INFLUENCE OF TARGET PREPARATION AND LASER IRRADIANCE IN MATRIX-ASSISTED UV LASER-DESORPTION OF PROTEINS

Results concerning the influence of laser irradiance and matrix:protei n molar ratio on the matrix-assisted UV (337 nm) laser desorption of m elittin, bovine insulin and alpha-lactalbumin are presented. Reproduci ble time-of-flight mass spectra of the proteins have been obtained wit h sinapinic acid which gives regular microcrystals well distributed ov er the target as observed by reflection optical microscopy. In additio n to the protonated protein peak, a neutral peak shifted towards high masses and exhibiting a high mass tail, has been generally observed. T he intensity of this peak which participates in the broadening of the molecular ion peak strongly increases with the laser irradiance but is independent of the matrix:protein molar ratio. A linear increase with the laser irradiance of the FWHM of the protonated protein peak, afte r correction of the neutral contribution, has been also observed. This neutral peak, whose relative intensity increases with the protein mol ecular mass, could originate from metastable or collision induced deco mposition of cationized proteins formed in the gas phase or resulting from a delayed emission.