Study Examines Impact of Zwitterionic Liquid Structures on Volatile Carboxylic Acid Separation in GC

In a recent study led by Iowa State University researchers, zwitterionic liquid (ZIL) chemicals’ structures were studied for their influences when used as stationary phases in gas chromatography (GC). Their findings were published in the Journal of Chromatography A (1).

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Gas chromatography (GC) is widely used for separating and quantifying free fatty acids (FAs) and volatile carboxylic acids (VCAs) in the food, pharmaceutical, biofuels, and cosmetic industries. However, due to their tendency to produce tailing chromatographic peaks, these molecules are often difficult to analyze using GC. Poor peak shapes typically stem from secondary interactions with interfacial silanol groups on the surface of capillary columns or other active sites. Further, severely tailing peaks are difficult to quantify because integrating peak areas can be affected, and sometimes, the true mass of an analyte or column may not be detected due to strong irreversible adsorptive interactions with active sites. Zwitterionic liquids (ZILs), which are ionic liquids (ILs) that have covalently linked cation and anion components, are promising GC stationary phase materials, as they feature improved inertness for acidic molecules (2).

In this study, imidazolium-based ZILs with sulfonate and triflimide anions were evaluated to understand the influence of ZILs’ chemical structures on polar analyte separation. Five VCAs were used as probe molecules to evaluate chromatographic retention, peak shapes, and selectivity. Higher VCA retention was found on imidazolium sulfonate ZIL stationary phases compared to 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 incorporating phenyl moieties to imidazolium rings increased retention of oligoether functionalized ZILs. VCA retention also decreased when anions were exchanged or if linker lengths connecting cation and anion groups were decreased.

The scientists found that the chemical natures of substituent groups and chain length appeared to be important for controlling VCA selectivity. Moreover, alkyl-substituted ZILs offered higher selectivity than the oligoether ZILs. While excellent peak asymmetry factors (<2) were found for most of the ZIL stationary phases, oligoether functionalized ZILs displayed limited analyte loading capacities for less polar VCAs. When ZIL stationary phases were coated on a salt-deactivated capillary, VCA retention increases were found alongside slightly higher peak asymmetry factors.

Overall, ZIL stationary phases containing oligoether functionalized substituents were found to be suitable compromises between VCA retention, peak shape, and thermal stability. To achieve better selectivity and higher VCA retention, and to increase the solubility of less polar VCAs, the scientists hope to investigate using longer substituent chain lengths. Exploring other surface roughening strategies can also help overcome salt’s effects on VCA peak shape, and alternative anions for ZIL chemical structures can have direct effects on stationary phase thermal stability and likely also VCA peak shape. However, further studies on comparing the uniformity of coated capillary surfaces should be organized. This study’s findings can provide guidance towards future development of new ZILs as highly polar and inert stationary phases, which can offer unique chromatographic retention and selectivity towards polar analytes.

References

(1) Zeger, V. R.; DeLair, J. F.; Thapa, B.; Shollenberger, D.; et al. Influence of Zwitterionic Liquid Chemical Structure on Separation of Volatile Carboxylic Acids When Used as Stationary Phases in Gas Chromatography. J. Chromatogr. A 2025, 1749, 465872. DOI: 10.1016/j.chroma.2025.465872

(2) Huang, H-H.; Jia, J.; Ren, L.; et al. A Zwitterionic Solution for Smart Ionic Liquids to Evade Cytotoxicity. J. Hazard. Mater. 2023, 453, 131430. DOI: 10.1016/j.jhazmat.2023.131430