Development of a new hybrid technique for rapid speciation analysis by directly interfacing a microfluidic chip-based capillary electrophoresis system to atomic fluorescence spectrometry

This paper represents the first study on direct interfacing of microfluidic chip-based capillary electrophoresis (chip-CE) to a sensitive and selective detector, atomic fluorescence spectrometry (AFS) for rapid speciation analysis. A volatile species generation technique was employed to convert the analytes from the chip-CE effluent into their respective volatile species. To facilitate the chip-CE effluent delivery and to provide the necessary medium for subsequent volatile species generation, diluted HCl solution was introduced on the chip as the makeup solution. The chip-CE-AFS interface was constructed on the basis of a concentric "tube-in-tube" design for introducing a KBH4 solution around the chip effluent as sheath flow and reductant for volatile species generation as well. The generated volatile species resulting from the reaction of the chip-CE effluent and the sheath flow were separated from the reaction mixture in a gas-liquid separator and swept into the AFS atomizer by an argon flow for AFS determination. Inorganic mercury (Hg(II)) and methylmercury (MeHg(I)) were chosen as the targets to demonstrate the performance of the present technique. Both mercury species were separated as their cysteine complexes within 64 s. The precision (relative standard deviation, RSD, n = 5) of migration time, peak area, and peak height for 2 mg.L(-1) Hg(II) and 4 mg.L(-1) MeHg(I) (as Hg) ranged from 0.7 to 0.9%, 2.1 to 2.9%, and 1.5 to 1.8%, respectively. The detection limit was 53 and 161 microg.L(-1) (as Hg) for Hg(II) and MeHg(I), respectively. The recoveries of the spikes of mercury species in four locally collected water samples ranged from 92 to 108%.     

Adsorption characteristics of as(III) and as(V) with titanium dioxide loaded Amberlite XAD-7 resin.

The adsorption characteristics of As(V) and As(III) on titanium dioxide loaded Amberlite XAD-7 resin have been studied. The resin was prepared by impregnation of Ti(OC2H5)4 followed by hydrolysis with ammonium hydroxide. Batch adsorption experiments were carried out as a function of the pH, shaking time and the concentration of As(V) and As(III) ions. The resin showed a strong adsorption for As(V) from pH 1 to 5 and for As(III) from pH 5 to 10. The adsorption isotherm data for As(V) at pH 4 fitted well to a Langmuir equation with a binding constant of 59 dm3 mol(-1) and a capacity constant of 0.063 mmol g(-1). The data for As(III) at pH 7 also fitted well to a Langmuir equation with a binding constant of 5.4 dm3 mol(-1) and a capacity constant of 0.13 mmol g(-1). The effect of diverse ions on the adsorption of arsenic was also studied. Column adsorption experiments showed that the adsorption of As(III) is more favorable compared to As(V), due to both the faster adsorption and larger capacity for As(III) than As(V).     

Speciation of arsenic(III) and arsenic(V) by inductively coupled plasma-atomic emission spectrometry coupled with preconcentration system

A novel and simple flow-based method was developed for the simultaneous determination of As(III) and As(V) in freshwater samples. Two miniature columns with a solid phase anion exchange resin, placed on two 6-way valves were utilized for the solid-phase collection/concentration of arsenic(III) and arsenic(V), respectively. As(III) could be retained on the column after its oxidation to As(V) species with an oxidizing agent. The collected analytes were then sequentially eluted by 2 M nitric acid and introduced into ICP-AES. Potassium permanganate was examined as potential oxidizing agent for conversion of As(III) to As(V). The standard deviation of the analytical signals (peak height) for the replicate analysis (n = 5) of 0.5 μg l⁻¹ solution were 3 and 5% for As(III) and As(V), respectively. The limit of detection (3σ) for both As(III) and As(V) were 0.1 μg l⁻¹. The proposed system produced satisfactory results on the application to the direct analysis of inorganic arsenic species in freshwater samples.     

