FORMATION OF GAS-PHASE PEPTIDE IONS AND THEIR DISSOCIATION IN MALDI: INSIGHTS FROM KINETIC AND ION YIELD STUDIES

Insights on mechanisms for the generation of gas-phase peptide ions and their dissociation in matrix-assisted laser desorption ionization (MALDI) gained from the kinetic and ion yield studies are presented. Even though the time-resolved photodissociation technique was initially used to determine the dissociation kinetics of peptide ions and their effective temperature, it was replaced by a simpler method utilizing dissociation yields from in-source decay (ISD) and post-source decay (PSD). The ion yields for a matrix and a peptide were measured by repeatedly irradiating a region on a sample and collecting ion signals until the sample in the region was completely depleted. Matrix- and peptide-derived gas-phase cations were found to be generated by pre-formed ion emission or by ion-pair emission followed by anion loss, but not by laser-induced ionization. The total number of ions, that is, matrix plus peptide, was found to be equal to the number of ions emitted from a pure matrix. A matrix plume was found to cool as it expanded, from around 800-1,000K to 400-500K. Dissociation of peptide ions along b/y channels was found to occur statistically, that is, following RRKM behavior. Small critical energy (E-0=0.6-0.7eV) and highly negative critical entropy (S-double dagger=-30 to -25eu) suggested that the transition structure was stabilized by multiple intramolecular interactions. (c) 2014 Wiley Periodicals, Inc. Mass Spec Rev 34: 94-115, 2015.