THERMALLY STABILIZED IRIDIUM ON AN INTEGRATED, CARBIDE-COATED PLATFORM AS A PERMANENT MODIFIER FOR HYDRIDE-FORMING ELEMENTS IN ELECTROTHERMAL ATOMIC-ABSORPTION SPECTROMETRY .2. HYDRIDE GENERATION AND COLLECTION, AND BEHAVIOR OF SOME ORGANOELEMENT SPECIES

The in situ collection of volatile hydrides in an electrothermal atomi zer with an integrated platform pre-treated with 110 mu g of Zr or 240 mu g of W and 2 mu g of Ir for permanent modification was studied. An optimization;study of the performance characteristics of an automated FI-HG-ETAAS system based on an FI hydride generator interfaced with a transverse-heated graphite atomizer and longitudinal Zeeman-effect ba ckground correction was elaborated. The HG step for As-III, As-V, Bi-I II, Sb-III, Sb-V, Se-IV, Sn-IV and Te-IV, as well as for several alkyl ated species of As and Sn, was optimized by means of a full factorial 3(2) design, the factors being the concentrations of acid and tetrahyd roborate (or their supply rates in pmol s(-1)). The corresponding regr ession equations are tabulated, and representative response surfaces a nd contour diagrams are plotted. All inorganic hydrides except for SnH 4 are generated and collected with high efficiency at tetrahydroborate concentrations of 0.25-0.4% m/v, sample acidity of 1.5-3 mol l(-1) HC l, trapping temperatures of 400 degrees C and a purge gas flow of argo n of 100-130 ml min(-1). The optimum conditions for stannane and alkyl tin hydrides are: pH 1-4, tetrahydroborate concentrations of 0.2-0.4% m/v, trapping temperatures between 400 and 600 degrees C and argon flo w rates of 60-120 ml min(-1). Arsine, monomethylarsine and dimethylars ine are effectively collected on both coatings at temperatures between 400 and 500 degrees C and purge gas flow rates of 70-120 ml min(-1). Optimum HG conditions differ strongly for As-III, As-V, monomethylarso nate and dimethylarsinate species with this FI system, unless L-cystei ne is added. Organoelement species of As, Sn and Se are thermally stab ilized in a similar manner on both Ir-Zr- and Ir-W-treated platforms, the least stable species being selenomethionine and trimethylselenoniu m. The best levelling-off effect on the integrated absorbance for diff erent analyte species (isoformation) is observed for As and the worst for organotins, particularly for trialkylated species such as tributyl tin, trimethyltin and trimethylselenonium. Relatively better isoformat ion is achieved for organotins on Ir-W- and for organoselenium on Ir-Z r-treated platforms. The long-term stability of the Ir-Zr and Ir-W mod ifier coatings during at least 600-700 thermal cycles is demonstrated. The Ir-Zr treatment is preferred to Ir-W for hydride trapping, owing to lower atomization temperatures, longer lifetime of the atomizer and an absence of double peaks. Such peaks persist for Bi and Te on Tr-W- treated platforms. The best characteristic masses in integrated absorb ance measurements with Ir-Zr-treated platforms are close to those for the direct injection mode, viz., 35, 107, 83, 43, 104, 48, 31, 32, 153 , 146, 148, 145 and 152 pg for As-III, Bi-III, Sb-III, Se-IV, Sn-IV, T e-IV, monomethylarsonate, dimethylarsinate, monomethyltin, dimethyltin , trimethyltin, diethyltin and monobutyltin, respectively. Analytical results for As, Sb and Se in certified reference materials (water and autoclave-decomposed sediments) are in good agreement with the certifi ed contents.