Matrix‐free laser desorption/ionization of ions landed on plasma‐treated metal surfaces

We report new experiments in which laser desorption/ionization time‐of‐flight mass spectrometry (LDI‐TOF‐MS) was applied to detection and characterization of gramicidin S and IgG pentapeptide (DSDPR) that were reactively landed on plasma‐treated stainless steel surfaces. The distributions of [M + H] + , [M + Na] + and [M + K] + ion species in LDI‐TOF for gramicidin S and IgG pentapeptide (DSDPR) were found to be markedly different from those in conventional MALDI‐TOF spectra of the same samples. LDI‐TOF mass spectra showed a strong preference for [M + K] + adducts even in the presence of a large excess of sodium cations, or following surface treatment with trifluoroacetic acid. Alkali metal cations (K + and Cs + ) can be exchanged in reactively landed peptide samples to provide the corresponding cationized peptide ions by LDI. Multiple charged trypsin cations were reactively landed into a layer of 2‐(4‐hydroxyphenylazo)benzoic acid and ionized by LDI. The ionization mechanisms for LDI of surface‐deposited peptides are briefly discussed. Copyright © 2008 John Wiley & Sons, Ltd.