Electron impact, chemical ionization and collisional activation mass spectrometry of methylO-acetyl-β-D-xylopyranosides

Abstract

The mass spectrometric behaviour of methyl O-acetyl-β-D-xylopyranosides has been studied using electron impact, methane and ammonia chemical ionization and collisional activation mass spectrometry. Based on this study the fragmentation of the compounds studied has been described. The structures of ions occurring after elimination of ketene in the fragmentation reactions of acetylated saccharides were studied by using the MNDO semi-empirical quantum chemical method. Calculated geometrical parameters (bond lengths, bond and torsional angles), distribution of net atomic charges and the difference in heat of formation (70 kJ mol−1) between the keto and enol isomers of [C5H7O3]+ ions studied predict that the enol structure is more stable thermodynamically than its corresponding keto isomer. The electron impact and chemical ionization mass spectra do not provide complete information for determination of the location of acetyl groups in methyl O-acetylpentopyranosides. The number and position of the acetyl groups however, can be determined unambiguously from collisional activation mass spectra of the [M - OCH3]+ ions. The reproducibility of the mass spectral data, obtained by the applied mass spectral methods was evaluated by calculation of discrepancy factors. From this point of view the collisional activation method is approximately five times more precise than electron impact and 10 times more precise than chemical ionization methods.