Ion formation mechanisms

Okuyama, F. Shen, G.-H. Ion Formation Mechanisms in Field-Desorption Mass Spectrometry of Semirefractory Metal Elements. Int. J. Mass Spectrom. Ion Phys. 1981, 39,327-337. 

From the above discussion of the characteristics of the spectra and ion formation mechanisms, it is obvious that, though there can be no doubt about the usefulness of laser mass spectrometry for a large variety of analytical tasks, more research is needed for a better understanding. This is particularly true for the transition from thermal evaporation to desorption and the desorption mode itself. In the following, a few first results of such experiments, conducted recently in the author s group, will be reported. 

Penning ionization is a dominant reaction when nitrogen or neon is used in the DART source. Nitrogen or neon ions are effectively removed by electrostatic lenses and are not observed in the background mass spectrum. When helium is used, the dominant positive-ion formation mechanism involves the formation of ionized water clusters followed by proton transfer reactions. Negative-ion formation occurs by production of electrons by Penning ionization or by surface Penning ionization ... 

This instrument has allowed several studies that provide information not obtainable by other means to be conducted. Four examples are presented as follows The first example concerns the question of the mechanism of emission of potassium ions from potassium zeolite [7]. Earlier studies had made the assumption that this was an S-L type of ion formation mechanism [8], implying that there was a neutral potassium atom flux accompanying the flux of atomic potassium cations. Experiments performed on this instrument clearly showed that this is not the case there was no detectable neutral atomic potassium flux accompanying the cation flux. Thus this instrument was used to answer a long-standing question with an experiment conducted in one afternoon and allowed the conclusion to be reached that the mechanism is potassium ions in the solid state subliming into the gas phase. 

Ion formation mechanisms for silica gel matrices have never been studied for those elements that are not readily reducible to the metal. The solvation/desol-vation mechanism hypothesized previously may have a role in enhancing ion emission from these materials, but it would not be expected that an alkaline earth element could exist in the zero oxidation state in these glass matrices, which are oxide based. The species in the molten glass would be expected to be in the standard +2 oxidation state, but the experimentally observed species is +1. Indeed, there has never been a +2 species reported from thermal ionization, so there is the question of how the +2 species in the molten glass is converted to and emitted as a +1 ion. 

Ionization reactions can occur under vacuum conditions at any time during this process but the origin of ions produced in MALDI is still not fully understood [27,28], Among the chemical and physical ionization pathways suggested for MALDI are gas-phase photoionization, excited state proton transfer, ion-molecule reactions, desorption of preformed ions, and so on. The most widely accepted ion formation mechanism involves proton transfer in the solid phase before desorption or gas-phase proton transfer in the expanding plume from photoionized matrix molecules. The ions in the gas phase are then accelerated by an electrostatic field towards the analyser. Figure 1.15 shows a diagram of the MALDI desorption ionization process. 

Proton abstraction is a common ion formation mechanism in the negative ion (NI) FD-MS mode. It is not often reported in the literature because little NI-FD-MS work is done. (Field electron emission has to be contended with in NI-FD-MS.) Polar organics in the NI mode will often show (M - H) ions, but polymers to date have only been analyzed in the positive ion mode. It is interesting to note tiiat mixtures of poly(ethylene glycol) (PEG) and water are typically used as a viscous solvent in NI-FD-MS studies of organic molecules. In this case, however, no NI-FD signals are observed from the PEG. 

All of the ion formation processes in SIMS are known for other forms of mass spectrometry. The fimdamental difference is that several ion formation mechanisms may occur simultaneously in competition with each other often one does not have control over which ion formation process will dominate. Also, the dominant ionization process may vary with the parhcular type of polymer involved. One great advantage of SIMS is that both positive and negative ions are formed, often in comparable yields. 

Proposed interchain ion formation mechanism. See text for details. 

Li, J.-X., GardeUa, J.A. Jr., McKeown, P.J. (1995) A quantitative time-of-flight secondary ion mass spectrometry study of ion formation mechanisms using acid-base alternating Langmuir-Blodgett films. Appl. Surf Sci., 90, 205-215. 

Leggett, G.J., Davies, M.C., Jackson, D.E., Tender, S.J.B. (1993) Chemisorption of thiol compounds onto gold surfaces studied by static secondary ion mass spectrometry and relevance of data to ion formation mechanisms during sputtering. J. Chem. Soc. Earaday Trans., 89, 179-180. 

Knochenmuss R. Ion formation mechanisms in UV-MALDI. Analyst. 2006 131 966-86. Dreisewerd K. The desorption process in MALDI. Chem Rev. 2003 103 395-426. 

LMMS results critically depend on the procedural details. This holds true for the basic parameters, e.g. resolution and calibration, and the fundamental aspects, such as major ion-formation mechanism, degree of fragmentation, etc. Van Vaeck et al. [330] have presented a standard procedure for singleparticle analysis with LMMS, which is found to determine the experimental conditions quite strictly. 

The following considerations needed to be addressed for initial source modification and LSI-IMS-MS applications. The mechanism for formation of highly charged ions by LSI is proposed to be initial formation of charged clusters or liquid droplets of matrix/analyte by laser ablation followed by matrix evaporation similar to the ion formation mechanism in ESI. Increasing the temperature of the AP to vacuum... 

Electrospray ionization is an ionization process by which analyte molecules or ions present originally in solution are transferred to the gas phase through either solvent or ion evaporation. Although the experimental setup is relatively simple, the ion-formation mechanisms are still under systematic studies [24-26]. A scheme for an electrospray source is shown in Fig. 9, while a simplified ion-formation mechanism is indicated in Fig. 10. 

Hoppilliard, Y., Y. Le Beyec, and S. Della-Negra. 1993. Particle-induced desorption-ionization processes for organic and bioorganic molecules ion formation mechanisms. J. Chem. Phys. 90 1367-1398. 

The ion formation mechanisms in positive chemical ionization are presented in Chapter 3. The M+" ions issued from charge exchange almost do not fragment in the conditions under which they are formed in GC-MS (unlike the M+ ions formed by electron ionization). Methane chemical ionization often directly supplies fragment ions by removing a hydride or alkyl anion. 

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