Laser vapourisation

Figure 10.36. Schematic diagram of the laser vapourisation source and pulsed Fourier transform microwave spectrometer developed to study rare earth oxides [88].

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]. 

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. 

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