INVESTIGATION OF THE IONIZATION PROCESSES OCCURRING IN LIQUID-ASSISTED SECONDARY-ION MASS-SPECTROMETRY OF DERIVATIZED MONOSACCHARIDES

Several derivatized monosaccharides, the 2-deoxy-D-ribofuranoses, have been studied by liquid-assisted secondary ion mass spectrometry (LSIM S) in order to gain insight into the factors affecting ionization in F AB/LSIMS. Examination of the mass spectra for these compounds obtained in eight liquid matrices (diethanolamine, ethylene glycol, glycerol, 2-hydroxyethyl disulfide, 2-hydroxyphenethyl alcohol, 3-nitrobenzyl al cohol, sulfolane and thioglycerol) reveals that in all cases the anoma lous [M - H]+ ion is the predominant species in the molecular ion regi on and that [M + Na]+ species are observed in the presence of Na+. The analysis of these compounds by chemical ionization with ammonis shows [M + H]+ as the major species while [M - H]+ is essentially absent. T his indicates that the ionization processes occurring in the two techn iques are not analogous. Thermodynamic considerations based on the gas -phase hydride ion affinities of the protonated matrices do not suppor t a predominant gas-phase mechanism for the formation of [M - H]+ in L SIMS. However, it is possible using solvation energies to rationalize the formation of [M - H]+ in terms of condensed-phase ionization proce sses which take place either in the liquid matrix or in the dense selv edge region immediately above the surface where extensive solvation is present. Electrospray data obtained for one of the derivatized monosa ccharides indicates that the [M - H]+ is not performed in the condense d phase in LSIMS and that it is the product of fast ion beam-induced p rocesses. While the nature of the matrix is seen to have little effect on the intensities of [M - H]+ and [M + H]+ it is observed to be an i mportant factor for the intensity of M '' for one of the monosaccharid es. This effect can be related to the electron-scavenging properties o f the matrices and reinforces the hypothesis that condensed phase proc esses are significant in ionization.