6.5 Evidence of Poor Maldi Sample Preparation

As described above, successful MALDI sample preparation involves creating inti­ mate contact between the matrix (which may consist of a few different molecules) and the analyte. The experiment requires each of the matrix roles to be completed to obtain a usable mass spectrum. To demonstrate the importance of the matrix roles, Figures 6.7 and 6.8 show two examples where a specific role of the matrix was not fulfilled. Example 1: Figure 6.7 shows a problem with the role of desorber. Here a PEG 1000 sample is prepared for analysis using DHB (panel a) and the potassium salt of DHB (KDHB) in panel b as the matrix. In panel a, strong PEG peaks cationized with Na+ are observed (by adventitious Na+ from the matrix). While ample cation is present in the sample shown in panel b, the KDHB is ineffective at desorbing the PEG analyte from the MALDI sample when illuminated with the laser beam. Example 2: Figure 6.8 shows a problem with the role of ionization agent. Here a PS 5050 sample is prepared for analysis using IAA as a matrix both without (panel a) and with (panel b) the addition of AgTFA as a cationization reagent. Note that as both the proton and the alkali ion affinity of PS is low, only Ag+ cationized peaks are observed in panel b. When no cationization agent is present in the sample, no sample related peaks are observed. Interestingly, in the case whose data are shown