH 2.0, 25 mM) in a volume ratio of 1:9. The results indicated that REPSM enhanced all analyte signals except for melamine, which could be concentrated only by the CSI. The detection limit was reduced from 1.7 mg L⁻¹ to 2.8 μg L⁻¹ for melamine by the optimal CSI step, and from 0.23-1.2 mg L⁻¹ to 2.4-5.0 μg L⁻¹ for the other three analytes by the optimal REPSM step. Tableware made of melamine and samples of flour were used as test samples, and the results indicated that the proposed MEKC methods can successfully determine contaminations from melamine. The study also indicated that when the plastic made of melamine was exposed only once to an acidic solution (acetic or phosphoric acid) at 80 °C for 30 min, melamine continuously leached out from the test sample even without any further treatment with an acidic solution.     

Catalytic Reduction of Bisulfite by Myoglobin/Surfactant Films

The voltammetry of bisulfite at a film formed with myoglobin was studied in aqueous solutions. A broad wave was observed for the reduction of bisulfite. Using controlled potential electrolysis, the reduction at potentials positive of the Fe^II/Fe^I wave formed dithionite exclusively. As the potential approached the region for the Fe^II/Fe^I reduction, bisulfite was reduced primarily to HS⁻. Even at the negative potentials, some dithionite was still formed, which could then be electrochemically reduced to thiosulfate. Analysis of the formation of HS⁻, dithionite and thiosulfate during the electrolysis was consistent with the parallel formation of HS⁻ and dithionite, the latter of which was reduced to thiosulfate. Thiosulfate was verified by chemical analysis of the products from controlled potential electrolysis of the solution, and dithionite was observed spectroscopically using spectroelectro−chemistry.     

Determination of trace elements in biological samples treated with formic acid by inductively coupled plasma mass spectrometry using a microconcentric nebulizer

A simple and fast method for the determination of As, Ba, Cd, Co, Cu, Fe, Ga, Mn, Mo, Ni, Pb, Rb, Se, Sr, Tl, U, V and Zn in biological samples by inductively coupled plasma mass spectrometry (ICP-MS), after sample solubilization with formic acid and introduction by a microconcentric nebulizer, is proposed. The sample is mixed with formic acid, kept at 90 °C for one hour and then diluted with nitric acid aqueous solution to a 50% v/v formic acid and 1% v/v nitric acid final concentrations. The final sample solution flow rate for introduction into the plasma was 30 μL min⁻¹. The optimized and adopted nebulizer gas flow rate was 0.7 L min⁻¹ and RF power was 800 W. These conditions are very different than those normally used when a conventional nebulizer is employed. Rodhium was used as internal standard. External calibration against aqueous standard solutions, without formic acid, could be used for quantification, except for As, Se and Zn. However, external calibration with 50% formic acid allows the determination of all analytes with high accuracy and it is recommended. The detection limits were between 0.0005 (Tl) and 0.22 mg kg⁻¹ (Fe) and the precision expressed by the relative standard deviations (RSD) were between 0.2% (Sr) and 3.5% (Ga). Accuracy was validated by the analysis of four certified reference biological materials of animal tissues, comparing the results by linear regressions and by the t-test at a 95% confidence level. The recommended procedure avoids plasma instability and carbon deposit on the cones.     

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.     

Raman sensitivity enhancement for aqueous absorbing sample using Teflon-AF 2400 liquid core optical fibre cell

The compatibility Teflon-AF 2400 liquid core optical fibre with resonance Raman spectroscopy (RRS-LCOF) was used to detect aqueous biomolecules. The maximum sensitivity enhancement factor for concentrations greater than the detection limit in a conventional cell was 10, and detection limit reduction of about 1000-fold have been achieved for the measurement of aqueous absorbing sample using Teflon-AF 2400 fibre Raman cell compared to the conventional cell. We were able to collect spectra of 2.5 × 10⁻⁹ and 2.5 × 10⁻¹⁰ M aqueous β-carotene using 16.2 mW of laser power and 10 s integration time. This volume of a 2.5 × 10⁻¹⁰ M aqueous solution corresponds to only 1.5 fmol or 830 fg of β-carotene. The results of this preliminary study indicate that RRS-LCOF has potential in bioanalytical and biomedical applications.     

