Surface-assisted laser desorption ionisation

As evident from Scheme 7.13, most modern ionisation techniques have been used for TLC-MS, and no single ionisation method is used exclusively with TLC-MS. Various ionisation methods may be applied that avoid the need to evaporate the sample into an El or Cl source these are based in particular on sputtering (FAB, SIMS) or laser desorption. Several sputtering methods of ionisation do not require the use of a liquid matrix, e.g. TLC-SIMS [797], Recent developments include the use of matrix-assisted laser desorption ionisation (MALDI) and surface-assisted laser desorption ionisation (SALDI). It is obvious that TLC-MS is complemented with TLC-MS11 [800] and TLC-HRMS techniques. Table 7.82 lists the general characteristics of TLC-MS. 

Matrix assisted laser desorption/ionisation (MALDI) For laser desorption methods a pulsed laser is used to desorb species from a target surface. Therefore, a mass analyser compatible with pulsed ionisation methods has to be used. Typically, time-offlight (TOF) analysers are employed, but several hybrid systems (Q-TOF) and, recently, high resolution Fourier transform ion cyclotron resonance (FT-ICR) analysers have been successfully adapted (see Section 10.2.4). Direct laser desorption rehes on the very rapid heating of the sample or sample substrate to vapourise molecules without decomposition. The more recent development of MALDI relies on the absorption of laser energy by a solid, microcrystalline matrix compound such as a-cyano-4-hydroxy ciimamic acid or sinapinic acid [8, 34]. MALDI has become an extremely popular method for the rapid and sensitive analysis of high-molecular-weight compounds [4]. 

The different stages of the preparation of peptide surfaces can be confirmed with surface-sensitive physical and chemical analysis techniqnes. For gold-based SAMs, Mrksich and co-workers have introduced a matrix-assisted laser desorption ionisation time-of-ftight (MALDI-TOF) mass spectrometry-based analysis procedure with which they are able to identify the presence of various surface functional groups via their mass (Yeo Mrksich, 2006 Yeo et al., 2003). Although this method is applicable to SAMs, it is not strictly a surface sensitive technique, as the desorption process in MALDI is not confined to the uppermost layer of a material. 

Parallel to this, new ionisation methods have been developed that are based on the direct desorption of ions from polymer surfaces. With the introduction of desorption/ionisation techniques, it has become possible to eject large molecules into the gas phase directly from the sample surface, and thereby mass spectra of intact polymer molecules have been produced. The term desorption/ionisation refers to a method in which the desorption/vapourisation and ionisation steps occur essentially simultaneously. Much progress to date has been made using matrix-assisted laser desorption/ionisation (MALDI-MS), which is capable of generating quasimolecular ions in the range of a million Daltons and beyond. 

For non-volatile sample molecules, other ionisation methods must be used, namely desorption/ionisation (DI) and nebulisation ionisation methods. In DI, the unifying aspect is the rapid addition of energy into a condensed-phase sample, with subsequent generation and release of ions into the mass analyser. In El and Cl, the processes of volatilisation and ionisation are distinct and separable in DI, they are intimately associated. In nebulisation ionisation, such as ESP or TSP, an aerosol spray is used at some stage to separate sample molecules and/or ions from the solvent liquid that carries them into the source of the mass spectrometer. Less volatile but thermally stable compounds can be thermally vaporised in the direct inlet probe (DIP) situated close to the ionising molecular beam. This DIP is standard equipment on most instruments an El spectrum results. Techniques that extend the utility of mass spectrometry to the least volatile and more labile organic molecules include FD, EHD, surface ionisation (SIMS, FAB) and matrix-assisted laser desorption (MALD) as the last... 

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