Utilization of electrospray ionization to gain information on the processes involved in [M − H] + formation in FAB/LSIMS

Abstract

Some cyclic acetals and derivatized monosaccharides which yielded anomalous [M − H] + in FAB/LSIMS were analyzed by electrospray ionization (ESI) to gain further information on the condensed phase processes suggested to be responsible for the formation of [M − H] + in FAB/LSIMS. For all compounds, [M + H] + was the only significant ion present in the molecular ion region in ESI. Since ionization in ESI is believed to reflect predominantly condensed phase equilibrium processes, the absence of [M − H] + in ESI suggests that these processes are not responsible for hydride abstraction in FAB/LSIMS and that processes other than the pre-formation of ions in solution have to be invoked. Fast atom/ion-induced processes are thus considered the most likely origin of [M − H] + in FAB/LSIMS and would explain the difference in ESI and FAB/LSIMS relative molecular ion intensities. The proposed occurrence of particle-induced ionization processes in the condensed phase in FAB/LSIMS is consistent with previous experimental evidence from this laboratory, and many chemical reactions which take place in FAB/LSIMS have been explained in like manner. The [M + H] + peak in ESI for the compounds studied is attributed to the presence of NH 4 + in solution as an impurity and the resulting condensed phase equilibrium processes. The presence of [M + NH 4] + in all ESI mass spectra is consistent with this interpretation. An [M + Na] + adduct is also observed in ESI for the derivatized monosaccharides which suggests that condensed phase equilibrium processes are responsible for this adduct when observed in FAB/LSIMS.