M. Baltz-Knorr, D. R. Ermer, M. R. Papantonakis, R. F. Haglund, Jr.
Department of Physics and Astronomy, Vanderbilt University, Nashville TN 37235
Matrix-Assisted Laser Desorption and Ionization (MALDI) mass spectrometry (MS) using tunable, ultrashort pulses from a mid-infrared (IR) free-electron laser (FEL) has been demonstrated in several contexts. However, the mechanism for this process has remained elusive. Recent velocity-distribution measurements desorbed ions and photoelectrons suggest that the ionization of these macromolecules may proceed in two steps: desorption of matrix and analyte molecules, followed by photoelectron-impact ionization. Moreover, by tuning the FEL across different absorption bands, we can show that the ionization efficiency is not well-correlated with the linear absorption coefficient. New measurements of desorption and ionization thresholds provide additional evidence for the operation of nonlinear mechanisms in MALDI with the IR-FEL. This enhanced mechanistic understanding of MALDI with the IR-FEL facilitates applications in both environmental and biomedical research, ranging from the characterization of mixed high-level wastes to the identification of proteins embedded in solid and liquid matrix materials.
