Xiao-Chun Le

SPECIATION OF ARSENIC COMPOUNDS BY HPLC WITH HYDRIDE GENERATION ATOMIC ABSORPTION SPECTROMETRY AND INDUCTIVELY COUPLED PLASMA MASS SPECTROMETRY DETECTION

An arsenic specific detection system utilizing on-line microwave digestion and hydride generation atomic absorption spectrometry (MD/HGAAS) is described for arsenic speciation by using high performance liquid chromatography (HPLC). Both ion exchange chromatography and ion pair chromatography have been studied for the separation of arsenite, arsenate, monomethylarsonic acid (MMAA), dimethylarsinic acid (DMAA), and arsenobetaine (AB). When the commonly used mobile phases, phosphate and carbonate buffers at pH 7.5, are used on an anion exchange column, arsenite and AB co-elute. However, selective determination of these two arsenic compounds can be achieved by using the new detection system. Partial separation between arsenite and AB can be achieved by increasing the mobile phase pH to 10.3 and by using a polymer based anion exchange column. The detection limit obtained by using anion exchange chromatography with MD/HGAAS detection is approximately 10 ng/ml (or 200 pg for a 20-mul sample injection) for arsenite, DMAA and AB, 15 ng/ml (or 300 pg) for MMAA, and 20 ng/ml (or 400 pg) for arsenate. Complete separation of the five arsenic compounds is achieved on a reversed phase C18 column by using sodium heptanesulfonate as ion pair reagent. Comparable resolution between chromatographic peaks is obtained by using MD/HGAAS detection and inductively coupled plasma mass spectrometry (ICPMS) detection.

Effect of cysteine on the speciation of arsenic by using hydride generation atomic absorption spectrometry

Abstract The use of hydride generation techniques for arsenic species is pH dependent. Identical responses can not be obtained from arsenite, arsenate, monomethylarsonic acid (MMAA) and dimethylarsinic acid (DMAA) when the same acid concentration is used. Thus some methods commonly used for the direct determination of total arsenic under compromised conditions are subject to error. This error is eliminated by the addition of 2% cysteine to samples prior to hydride generation. In the presence of cysteine the optimum condition for the determination of these arsenic species is in the same range, and a single arsenic species can be used for calibration. This finding is applied to the determination of arsenic in human urine by using flow injection, hydride generation, and atomic absorption spectrometry methodology. The effect of cysteine and thioglycerol is reported in detail, and it is proposed that arsenate. MMAA and DMAA all in the As(V) state, are reduced to the As(III) state as organo-sulfur-arsenic(III) compounds through the reaction between the arsenic species and the thiol. These products, organosulfur derivatives of arsenic(III), easily react with tetrahydroborate(III) under similar conditions to afford the arsines without interference from cysteine. Non-thiol-containing amino acids such as methionine, glycine and histidine do not react with arsenic species in this way, and therefore they do not affect the pH dependence of the generation of arsenic hydride species.

Speciation of arsenic compounds by using ion-pair chromatography with atomic spectrometry and mass spectrometry detection

Abstract Chromatography separation of arsenic compounds is important for chemical speciation studies; yet a complete separation of several biologically and environmentally important arsenic compounds has been difficult to achieve on a single column. We report here baseline resolution for arsenate, arsenite, monomethylarsonic acid, dimethylarsinic acid, arsenobetaine, arsenocholine, and tetramethylarsonium ion, by using mixed ion-pair reagents containing 10 mM hexanesulfonate and 1 mM tetraethylammonium hydroxide, as mobile phase. The complete separation of these anionic, cationic and neutral arsenic species on a reversed-phase C18 column took only 12 min. Inductively coupled plasma mass spectrometry, hydride generation atomic absorption spectrometry and atomic fluorescence spectrometry systems were used for element-specific detection. The speciation technique was successfully applied to studies of urinary excretion of arsenic compounds following one-time ingestion of shrimp. Arsenobetaine ingested from the consumption of shrimp was excreted into urine in its original form; and approximately 70% of the total arsenobetaine ingested was excreted into urine within 37 h after ingestion.

A new continuous hydride generator for the determination of arsenic, Antimony and tin by hybride generation atomic absorption spectrometry

Abstract A new continuous hybride generator is described. Its characteristics are compared with a conventional hybride generator for the determination of arsenic, antimony and tin by using flame atomic absorption spectrometry. This simple hybride generator combines hybride production and gas-liquid separation in one unit, obviating the use of a separate mixing/reaction coil and gas-liquid separator. Radioactive tracer studies show that a hybride generation efficiency of over 95% is achieved with the new hybride generator and that approximately 90% of monomethylarsonic acid (MMA) is converted to its hybride (methylarsine) within the first 10 s of the hybride generation reaction. Compared to a conventional hybride generator, an improved signal-to-noise ratio is achieved and comparable or less interference from transition metal ions is observed. Cysteine (1% in the sample solution) is effective in reducing interferences during the hybride generation processes. In the presence of cysteine, 10–1000 fold higher amounts of Ni2+, Co2+ and Cu2+ are tolerated in the determination of arsenic, antimony and tin. Cysteine also enhances the signals of arsenic, antimony and tin by 60, 200 and 40%, respectively. Reaction conditions were optimized by using both the simplex optimization method and one-factor-at-a-time method and results are in good agreement.

Determination of urinary arsenic and impact of dietary arsenic intake.

