Comparative study of photodissociation and surface-induced dissociation by laser desorption Fourier transform mass spectrometry.

Photodissociation (PD) and surface-induced dissociation (SID) are compared for structural analysis of several nonvolatile compounds analyzed by laser desorption Fourier transform mass spectrometry (LD/FTMS). SID and PD of a porphyrin and two metalloporphyrins were investigated using a variety of experimental conditions. Optimum structural information is obtained from PD when parent ions are irradiated for relatively long times (10-30 s) using 575-nm radiation and short times (0.5-1 s) using 308- or 388-nm radiation. Shorter irradiation times in the visible region resulted in less efficient production of structurally significant product ions, while longer times in the ultraviolet region produced more nonspecific fragment ions, apparently at the expense of more structurally significant fragment ions. SID conversion efficiencies for the porphyrins are estimated for collision energies from 25 to 360 eV, with maximum conversion efficiency found using 62- and 115-eV collision energies for the two porphyrins studied. Results from a concurrent study on the combined use of PD and SID for MS/MS/MS are discussed in the context of these results. The MS3 ion spectra generated by the two dissociation techniques differ more significantly than MS2 product ion spectra. These data suggest some general guidelines for MSn studies of nonvolatile compounds analyzed by LD/FTMS, employing PD and SID for ion activation.