Californium-252 plasma desorption mass spectrometry of oligosaccharides and glycoconjugates: control of ionization and fragmentation.

The fragmentation of underivatized and peracetylated maltooligosaccharides, ranging in length from four to seven glucose residues, has been investigated by 252Cf plasma desorption mass spectrometry (PDMS). Investigations are made of the effects on the mass spectra of (1) peracetylation, (2) sample preparation by electrospray deposition onto metallic substrates versus adsorption onto nitrocellulose films, and (3) sodium addition to or elimination from the sample. Peracetylation enhances the mass spectrometric response of the oligosaccharides and also enables the efficient removal from the sample of water-soluble components such as sodium salts. A rapid and simple method has been developed for controlling the amount and type of fragmentation of peracetylated maltooligosaccharides. The method involves control of the amount of sodium in the sample introduced into the mass spectrometer. When a large molar excess of sodium is added to the sample, in the form of sodium chloride, the positive-ion mass spectrum is dominated by a peak corresponding to the sodium-cationized molecule. On the other hand, when the sample is completely depleted of sodium, the spectrum shows no quasimolecular ions [(M + Na)+ or (M + H)+] whatsoever and is instead totally dominated by fragment ion species. Thus, it proves feasible to alternate between a spectrum dominated by the (M + Na)+ ion peak and one dominated by fragment ions. This method has also been found to be useful in controlling the fragmentation of permethylated gangliosides.