Adsorption of arsenite and arsenate onto muscovite and biotite mica

Arsenite and arsenate sorption was studied on two silt-sized phyllosilicates, namely muscovite and biotite, as a function of solution pH (pH 3-8 for muscovite, and 3-11 for biotite) at an initial As concentration of 13 microM. The amount of arsenic adsorbed increases with increasing pH, exhibiting a maximum value, before decreasing at higher pH values. Maxima correspond to 3.22+/-0.06 mmol kg-1 As(V) at pH 4.6-5.6 and 2.86+/-0.05 mmol kg-1 As(III) at pH 4.1-6.2 for biotite, and 3.08+/-0.06 mmolkg-1 As(III) and 3.13+/-0.05 mmol kg-1 As(V) at pH 4.2-5.5 for muscovite. The constant capacitance surface complexation model was used to explain the adsorption behavior. Biotite provides greater reactivity than muscovite toward arsenic adsorption. Isotherm data obeyed the Freundlich or Langmuir equation for the arsenic concentration range 10(-7)-10(-4) M. Released total Fe, Si, K, Al, and Mg in solution were analyzed. Calculation of saturation indices by PHREEQC indicated that the solution was undersaturated with respect to aluminum arsenate (AlAsO42H2O), scorodite (FeAsO42H2O), and claudetite/arsenolite (As4O6).     

Electrodeposition and stripping voltammetry of arsenic(III) and arsenic(V) on a carbon black–polyethylene composite electrode in the presence of iron ions

In this work, a new sensor is proposed for the stripping voltammetric determination (anodic stripping voltammetry—ASV) of total arsenic(V) or arsenic(III). The sensor is based on an Fe-modified carbon composite electrode containing 30 % carbon black–high-pressure polyethylene (CB/PE). The modification with iron is achieved by the addition of Fe(III) or Fe(II) ions to the sample solution and co-electrodeposition of iron and arsenic on the CB/PE electrode. In anodic stripping voltammetry, two peaks are observed: an Fe peak at ?0.45 or ?0.29 V and a peak at 0.12?±?0.07 V which depends on the arsenic concentration and corresponds to the As(0) → As(III) oxidation, as is the case with other solid electrodes. The optimum conditions proposed for ASV determination of As(V) and As(III) in solutions in the presence of dissolved oxygen are the following: the background electrolyte is 0.005 M HCl containing 0.5–1 mg/?L Fe(III) for As(V) and containing 1.0–1.5 mg/?L Fe(III) for As(III), respectively; E _dep?=??2.3 V; rest period at ?0.10 V for 3–5 s before the potential sweep from ?0.2 to +0.4 V; scan rate is 120 mV/?s. The detection limit (LOD, t?=?120 s) for As(III) and As(V) is 0.16 and 0.8 μg/?L, respectively. Various hypotheses on the effect of Fe ions and atoms on the electrodeposition and dissolution of arsenic are considered. The new method of determination of As(III) and As(V) differs from known analogues by its simplicity, low cost, and easy accessibility of the electrode material. It allows the voltammetric determination of total arsenic after chemical reduction of all its forms to As(III) or after their oxidation to As(V).     

Differential determination of arsenic (III) and total arsenic with L-cysteine as prereductant using a flow injection non-dispersive atomic absorption device

Speciation of arsenic in environmental samples gains increasingly importance, as the toxic effects of arsenic are related to its oxidation state. A method was developed for the determination of trace amounts of arsenic (III) and total arsenic by flow injection hydride generation coupled with an in-house made non-dispersive AAS device. The total arsenic is determined after prereduction of arsenic (V) to arsenic (III) with L-cysteine in a low concentration of hydrochloric, acetic or nitric acid. The conditions for the prereduction, hydride generation and atomization were systematically investigated. A quartz tube temperature of 800 degrees C was found to be optimum in view of peak shape and baseline stability. Pb(II), Ni(II), Fe(III), Cu(II), Ag(I), Al(III), Ga(II), Se(IV), Bi(III) were checked for interfering with the 2 microg/L As(V) signal. A serious signal depression was only observed for Se(IV) and Bi(III) at a 150-fold excess. With the above system, arsenic was determined at a sampling frequency of about 1/min with a detection limit (3sigma) of 0.01 microg/L using a 0.5 mL sample. The reagent blank was 0.001+/-0.0003 absorbance units and the standard deviation of 10 measurements of the 2 microg/l As signal was found to be 1.2%. Results obtained for standard reference materials and water samples are in good agreement with the certified values and those obtained by ICP-MS     

