Tandem mass spectrometry of polychlorodibenzo-p-dioxin and polychlorodibenzofuran in a quadrupole ion trap .1. Comparison of single-frequency, secular frequency modulation, and multifrequency resonant excitation modes

An investigation has been carried out on three modes of resonant excitation in the development of an analytical protocol for the determination of polychlorodibenzo-p-dioxins (PCDDs) and polychlorodibenzofurans (PCDFs) by ion trap tandem mass spectrometry. The resonant excitation modes investigated are single-frequency irradiation (SFI), secular frequency modulation (SFM), and multifrequency irradiation (MFI) of isolated molecular ion species. Each excitation mode was examined for fragmentation channel selectivity so as to optimize instrument sensitivity, Collision-activated dissociation mass spectra obtained with each excitation mode are compared with those obtained by triple-stage quadrupole mass spectrometry (TSQMS). While the same reaction channels (corresponding to losses of Cl-., COCl., and 2COCl(.) for PCDDs and Cl-., COCl., and COCl2 and COCl3. for PCDFs) were observed for each excitation mode, the fragment ion relative abundances differed among the resonant excitation modes and showed that internal energy deposition in the precursor ion was in the order MFI > SFI > SFM, In each resonant excitation mode, the observed conversion efficiency for loss of COCl. exceeded that observed with TSQMS. The procedure of tuning SFI to ion secular frequencies was laborious, while the required duration of irradiation with SFM was excessively long compared to the gas chromatographic time scale, The tuning requirements of MFI using 1 and 2 kHz bandwidth pulses were less rigorous than those for SFI, and the duration of irradiation was compatible with the gas chromatographic time scale.