Multidimensional gas chromatography investigation of concentration and temperature effects of oxime interconversion on ionic liquid and poly(ethylene glycol) stationary phases

Abstract

This study develops analytical approaches using comprehensive heart-cut multidimensional gas chromatography (H/C MDGC) to experimentally investigate effects of concentration and temperature on peak shapes and on kinetic analysis of E/Z isomerisation of oximes which undergo various extents of interconversion on 1D and 2D separation columns. Different stationary phase combinations were investigated. An SLB-IL76 1D column results in good separation of the isomers but with negligible interconversion (1Dsep). Precise targeted heart-cuts (H/C; 3–15 s windows) and cryogenic trapping of selected sections eluting from this 1D column allows well controlled E/Z isomer ratios to be delivered to a 2D poly(ethyleneglycol) (PEG) phase column. This phase catalyses oxime interconversion, with separation; this is termed 2Dinter. This column set describes a 1Dsep × 2Dinter arrangement. The interconversion forward rate constant (k1UE, calculated by using DCXplorer) was found to be independent of the %E isomer (i.e. first order kinetics); k1UE increased at higher temperature (increased Gibbs activation energies, ΔGapp). In addition, a long 1D SLB-IL111 column gave oxime separation with interconversion (1Dinter). Comprehensive multiple H/C of the zone performed on the 2D PEG (2Dinter) at 40, 60 and 80 °C resulted in a 2D presentation (1Dinter × 2Dinter) with a unique rectangular display of interconversion from each H/C pulse with varied isomer concentration ratios. Data for k1UE from 1DGC analysis suggests underestimation of k1UE on 1Dinter SLB-IL111 from the 2D result. The k1UE value on 2Dinter PEG column is more reliably evaluated by summation of all the 2Dinter profiles rather than that obtained from the average value for the individual H/C pulses.