Atmospheric pressure matrix assisted laser desorption/ionization mass spectrometry

sA novel ionization source for biological mass spectrometry is described th at combines atmospheric pressure (AP) ionization and matrix-assisted laser desorption/ionization (MALDI), The transfer of the ions from the atmospheri c pressure ionization region to the high vacuum is pneumatically assisted ( PA) by a stream of nitrogen, hence the acronym PA-AP MALDI, PA-AP MALDI is readily interchangeable with electrospray ionization on an orthogonal accel eration time-of-flight (oaTOF) mass spectrometer, Sample preparation is ide ntical to that for conventional vacuum MALDI and uses the same matrix compo unds, such as alpha-cyano-4-hydroxycinnamic acid. The performance of this i on source on the oaTOF mass spectrometer is compared with that of conventio nal vacuum MALDI-TOF for the analysis of peptides, PA-AP MALDI can detect l ow femtomole amounts of peptides in mixtures with good signal-to-noise rati o and with less discrimination for the detection of individual peptides in a protein digest. Peptide ions produced by this method generally exhibit no metastable fragmentation, whereas an oligosaccharide ionized by PA-AP MALD I shows several structurally diagnostic fragment ions. Total sample consump tion is higher for PA-AP MALDI than for vacuum MALDI, as the transfer of io ns into the vacuum system is relatively inefficient. This ionization method is able to produce protonated molecular ions for small proteins such as in sulin, but these tend to form clusters with the matrix material. Limitation s of the oaTOF mass spectrometer for singly charged high-mass ions make it difficult to evaluate the ionization of larger proteins.