Thermally stabilized iridium on an integrated, carbide-coated platform as a permanent modifier for hydride-forming elements in electrothermal atomic absorption spectrometry .3. Effect of L-cysteine

The in situ collection of volatile hydrides in an electrothermal atomizer with an integrated platform pre-treated with 110 ?g of Zr or 240 ?g of W and 2 pg 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 background correction was elaborated. The HG step for AsIII, AsV, BiIII, SbIII, SbV, SeIV, SnIV and TeIV, as well as for several alkylated species of As and Sn, was optimized by means of a full factorial 32 design, the factors being the concentrations of acid and tetrahydroborate (or their supply rates in ?mol s-1). The corresponding regression equations are tabulated, and representative response surfaces and contour diagrams are plotted. All inorganic hydrides except for SnH4 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 HCl, trapping temperatures of 400°C and a purge gas flow of argon of 100-130 ml min-1. The optimum conditions for stannane and alkyltin hydrides are: pH 1-4, tetrahydroborate concentrations of 0.2-0.4% m/v, trapping temperatures between 400 and 600°C and argon flow rates of 60-120 ml min-1. Arsine, monomethylarsine and dimethylarsine are effectively collected on both coatings at temperatures between 400 and 500°C and purge gas flow rates of 70-120 ml min-1. Optimum HG conditions differ strongly for AsIII, AsV, monomethylarsonate and dimethylarsinate species with this FI system, unless L-cysteine is added. Organoelement species of As, Sn and Se are thermally stabilized in a similar manner on both Ir-Zr- and Ir-W-treated platforms, the least stable species being selenomethionine and trimethylselenonium. The best levelling-off effect on the integrated absorbance for different analyte species (isoformation) is observed for As and the worst for organotins, particularly for trialkylated species such as tributyltin, trimethyltin and trimethylselenonium. Relatively better isoformation is achieved for organotins on Ir-W- and for organoselenium on Ir-Zr-treated platforms. The long-term stability of the Ir-Zr and Ir-W modifier 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 Ir-W-treated platforms. The best characteristic masses in integrated absorbance 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 AsIII, BiIII, SbIII, SeIV, SnIV, TeIV, 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 certified contents.