Application of direct analysis in real time to the study of chemical vapor generation mechanisms: identification of intermediate hydrolysis products of amine-boranes

Abstract

In order to elucidate controversial results emerging in chemical vapor generation (CVG) for trace element determination, we conducted a series of experiments devoted to the identification of intermediates formed by acid hydrolysis of amine-boranes. For the first time, direct analysis in real time coupled with high-resolution mass spectrometry (DART-Orbitrap) was applied for detection of this class of compounds. Mass spectra of both solid amine-boranes and their aqueous solutions (pH ~\u20098, no hydrolysis) were acquired for understanding their ionization pathway. Mass spectra of aqueous solutions of t-BuNH2\ub7BH3 and Me2NH\ub7BH3 were acquired under conditions that are employed in CVG (0.017\u20134.0 mol L 121 HCl, 0.167\u20130.2 mol L 121 borane reagent). The results disclose a reactivity driven by pH of amine-boranes undergoing hydrolysis. At low acidity, the hydrolysis proceeds according to the currently accepted displacement mechanisms (i.e., R3N\ub7BH3 + H3O+ \u2192 R3NH+ + H2OBH3). At higher acidity, N-tert-butyl, cyclotriborazane, and bis(dimethylamino)boronium were identified, for the first time, during the hydrolysis of t-BuNH2\ub7BH3 and Me2NH\ub7BH3, respectively. Formation of these intermediates was ascribed to a hydrolysis pathway starting with the ionization of the amine-borane, (i.e., R3N\ub7BH3 + H3O+ \u2192 [(H2O)R3NBH2] + + H2). The new evidence explains the anomalous behavior observed in CVG by amine-borane derivatization, and updates the currently accepted mechanisms for the acid hydrolysis of amine-boranes.Peer reviewed: YesNRC publication: Ye