Investigation on Production of Cluster Ions Using Surface-Assisted Laser Desorption/Ionization Mass Spectrometry

Surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS) has become an indispensable tool in analysis of macromolecule and small molecule in solid and liquid samples. However, there were few studies focusing on the ionization mechanisms especially for the small molecules. In this work, the compounds of pyrene, coronene and rubrene with similar molecular structures were used to investigate the ionization mechanism via SALDI-MS. Additionally, the effect of laser power on product ions was also investigated. It was found that strong cluster ions peaks nM(+) (M = pyrene, coronene) were observed for pyrene and coronene, and daughter ions of coronene by loss of C2H2 were also observed. On the contrary, no cluster ion was obtained for rubrene, only daughter ions with the losses of nC(6)H(5) (n = 1, 2, 3) were acquired. Finally, the ionization mechanism was discussed. The formation of clusters of pyrene and coronene was attributed to the interaction of pi-pi bonds. For rubrene, the spatial barrier weakened the interaction of pi-pi bonds because the four phenyl groups were not on the same plane of skeleton structure, thus impeding the formation of cluster ions.