Arsenic hydrides cations

A novel HPLC-AAS interface based upon thermochemical hydride generation (THG) was developed for the determination of AB, AC and tetramethylarsonium cations, which had been separated by HPLC (Blais et al., 1990 Momplaisir et al., 1991). This has been described in Section 15.7. The detection limits of the three species were 13.3 ng (AB), 14.5 ng (AC) and 7.6ng (tetramethylarsonium cations). Advantages of this interface included low purchase and operating costs, low detection limits, and good reproducibility of results. The method was applied to determinations of arsenic species in seafoods and human urine. 

M. A. Suner, V. Devesa, I. Rivas, D. Velez, R. Montoro, Speciation of cationic arsenic species in seafood by coupling liquid chromatography with hydride generation atomic fluorescence detection, J. Anal. Atom. Spectrom., 15 (2000), 1501-1508. 

Another, relatively simple method, also applicable to plasma samples, is as follows. Bio-Rad AG 50W-x8, 100-200 mesh cation exchange resin (7,5 g) was packed into a 10-mL disposable glass pipette (0.85 x 16.5 cm) and conditioned with 0.5 M HCI (Tam et al., 1979). One millilitre of plasma, urine or arsenic reference solution was applied to the top of the column. Then, the column was eluted successively with 0.5 M HCI (As(lll) and As (V), deionized water (MMA). and first 5% (v/v). then 20% (v/v) ammonia (DMA). A modification of the elution procedure, using 0.5 M HCI, deionized water and 1.5 M perchloric acid, has been reported by Marafante et al. (1987). Arsenic may be detected by gamma counting for radioiabeiled arsenic or XRF or hydride AAS for nonradioactive arsenic compounds (Tam et al. 1979 Vahter, 1986). Separation of As(lll) and As(V) in the HCI fractions has been performed using a weakly basic resin (AG3-X4A, 100-200 mesh. Bio Rad) (Vahter and Envall, 1983). The HCI-fractions were neutralized with NaOH and applied to the column. As (III) was eluted with 0.01 M phosphate buffer, pH 6.9, and As(V) with 1 M HCI. 

In covalent hydrides, hydrogen and the metal are linked by a covalent bond. Aluminum, silicon, germaninm, arsenic, and tin are some of the metals whose covalent hydride structures have been stndied extensively. Some ionic hydrides, snch as LiH or MgH2, exhibit partial covalent character. The complex hydrides, such as lithium aluminum hydride and sodium borohydride, contain two different metal atoms, usually an alkali metal cation bound to a complex hydrido anion. The general formula for these compounds is M(M H4) , where the tetrahedral M H4 contains a group IIIA metal such as boron. 

In this conjunction, the integration of a hydride generator in front of the sample introduction system of the ICP-MS described by Roehl et al. [93] and Schlegel et al. [101] should also be mentioned. It increases sensitivity for various arsenic-and selenium compounds by a factor of 2 to 5. It is true that in hydride formation with the subsequent separation of gas and liquid, temperature has to be strictly controlled for reproducibility reason, but detection is much more specific because molecular interferences do not occur. Mattusch and Weimrich [102] also separated anionic, neutral, and cationic arsenic compounds such as arsenite, dimethylarsenic acid, arsenobetain, and arsenocholine on lonPac AS7, but used a very steep nitric acid gradient. Arsenobetain and arsenocholine are retained, above all, by hydrophobic interactions with the stationary phase. 

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