Equilibrium conditions in the MALDI plume: Thermodynamic evaluation of in-plume proton transfer reactions

Matrix-Assisted Laser Desorption Ionization Mass Spectrometry (MALDI-MS) has rapidly developed as a powerful tool for the mass analysis of large molecules. However, a comprehensive picture of the MALDI ion formation process remains elusive despite a variety of models having been proposed in recent years.;This project has investigated the equilibrium nature of the MALDI plume by studying the proton transfer reactions between MALDI matrices and amino acid/dipeptide analytes. The mass spectra obtained were evaluated with regard to the thermochemical properties of the matrix proton donor and the amino acid or dipeptide proton acceptor constituent species. The studies performed show that a plot of the natural logarithm of the proton acceptor to proton donor ion signal ratio versus the basicity of the proton acceptor is linear. This result demonstrates that there is a systematic trend in the proton acceptor to proton donor ratio as a function of the thermodynamic properties of the donor and acceptor. This trend supports the existence of a thermodynamic equilibrium in the laser desorption plume. Additional analysis of the experimental trends the experimental trends allows the determination of the gas-phase basicities of the matrix proton donor species and the effective temperature of the proton transfer reaction.