LIF IMAGING AND GAS-PHASE DIAGNOSTICS OF LASER-DESORBED MALDI-MATRIX PLUMES

The first gated laser-induced fluorescence (LIF) imaging and absorptio n spectroscopy have been performed on laser desorbed plumes from organ ic crystals which are commonly used as MALDI (matrix assisted laser de sorption ionization) matrices. These plasma diagnostic techniques, as well as ion probe measurements, were employed to investigate the desor bed products, densities, fractional ionization, and velocity distribut ions of the plume of ejecta which is typically encountered as the main desorption product in the mass spectrometry analysis of large biomole cules. Pulsed-ultraviolet (193 nm and 248 nm) desorption from 3-hydrox ypicolinic acid (3-HPA) crystals was studied. Gas-phase absorption spe ctra of 3-HPA measured here revealed very different absorption cross-s ections at these two wavelengths, and this was exploited to investigat e the effects of plume absorption of the desorbing laser pulse. In bot h cases, LIF imaging revealed two plume components: a fast (maximum si milar to 0.1 cm/mu s) low-intensity component which appear to be 3-HPA fragments, and a slower component of 3-HPA expanding at 0.05 cm/mu s. In the case of ArF-laser irradiation, optical absorption spectroscopy indicated a breaking of the intramolecular hydrogen bond in the gas-p hase matrix material. (C) 1998 Elsevier Science B.V.