Ionic liquids as stationary phases for gas chromatography—Unusual selectivity of ionic liquids with a phosphonium cation and different anions in the flavor, fragrance and essential oil analyses

Room-temperature ionic liquids (ILs) have been shown to be successful as stationary phases (SPs) for gas chromatography in several fields of applications because of their unique and tunable selectivity, low vapor pressure and volatility, high thermal stability (over 300°C), and good chromatographic properties. This study has been focused on two ILs based on a phosphonium cation (trihexyl(tetradecyl)phosphonium, P66614) combined with different anions, previously shown to be suitable as gas chromatography (GC) SPs. In particular, trihexyl(tetradecyl)phosphonium bis[(trifluoromethyl)sulfonyl]imide ([P66614+] [NTf2-]) and trihexyl(tetradecyl)phosphonium chloride ([P66614+] [Cl-]) were investigated, as the Abraham linear solvation energy relationship has shown their ability to interact with the solute(s) when tested with a set of 26 to 34 probe analytes. The chromatographic performance were investigated on narrow bore and conventional test columns using the following: i) Grob test, ii) a group of model mixtures of compounds characteristic of the flavor, fragrance and essential oil fields (FFMix), iii) a standard mixture of 29 volatile allergens (AlMix), and iv) two essential oils of different complexity (sage and vetiver essential oils). The columns coated with the investigated IL SPs were characterized by similar polarity (Polarity Number (PN): 37 for [P66614+] [Cl-] and 33 for [P66614+] [NTf2-]), high efficiency and highly satisfactory inertness. The two IL SPs also exhibited a completely different separation performance, with [P66614+] [Cl-] test columns mainly characterized by high retention and selectivity based on the analyte functional groups, and [P66614+] [NTf2-] test columns featured by short retention and selectivity mainly related to the analyte volatility and polarity. These results were also confirmed with the analysis of sage and vetiver essential oils.