Chemical Vapor Generation by Aqueous Boranes: from Anaytical Applications to Mechanistic Investigations

Since1971, Chemical Vapor Generation (CVG) by aqueous borane reagents has been employed as an analytical derivatization technique for the conversion of aqueous ions of several metals and semi-metals to their volatile derivatives. At present CVG is a popular technique and it has been widely employed in the determination and speciation of several elements at ultratrace level by atomic and mass spectrometry. More recently, starting since 2003, dedicated studies were developed with the aim to clarify the mechanisms governing the chemistry of CVG. In a first phase, the attention was focused on the removal of erroneous concepts and the clarification of controversial aspects of CVG of volatile hydrides. The studies developed during this step served to reconcile CVG with the evidence reported in the fundamental chemistry literature concerning the mechanisms of hydrolysis of boranes, NaBH4 (THB) and amine-boranes (ABs), and the mechanism of hydrogen transfer from borane to analytical substrate [1,2]. Due to the particular conditions in which analytical CVG operates i.e., elevated acidities, high borane to analyte molar ratios, presence of foreign species and chemical additives, the use of dedicated experiments is necessary, in particular to investigate the mechanism of action of additives and the mechanisms of hydrolysis of ABs [3,4]. Some of the evidences obtained under analytical conditions for the hydrolysis of amine-boranes cannot be explained in the light of the current knowledge and suggests the needing of a revision of the mechanism of acid-catalyzed hydrolysis which is currently accepted in the literature for ABs.