MALDI at Elevated Pressure

An elevated pressure of typically a few millibars of inert gases such as He, Ar, or N2 in the ion source, can cool the ions and thereby increase their stability. If the MALDI ions coUide with these thermal gas molecules, some of their vibrational energy will be removed via a process known as collisional cooling.  

The vibrational cooling effect has been extended to the analysis of ions desorbed at atmospheric pressure [15-20]. This arrangement is extremely simple, and involves merely placing the MALDI sample directly in front of an electrospray ionization source orifice. This proved to be effective, and Cotter et al. shortly thereafter duplicated the ganghoside investigations of O Connor et al., thus proving that the AP-MALDI source was also a coUisionaUy cooled ion source design [21, 

However, compared to operation at a few millibars, ionization at atmospheric pressure requires that the ions be extracted down a small tube and through a molecular beam skimmer in order to move them into the vacuum where they can be analyzed. Considering the losses expected on the capillary walls and on the skimmer, it is unlikely that the AP-MALDI method will become as sensitive as the vacuum or low-pressure MALDI experiments. 

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Figure 6.12. The first exploration of MALDI at elevated pressure showed reduced fragmentation of myoglobin and increased matrix adduction. (Reproduced from Ref. 121, with permission.)...

On-hne high-performance TLC (HPTLC) MALDI-FT-MS at elevated pressures was described in the direct analysis of gangliosides from TLC plates using soHd matrices [176]. Gangliosides are glycosphingolipids that contain one or more siahc... 

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