Liquid Atmospheric Pressure Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry Using a Commercial Ion Source and Orbitrap Mass Analyzer.

A liquid atmospheric pressure-matrix-assisted laser desorption/ionization (LAP-MALDI) method has been developed and applied to a commercial AP-MALDI source on a hybrid orbitrap mass analyzer. It is shown that electrospray ionization (ESI)-like mass spectra of a range of peptides and proteins can be acquired by LAP-MALDI mass spectrometry (MS) as previously demonstrated on a homemade LAP-MALDI-Q-TOF setup but without the need of any modification to the commercially available MS equipment used. Multiply charged peptide ions were recorded with a resolution of around 100,000 and a mass accuracy of less than 5 ppm. The higher resolution and mass accuracy of the orbitrap analyzer compared with previously employed Q-TOF instrumentation provided high confidence in bacterial proteoform and species identification by top-down protein analysis.