Characterization of seven antihistamines, theirN-oxides and related metabolites by fast atom bombardment mass spectrometry and fast atom bombardment tandem mass spectrometry

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Abstract
We have examined the synthetic N‐oxides of five ethylenediamine‐type antihistamines using fast atom bombardment (FAB) mass spectrometry and FAB tandem mass spectrometry (MS/MS). Fragmentation of the protonated molecule in the normal and collisionally activated spectra appeared to be characteristic for this class of antihistamine N‐oxide. Spectra were also acquired from an ethanolamine and a propylamine antihistamine N‐oxide for comparison. These results were very similar to those obtained from biologically produced antihistamine N‐oxides, as well as isomeric metabolites, which were readily distinguished from the N‐oxides by characteristic fragmentation. In addition, a prominent ion 16 daltons lower in mass, which has been attributed to loss of elemental oxygen from the protonated N‐oxide in chemical ionization mass spectral studies, was shown to be a matrix‐dependent product of the solution‐phase reduction of the antihistamine N‐oxide to the parent antihistamine during FAB ionization. These results demonstrate that with a non‐reducing matrix such as glycerol, FAB mass spectrometry and FAB MS/MS are excellent methods for the characterization of the non‐conjugated antihistamine metabolites such as the N‐oxides.