Speciation of Arsenic by Potentiometric Stripping Analysis Using Gold(III) Solution as Chemical Reoxidant and a Wall‐Jet Flow Cell

A simple procedure is described for the potentiometric stripping of arsenic with a wall‐jet cell by means of potentiostatic co‐deposition of gold and arsenic at a glassy‐carbon electrode and subsequent chemical stripping with Au(III). Optimum medium containing 160 mg L^?1 of Au(III) in HCl 0.1 M, where it is possible to speciate As(III) and As(V). As(V) was electrodeposited directly without prior chemical reduction at working electrode. As(III) was first determined at an electrodeposition potential of ?0.1 V. Afterwards, total arsenic was determined by an electrodeposition potential of ?0.7 V, from the area of peak obtained of the differential stripping potentiogram by using the standard addition method. The original As(V) concentration in the sample was calculated by difference. The possibilities of the optimized method were demonstrated by determinations of As(III), As(V) and total arsenic in samples of polluted water.     

液相色谱-双通道原子荧光检测联用法同时测定砷和硒的形态

建立了一种利用高效液相色谱-双通道原子荧光检测联用同时进行砷和硒形态分析的方法。以10 mmol/L NH4H2PO4溶液(pH 5.6)(添加2.5%(体积分数)的甲醇)为流动相,在12 min内同时分离了三价砷(As(III))、一甲基砷(MMA)、二甲基砷(DMA)、五价砷(As(V))、硒代胱氨酸(SeCys)、硒代蛋氨酸(SeMet)和四价硒［Se(IV)］等化合物。As(III)、DMA、MMA、As(V)、SeCys、SeMet和Se(IV)的检出限分别为1,3,2,3,4,18和3 μg/L (进样量为200 μL),5次测定的相对标准偏差为1.9%～6.1%(As 100 μg/L, Se 300 μg/L)。应用该方法对人体尿样及硒酵母片中砷和硒的形态进行了分析,目标物在尿样中的加标回收率为83%～108%,在硒酵母片中的加标回收率为88%～105%。实验结果表明,该方法可用于尿样及药品中砷和硒形态的日常分析。该方法减少了样品的分析时间和试剂用量,降低了工作强度,提高了工作效率。     

Selective determination of arsenic(III) and arsenic(V) with ammonium pyrrolidinedithiocarbamate, sodium diethyldithiocarbamate and dithizone by means of flameless atomic-absorption spectrophotometry with a carbon-tube atomizer

The extraction behaviour of arsenic(III) and arsenic(V) with ammonium pyrrolidinedithiocarbamate, sodium diethyldithiocarbamate and dithizone in organic solvents has been investigated by means of nameless atomic-absorption spectrophotometry with a carbon-tube atomizer. The selective extraction of arsenic(III) and differential determination of arsenic(III) and arsenic(V) have been developed. With ammonium pyrrolidinedithiocarbamate and methyl isobutyl ketone or nitrobenzene, when the aqueous phase/solvent volume ratio is 5 and the injection volume in the carbon tube is 20 μl, the sensitivities for 1% absorption are 0.4 and 0.5 part per milliard of arsenic, respectively. The relative standard deviations are ca. 3%. Interference by many metal ions can be prevented by masking with EDTA. The proposed methods are applied satisfactorily for determination of As(III) and As(V) in various types of water.     

Determination of inorganic arsenic(III) in ground water using hydride generation coupled to ICP-AES (HG-ICP-AES) under variable sodium boron hydride (NaBH4) concentrations