An analytical method based on microwave decomposition and flow injection analysis (FIA) coupled to hydride generation atomic absorption spectrometry (HGAAS) is described. This is used to differentiate arsenite [As(III)], arsenate [As(V)], monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA) from organoarsenic compounds usually present in seafood. Without microwave digestion, direct analysis of urine by HGAAS gives the total concentration of As(III), As(V), MMA and DMA because organoarsenic compounds such as arsenobetaine, usually found in most seafood, are not reducible upon treatment with borohydride and therefore cannot be determined by using the hydride generation technique. The microwave oven digestion procedure with potassium persulfate and sodium hydroxide as decomposition reagents completely decomposes all arsenicals to arsenate and this can be measured by HGASS. Microwave decomposition parameters were studied to achieve efficient decomposition and quantitative recovery of arsenobetaine spiked into urine samples. The method is applied to the determination of urinary arsenic and is useful for the assessment of occupational exposure to arsenic without intereference from excess organoarsenicals due to the consumption of seafood. Analysis of urine samples collected from an individual who ingested some seafood revealed that organoarsenicals were rapidly excreted in urine. After the ingestion of a 500-g crab, a 10-fold increase of total urinary arsenic was observed, due to the excretion of organoarsenicals. The maximum arsenic concentration was found in the urine samples collected approximately between 4 to 17 hr after eating seafood. However, the ingestion of organoarsenic-containing seafoods such as crab, shrimp and salmon showed no effect on the urinary excretion of inorganic arsenic, MMA and DMA.

Analysis by capillary electrophoresis-laser-induced fluorescence detection of oligosaccharides produced from enzyme reactions.

Six structurally similar, fluorescently labeled oligosaccharides were baseline resolved by capillary electrophoresis (CE); laser induced fluorescence (LIF) detection gave detection limits of 50 molecules for the oligosaccharides. A simple design of the LIF detector that incorporates the advantages of high sensitivity, stability and ease of operation is described. The system was used to monitor enzyme products formed during the incubation of yeast cells with alpha-D-Glc(1-->2)alpha-D-Glc(1-->3)alpha-D-glc-O(CH2)8CONHCH2CH2NHCO - tetramethylrhodamine. This fluorescent trisaccharide is enzymatically hydrolyzed to fluorescent disaccharide, monosaccharide and the free linker arm that is used to conjugate the saccharides with the fluorophore tetramethylrhodamine.

Low pressure chromatographic separation of inorganic arsenic species using solid phase extraction cartridges

Routine water analysis of arsenic species requires simple, inexpensive, rapid and sensitive methods. To this end, we have developed two methods, which are based on the use of inexpensive solid phase extraction (SPE) cartridges as low pressure chromatographic columns for separation and hydride generation atomic absorption spectrometry (HGAAS) and hydride generation atomic fluorescence spectrometry (HGAFS) for detection of arsenic. Both anion exchange and reverse phase cartridges were successfully used to separate arsenite [As(III)] and arsenate [As(V)]. The composition, concentration, and pH of eluting buffers and the effect of flow rate were systematically investigated. Speciation of inorganic As(III) and As(V) were achieved within 1.5 min, with detection limits of 0.2 and 0.4 ng/ml, respectively. Both isocratic and step gradient elution techniques were suitable for the baseline resolution of As(III) and As(V) using anion exchange cartridges. Application of the methods to the speciation of As(III) and As(V) in untreated water, tap water, and bottled water samples were demonstrated. Results from the speciation of arsenic in a standard reference material water sample using these methods were in good agreement with the certified value and with inter-laboratory comparison results obtained using HPLC separation and inductively coupled plasma mass spectrometric detection (HPLC-ICPMS).

Migration time correction for the analysis of derivatized amino acids and oligosaccharides by micellar capillary electrochromatography

Migration-time reproducibility is essential in the use of capillary electrophoresis to identify components in mixtures. Two methods based on the migration time of either one or two reference markers are proposed for improving migration time reproducibility. These methods were evaluated to determine the migration time reproducibility for phenylthiohydantoin-amino acids, fluorescein thiohydantoin-amino acids, and tetramethylrhodamine labeled oligosaccharides. In the best case, the relative standard deviation of the migration time was reduced from >3% without correction to <0.04% with the two-marker correction.

Convenient method for the determination of trace amounts of germanium by hydride generation direct current plasma atomic emission spectrometry: interference reduction by L-cystine and L-cysteine

A convenient and rapid method for the determination of trace amounts of germanium has been developed. This method has the advantage that it is less sensitive to instrumental parameters than previously described methods. Interferences from transition elements are reduced and interferences from other hydride-forming elements are negligible. The detection limit for the determination of germanium is 20 pg ml–1(3σ). Germanium concentrations in metallurgical samples are easily determined and this method has been applied to the determination of germanium in copper and iron.

Study of the enzymatic transformation of fluorescently labeled oligosaccharides in human epidermoid cells using capillary electrophoresis with laser-induced fluorescence detection

Isomeric oligosaccharides of both beta Gal(1-->3)beta GlcNAc (type I) series and beta Gal(1-->4)beta GlcNAc (type II) series were studied by using capillary electrophoresis (CE) with laser-induced fluorescence (LIF) detection. A mixture of phenylboronic acid and sodium tetraborate was used in the CE running buffers to improve the electrophoretic separation of the oligosaccharides. Both series of the tetramethylrhodamine (TMR)-labeled substrates [beta Gal(1-->3)beta GlcNAc-O-TMR and beta Gal(1-->4)beta GlcNAc-O-TMR) and their potential enzymatic products were baseline resolved using CE. The high resolution provided by CE and the excellent detection limit (8.10(-23) mol, or 50 molecules) by LIF allowed for the determination of minor enzyme products in the presence of excess unreacted substrate. The action of competing enzymes acting on the common type I sequence was monitored after the incubation of human epidermoid carcinoma cells (A431) with a fluorescent substrate (beta Gal(1-->3)beta GlcNAc-O-TMR). The CE-LIF analyses showed the formation of both synthetic and hydrolytic products, suggesting the actions of glycosyltransferases and glycosidases in the cells.