BIOMEDICAL AND ENVIRONMENTAL MASS SPECTROMETRY, VOL. zyxwv 18, 157-167 (1989) Characterization of Seven Antihistamines, their N-Oxides and Related Metabolites by Fast Atom Bombardment Mass Spectrometry and Fast Atom Bombardment Tandem Mass Spectrometry J. 0. Lay zyxwvutsrqp Jr,? C. L. Holder and W. M. Cooper Division of Biochemical Toxicology, National Center for Toxicological Research, Jefferson, Arkansas 72079, USA We have examined the synthetic N-oxides of five ethylenediamine-type antihistamines using fast atom bombard- ment (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 anti- histamine 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 fragmenta- tion. 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 zyxw be a matrixdependent 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 nonKonjugated antihistamine metabolites such as the N-oxides. INTRODUCTION Most antihistamines can be grouped into three structur- ally similar classes, namely ethylenediamine derivatives, ethanolamine derivatives and propylamine derivatives.' The ethylenediamine-type antihistamines are the most frequently used, in both prescription and non- prescription drugs used for allergies, sleep-aids, and cold remedies.' Unfortunately, two ethylenediamine antihistamines have been shown recently to have unde- sirable properties. Methapyrilene appears to be a sig- nificant rat hepato~arcinogen,'.~ while pyrilamine maleate induces unscheduled DNA synthesis."6 For this reason, several mass spectral methods have been applied to the analysis of antihistamine metabolite^.^^' Desorption chemical ionization (DCI) has been shown to be a powerful method for the analysis of relatively volatile, non-conjugated metabolites of several anti- histamine~.~~~' The more polar, conjugated metabolites of doxylamine have been studied using fast atom bom- bardment (FAB) mass spectrometry." Properly deriva- tized metabolites of doxylamine have also been studied using electron impact (EI) mass spectrometry.' In this paper we report the analysis of five ethyl- enediamine antihistamines and their corresponding N- oxides by FAB mass spectrometry and by FAB tandem mass spectrometry (MS/MS) using collisional activation (CA) of mass-selected [MH] zyxwvutsrq + ions. Neutral losses and fragment-ion masses which characterize these com- t Author to whom correspondence should be addressed. 0887-6134/89/030157-11 $05.50 zyxwvutsr KJ 1989 by John Wiley & Sons, Ltd. pounds are detailed. These results are compared with the fragmentation observed from ethanolamine and propylamine antihistamines. We also report the use of CA and FAB MS/MS to distinguish between isomeric metabolites of antihistamines having the same elemental composition and for the identification of metabolite components in mixtures. EXPERIMENTAL Mass spectrometry FAB mass spectra were acquired with a Kratos MS-50 high-resolution mass spectrometer equipped with an M-SCAN fast atom gun. FAB ionization employed 10 keV xenon atoms impinging on a copper target. The spectra were recorded in 30 s scans downfield. The accelerating voltage was 8 keV. The FAB MS/MS data were acquired using an MS-50 triple-analyzer instrument (described previously in Ref. 13) equipped with an Ion Tech fast atom gun. Measurements were obtained by mass selecting the [MH] ion for CA measurements using mass-analyzed ion kinetic energy (MIKE) spectral scans. The collision gas was helium, and the gas pressure was set to reduce the abundance of the [MH]' ion by 50%. The acceler- ating voltage was set to 8 keV. Samples were dissolved in methanol (1-5 pg p1-I) and about 1 pl was applied to the probe tip along with about 1-2 p1 of glycerol or thioglycerol. zyx Received 6 April 1988 Accepted zyxw 22 August I988