Determination of arsenate in natural pH seawater using a manganese-coated gold microwire electrode

Direct electrochemical determination of arsenate (As^V) in neutral pH waters is considered impossible due to electro-inactivity of As^V. As^III on the other hand is readily plated as As⁰ on a gold electrode and quantified by anodic stripping voltammetry (ASV). We found that the reduction of As^V to As^III was mediated by elemental Mn on the electrode surface in a novel redox couple in which 2 electrons are exchanged causing the Mn to be oxidised to Mn^II. Advantage is taken of this redox couple to enable for the first time the electrochemical determination of As^V in natural waters of neutral pH including seawater by ASV using a manganese-coated gold microwire electrode. Thereto Mn is added to excess (∼1 μM Mn) to the water leading to a Mn coating during the deposition of As on the electrode at a deposition potential of −1.3 V. Deposition of As⁰ from dissolved As^V caused elemental Mn to be re-oxidised to Mn^II in a 1:1 molar ratio providing evidence for the reaction mechanism. The deposited As^V is subsequently quantified using an ASV scan. As^III interferes and should be quantified separately at a more positive deposition potential of −0.9 V. Combined inorganic As is quantified after oxidation of As^III to As^V using hypochlorite. The microwire electrode was vibrated during the deposition step to improve the sensitivity. The detection limit was 0.2 nM As^V using a deposition time of 180 s.     

Electrochemical sensors based on binuclear cobalt phthalocyanine/surfactant/ordered mesoporous carbon composite electrode

A new ordered mesoporous carbon (OMC) composite modified electrode was fabricated for the first time. Binuclear cobalt phthalocyaninehexasulfonate sodium salt (bi-CoPc) can be adsorbed onto didodecyldimethylammonium bromide (DDAB)/OMC film by ion exchange. UV-vis spectroscopy, scanning electron microscopy (SEM) and electrochemical methods were used to characterize the composite film. The cyclic voltammograms demonstrate that the charge transfer of bi-CoPc is promoted by the presence of OMC. Further study indicated that bi-CoPc/DDAB/OMC film is the excellent electrocatalyst for the electrochemical reduction of oxygen in a neutral aqueous solution and hemoglobin (Hb) at lower concentrations. Additionally, as an amperometric 2-mercaptoethanol (2-ME) sensor, this modified electrode shows a wider linear range (2.5 × 10⁻⁶ to 1.4 × 10⁻⁴ M), high sensitivity (16.5 μA mM⁻¹) and low detection limit of 0.6 μM (S/N = 3). All these confirm the fact that the new composite film may have wide potential applications in biofuel cells, biological and environmental sensors.     

Synthesis, crystal structure and charge distribution of Na7As11O31: An oxygen-deficient layered sodium arsenate

A new sodium arsenate with layer structure has been synthesized and its crystal structure solved and refined by single-crystal X-ray diffraction. The crystal is trigonal, space group , a=11.199(3) Å, c=5.411(2) Å, V=587.80(3) Å³, Z=1; the refinement converged to R=0.0282 and wR=0.0751 for 590 reflections with (I)>2sigma(I). The structural model gives the formula Na₇As₁₁O₃₂, which would be non-neutral; besides, the structural model is not validated by the charge distribution (CD) analysis, which gives an unsatisfactory agreement on the computed charges of the cations. The CD analysis suggest incomplete (5/6) occupation of the O5 site, which leads to the deficiency of an oxygen atom per unit cell and to formula Na₇As₁₁O₃₁: this new structural model corresponds to a neutral compound, is validated by the CD analysis, and results in better displacement parameters for O5 than its non neutral counterpart. The (001) anionic layers are built up from corner and edge sharing of As1 and As2 distorted octahedra and As3 distorted tetrahedra, the sodium cations playing the role of interlayer cations. The effects of the oxygen deficiency on the crystal structure are discussed.     

Determination of arsenite,arsenate, monomethylarsonic acid and dimethylarsinic acid in cereals by hydride generation atomic fluorescence spectrometry

A fast, sensitive and simple non-chromatographic analytical method was developed for the speciation analysis of toxic arsenic species in cereal samples, namely rice and wheat semolina. An ultrasound-assisted extraction of the toxic arsenic species was performed with 1 mol L^− 1 H₃PO₄ and 0.1% (m/v) Triton XT-114. After extraction, As(III), As(V), dimethylarsinic acid (DMA) and monomethylarsonic acid (MMA) concentrations were determined by hydride generation atomic fluorescence spectrometry using a series of proportional equations corresponding to four different experimental reduction conditions. The detection limits of the method were 1.3, 0.9, 1.5 and 0.6 ng g^− 1 for As(III), As(V), DMA and MMA, respectively, expressed in terms of sample dry weight. Recoveries were always greater than 90%, and no species interconversion occurred. The speciation analysis of a rice flour reference material certified for total arsenic led to coherent results, which were also in agreement with other speciation studies made on the same certified reference material.