MALDI experimental approaches

Surface layer-MALDI-MS has been developed specifically to identify proteins adsorbed onto biomaterial surfaces. Although the experimental approach for this technique is analogous to traditional MALDI-ToF MS, surface layer-MALDI-MS does not require protein isolation from the biomaterial surface, because the protein-adsorbed surface is submerged directly in a matrix solvent for crystallization (Griesser et al., 2004). For example, surface-MALDI-MS can be used to identify which proteins from blood plasma adsorb onto a biomaterial surface directly off the original material surface, such as polyurethane, as shown in Fig. 5.14 (Oleschuk et al., 2000). The acquired spectra show clear peaks, indicating different proteins with a distinct mass. Each of these proteins can be identified by comparing the experimentally determined masses with those in the literature. 

Different approaches for resistance detection by MALDI-TOF MS have been described. The most simple, straightforward way would be to predict resistance from the protein profile used for identification. However, reports to achieve this for MRSA (e.g., [72]) could not be confirmed and there is strong experimental... 

Ionization at atmospheric pressure was for many years considered to be an experimental curiosity despite excellent early work (Homing 1974, 1974a Carroll 1975). However, API techniques nowadays dominate on-line LC-MS work. There are currently three kinds of API sources in common use for trace quantitative analyses, namely, atmospheric pressure chemical ionization (APCI), atmospheric pressure photoionization (APPI) and electrospray ionization (ESI). In addition, atmospheric pressure MALDI has been investigated but not as an on-line approach to quantitative LC-MS. Since the highly practical discussions of Chapters 9 and 10 will be concerned mainly with LC-MS analyses (since GC-MS is now almost routine), an extended discussion of API methods is appropriate here. 

Unlike conventional ionization techniques (e.g., electron or chemical ionization), soft ionization techniques (e.g., ESI and MALDI) (see above) yield minimal in-source fragmentation under appropriate experimental conditions. In-source fragmentation can provide structural information when used properly, but generally leads to complication of lipid analysis since frequently multiple lipid species enter the ion source simultaneously even when coupled with chromatographic separation (i.e., an LC-MS lipidomics approach) (see Chapter 3). Therefore, the absence of in-source fragmentation becomes a big advantage of these ionization techniques for lipid analysis in lipidomics. 

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