Desorption ionization mass spectrometry: Secondary ion mass spectra of phosphonium salts

Abstract

Secondary ion mass spectra are obtained for involatile phosphonium salts not analyzable by conventional ionization methods. The spectra typically exhibit an abundant intact cation and fragment ions which result from predictable losses of stable neutral species from the cation. A systematic pattern of fragmentation occurs in simple salts which serve as model compounds, and also more complex structures which are of pharmaceutical interest. Close parallels with the behavior of related volatile compounds in electron and chemical ionization mass spectrometry are observed in the presence of characteristic fragment ions and a tendency for rearrangement with elimination of stable molecules. Doubly charged salts undergo loss of a charged fragment such as a proton to give stable singly charged species. Sensitivity is such that a few nanograms of phosphonium salt suffices to provide a spectrum observable for ten to twenty minutes. The preferential desorption of precharged species makes possible the direct analysis of these salts in Chromatographic media, including electrophoresis membranes, and also the direct desorption of these salts from vacuum compatible solutions.