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| - Atmospheric Pressure Matrix-Assisted LaserDesorption/Ionization Mass Spectrometry
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| Abstract
| - A novel ionization source for biological mass spectrometryis described that combines atmospheric pressure (AP)ionization and matrix-assisted laser desorption/ionization(MALDI). The transfer of the ions from the atmosphericpressure ionization region to the high vacuum is pneumatically assisted (PA) by a stream of nitrogen, hence theacronym PA-AP MALDI. PA-AP MALDI is readily interchangeable with electrospray ionization on an orthogonalacceleration time-of-flight (oaTOF) mass spectrometer.Sample preparation is identical to that for conventionalvacuum MALDI and uses the same matrix compounds,such as α-cyano-4-hydroxycinnamic acid. The performance of this ion source on the oaTOF mass spectrometeris compared with that of conventional vacuum MALDI-TOF for the analysis of peptides. PA-AP MALDI can detectlow femtomole amounts of peptides in mixtures with goodsignal-to-noise ratio and with less discrimination for thedetection of individual peptides in a protein digest.Peptide ions produced by this method generally exhibitno metastable fragmentation, whereas an oligosaccharideionized by PA-AP MALDI shows several structurallydiagnostic fragment ions. Total sample consumption ishigher for PA-AP MALDI than for vacuum MALDI, as thetransfer of ions into the vacuum system is relativelyinefficient. This ionization method is able to produceprotonated molecular ions for small proteins such asinsulin, but these tend to form clusters with the matrixmaterial. Limitations of the oaTOF mass spectrometer forsingly charged high-mass ions make it difficult to evaluatethe ionization of larger proteins.
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