Enantioselective gas chromatography with functionalized cyclodextrins as chiral selectors. Fundamentals of the measurement of absolute association constants using capillary columns

- We propose a reliable GC method for determining enantiomeric association constants.
- Capillary GC columns with a g-CD derivative as a chiral selector were constructed.
- Absolute association constants of amino acid derivatives with a g-CD were obtained.
- True enantioselectivity and apparent enantioselectivity are critically discussed.

Chiral capillary GC columns containing different amounts of octakis(6-O-tert-butyldimethylsilyl-2,3-di-O-acetil)-γ-cyclodextrin as chiral selector dissolved in a polymeric matrix were constructed with the aim of determining enantiomeric association constants between a group of well resolved chiral N-trifluoroacetyl amino acid methyl esters and this specific selector at different temperatures. The most relevant sources of uncertainties in the experimental data (hold-up and retention times, and column phase ratios at each temperature) were assessed. These cyclodextrin-based columns are known to enantioseparate a wide variety of chemical compounds, thus, the measurement of the absolute enantioselective constants of a group of solutes with this selector can be useful for systematic studies aimed to a general understanding about how these selectors work. These absolute association constants were estimated from data collected from very simplified experimental systems, and by using the fundamental gas-liquid chromatography equations.