Capillary Gas Chromatographic Separation of Carboxylic Acids Using an Acidic Water Stationary Phase

Influence of zwitterionic liquid chemical structure on separation of volatile carboxylic acids when used as stationary phases in gas chromatography

Volatile carboxylic acids (VCAs) are important chemicals and synthetic building blocks in various industrial fields ranging from foods to polymers and pharmaceuticals. The analysis of VCAs using gas chromatography (GC) is challenging due to poor peak shapes on many traditional polar stationary phases. Zwitterionic liquids (ZILs) are promising GC stationary phase materials, featuring improved inertness for acidic molecules. In this study, imidazolium-based ZILs with sulfonate and triflimide anions are evaluated to understand the influence of ZIL chemical structure on the separation of polar analytes. Five VCAs were used as probe molecules to evaluate chromatographic retention, peak shapes, and selectivity. Higher VCA retention was observed on imidazolium sulfonate ZIL stationary phases compared to a commercially available wax-type or polyethylene glycol-based columns. ZILs containing either octyl or decyl substituents provided higher VCA retention than oligoether functionalized ZILs, but incorporation of a phenyl moiety to the imidazolium ring increased retention of oligoether functionalized ZILs. VCA retention also decreased when either the anion was exchanged or linker length connecting the cation and anion groups was decreased. The chemical nature of substituent groups and chain length appears to play important roles in controlling VCA selectivity, where alkyl-substituted ZILs offered higher selectivity than the oligoether ZILs. Excellent peak asymmetry factors (< 2) were observed for most ZIL stationary phases; however, oligoether functionalized ZILs exhibited limited analyte loading capacity for less polar VCAs. When ZIL stationary phases were coated on a salt-deactivated capillary, an increase in VCA retention was observed along with slightly higher peak asymmetry factors. For an oligoether ZIL stationary phase on salt-deactivated capillary, more prominent fronting peak shapes were observed due to reduced solubility of less polar VCAs in the stationary phase.

Sulfolane as a novel stationary phase for analytical separations by gas chromatography

Sulfolane is explored as a novel stationary phase for use in analytical separations by capillary column gas chromatography with flame ionization detection (GC-FID). Stainless steel capillaries were found to provide a good substrate for coating and retaining a sulfolane phase, whereas fused silica tubing did not perform well for this. In general, the phase was found to be stable for several hours of use when using elevated carrier gas pressures (90 psi) and a small restriction (25 μm I.D. tubing) at the outlet. This normally provided good performance at temperatures up to about 200 °C with very little background interference in the FID. Given its separation properties, a short 2 m × 100 μm I.D. column was found to be preferable for most separations in this study. Measurements indicated the coating procedure yielded a sulfolane film near 4 μm thick on this column, which produced 4400 plates for benzene with a sample capacity near 30 μg. The sulfolane phase yielded good retention and peak shape for many analytes including alkanes, aromatics, alcohols, bases, sulfides, phosphites, thiols, and others. Compared to longer conventional GC columns, the relatively short sulfolane column was found to offer improved selectivity in the separation of unsaturated, aromatic, and alkane test analytes. As such the method was successfully applied to the analysis of aromatics in gasoline headspace. Results suggest that sulfolane could be a potentially useful stationary phase to further explore in GC separations.

Dynamic Control of Gas Chromatographic Selectivity during the Analysis of Organic Bases

A novel method for controlling selectivity during the gas chromatographic (GC) analysis of organic bases is presented. The technique employs tandem stainless steel capillary columns, each coated with a pH adjusted water stationary phase. The first is a 0.5 m trap column coated with a pH 2.2 phase, while the second is an 11 m analytical column coated with a pH 11.4 phase. The first column traps basic analytes from injected samples, while the remaining components continue to elute and separate. Then, upon injection of a volatile aqueous ammonia solution, the basic analytes are released as desired to the analytical column where they are separated and analyzed. Separations are quite reproducible and demonstrate an average RSD of 1.2% for analyte retention times in consecutive trials. Using this approach, the retention of such analytes can be readily controlled and they can be held in the system for periods of up to 1 h without significant erosion of peak shape. As such, it can provide considerable control over analyte selectivity and resolution compared to conventional separations. Further, by employing a third conventional GC column to the series, both traditional hydrocarbon and enhanced organic base separations can be performed. The method is applied to the analysis of complex mixtures, such as gasoline, and much less matrix interference is observed as a result. The findings indicate that this approach could be a useful alternative for analyzing such samples.