The determination of inorganic arsenic species in ground water matrices using hydride generation coupled online to ICP-AES (HG-ICP-AES) is suggested on the fact that the As(III)-species shows significantly higher signal intensities at low sodium boron hydride (NaBH₄) concentrations than the As(V)-species. The sodium boron hydride concentration used for the determination of As(III) without any considerable interferences of As(V) was at 13.2 mmol/L NaBH₄ (0.05 wt/v%), whereas the concentration for the total As determination was at 158.4 mmol/L NaBH₄ (0.6 wt/v%). The interferences of As(V) during the As(III) measurements were very small: at concentrations below 100 μg/L of total arsenic, the interferences of As(V) were smaller than 2%. An amount of As(III) higher than 10% of the total As amount could be determined exactly and reliably. The total amount of arsenic is measured after reducing the sample with 20 mmol/L L-cysteine (C₃H₇NO₂S). Finally, the amount of the As(V)-species is calculated by the difference between the As(III)-species and the total arsenic. Therefore, this analytical method requires the absence of organic arsenic species, but if they still appear, they could be frozen out with liquid nitrogen after the hydride generation system. The linearity of calibration reaches from 2 μg/L up to 1000 μg/L with a detection limit routinely of about 1 μg/L for each species. The advantages of this method in comparison to AAS measurements are the higher extent of the linear calibration range (3 orders of magnitude) and a higher sensitivity. Additional merits of the method developed are easy handling and high sampling rates.     

New considerations about the separation and quantification of antimony species by ion chromatography-hydride generation atomic fluorescence spectrometry

A new method for the speciation of inorganic [Sb(III) and Sb(V)] and organic (Me3SbCl2) antimony species by using a polystyrene-divinylbenzene-based anion-exchange HPLC column (Hamilton PRP-X100) coupled to hydride generation atomic fluorescence spectrometry (HG-AFS) is presented. Several mobile phases were tested for the baseline separation of these three antimony species, investigating in detail experimental parameters such as concentration and pH. The best efficiency and resolution was achieved by using a gradient elution between diammonium tartrate 250 mmol l(-1) pH 5.5 (A) and KOH 20 mmol l(-1) pH 12 (B). The gradient programme used was 100% B for 1.5 min, decreasing to 0% B in 0.1 min and maintained the elution with 100% A for 5.5 min. Analysis time was less than 7 min. Equilibration of the column with the complexing mobile phase was found to be critical in order to avoid Sb(III) double peak formation. Dilution in diammonium tartrate medium was necessary in order to avoid Sb(III) oxidation at microg l(-1) concentration level. Detection limits of 0.06 microg l(-1) for Sb(V), 0.09 microg l(-1) for Me3SbCl2 and 0.04 microg l(-1) for Sb(III) as well as repeatability and reproducibility better than 5% R.S.D. (n = 10) and 9% R.S.D. (n = 30) (for 1 and 5 microg l(-1) of Sb(V) and Sb(III) and 5 and 10 microg l(-1) of Me3SbCl2) were obtained. Accuracy and recovery studies were carried out by analysing one river freshwater sample and two water certified reference materials. The proposed methodology can be considered reliable and straightforward for antimony speciation in fresh water samples.     

The speciation of arsenic(V) and arsenic(III), by ion-exclusion chromatography, in solutions containing iron and sulphuric acid

Arsenic(V) and arsenic(III) can be separated, by ion-exclusion chromatography, in solutions containing iron and sulphuric acid. Iron is removed by ion-exchange before the speciation of arsenic, with phosphoric acid as the eluent. The separated arsenic(V) and arsenic(III) are measured spectrophotometrically in the ultraviolet region at a wavelength of 195 mn. Arsenic(V) and arsenic(III) can be determined at concentrations > or = 3 mg/1. The relative standard deviations are 1.3% for arsenic(V) and 0.9% for arsenic(III), at the 10 mg/1. level. The time required for the separation of the inorganic arsenic species is 11 min.     

Electrochemical measurement and speciation of inorganic arsenic in groundwater of Bangladesh

