Chemical vapor generation by aqueous boranes

Chemical vapor generation (CVG) concepts of volatile species, mainly hydrides, have been revised based on the current state of knowledge of the mechanisms governing this derivatization system. Attention has been focused on the two most important reaction pathways: hydrolysis of aqueous borane reagent and hydrogen transfer from the reaction environment to the analyte substrate. Both pathways are quite complex and take place stepwise, passing through the formation of intermediates. Aqueous boranes undergo hydrolysis through the stepwise formation of hydridoboron intermediates, [BH n X 4- n ] z (BH), where n =1–3, X is a ligand/donor species (X=H 2 O, OH − , etc.), and z =0, ±1, is the charge. Final product hydrides, EH m , are also formed stepwise, passing through hydrido–metal(loid) complexes, H x EY n- x (1≤ x ≤n−1) (HMC). The final hydrides and HMC intermediates arise by direct hydrogen transfer from borane or BH intermediates to analyte substrates through analyte–borane complex intermediates. Solution composition, in terms of pH and the presence of ligand/donor species can exert dramatic control of the derivatization reaction through modification of the coordination sphere of the analyte and/or the borane. The mechanism of action of additives employed to improve generation efficiency or for the control of interferences, and the mechanism of interference by foreign species, which is the major problem encountered with analytical applications, have been discussed in the light of most recent evidence. The role played by additives and the mechanism of interferences are not yet sufficiently understood and will require further dedicated investigations. A more general reaction model has been presented, which represents the first attempt to include the reactivity observed in a real CVG system in a single framework.