Abstract

Comparison of the mass spectral techniques of Laser (LMS), Californium-252 Plasma Desorption (²⁵²Cf-PDMS), Secondary Ion (SIMS), Fast Atom Bombardment (FAB), and Field Desorption (FD) Mass Spectrometry is presented for the positive- and negative-ion spectra of a series of internal salts (zwitterions) known as the sultaines. The spectral characteristics compared are: quasimolecular ions, fragment ions and their implication for structure determination, and cluster-ion formation. Quasimolecular ions corresponding to (M+H)⁺ (all techniques); (M+CH₃)⁺ (LMS, PDMS, SIMS); (M+R)⁺ (R = long alkyl chain up to C₁₂, LMS); (M+Na)⁺ (SIMS, ²⁵²Cf–PD); (M+Ag)⁺ (SIMS), (M–H)⁻ (FAB); (M–CH₃)⁻; and (M-R)⁻ were observed (negative-ion FD mass spectra were not obtained). Extensive fragmentation is observed for PDMS and LMS, providing the highest level of structural information. Also, the possibilities for structure elucidation by SIMS are very good. Minimal fragmentation is observed for FAB; however, complete structure elucidation is possible by using both the positive- and negative-ion FAB spectra. FD is a less favored technique for the structural analysis of the sultaines. The negative-ion spectra of sultaines obtained by LMS, PDMS, SIMS, and FAB showed identical peaks in the lower mass ranges (<i>m/z</i> < (M–CH₃)⁻), although of varying intensities. Cluster ions and solvated (glycerol) cluster ions are observed in negative FAB in the higher mass ranges (<i>m/z</i> > (M–CH₃)⁻). FAB is unique with respect to cluster-ion formation; cluster ions of high order [(<i>n</i>M+H)⁺, <i>n</i> = 2-14; (<i>n</i>M–CH₃)⁻, (<i>n</i>M–R)⁻, <i>n</i> = 2-9; (<i>l</i>M+<i>m</i>G–H)⁻, <i>l</i> = 1-4, <i>m</i> = 1-3] are observed. Other techniques show cluster ions such as (<i>n</i>M+H)⁺, <i>n</i> = 2-3 (SIMS, PDMS, FD); (<i>n</i>M+Na)⁺, <i>n</i> = 2 (SIMS and PDMS); and (2M+Ag)⁺ (SIMS). No cluster ions were observed in LMS.

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