External MALDI-FTMS

Figure 11.17. Schematic representations of external MALDI-FTMS systems. (A) vacuum MALDI-FTMS system where ions are generated in the external source and transferred to the analyzer cell via an RF-only multipole ion guide. (B) IP-MALDI-FTMS system. Argon is introduced in a short burst via a pulsed valve to briefly elevate the pressure within the source region. Ions are first stored in the source hexapole and undergo collisional cooling before transfer to the analyzer cell via an RF-only ion guide.

The abihty to desorb and cool ions even from such a hot matrix as a-cyano-4-hydroxycinnamic acid suggests that another approach may also work. While only a half dozen matrix preparation methods are routinely used in MALDl, all of these techniques have been developed initially for MALDl-TOF instruments. Since MALDl-TOFMS requires very flat desorption surfaces for the best resolving power, many potential matrices have not been explored. However, systems where the external ion source is decoupled from the mass analyzer, as is the case with MALDI-QTOF, MALDI-QIT, and MALDl-FTMS instruments, do not have this limitation. Indeed, some of the best signals from MALDl-FTMS systems are generated from large, irregular DHB crystals. Thus, there is a high probability that exploration of matrices with the decoupled MALDl sources will identify new matrices with favorable properties, such as improved ion abundance or reduced matrix adduction. 

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