End Column Reverse Chromatography as a Novel Approach for Enhanced Separation: A Pilot Study.

Background: Currently, the most popular technique in gas chromatography (GC) is "temperature programming," where the temperature increases from the start of the injection. This leads to faster elution of analytes compared to isothermal methods. However, isothermal methods are considered optimal for separating compounds with similar retention times. Another interesting technique that provides higher resolution is dynamic thermal gradient gas chromatography (TGGC), where separations are achieved as a decreasing thermal gradient. This gradually decreases the positive gas velocity. Nevertheless, it was proven that GC techniques with negative velocity gradients do not improve the resolution of compounds with nearly identical retention times.

Objective: Optimizing a new GC approach to combine both the short time from positive temperature ramps programming, and the enhanced separation of the negative ramps of the TGGC, a model under the name of "end column reverse chromatography" (ECRC).

Methods: The process simply consists of two steps: the first is a normal positive ramp from the start of the injection, and the second step is a negative thermal ramp at a time that is around the retention time of the first eluting peak. This will decrease the solute velocity almost solely for the second compound, leading to relatively enhanced separation.

Results: The optimized ECRC method increased the resolution of two isomers (trans- and cis-chlordane) from 1 (slightly overlapping) in the case of temperature programming to 2.78 as shown in this study. This comes at the expense of the width and intensity of the peaks, where the intensity decreased about 17 and 12% for cis- and trans-chlordane, and the peak width increased with 37 and 77% for the same compounds, respectively.

Conclusions: ECRC is a novel model for enhanced separation that comes with some drawbacks.

Highlights: It can be an alternative approach to get a fast GC method with enhanced separation for isomers.