Fundamentals of matrix-assisted laser desorption/ionization mass spectrometry with pulsed infrared lasers

The fundamental work and current knowledge on the mechanisms of matrix-assisted laser desorption ionization with pulsed infrared lasers is reviewed and discussed, with a special emphasis on the work performed in the laboratory of the authors in the recent past. The role of the different relevant irradiation parameters (laser wavelength, laser pulse duration, irradiated area, and laser fluence) and matrix properties (absorption, state of aggregation) in the MALDI process is summarized as well as results on the dynamic parameters of the expanding desorption plume. New results include an extension of former wavelength studies to the 10 mum wavelength range. Furthermore, a detailed analysis of matrix ion signals generated at infrared laser wavelengths around 3 and 10 mum is presented and compared to those generated by UV-MALDI, revealing a complex picture of the hitherto hardly understood IR-MALDI ionization mechanisms. Preliminary results from a photoacoustic study of the desorption process point to a prolonged material evaporation in case of liquid matrices and pulsed Er:YAG laser excitation. The combined description of the various fundamental work promotes the understanding of the IR-MALDI desorption and ionization processes and allows an assessment of possible solutions for current mass spectrometric limitations of the IR-MALDI method. (C) 2002 Elsevier Science B.V. All rights reserved.