Hydride Generation Atomic Fluorescence Detection Research Articles

On-line pre-reduction of Se(VI) by thiourea for selenium speciation by hydride generation

In this study, thiourea (TU) was novelly developed as a reduction reagent for on-line pre-reduction of selenium(VI) before conventional hydride generation (HG) by KBH 4/NaOH–HCl. After TU on-line pre-reduction, the HG efficiency of Se(VI) has been greatly improved and because even higher than that of the same amount of Se(IV) obtained in the conventional HG system. The possible pre-reduction mechanism is discussed. The detection limit (DL) of selenate reaches 10 pg mL − 1 when using on-line TU pre-reduction followed by HG atomic fluorescence detection. When TU pre-reduction followed by HG is used as an interface between ion-pair high performance liquid chromatography and atomic fluorescence spectrometry, selenocystine, selenomethionine, selenite and selenate can be measured simultaneously and quantitatively. The DLs of these are 0.06, 0.08, 0.05 and 0.04 ng mL − 1 , respectively, and the relative standard deviations of 9 duplicate runs for all the 4 species are less than 5%. Furthermore, it was successfully applied to Se speciation analysis of cultured garlic samples, and validated by determination of total selenium and selenium species in certified reference material NIST 1946.

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 the immediately mobilisable As(III), As(V), MMA and DMA in river sediments by high performance liquid chromatography–hydride generation–atomic fluorescence spectrometry following ultrasonic extraction

In this work, a fast method is developed for the speciation of As(III), As(V), MMA and DMA in the immediately mobilisable fraction of river sediments (i.e. water-soluble and phosphate-exchangeable) by high performance liquid chromatography–hydride generation–atomic fluorescence detection (HPLC–HG–AFD) after extraction using focused ultrasound. The influence of relevant parameters influencing an ion-pairing chromatographic separation following isocratic elution (i.e. amount of MeOH in the mobile phase, ion pair reagent concentration, pH, flow rate) was studied. Focused ultrasound transmitted from an ultrasonic probe provided the same extractable contents as conventional extraction with no changes in the species distribution. The effect of the drying step over extraction of As species was investigated. The following drying procedures were compared: freeze-, oven-, microwave- and air-drying. No influence of the drying operation on the water-extractable fraction was observed. However, freeze- and air-drying yielded significantly higher phosphate-extractable amounts of As(III) and As(V) as compared to oven and microwaves. Detection limits for the As species were in the range 1.3–4.1 ng/g for the water-soluble fraction and 1.6–4.8 ng/g for the phosphate buffer exchangeable fraction. The method was applied to the speciation of immediately mobilisable As(III), As(V), DMA and MMA in 11 sediment samples collected along the beds of the Louro River (southern Galicia, Spain).

Antimony determination and speciation by multisyringe flow injection analysis with hydride generation-atomic fluorescence detection

A new analytical procedure for determination of inorganic antimony and speciation of antimony(III) and antimony(V) is presented. For this purpose, a software-controlled time-based multisyringe flow injection system, which contains a multisyringe burette provided with a multi-port selection valve, was developed. Hydride generation-atomic fluorescence spectrometry was used as a detection technique. A 0.3% (w/v) reducing sodium tetrahydroborate solution, hydrochloric acid (2 M), an antimony solution and a pre-reducing solution of 10% (w/v) KI and 0.3% (w/v) ascorbic acid are dispensed simultaneously into a gas–liquid separation cell with further propulsion of the reaction product into the flame of an atomic fluorescence spectrometer using argon flow. A hydrogen flow was employed to support the flame. The linear range and the detection limit (3s b/S) of the proposed technique were 0.2–5.6 μg l −1 and 0.08 μg l −1, respectively. A sample throughput of 18 samples per hour (corresponding to 80 injections per hour) was achieved. The relative standard deviation for 18 independent measurements was 4.6%. This technique was validated by means of reference solid and water materials with good agreement with the certified values. Satisfactory results for speciation of Sb(III) and Sb(V) by means of the developed technique were obtained.

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 KH 2PO 4 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 (3 σ) 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 arsenic metabolic complex excreted in human urine after administration of sodium 2,3-dimercapto-1-propane sulfonate.

