SECONDARY-ION GENERATION FROM LARGE KEV MOLECULAR PRIMARY IONS INCIDENT ON A STAINLESS-STEEL DYNODE

Positive and negative secondary ions from large (up to 40 kDa) peptide and protein primary ions incident at 5-40 keV collisional energies on a stainless-steel (mesh) conversion dynode have been investigated in a tandem (reflectron) time-of-flight arrangement. Negative secondary-ion spectra consisted mainly of H- and C2H-, C2H2- ions; positive secondary-ion spectra were more complex and contained prominent signals due to H+, Na+ and K+, and further signals at m/z 28, 41, 43, 45, 73 up to m/z 326. The secondary mass spectra were qualitatively very similar amongst the various peptide primaries and not different from those of other organic projectiles such as, for example, coronene or cyclodextrin. With increasing mass of the primary ion, ratios of the secondary-ion abundances changed drastically in the negative secondary-ion spectra (increase of higher m/z signals, decrease of H-). Since most of the positive secondary ions could be attributed to a silicon-organic surface contamination of the dynode (vacuum grease), it is concluded that the formation of such secondary ions is due to sputtering rather than surface-induced dissociation.