The collisionally activated decomposition of [M + H]+ ions, generated by fast atom bombardment (FAB) of glutathione conjugates, has been studied by tandem mass spectrometry (MS/MS) using hybrid sector/quadrupole instruments. Abundant fragments of diagnostic utility were present in the daughter ion spectra. Common fragmentation modes were observed but their relative importance was strongly dependent on the nature of the conjugated species. As an example of a general approach to the characterization of glutathione conjugates in biological samples, the acetaminophen-glutathione conjugate was identified in rat bile, following coadministration of (2H0)- and (2H3)acetaminophen, using the experimental sequence: (i) conventional FAB mass spectrometric analysis, (ii) MS/MS using constant neutral loss (129 u) scanning to identify parent ions corresponding to glutathione conjugates, (iii) MS/MS to yield daughter ion spectra of parents so identified and corresponding to (2H0)- and (2H3)-labeled conjugates.

Product-ion spectra produced by high- and low-energy collisionally activated dissociation (CAD) of [M + H]+ ions of a series of peptides (Mr 550-2500) have been compared on four-sector and hybrid tandem mass spectrometers, respectively. The fast atom bombardment product-ion spectra obtained for the smallest peptide analyzed (methionine-enkephalin) were remarkably similar, but substantial differences in fragmentation were observed for the heavier analytes. For peptides with Mr greater than 1000, more complete sequence information was obtained from high-energy CAD on the four-sector instrument. Nevertheless, low-energy CAD on the hybrid mass spectrometer was able to produce partial sequence information even for the largest of the peptides compared. Limits of analysis, defined as the least quantities of analyte for which product-ion spectra of essentially uncompromised quality could be obtained, were similar (ca. 15 pmol) for small peptides, but lower limits were achieved for larger peptides (Mr greater than 1000) with the four-sector instrument. High-energy CAD spectra were found to be highly reproducible, with qualitatively similar spectra obtained over a wide range of operating conditions. In contrast, it was necessary to carefully control collision gas pressures and collision energies in order to obtain good reproducible data for low-energy CAD. Experimental procedures for obtaining reproducible spectra with good sensitivity for peptides on the hybrid instrument are presented.

Analyses are described of small peptides and related compounds using a tandem hybrid mass spectrometer of BEQQ geometry. Collisionally activated decomposition of [M + H]+ ions, generated by fast atom bombardment, was performed in the radio frequency (rf)-only quadrupole. Interpretation of fragmentation was greatly facilitated by analysis of labeled analogs, obtained by 18O exchange of carboxyl oxygens. N-Acetylation was also valuable although significant changes in fragmentation resulted from derivatization. Daughter ion spectra of [M + H]+ ions of angiotensin III showed diagnostic fragmentations throughout the peptide chain.

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Scheme 5 group and ring oxidation reactions, PAH with methyl groups substituted at a site of oxidation show ions resulting from methyl group displacement reactions. These latter ions appear at [M + 16]-' or [M + 21-' in the spectra of methyl-anthracenes and benz[a]anthracenes. Isomeric benz[aJanthracenes with methyl substituents in the 1or 12-positions show ions that result from oxidative methyl-groupfring interactions. These ions can be used, in conjunction with methyl group displacement reactions, to differentiate 1-, 7-, and 12methylbenz[a]anthracene. Further studies are being REFERENCES