Switchable enantioselective three- and four-dimensional dynamic gas chromatography−mass spectrometry: example study of on-column molecular interconversion

A novel hybrid online enantioselective four-dimensionaldynamic GC (e4D-DGC) approach to study reversible molecular interconversion through specific isolation of a diastereo and enantiopure oxime, 2-phenylpropanaldehyde oxime, from prior multidimensional separation, is described. It incorporates a pre-enantioseparation step that applies comprehensive two-dimensional GC (GC × GC), prior to multiple microfluidic (Deans) switching for selection of components of a diastereomeric (E,Z) and enantiomeric (R,S) oxime into a third reactor column where isomerization occurs. This is followed by E/Z separation in a fourth analytical column. The enantioselective first dimension (¹D_enant)yields enantioseparation of E(R), Z(R), E(S), and Z(S) isomers,with a characteristic interconversion zone between the E and Z isomers. However, these are contaminated with underlying stereoisomers. Selected separation regions were then modulated and separated using a second dimension (²D) column via GC × GC, resolving the interfering stereoisomers. Individual pure enantiomers were then selectively heart-cut from within the 2D separation space, cryofocused, then eluted on a 3D reactor column for E ⇌ Z isomerization under controlled oven temperature and flow. Heart-cuts taken over the resulting interconversion distribution were cryotrapped at the inlet of a ⁴D column, on which achiral separation allows precise quantification of each E and Z isomer of the enantiomer. From peak areas and isomerization time, the forward and backward rate constants (k_E→Z and k_Z→E) were determined. The described methodology is suited to other configurationally labile molecules (for instance, hydrazones and imines), which exhibit isomerization, and can be used to isolate individual compounds from multicomponent samples, without requiring pure compound synthesis, or complex mathematical models or in-silico simulations.