Vacuum MALDI

Kenny, D., Snel, M., Brown, J., Bateman, B., Coleman, J., Petrie, J. R., Laidlaw, H., and Ashford, M. (2005). Evaluation of a new aerosol matrix deposition method for atmospheric pressure and vacuum MALDI ion imaging. In Proceedings of the 53rd ASMS Conference on Mass Spectrometry and Allied Topics, San Antonio, TX. 

The AP-MALDI source is illustrated in Figure 1.17. It works in a similar manner to the conventional MALDI source. The same sample preparation techniques and the same matrices used for conventional vacuum MALDI can be used successfully for AP-MALDI. The main difference is the pressure conditions where ions are produced. Conventional MALDI is a vacuum ionization source where analyte ionization takes place inside the vacuum of the mass spectrometer whereas AP-MALDI is an atmospheric ionization source where ionization occurs under atmospheric pressure conditions outside of the instrument vacuum. 

Next to conventional (vacuum) MALDI, atmospheric-pressure MALDI interfaces have been described, especially to enable MS-MS on MALDI-generated ions by ion-trap and (J-TOF instmments [145-146]. Atmospheric-pressure MALDI sources are commercially available from all major instrument manufacturers. First results on-line LC-atmospheric-pressure MALDI were reported as well [147]. 

Schneider BB, Lock C, Covey TR. AP and vacuum MALDI on a QqLIT instrument. J Am Soc Mass Spectrom 2005 16(2) 176—182. 

FIGURE 9.2 Comparison of mass spectra obtained by vacuum MALDI-MS (a and c) and AP-MALDI-MS (b and d) recorded on monolithic silica UTLC plates (10 nmol a-cyano-4-hydroxycinnamic acid sprayed over triazole (a and b) and midazolam (c and d), 1 nmol each). (From Salo, P.K. et ah, J. Am. Soc. Mass Spectrom., 16 906-915, 2005. Copyright 2005. With kind permission from Springer Science+Business Media.)... 

Typically ionization is performed at the pressure of the DTIM (e.g., 1-10 torr), which results in moderate pressure MALDI. Thus some collisional cooling typically takes place after ionization and can result in matrix-adducted and cluster species. These can be dissociated prior to DTIM by performing injected ion experiments. Furthermore, matrix optimization may be required. One effect of moderate pressure MALDI that we have observed is that higher ion currents can be achieved at slightly lower matrix-to-analyte ratios (i.e., 1,000-100 1) than those used in high-vacuum MALDI (i.e., 10,000-1,000 1). 

In early times, the MALDI ion source was under a high vacuum. Lately, atmospheric pressure (AP) MALDI has been developed, which in contrast to vacuum MALDI operates at a normal atmospheric environment [51]. The mechanism of AP-MALDI ion production is similar to that of conventional MALDI. The main difference between vacuum MALDI and AP-MALDI is that AP-MALDI produces ions under atmospheric pressure conditions outside of the instrument vacuum housing. In vacuum MALDI, ions are typically produced at lOmTorr or less, whereas in AP-MALDI, ions are formed at the atmosphere pressure. 

The main disadvantage of AP-MALDI relative to vacuum MALDI was limited sensitivity however, significant changes regarding ion transferring have been made, leading to profound improvement of detection limits [53]. 

Introduction of atmospheric pressure (AP) MALDI greatly improves the IMS analysis of lipids. One of the main advantages of AP MALDI relative to conventional vacuum MALDI is the collisional cooling of ions (i.e., the facile removal of ion internal energy by collisions with a neutral gas) during transfer at atmospheric pressure. This results in less fragmentation than conventional MALDI. 

Nevertheless, AP MALDI is generally less sensitive than vacuum MALDI due to the ion loss occurred during transferring from the ambient environment into the... 

In review of the data in Table 9.3, it is clear that MS is a sensitive technique and with a reasonable introduction technique, whether at atmospheric pressure (cap-LC/ESI, AMALDI) or not (vacuum-MALDI), the ion source mass analyzer combinations mentioned in this chapter offer single-digit femtomole detection in all but the most difficult samples with special separation techniques, low-attomole detection levels are achievable. 

Figure 11.12. Relationship of analyte ion kinetic energy to mass analyte mass for ions produced by vacuum MALDI. Ions below Une A may be captured by a 10-V trapping potential, while ions below line B require a 60-V trapping potential to be captured. (Adapted from Ref. 42.)...

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 analysis of oligosaccharides is particularly challenging due to the lability and complexity of these molecules. While vacuum MALDI-FTMS of oligosaccharides may... 

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