Chemical vapor generation by tetrahydroborate(III) and other borane complexes in aqueous media: A critical discussion of fundamental processes and mechanisms involved in reagent decomposition and hydride formation

The mechanisms involved in chemical vapor generation (CVG) are critically reviewed on the basis of the experimental evidence reported both in the analytical chemistry and in the fundamental inorganic and organic chemistry literature. The most important topics discussed are the mechanisms of hydrolysis of tetrahydroborate(III) (THB) and borane complexes, and the mechanism of hydride generation using THB and other borane complexes. The influence of aqueous matrix composition (pH, type of acid, and the presence of additives) on these two fundamental processes is discussed in detail. Considerations and conclusions developed by this discussion contribute to the clarification of several controversial points in the CVG literature—among them the fundamental role played by numerous hydroboron intermediates (X-BHnY3−n; n=1, 2, 3, with X and Y neutral or anionic ligands/donor species), which can be formed during the derivatization process, and the incompatibility of the “nascent hydrogen” mechanism with the chemistry of THB and boranes. A view of CVG of hydrides as a complex reaction system in which the boron-to-analyte hydride transfer process is controlled by the chemistry of both the analyte substrate and the hydroboron species is proposed.