The effects of laser fluence on ion formation in MALDI was studied using a tandem TOF mass spectrometer using a Nd-YAG laser and alpha-cyano hydrocinnamic acid matrix. Leucine enkephalin ionization and fragmentation were followed over laser fluence ranging from the threshold of ion formation to the maximum available, i.e., approximately 280 - 930 mJ/mm2. The most notable finding was the appearance of immonium ions at fluence values close to threshold, increasing rapidly and then tapering in intensity with the appearance of typical backbone fragment ions. The data suggest the presence of two distinct environments for ion formation. One is associated with molecular desorption at low values of laser fluence that leads to extensive immonium ion formation. The second becomes dominant at higher fluences, is associated initially with backbone type fragments, but progresses at the highest values of fluence to immonium fragments. This second environment is suggestive of ion desorption from large pieces of material ablated from the surface. Arrhenius rate law considerations were used to estimate temperatures associated with the onset of each type of process.
Citation: Journal of the American Society for Mass Spectrometry
Pub Type: Journals
ions, kinetics, MALDI, mass spectra, peptides, thermal decomposition