Gas phase ion/molecule reactions in laser desorption mass spectrometry

Abstract

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...

Cationization by Alkali Ion Attachment in Laser Desorption Mass Spectrometry

“…In these experiments the [M + K] + ions could not be detected prior to the onset of the K + emission and the onset of evaporation of neutral molecules. Evidence for a gas phase ionization mechanism has recently been obtained by these authors [26] and is also reported in the present paper. It has to be added that the appearance of [M +Ag]+ ions of sucrose in the gas phase upon irradiation of a sample layer by short laser pulses of 10 8 W cm -2 [17,27] also points to an ionization mechanism where M and Ag + combine in the gas phase close to the surface because the temperatures required for the release of Ag + from the substrate are by far too high to leave organic molecules intact.…”

“…Applying electrical heating instead of laser heating for the evaporation of sucrose and for the emission of K + ions (Fig. 2 b) [26]. However, these ions are always observed if the evaporation of sucrose is performed by CCVlaser irradiation i.e.…”

Lasers and Mass Spectrometry

“…sample or substrate following a single pulse. The evidence for thermal mechanisms includes the observation of thermal kinetic energy spreads for the ions formed (63,64), ion formation by gas phase cationization between codesorbed neutral molecules and thermionically emitted alkali ions (65), and broad desorption times following the laser pulse (66,67). Figure 8 shows that the desorption of quaternary ammonium ions from tetramethylammonium chloride continues for some time (microseconds) following the laser pulse but that the desorption of these "molecular ions" is enhanced at the early stages, while decomposition is generated in a longer time frame (66).…”

“…In the negative-ion mode, Hercules et al (74) have used reagent compounds containing the nitro group to produce (M -H + Ο)ions of several aromatic compounds. In the most general sense, cationization of any kind might be viewed as laser desorption chemical ionization, particularly in those cases where a gas phase ion-molecule pro cess can be invoked (65). The combi nation techniques are important in view of the abundance of neutral species that are desorbed, and one can expect to see additions of other re agent materials to enhance their ion ization in future work.…”

Laser resonance photoionization spectroscopy of molecules

“…Один из механизмов образования ионов органических молекул заключается в следующем: импульс лазерного излучения нагревает поверхность, интенсивное испарение вещества приво-дит к образованию достаточно плотного облака пара, в котором протекают химические реакции (перенос протона, присоединение катиона щелочного металла и др. ), приводящие к образованию ионов 88 ' 89 . В масс-спектре наблю-даются как положительные, так и отрицательные ионы, (М + Н) + , (М-Н)~, (М + Alkali) и осколочные ионы.…”