The presence of arsenic in groundwater above the maximum permissible limit of 50 mug l(-1) has threatened the health of more than 50 million people in Bangladesh and neighboring India. We report here the development of an inexpensive anodic stripping voltammetric (ASV) technique for routine measurement and speciation of arsenic in groundwater. The measurements are validated by more expensive atomic absorption, atomic emission and other techniques. To understand the present situation in Bangladesh, we measured As(III) in 960 water samples collected from 18 districts. A random distribution of 238 samples was used to measure both As(III) and As(V). The results from the present study indicate that most toxic form of inorganic arsenic, As(III), has the broad range of 30-98%. It shows 60% of the samples have 10 mug l(-1) and 44% of the samples have 50 mug l(-1) or more As(III). The fractional distribution pattern shows significant skew towards high percent occurrence which may indicate a progressive reduction process with a single source or a single mechanism for the formation of As(III). For direct consumption, this is possibly one of the most toxic groundwater known today. Speciation distribution at groundwater pH value shows H(3)AsO(3) is the predominant species including H(2)AsO(4)(-) and H(2)AsO(4)(2-) whose distribution is significantly pH dependent. This is also supported by E(h)-pH measurements. The depth distribution for Kushtia shows most of the As(III) is located within 100-200 ft deep aquifers. Similar fractional distribution of As(III) is found in deeper aquifers and may indicate contamination by leakage from upper aquifer. This study clearly demonstrates the aquifer environment is reductive and conducive to the formation of As(III) species.     

Differential determination of arsenic(III) and total arsenic using flow injection on-line separation and preconcentration for graphite furnace atomic absorption spectrometry

Arsenic(III) can be quantitatively extracted using sodium diethyldithiocarbamate (NaDDTC) as the complexing agent and C18 reversed phase packing as the column material for solid phase extraction. Arsenic(V) must be reduced to its trivalent oxidation state prior to extraction. A mixture of sodium sulphite, hydrochloric acid, sodium thiosulphate and potassium iodide was found to be optimum for on-line reduction. When the sorbent extraction is carried out without and with the addition of the reduction mixture, arsenic(III) and total arsenic can be determined sequentially by graphite furnace atomic absorption spectrometry with detection limits (3 σ) of 0.32 ng for As(III) and 0.43 ng for total arsenic. A 7.6-fold enhancement in peak area compared to direct injection of 40 μl samples was obtained after 60 s preconcentration. Results obtained for sea water standard reference materials, using aqueous standards for calibration, agree well with certified values. A precision of 5.5% RSD was obtained for total arsenic in a sea water sample (1.65 As). Results obtained for synthetic mixtures of trivalent and pentavalent arsenic agreed well with expected values.     

Flow-injection hydride generation atomic absorption spectrometric study of the automated on-line pre-reduction of arsenate, methylarsonate and dimethylarsinate and high-performance liquid chromatographic separation of their l-cysteine complexes

An automated on-line pre-reduction of arsenate, monomethylarsonate (MMA) and dimethylarsinate (DMA) using flow injection hydride generation atomic absorption spectrometry (FI-HGAAS) is feasible. The kinetics of pre-reduction and complexation depend strongly on the concentration of l-cysteine and on the temperature in the following increasing order: inorganic As(V)相似文献   

Detection of As(III) via oxidation to As(V) using platinum nanoparticle modified glassy carbon electrodes: arsenic detection without interference from copper

The electrochemical detection of As(III) was investigated on a platinum nanoparticle modified glassy carbon electrode in 1 M aqueous HClO4. Platinum nanoparticle modified glassy carbon electrodes were prepared by potential cycling in 0.1 M aqueous KCl containing 1 mM K2PtCl6. In each potential cycle, the potential was held at + 0.5 V for 0.01 s and at -0.7 V for 10 s. 25 cycles were optimally used to prepare the electrodes. The resulting electrode surfaces were characterized with AFM. The response to arsenic(III) on the modified electrode was examined using cyclic voltammetry and linear sweep voltammetry. By using the As(III) oxidation peak for the analytical determination, there is no interference from Cu(II) if present in contrast to the other metal surfaces (especially gold) typically used for the detection of arsenic; Cu(II) precludes the use of the As(0) to As(III) peak for quantitative anodic stripping voltammetry measurements due to the formation of Cu3As2 and an overlapping interference peak from the stripping of Cu(0). After optimization, a LOD of 2.1 +/- 0.05 ppb was obtained using the direct oxidation of As(III) to As(V), while the World Health Organization's guideline value of arsenic for drinking water is 10 ppb, suggesting the method may have practical utility.     

