SIMULTANEOUS DETECTION OF IONS AND NEUTRALS PRODUCED BY MATRIX-ASSISTED LASER-DESORPTION

Ionic and neutral products of matrix-assisted laser desorption were simultaneously detected in a time-of-flight mass spectrometer. The neutrals were photoionized with coherent vacuum ultraviolet radiation. Mass spectra were obtained for primary and secondary alkyl amines in various absorbing matrices. Based upon relative signal intensities of directly formed analyte ions and photoionized analyte neutrals, the ratio of neutrals-to-ions ejected was found to be of the order of 10 000 or greater with desorption irradiances near the threshold for ion formation. The ratio decreased with increasing irradiance of the desorption laser beam. This dependence is consistent with a collisional mechanism for analyte-ion formation during the desorption event that is initiated by photoionization of the matrix. Matrices which gave large signal intensities of directly formed analyte ions also gave large signal intensities of photoionized analyte neutrals. Conversely, matrices which gave small signal intensities of directly formed analyte ions also gave small signal intensities of photoionized analyte neutrals. These results suggest that the efficiency of analyte vaporization by matrix-assisted laser desorption is an intrinsic property of the matrix-analyte pair, independent of the ion-formation process.