Credit: © 2010 Wiley

Desorption electrospray ionization (DESI) used in conjunction with mass spectrometry (MS) has become an important technique for chemical analysis in ambient conditions. Mass spectrometry requires the analyte to be charged and this is achieved in DESI by aiming an electrically charged aqueous mist at an analyte deposited on a surface. This extracts the analyte from the surface, creating ionized droplets from which detectable gas-phase ions eventually form. Now Richard Zare and colleagues at Stanford University have used the short desorption and ionization timescales involved in DESI to detect transitory reaction intermediates1.

To test the technique the researchers studied the synthesis of ruthenium catalysts, which are used for the asymmetric reduction of ketones through transfer hydrogenation. One such catalyst can be synthesized through the reaction of the diruthenium complex [{RuCl2(p-cymene)}2] with 1-amino-2-indanol. This reaction was examined by depositing the former on a surface and including the latter in the charged mist used to ionize the analyte. The reactions of three other amino alcohols — all ethyl benzene derivatives — were also studied. Significant mass spectra signals were observed that correspond to short-lived coordination species between the diruthenium complex and the two β-amino alcohols studied, but equivalent signals were not observed for those amino alcohols in which the amine and hydroxyl groups were further apart.

The researchers could not unambiguously determine the structures of the intermediates, but the spectra were enough to highlight the importance of the relative positions of the amine and hydroxyl groups to the reactivity of the amino alcohols and, more generally, the potential of the DESI technique for studying reaction intermediates.