Journal of the Mass Spectrometry Society of Japan

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Matrix effects on the competitive formation and extents of fragmentation of M^+· and [M+H]⁺ ions produced under fast atom bombardment (FAB) conditions, and on the formation of complex [M+matrix+H]⁺ and [M+matrix-H]^- ions under FAB conditions have been described using various organic compounds. The use of hydrophilic matrices such as glycerol and thioglycerol resulted in the preferential formation of [M+H]⁺ to M^+· ions compared to the use of hydrophobic matrices such as m-nitrobenzyl alcohol and o-nitrophenyloctyl ether. The use of alcoholic-hydrophilic matrices such as glycerol and pentamethylene glycol resulted in the preferential formation of [M+H]⁺ to M^+· ions compared to the use of thiol-containing hydrophilic matrices such as thioglycerol and dithiothreitol. The latter matrix effect was explained on the basis of the distinction of hydrogenbonding ability between hydroxyl -OH and thiol -SH groups, and it was proposed that the hydrogen-bonding interaction between the hydroxyl group(s) of matrix and the basic site(s) of analyte molecules in matrix solution, which was named as a ‘quasi-preformed state’, is advantageous for the formation of [M+H]⁺ to M^+· ions. It was confirmed that the fragmentation of [M+H]⁺ and M^+· ions produced under FAB conditions occurs independently to each other and that the extent of fragmentation of M^+· ions is often comparable to that of fragmentation of M^+· ions formed at 70 eV or above in the electron impact ionization method. The extent of fragmentation of M^+· ions under FAB conditions depended upon the matrix materials used. Further, the extent of fragmentation of [M+H]⁺ ions under FAB conditions was larger than that of fragmentation of [M+H]⁺ ions formed by the isobutane chemical ionization method. The complex ions, [M+matrix+H]⁺ and [M+matrix-H]^-, were formed by the use of a certain prenylated flavone and thiol-containing matrices, while the complex ions were not formed by the use of the flavone lacking a prenyl group or glycerol matrix. The suitability of matrix materials for positive- and negative- ion FAB mass spectrometry has been discussed.

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The unimolecular dissociation of the cycloglycylglycine ion has been investigated using the ab initio MO method. The optimized geometries of possible molecular ion conformers, reaction intermediates and fragments have been calculated at the HF/6-31G//3-21G and HF/3-21G//3-21G levels. The potential energy curves for CO and NHCO eliminations from cycloglycylglycine ions have been calculated at low energy. The present study indicates that [M-CO]^+· and [M-NHCO]^+· are produced via proton transfers, which result in stable intermediates with a three-membered ring and a five-membered ring, respectively.

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