MALDI typically utilizes a nitrogen laser at 337 nm as the ionization source. The sample is mixed with a matrix, and allowed to dry prior to insertion into the mass spectrometer. Crystallization of the sample within the matrix is an important component of successful MALDI analysis. A variety of matrices, present in great excess relative to the sample amount, are used to span the range of compound classes amenable to MALDI mass analysis. Formation of sample ions, upon laser irradiation, involves a proton transfer reaction involving the matrix (which absorbs the UV photon) and the analyte. The ions are then accelerated into a time-of-flight mass analyzer for mass analysis. Typical matrices include a-cyano-4-hydroxycinnamic acid, sinapinic acid and 2,5-dihydroxybenzoic acid. MALDI can be done routinely to m/z 100,000 and there are many examples of analyses going well above this mass range. Figure 5-5A shows a MALDI mass spectrum of grape anthocyanins over the mass range m/z 450 to 550 (Sugui et al., 1999).
Two series of ions are evident, representing monoglucosides (1-5) and acetylglucosides (6-10) of the following aglycones: cyanidin (1, 6; 1.40), peonidin (2, 7; 1.41), delphinidin (3, 8; 1.42), petunidin (4, 9; 1.43) and malvidin (5, 10; 1.44). These results were obtained with a first-generation (continuous mode) MALDI mass spectrometer that did not have delayed extraction capabilities. Figure 5-5B shows a similar mass range for the analysis of anthocyanins from a different grape variety, with improved mass resolution using a MALDI mass spectrometer having delayed extraction capabilities. Note the presence of the monoglucosides of cyanidin (m/z 449), peonidin (m/z 463), petunidin (m/z 479) and malvidin (m/z 493). Both MALDI mass spectra were obtained in the reflector mode.
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