LIF imaging and gas-phase diagnostics of laser desorbed MALDI-matrix plumes

Abstract

The first gated laser-induced fluorescence (LIF) imaging and absorption spectroscopy have been performed on laser desorbed plumes from organic crystals which are commonly used as MALDI (matrix assisted laser desorption ionization) matrices. These plasma diagnostic techniques, as well as ion probe measurements, were employed to investigate the desorbed products, densities, fractional ionization, and velocity distributions of the plume of ejecta which is typically encountered as the main desorption product in the mass spectrometry analysis of large biomolecules. Pulsed-ultraviolet (193 nm and 248 nm) desorption from 3-hydroxypicolinic acid (3-HPA) crystals was studied. Gas-phase absorption spectra of 3-HPA measured here revealed very different absorption cross-sections at these two wavelengths, and this was exploited to investigate the effects of plume absorption of the desorbing laser pulse. In both cases, LIF imaging revealed two plume components: a fast (maximum∼0.1 cm/ μs) low-intensity component which appear to be 3-HPA fragments, and a slower component of 3-HPA expanding at 0.05 cm/ μs. In the case of ArF-laser irradiation, optical absorption spectroscopy indicated a breaking of the intramolecular hydrogen bond in the gas-phase matrix material.