Time-resolved cryogenic modulation reveals isomer interconversion profilesin dynamic chromatography

The dynamic chromatographic study of interconversion of E and Z forms of ox imes has been investigated by using a novel cryogenic modulation method in a two-dimensional gas chromatographic array. The primary column is a conven tional capillary GC column on which the molecular interconversion proceeds. In this case, the molecular dynamical process leads to a peak profile desc ribing the kinetics and thermodynamics of the interconverting molecules dur ing its chromatographic elution. Thus an interconversion region intercedes the elution of the individual stereoisomers of the reaction. Since the mole cules are isomers, classical molecular identification methods such as gas c hromatography-mass spectrometry are unable to study the individual instanta neous amounts of each of the compounds. Hence the infinitesimal profiles of interconversion along the entire column have never been experimentally obs erved; rather the total profile is normally subjected to mathematical model ling studies in order to match experiment with theory, and to gain the kine tic parameters of the process. In the present study, an instantaneous ratio of the individual isomers can be found during the chromatographic elution by direct measurement. This is achieved by using a cryogenic zone focussing process, with rapid longitudinal modulation of a cold trap and continual p ulsing of collected zones into a fast-analysis high-resolution capillary co lumn on which isomer interconversion is minimized. The data can be displaye d as a two-dimensional contour plot to demonstrate the individual isomer pr ofiles. The two-dimensional analysis also allows easy measurement of the pe ak ratios of the two isomers which is an indicator of the extent of interco nversion that has taken place. Two model systems, acetaldoxime and butyrald oxime, were chosen to illustrate the use of the cryogenic modulation proced ure. It is anticipated that the procedure could be applied to other molecul es which exhibit gas-phase isomerizations or reactions. (C) 2001 Elsevier S cience B.V. All rights reserved.