Temperature-dependent changes in gas chromatographic separation metrics for trihexyl(tetradecyl)phosphonium-based...

To assess whether the trihexyl(tetradecyl)phosphonium chloride ([P₆₆₆₁₄⁺][Cl^-]) ionic liquid (IL), employed as a gas chromatographic stationary phase, exhibits temperature-dependent phase transitions or structural heterogeneity, retention thermodynamics for probe molecules of varied chemical structure were investigated by gas chromatography (GC) over the temperature range of 25 to 64 °C using van't Hoff analysis. All van't Hoff plots for n-alkanes (C8-C12) and other probe analytes were linear, indicating the absence of detectable phase-transition behavior. The [P₆₆₆₁₄⁺][Cl^-] IL was further compared with the [P₆₆₆₁₄⁺] bis[(trifluoromethyl)sulfonyl]imide ([P₆₆₆₁₄⁺][NTf₂^-]) IL and commercial polysiloxane stationary phases in terms of temperature-dependent resolution behavior and chromatographic selectivity over the temperature range of 28-52 °C. The resolution factors of analytes in mixtures obtained using gas chromatography-mass spectrometry (GC-MS) and the summation of resolution values decreased smoothly with temperature on both IL stationary phases and on the dimethylphenylcyano-substituted polymeric stationary phase (OV-1701), whereas the poly(50% diphenyl/50% dimethyl siloxane) stationary phase (SPB-50) exhibited irregularities in its temperature-dependent summation of resolution profile, showing a distinct maximum in the temperature range of 34-40 °C. Selectivity values for the IL stationary phases were largely determined by the anion, with the [P₆₆₆₁₄⁺][Cl^-] IL providing enhanced separation between polar and aromatic analytes, while the [P₆₆₆₁₄⁺][NTf₂^-] IL favored dispersive and π-π interactions that resulted in a reversal of elution order for some aliphatic-aromatic and alkyne-containing analyte pairs. Capillary columns prepared using different surface activation methods and coated with identical [P₆₆₆₁₄⁺][Cl^-] IL stationary phase showed indistinguishable temperature-dependent trends in summation of resolution value and selectivity. An evaluation of analyte-specific selectivity provides insight into the characteristic separation behavior of the stationary phases and enables identification of analyte classes that are more efficiently separated on the [P₆₆₆₁₄⁺]-based IL stationary phases compared to conventional stationary phase materials.