The detection of large molecules in matrix‐assisted UV‐laser desorption

Mechanisms related to high-mass ion detection were studied using matrix-assisted UV laser desorption for ion generation and a tandem (MS/MS) time-of-flight mass spectrometer for the analysis of secondary species formed at the conversion dynodes. Secondary-electron emission was found to be less pronounced with increasing molecular weight. Instead small secondary ions generated by collision of primary ions with the conversion dynodes appeared to be the dominant process for the detection of ions of masses up to several hundred thousand Da. The use of microchannel plate detectors has been shown to become less effective for high molecular weight applications. Mass spectra of secondary ions emitted from a conversion dynode bombarded with keV primary ions have been recorded. Regardless of the composition of the primary molecules, the secondary mass spectra were similar. Sputtering of surface adsorbates is assumed to be the major mechanism of secondary ion formation, rather than surface induced dissociation or fragmentation of the primary ions.