ELECTROSPRAY MASS-SPECTROMETRIC STUDIES OF SOME PHOSPHONIUM CATIONS

A series of methylphosphonium cations derived from mono- and polyphosphines were generated by interaction of the phosphine with methyl iodide. Electrospray mass spectra of the cations were obtained directly from dichloromethane-methanol solutions. In all cases the intact phosphonium cations were observed, often as the base peak, emphasizing the 'soft' nature of this type of ionization. Some of the polyphosphines showed additional peaks at m/z 16 and 32 units higher than the molecular ions due to oxidation of the non-methylated phosphine groups. Collisionally activated dissociation mass spectra show a consistent mode of fragmentation with the formation of alkene ions being dominant. Where comparisons are possible, there is a close correlation with the electron impact mass spectra of the neutral phosphines.