DIRECT COMPARISON OF INFRARED AND ULTRAVIOLET WAVELENGTH MATRIX-ASSISTED-LASER-DESORPTION IONIZATION MASS-SPECTROMETRY OF PROTEINS/

In an effort to gain an understanding of the processes governing matri x-assisted laser desorption/ionization (MALDI), we made a direct compa rison of ultraviolet (UV)- and infrared (IR)-MALDI linear time-of-flig ht mass spectra of proteins obtained from the same samples and matrice s (on the same sample surface), using two different lasers, each havin g short duration (<10-ns) pulses, i.e., a tunable wavelength Nd:yttriu m aluminum garnet (YAG) pumped optical parametric oscillator laser ope rating at 2.94 mu m and a Nd:YAG laser operating at 355 nm. We observe d that (1) the IR-MALDI and UV-MALDI spectra of a given protein from t he same matrix were strikingly similar; (2) protein ions produced by I R-MALDI experienced less fragmentation than those produced by UV-MALDI ; and (3) photochemical adducts produced during UV-MALDI were absent i n IR-MALDI. These results lead us to speculate on the mechanisms for t he ionization process in UV- and IR-MALDI. Because photons with a wave length of similar to 3 mu m are unlikely to effect electronic excitati on of the matrix at the irradiance used for MALDI, we propose that ion ization in IR-MALDI occurs as a natural consequence of the solid-to-ga s phase transition induced by the IR irradiation, and involves proton transfer reactions in the intermediate phase between solid and gas. Th e strikingly similar UV- and IR-MALDI mass spectra leads us to the add itional proposal that ionization in UV-MALDI may also be a natural con sequence of the phase transition and that electronic excitation may no t play a primary role in the ionization process. (C) 1998 American Soc iety for Mass Spectrometry.