Sodium 2,3-dimercapto-1-propane sulfonate (DMPS) has been used to treat acute arsenic poisoning. Presumably DMPS functions by chelating some arsenic species to increase the excretion of arsenic from the body. However, the excreted complex of DMPS with arsenic has not been detected. Here we describe a DMPS complex with monomethylarsonous acid (MMA(III)), a key trivalent arsenic in the arsenic methylation process, and show the presence of the DMPS-MMA(III) complex in human urine after the administration of DMPS. The DMPS-MMA(III) complex was characterized using electrospray tandem mass spectrometry and determined by using HPLC separation with hydride generation atomic fluorescence detection (HGAFD). The DMPS-MMA(III) complex did not form a volatile hydride with borohydride treatment. On-line digestion with 0.1 M sodium hydroxide following HPLC separation decomposed the DMPS-MMA(III) complex and allowed for the subsequent quantification by hydride generation atomic fluorescence. Arsenite (As(III)), arsenate (As(V)), monomethylarsonic acid (MMA(V)), dimethylarsinic acid (DMA(V)), MMA(III), and DMPS-MMA(III) complex were analyzed in urine samples from human subjects collected after the ingestion of 300 mg of DMPS. The administration of DMPS resulted in a decrease of the DMA(V) concentration and an increase of the MMA(V) concentration excreted in the urine, confirming the previous results. The finding of the DMPS-MMA(III) complex in human urine after DMPS treatment provides an explanation for the inhibition of arsenic methylation by DMPS. Because MMA(III) is the substrate for the biomethylation of arsenic from MMA(V) to DMA(V), the formation of DMPS-MMA(III) complex would reduce the availability of MMA(III) for the subsequent biomethylation.

Unstable trivalent arsenic metabolites, monomethylarsonous acid and dimethylarsinous acid

Two key arsenic metabolites, monomethylarsonous acid (MMAIII) and dimethylarsinous acid (DMAIII), have recently been detected in human urine. There is an increasing interest in the speciation of these metabolites in humans because of their demonstrated effects on cellular toxicity and DNA damage. However, there is no information on the oxidative stability of these arsenic species. It is not known whether and to what extent these trivalent metabolites are changed during sample handling and storage. The objective of this study was to demonstrate the oxidative conversion of these arsenic species during sample storage. We compared the effects of the storage temperature (25, 4, and −20 °C) and storage duration (up to 5 months) on the stability of MMAIII and DMAIII in de-ionized water and in human urine. We used HPLC with hydride generation atomic fluorescence detection for the speciation of arsenic. This method provided sub-µg L−1 to low-µg L−1 detection limits for each arsenic species. We found that the oxidation of MMAIII and DMAIII was matrix and temperature dependent. Low temperature conditions (4 and −20 °C ) improved the stability of these arsenic species over the room temperature storage condition. MMAIII in de-ionized water was relatively stable for almost 4 months, when stored at 4 or −20 °C with less than 10% of MMAIII oxidized to MMAV. In contrast, most of MMAIII (>90%) in urine was oxidized to MMAV over the 5 month period under the 4 or −20 °C storage condition. At 25 °C, MMAIII in urine was completely oxidized to MMAV within a week. DMAIII in de-ionized water was stable for only 2–3 days, being rapidly oxidized to DMAV. DMAIII in urine was completely oxidized to DMAV within a day at 4 or −20 °C. The conversion of DMAIII to DMAV in urine at 25 °C was complete in 17 h. These results show that MMAIII and DMAIII are much less stable than other arsenic species, and their stability depends on sample matrix and temperature.

Determination of Inorganic Sb(V) and Methylantimony Species by HPLC with Hydride Generation-Atomic Fluorescence Spectrometric Detection

An on-line method for the separation and analysis of Sb(V) and Me3Sb in the presence of Sb(III) in liquid samples is described. Inorganic and organic antimony species were separated using anion-exchange high-performance liquid chromatography (HPLC) coupled with hydride generation-atomic fluorescence detection (HG-AFS). Optimum conditions for the separation of antimony species by HPLC and the hydride generation conditions for the determination by HG-AFS were established. Matrix interference of the chromatographic determination was studied in relation to MgSO4 and NaCl. The method developed was applied to the separation and determination of antimony species in spiked and natural water samples. The suitability of the method for analysis in microbial growth media and physiological studies involving methylantimony species is discussed.

