Field desorption mass spectral fragmentation of monosaccharide isomers

Abstract

Field desorption ionization of selected monosaccharide isomers by potassium ion attachment in high fields is described. The ionization is obtained with a tungsten wire activated with carbon needles and covered with KH,PO, salt. In addition to the potassium cationized molecular ions, reproducible fragmentation was obtained at 10 mA and 20 mA typical of the various monosaccharides. For instance, D-glucose tends to lose a water molecule, whereas D-fructose loses primarily methanol under similar conditions. Methanol elimination is suggested to be characteristic of furanoidal ketoses and water elimination for pyranoidal aldoses, whereas glycol elimination occurs for furanoidal aldoses.Field ionization (FI) and field desorption mass spectrometry (FDMS) of some monosaccharides such as D-glucose, D-fructose and D-ribose have been reported by several authors since 1973.'*2 The occurrence of distinct fragmentation patterns in the FD mass spectra of monosaccharides could provide a reliable tool for the characterization of these compounds. However, the FD mass spectra of the sugars studied showed mainly quasimolecular ions with very little fragmentation.An advance towards the characterization of carbohydrates by FDMS was made by ionization of carbohydrates by alkali ion attachment on the surface of salt layers, which had been deposited on emitter^.^-^ This technique increased the sensitivity of the FDMS method and also gives some fragmentation patterns. Of special interest is the fact that carbohydrates can produce cationized species with cations from the emitters previously treated with alkali solutions.6This study focuses on the ability of carbohydrate isomers to give potassium cationized molecular ions with emitters pretreated with a saturated solution of KH,PO,. With this treatment, the carbohydrates gave reproducible structure dependent fragmentation at different wire currents, as well as the usual quasimolecular ions.A comparison of the F D mass spectra of D-glucose obtained with a treated emitter at various wire currents (Fig. l) shows striking differences. In addition to the protonated molecular ion at m/z 181 a potassium cationized ion shifted by 38 u appears with high abundance at m/z 219. This peak is accompanied by the 4'K isotope peak. An increase in the wire current to 15 mA changes the spectrum, giving rise to an additional peak at m/z 163 due to elimination of water. At 2 0 m A three more peaks appear at mlz 61, 91 and 119. This fragmentation is readily reproducible.The FD mass spectrum of D-fructose under the same conditions is shown in Fig. 2. The major fragmentation at 1 0 m A leads to a peak at m/z 149 corresponding to elimination of a neutral fragment with the composition of methanol (CH,O). The peak at m/z 163 found in the FD mass spectrum of Dglucose, corresponding to water elimination, is absent or of lower abundance in the spectrum, and is replaced by the m/z 149 peak. An increase in the wire current gives more fragmentation which qualitatively resembles the fragmentation of D-glucose. Similar beh...

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