Stereochemical Analysis of Monodeuterated Isomers By GC/MS/MS

GC/MS/MS techniques to quantitate mixtures of acyclic stereoisomers are explored for a set of secondary n-alkyl phenyl ethers vicinally substituted with deuterium. Examination of a set of test compounds using a commercial quadrupole ion trap instrument (GC/QIT) reveals large differences between threo and erythro diastereomers when trapped molecular ions expel neutral alkene as a result of collisionally activated dissociation (CAD). All of the monodeuterated compounds give PhOH•+ (m/z 94) and PhOD•+ (m/z 95) as the principal fragments under GC/QIT/CAD conditions. The threo isomers give consistently larger values of the ion intensity ratio r ) m/z 95/m/z 94 than do the corresponding erythro, with a ratio of ratios rt/re as high as 3. A nonlinear calibration curve shows that the threo decomposes to a lesser extent than the erythro in mixtures of the two stereoisomers; hence, the precision of quantitation is sensitive to experimental error. Sustained off-resonance irradiation experiments in an FT-ICR give virtually complete dissociation, which results in a linear calibration curve. Ion-molecule reactions of the fragment ions indicate that alkene expulsion does not take place via a six-member cyclic transition state, while the stereochemical selectivity appears to rule out the intermediacy of ion-neutral complexes. Threoand erythro-2-phenoxy-3-deuterio-n-octane (and their mixtures) are proposed as test compounds for evaluating the capability of GC/MS/MS instrumentation to perform stereochemical analyses on subnanomole samples of isotopically labeled molecules.