The desorption of neutrals, alkali ions and quasimolecular ions of sucrose was studied as function of substrate temperature in laser desorption mass spectrometry. These phenomena were also invesbigated in thermal desorption experiments. It was concluded that in these experiments gas phase cationization is the major ionization process.
%OIDUCTIONLaser desorption (LD) mass spectrometry has been introduced as an analytical technique for the analysis of so-called non-volatile organic compounds. l4 Representatives oE various classes of biomolecules, e.g. oligosaccharides, oligonucleotides and oligopeptides have been used frequently to demonstrate the potential of the method. The phenomena encountered in LD mass spectrometry e.g. the occurrence of ions indicative of the molecular weight of 'non-volatiles', were quite unexpected. Initially a rationalization of the physicochemical processes involved could not be given. This challenged various scientists' imagination to advance new theories on the desorption of organic ions.' S6As new observations on desorption phenomena crowded i n: -' * various features in LD mass spectrometry were recognized, which pointed to ion/molecule reactions and unimolecular dissociations. A similar situation existed in ion induced desorption mass spectrometry1g-22 and in fact Cooks et al.22*23 pointed out that the interpretation of the desorption spectra can be facilitated greatly by using the available knowledge on gas phase ion chemistry. However, details of the desorption and ionization processes involved in the ion formation and release of 'nonvolatile' sample molecules have not appeared previously. Surface ionization and consecutive ionic emission as well as gas phase ionization of neutral molecules desorbed intact have been proposed," but no evidence has been presented which favours one of these processes. Of course, the 'non-volatility' of the samples investigated should exclude a gas phase ionization model; however, it has been argued that under specific conditions, for example high heating rates, molecular evaporation can be enhanced over molecular dissociation.'o~1'~18~24. 25 LD mass spectra of various organic samples have in common the predominant occurrence of quasimolecular ions formed either by protonation or attachment of metal ions. Alkali ion attachment has been observed most frequently, although also cationization by other ? Guest scientist from Fukui University, Fukui, Japan. metal ions has been r e p~r t e d .~~.~~ Furthermore, abundant contributions of single cations involved in the cationization process are found to accompany the quasimolecular ion yields. Although the formation of ions can result from single and multiphoton absorption processes in principle, the cation production observed under low power C 0 2 laser radiation9-l6 can hardly be attributed to processes of this type. The conditions rather point to a thermal process. Indications for cation production by thermal ionization2* in LD mass spectrometry can be extracted from several reports. First, with low power l...
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