Reproducibility and extent of fragmentation of laser-desorbed ions : Relation to mechanism of desorption

The reproducibility of decomposition patterns of laser desorbed propyltriphenyl-phosphonium ions and (M + Na)⁺ ions of sucrose is evaluated. These compounds were chosen because they desorb by two different mechanisms commonly ascribed to “thermal” desorption. The phosphonium ions desorb directly whereas sucrose requires cationization with an alkali metal ion. Samples were either pipetted or electrosprayed onto both “smooth” and “rough” copper probe tips. The precision of two ratios of ion abundances are slightly worse for laser desorption of the phosphonium ions than for ions produced by electron ionization of toluene and 2,5-dibromotoluene. Little improvement was obtained by turning to electrospray as a means of depositing the sample. Worse precision was found for ion abundance ratios obtained from the laser desorption of (M + Na)⁺ of sucrose which hadbeen pipetted onto the probe. The precision was improved by using the electrospray deposition method. Fragmentation of the phosphonium ions was greater if desorption was from a “rough” probe tip whereas less decomposition of the (M + Na)⁺ of sucrose occurred with desorption from a “rough” tip. The trends in precision and extent of fragmentation are rationalized in terms of mechanisms of ion desorption. Chemical ionization of laser-desorbed neutral species was found to be impossible under the experimental conditions used.