Determination of arsenic species in oyster tissue by microwave‐assisted extraction and liquid chromatography–atomic fluorescence detection

AbstractA method for the determination of arsenic species in oyster tissue is established. The extraction of arsenic species is carried out by using low‐power microwaves. Quantitative extraction is obtained at a power of 40 W, and in 5 min, using the extracting agent methanol/water (1 + 1). The measurements are carried out using liquid chromatography–UV irradiation–hydride generation–atomic fluorescence detection (LC–UV–HG–AFS). Three arsenic species were detected in oyster tissue: arsenobetaine (AsBet) (87%), a probable arsenosugar (AsS) (4.9%), and dimethylarsinate (DMA) (4.7%). No influence of the clean‐up, the microwave field or the IR drying system on the stability of the arsenic compounds was observed. The extracts can be kept stable up to 3 days at 4 °C. The performance of the method is proved on fresh samples, as they are usually analysed in routine laboratories. Copyright © 2001 John Wiley &amp; Sons, Ltd.

Determination of Arsenic in Seafood by Focused Microwave Digestion and Hydride Generation–Atomic Fluorescence Detection

A new method was developed for total arsenic determination in seafood products such as oysters, mussels, tuna fish, and algae. Matrix decomposition and oxidation to arsenate of all the arsenic compounds in the product were completed in 25 min by using a 3-step program of focused microwaves (40-120 W) with nitric and sulfuric acids. Quantitation was performed by hydride generation-atomic fluorescence detection (HG-AFS). Results of method optimization are presented and discussed. A detection limit <125 microg/kg arsenic was obtained; the quantitation limit was close to 400 microg/kg, with repeatability and reproducibility <5% relative standard deviation. Validation was performed by analyzing 4Reference Materials (arsenic concentration expressed as mg/kg): The National Institute of Standards and Technology SRM 1566a Oyster tissue (14.0 +/- 1.2); the Bureau Community of Reference (now Standard Measurements & Testing Program) CRM 278 Mussel tissue (5.9 +/- 0.2) and CRM 627 Tuna fish (4.8 +/- 0.3); and the International Atomic Energy Agency RM 140 Fucus sample (44.3 +/- 2.1).

Improvements of hydride generation for the speciation of arsenic in natural freshwater samples by HPLC-HG-AFS

A new gas–liquid separator was used to improve the hydride generation step of a hyphenated HPLC-HG-AFS system applied to arsenic speciation studies. Optimisation of the hydride generation atomic fluorescence detection with this new device produced by SpectraFrance led to a significant improvement in performance. Absolute detection limits were as low as 5–7 pg As for the four arsenic species usually encountered in water samples: As(III), As(V), monomethylarsonic acid (MMAA) and dimethylarsinic acid (DMAA). Using a 100 µl injection volume the concentration detection limits were in the range 0.05–0.07 ng ml−1. The optimised procedure was applied to determine arsenic speciation in a wide range of natural fresh water samples.

Speciation of key arsenic metabolic intermediates in human urine.

Biomethylation is the major human metabolic pathway for inorganic arsenic, and the speciation of arsenic metabolites is essential to a better understanding of arsenic metabolism and health effects. Here we describe a technique for the speciation of arsenic in human urine and demonstrate its application to the discovery of key arsenic metabolic intermediates, monomethylarsonous acid (MMAIII) and dimethylarsinous acid (DMAIII), in human urine. The study provides a direct evidence in support of the proposed arsenic methylation pathway in the human. The finding of MMAIII and DMAIII in human urine, along with recent studies showing the high toxicity of these arsenicals, suggests that the usual belief of arsenic detoxification by methylation needs to be reconsidered. The arsenic speciation technique is based on ion pair chromatographic separation of arsenic species on a 3-micron particle size column at 50 degrees C followed by hydride generation atomic fluorescence detection. Speciation of MMAIII, DMAIII, arsenite (AsIII), arsenate (AsV), monomethylarsonic acid (MMAV), and dimethylarsinic acid (DMAV) in urine samples is complete in 6 min with detection limits of 0.5-2 micrograms/L. There is no need for any sample pretreatment. The capability of rapid analysis of trace levels of arsenic species, which resulted in the findings of the key metabolic intermediates, makes the technique useful for routine arsenic speciation analysis required for toxicological and epidemiological studies.

