Mechanisms in chemical vapor generation for trace element determination. Focus on Cadmium

Chemical vapor generation (CVG) of cadmium by aqueous boranes has been investigated by using continuous flow reaction system coupled with quartz tube atomizer and atomic absorption spectrometry, with the aim to clarify some of the mechanisms controlling the generation of volatile species. Experimental evidence collected in the present study indicates that BH3OH- is the most likely effective species in the generation of volatile Cd species. It can be synthesized on-line by quenching the acid hydrolysis of BH4 - by NaOH, according the following reactions [1]: BH4 - + H3O+ -> BH3(H2O) + H2 BH3(H2O) + OH- -> BH3OH- + H2O The use of BH3OH- in alkaline conditions increases sensitivity of about a factor 2.2 with respect to BH4 -, indicating an improved generation efficiency. A further parameter controlling dramatically the sensitivity was the presence of dissolved oxygen in the reagents. Removal of oxygen from analyte and reductant solutions, by argon purging, resulted in a sensitivity improvement of about 8 fold and 6 fold by using BH4 - and BH3OH- reductant, respectively. Oxygen gas added between gas-liquid separator and the atomizer hardly affects the sensitivity, indicating that the oxygen interferes mainly in the liquid phase, during the generation step of volatile Cd species. The use of BH3OH- under oxygen free conditions resulted about 13 fold improved LODs (about 10 ng L-1, 3s). The use of additives as thiourea (up to 1%) in the presence of NiII (10 ng mL-1), which are typically employed as signal enhancers in CVG of Cd [2], resulted in a marked signal depression for concentration of thiourea > 0.1 g L-1. Signal enhancement (+10%) was observed with Ni + 0.05 g L-1 thiourea, the use of which is mandatory to keep under control interference effects in Cd determination in some natural water samples.