STEREOCHEMICAL ANALYSIS OF MONODEUTERATED ISOMERS BY GC MS/MS/

GC/MS/MS techniques to quantitate mixtures of acyclic stereoisomers ar e explored for a set of secondary n-alkyl phenyl ethers vicinally subs tituted with deuterium. :Examination of a set of test compounds using a commercial quadrupole ion trap instrument (GC/QIT) reveals large dif ferences between three and erythro diastereomers when trapped molecula r ions expel neutral alkene as a result of collisionally activated dis sociation (CAD). All of the monodeuterated compounds give PhOH.+.(m/z 94) and PhOD.+ (mit 95) as the principal fragments under GC/QIT/CAD co nditions, The three 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 r(t)/r(e) as high as 3. A nonlinear calibration curve shows that the three decomposes to a lesser extent than the ery thro in mixtures of the two stereoisomers; hence, the precision of qua ntitation is sensitive to experimental error. Sustained off-resonance irradiation experiments in an FT-ICR give virtually complete dissociat ion, which results in a linear calibration curve. Ion-molecule reactio ns of the fragment ions indicate that alkene expulsion does not take p lace via a six-member cyclic transition state, while the stereochemica l selectivity appears to rule out the intermediacy of ion-neutral comp lexes. Threo- and erythro-2-phenoxy-3-deuterio-n-octane (and their mix tures) are proposed as test compounds for evaluating the capability of GC/MS/MS instrumentation to perform stereochemical analyses on subnan omole samples of isotopically labeled molecules.