Formation of (M-H)- Ions under Fast-atom Bombardment Conditions: Which is reflected, gas-phase or solution chemistry?

Abstract

The formation of deprotonated molecules [M-H]- of Trolox having a carboxyl and a phenolic hydroxyl group and the derivatives, under fast-atom bombardment (FAB) conditions, has been examined from the viewpoint of proton-transfer reaction in gas-phase or solution. The negative-ion FAB mass spectra of the derivatives strongly suggested that deprotonation mainly occurs on the carboxyl group through the electrolytic dissociation in matrix solution. The use of higher proton acceptability matrices such as glycerol, pentamethylene glycol, and diethanolamine was favorable for the detection of [M-H]- ions of a Trolox methyl ester, while the use of lower proton acceptability matrices such as m-nitrobenzyl alcohol and dithiothreitol was unfavorable for the [M-H]- formation. While the result obtained suggested that the formation of [M-H]- ions reflects the gas-phase basicities of matrices used, the results from the negative-ion FAB mass spectra of the Trolox methyl ester and α-tocopherol suggested that the [M-H]- formation reflects solvation states such as hydrophobic and hydrophilic interactions between analyte and matrix molecules in matrix solution. The site of deprotonation of the compounds used has been examined by using collision-induced dissociation (CID) technique, and it was shown that the CID characteristics of [M-H]- ions depend markedly on the site of deprotonation. It was presumed, consequently, that the [M-H]- ion formation under FAB conditions is governed by gas-phase thermochemistry and/or solution chemistry according to the nature of analyte and matrix used.