Thermodynamic control of final ion distributions in MALDI: in-plume proton transfer reactions

Gas-phase thermochemical data on matrix-assisted laser desorption/ionization (MALDI) matrix species were used to calculate the energetics of possible proton transfer reactions. These were investigated for the MALDI matrices 2,4,6-trihydroxy-acetophenone and sinapic acid, with the tripeptide glycyl-glycyl-histidine as analyte. MALDI proton transfer product ion distributions were found to be predicted by the energetics of possible secondary ion-molecule reactions, at all laser fluences sufficient to generate a dense plume. Near the ion generation fluence threshold, the mass spectra deviate from the thermodynamic predictions. This shows that the MALDI plume exhibits both thermodynamically and kinetically controlled regimes, depending on desorption conditions. Possible thermal ground state proton transfer primary ionization pathways were also considered, and found to be inconsistent with the data. (C) 2002 Elsevier Science B.V. All rights reserved.