Comparisons between electron impact and fast atom bombardment mass spectra of some triphenylphosphonium and triphenylarsonium salts

The electron impact (EI) mass spectra of alkyltriphenylphosphonium and alkyltriphenylarsonium salts were compared with fast atom bombardment (FAB) mass spectra in a 3-nitrobenzyl alcohol matrix. It is clearly demonstrated that FAB is an effective ionization method compared with EI for analysis of these non-volatile and thermally labile compounds, particularly triphenylarsonium salts. In FABMS, the spectra generally show the intact cations (C+) as the base peaks, in addition to the singly charged cluster ions (C2A)+. High-resolution mass spectrometry was used to determine the elemental composition of the ions at m/z 277 for some phosphonium salts in the EI mode and the results show that the formation of (O(double bond)PPh3 − 1)+ (m/z 277) involves an oxygen rearrangement as a possible reaction pathway. B/E linked scans of the intact cations of triphenylarsonium salts generated by FABMS were studied in detail both to illustrate the dissociation pathways and to compare the reaction mechanism with that occurring for triphenylphosphonium salts.