Immobilization of phosphonium-based ionic liquid stationary phases extends their operative range to routine applications in the flavor, fragrance and natural product fields

Abstract

Phosphonium-based ionic liquids (ILs) have proven to be successful stationary phases (SPs) for gas chromatography (GC) in several fields of application because of their unique selectivity and good chromatographic properties. This study focuses on the use of two ILs as GC SPs that are based on the phosphonium derivatives trihexyl(tetradecyl)phosphonium chloride ([P₆₆₆₁₄⁺] [Cl^-]), and trihexyl(tetradecyl)phosphonium bis[(trifluoromethyl)sulfonyl]imide ([P₆₆₆₁₄⁺][NTf₂^-]), which have previously been shown to be complementary in terms of chromatographic selectivity and retention. Their application in routine analysis has been limited by their lower maximum allowable operating temperatures (MAOT) (200 °C for the [P₆₆₆₁₄⁺][Cl^-] IL and 180 °C for [P₆₆₆₁₄⁺][NTf₂^-]), which restricts their use to samples that consist of analytes with relatively high volatility. A previous study carried out in the Authors' laboratory focused on extending the use of the [P₆₆₆₁₄⁺][Cl^-] IL SP to the analysis of samples with analytes of medium-to-low volatility by optimizing column characteristics and operative conditions. This study addresses the immobilization of both the [P₆₆₆₁₄⁺][Cl^-] and [P₆₆₆₁₄⁺][NTf₂^-] ILs to the inner wall of fused silica columns to increase their MAOT under soft and hard reaction conditions. The resulting MAOT depended on more or less drastic immobilization conditions, and reached 220 °C for soft immobilization (So-Im) and 240 °C for hard immobilization (Ha-Im) in the [P₆₆₆₁₄⁺][Cl^-] IL columns, and 200 °C for So-Im and 220° for Ha-Im in columns coated with the [P₆₆₆₁₄⁺] [NTf₂^-] IL. The influence of immobilization on the separation power and performance of all the columns has been evaluated using i) the Grob test, ii) a model mixture of 41 compounds of different polarity, structure, and with different organic functional groups representative of the flavor and fragrance field, iii) a standard mixture of 37 fatty acid methyl esters, iv) the peppermint essential oil, v) two mixtures of sesquiterpenic alcohols (farnesols and santalols), and vi) a standard mixture of 16 pesticides. These test samples were also used to demonstrate the complementarity of the two phosphonium-based IL SPs in terms of selectivity and retention.

Keywords: Column immobilization; Flavors; Fragrances and natural products; Gas chromatography; Ionic liquid stationary phases; Operative conditions; Phosphonium-based ionic liquids.