Simultaneous determination of arsenic species and chromium(VI) by high-performance liquid chromatography-inductively coupled plasma-mass spectrometry

The simultaneous determination of As(III), As(V), monomethylarsenic acid (MMA), dimethylarsinic acid (DMA) and Cr(VI) in fresh water has been carried out by coupling an anion-exchange column to an inductively coupled plasma-mass spectrometer. Optimisation of chromatographic conditions led to baseline separation of signals from the five species in approximately 9 min using gradient elution. Detection limits were 0.02-0.05 microg As l(-1) and 5.5 microg Cr l(-1). Repeatability was 2-3% for arsenic species and higher, i.e., 8%, for Cr(VI) due to the higher background for this species. Arsenic species and hexavalent chromium stability in surface water samples was evaluated, and storage conditions were set to 1 day at 4 degrees C in polyethylene flasks (without acidification) in order to avoid As(III)-As(V) conversions. The method was applied to the analysis of surface water.     

Inorganic arsenic speciation in various water samples with GFAAS using coprecipitation

A simple, economic and sensitive method for selective determination of As(III) and As(V) in water samples is described. The method is based on selective coprecipitation of As(III) with Ce(IV) hydroxide in presence of an ammonia/ammonium buffer at pH 9. The coprecipitant was collected on a 0.45 µm membrane filter, dissolved with 0.5 mL of conc. nitric acid and the solution was completed to 2 or 5 mL with distilled water. As(III) in the final solutions was determined by graphite furnace atomic absorption spectrometry (GFAAS). Under the working condition, As(V) was not coprecipitated. Total inorganic arsenic was determined after the reduction of As(V) to As(III) with NaI. The concentration of As(V) was calculated by the difference of the concentrations obtained by the above determinations. Both the determination of arsenic with GF-AAS in presence of cerium and the coprecipitation of arsenic with Ce(IV) hydroxide were optimised. The suitability of the method for determining inorganic arsenic species was checked by analysis of water samples spiked with 4–20 µg L^?1 each of As(III) and As(V). The preconcentration factor was found to be 75 with quantitative recovery (≥95%). The accuracy of the present method was controlled with a reference method based on TXRF. The relative error was under 5%. The relative standard deviations for the replicate analysis ( n?=?5) ranged from 4.3 to 8.0% for both As(III) and As(V) in the water samples. The limit of detection (3σ) for both As (III) and As(V) were 0.05 µg L^?1. The proposed method produced satisfactory results for the analysis of inorganic arsenic species in drinking water, wastewater and hot spring water samples.     

Optimization of a flow injection system with amperometric detection for arsenic determination.

A selective and sensitive analytical procedure for rapid arsenic determination by gas-diffusion flow injection analysis with amperometric detection was developed. The method is based on the arsenite reduction by NaBH(4). Derived arsine diffuses through a PTF membrane into the acceptor flow stream and is amperometrically determined on a platinum working electrode. The limit of detection (3 sigma) at room temperature was 5 microg/dm(3) of As(III). The relative standard deviation for a 1 mg/dm(3) As(III) standard was 1.96% for six repetitive injections. Arsenic(V) was determined after its prereduction with potassium iodide. Arsenic determination was not interferred with by 1 mg/dm(3) Sb(III), 5 mg/dm(3) Sn(II), 10 mg/dm(3) Se(IV), 1 mg/dm(3) As(V), 1 mg/dm(3) hydrasine, 1 mg/dm(3) Fe(II) or 0.5 mg/dm(3) Fe(III) solution. The throughput of this method was 60 analyses per hour. This method was successfully applied to arsenic determination in some power plant waste water samples.     

Kinetic-spectrophotometric determination of trace amounts of As(III) based on its inhibitory effect on the redox reaction between bromate and hydrochloric acid

A simple, sensitive, rapid and reliable method has been developed for spectrophotometric determinations of As(III) in the presence of As(V) based on its inhibition effect on the redox reaction between bromate and hydrochloric acid. The decolorization of methyl orange by the reaction products was used to monitor the reaction spectrophotometrically at 525 nm. The method allows the determination of arsenic in the range of 6-1000 mug l(-1). The relative standard deviation for 10 determinations of 40 mug l(-1) of As(III) was 1.43% and the limit of detection, corresponding to a signal to noise ratio of three, was 3.4 mug l(-1). The proposed method was applied to the determination of As(III) in water samples with satisfactory results.