Speciation of cationic arsenic species in seafood by coupling liquid chromatography with hydride generation atomic fluorescence detection

A method was developed for determining arsenobetaine (AB), arsenocholine (AC), trimethylarsine oxide (TMAO) and tetramethylarsonium ion (TMA+) in seafood products. The arsenic species were extracted from the matrix by methanol–water and the extracts were quantified by high-performance liquid chromatography coupled with thermo-oxidation hydride generation atomic fluorescence spectrometry (HPLC–thermo-oxidation–HG-AFS). The variables affecting each stage of the methodology were optimized. The analytical features of the method (recovery, precision, limit of detection and linearity range) were calculated for each arsenical species. The lowest limit of detection was obtained for TMAO (0.0009 µg g−1, dry mass), whereas AC was the arsenic species with the highest LOD (0.0063 µg g−1, dry mass). The precision of the method varied between 0.7% for AB and 8.4% for TMA+. The recovery percentage was greater than 97% for all species. The proposed procedure was applied to reference materials: DORM-2 (Dogfish muscle, National Research Council of Canada), NFA-Shrimp and NFA-Plaice (National Food Agency of Denmark). The results were compared with the values obtained by other authors.

Sample Preparation and Storage Can Change Arsenic Speciation in Human Urine

Stability of chemical speciation during sample handling and storage is a prerequisite to obtaining reliable results of trace element speciation analysis. There is no comprehensive information on the stability of common arsenic species, such as inorganic arsenite [As(III)], arsenate [As(V)], monomethylarsonic acid, dimethylarsinic acid, and arsenobetaine, in human urine. We compared the effects of the following storage conditions on the stability of these arsenic species: temperature (25, 4, and -20 degrees C), storage time (1, 2, 4, and 8 months), and the use of additives (HCl, sodium azide, benzoic acid, benzyltrimethylammonium chloride, and cetylpyridinium chloride). HPLC with both inductively coupled plasma mass spectrometry and hydride generation atomic fluorescence detection techniques were used for the speciation of arsenic. We found that all five of the arsenic species were stable for up to 2 months when urine samples were stored at 4 and -20 degrees C without any additives. For longer period of storage (4 and 8 months), the stability of arsenic species was dependent on urine matrices. Whereas the arsenic speciation in some urine samples was stable for the entire 8 months at both 4 and -20 degrees C, other urine samples stored under identical conditions showed substantial changes in the concentration of As(III), As(V), monomethylarsonic acid, and dimethylarsinic acid. The use of additives did not improve the stability of arsenic speciation in urine. The addition of 0.1 mol/L HCl (final concentration) to urine samples produced relative changes in inorganic As(III) and As(V) concentrations. Low temperature (4 and -20 degrees C) conditions are suitable for the storage of urine samples for up to 2 months. Untreated samples maintain their concentration of arsenic species, and additives have no particular benefit. Strong acidification is not appropriate for speciation analysis.

Speciation of Arsenic Compounds Using High-Performance Liquid Chromatography at Elevated Temperature and Selective Hydride Generation Atomic Fluorescence Detection

Understanding arsenic toxicity and metabolism requires quantitation of individual arsenic species. However, it has been difficult to separate many biochemically and environmentally important arsenic species on a single chromatography column. We have studied the separation of 11 arsenic compounds by using ion pair chromatography at 30, 50, and 70 °C column temperatures. The use of elevated column temperature improved separation efficiency and dramatically reduced chromatographic retention time for arsenobetaine, arsenocholine, tetramethylarsonium, and arsenosugars. On-line microwave derivatization combined with hydride generation and atomic fluorescence spectrometry (HGAFS) was used for detection, which was able to differentiate more toxic from less toxic arsenic species. The speciation technique was successfully applied to a study of metabolism of arseno sugars present in commercial seaweed products. Two uncharacterized arsenic-containing metabolites were detected in urine samples collected 20−33 h after the consumption of the seaweed product. These metabolites did not form hydride without microwave digestion. Up to 90 ng/mL of dimethylarsinic acid was detected in the urine samples collected 25−35 h after the consumption of seaweed, compared to a background level of less than 15 ng/mL in urine samples collected before the ingestion of seaweed. The combined high-temperature ion pair chromatography with selective arsenic detection provided a rapid approach to monitoring metabolism of arsenic compounds.

Investigation into the kinetics of selenium(VI) reduction using hydride generation atomic fluorescence detection

Utilizing the rapid response times that are possible with atomic fluorescence spectrometers, the activation energy for the reduction of selenium(VI) to selenium(IV) has been calculated and found to be 90.4 kJ mol–1 using 6 mol l–1 HCl. Results from the studies indicate that full reduction of selenium(VI) to selenium(IV) may be obtained in as short a time as 6 min at 70 °C.