Fluorophilic Interactions in GC: Advancing Ionic Liquid Stationary Phases for the Analysis of Persistent PFAS Pollutants

Per- and polyfluoroalkyl substances (PFAS) are widely used in industrial applications and pharmaceuticals due to their exceptional chemical stability, hydrophobicity, and lipophobicity. However, these same properties contribute to their persistence in the environment, leading to their classification as “forever chemicals” and raising concerns about bioaccumulation and human health risks. While analytical techniques such as high-performance liquid chromatography (HPLC) are well established for detecting ionizable PFAS, they are often ineffective for neutral, volatile PFAS like fluorotelomer alcohols. As industries shift toward these alternative PFAS, there is an increasing need for analytical methods capable of effectively separating and detecting them, with gas chromatography (GC) emerging as a more suitable approach.

The effectiveness of gas chromatography (GC) analysis depends heavily on the stationary phase, yet conventional phases often lack sufficient selectivity for PFAS, particularly short-chain volatile compounds. To address this limitation, a study conducted by members of the Ames National Laboratory, Iowa State University (both in Ames, Iowa) and the University at Buffalo (Buffalo, New York) explored the use of ionic liquid (IL) stationary phases incorporating fluorinated moieties to enhance fluorophilic interactions. Novel IL stationary phases with linear and branched perfluoroalkyl groups were developed and systematically evaluated alongside a non-fluorinated control. Using both one-dimensional GC and comprehensive two-dimensional GC (GC × GC), the results demonstrate improved retention and separation of fluorinated analytes, highlighting the potential of fluorinated IL stationary phases as advanced tools for analyzing persistent PFAS pollutants.

LCGC International spoke to the four authors of the paper resulting from this study and published in Analytica Chimica Acta,¹ Donghyun Ryoo (lead author), SeongSoo Lee, Emanuela Gionfriddo, and Jared Anderson, about their work.

How do the chemical properties of PFAS contribute to environmental persistence?
The environmental persistence of PFAS is fundamentally driven by their chemical structure, characterized by their highly fluorinated carbon chains in which the strength of the carbon–fluorine bond imparts exceptional thermal and chemical stability. This stability is coupled with amphiphilic behavior arising from the co-existence of a hydrophobic fluorinated tail and a polar functional headgroup, which governs their partitioning across environmental compartments. For volatile and neutral PFAS, this same chemical stability is paired with sufficient vapor pressure to enable gas-phase transport, allowing them to undergo long-range atmospheric dispersion and redistribute between air, water, and surfaces. These properties not only hinder conventional degradation pathways but also enhance mobility and interfacial accumulation, contributing to their widespread environmental presence.

How does the widespread industrial and pharmaceutical use of fluorinated compounds contribute to global contamination of soil and water systems?
The widespread industrial and pharmaceutical use of fluorinated compounds drives global contamination of soil and water systems through continuous release of PFAS and their precursors across the entire life cycle. Industrial sources, including manufacturing, the use of aqueous film-forming foams, and the disposal of PFAS-containing materials in landfills or biosolids, introduce these compounds directly into environmental matrices. In addition, an often overlooked but significant contribution comes from mono- and polyfluorinated pharmaceuticals, which are often not effectively removed during conventional wastewater treatment. In fact, these compounds have been reported to account for up to 75% of the extractable organofluorine (EOF) in both influent and effluent of municipal wastewater treatment facilities.²

Why are neutral PFAS (such as fluorotelomer alcohols) more challenging to detect and monitor compared to ionizable PFAS?
The legacy PFAS, such as PFOA and PFOS, are negatively ionized in aqueous solution, making high-performance liquid chromatography (HPLC) amenable to their analysis. Therefore, analytical methodologies for ionizable PFAS based on HPLC mass spectroscopy (MS) have been widely investigated and validated by regulatory agencies. However, neutral PFAS lack charge and often exhibit poor ionization efficiency in electrospray ionization, leading to reduced sensitivity. In addition, their higher volatility and lower polarity promote partitioning into the gas phase, making them susceptible to loss during sampling, storage, and sample preparation, and requiring alternative analytical platforms, such as gas chromatography (GC)-based methods.

Explain how incomplete thermal degradation of PFAS can lead to secondary environmental pollution. What types of products are formed?
Incomplete thermal degradation of PFAS can lead to the formation of perfluorinated gases, which may contribute to greenhouse effects, as well as perfluoroalkanes and persistent short-chain perfluoroalkyl carboxylic acids, such as trifluoroacetic acid (TFA). Therefore, it is critical to establish effective separation techniques to monitor these degradation products and accurately assess thermal degradation processes efficiency and prevent the generation of secondary pollutants.³ 

Compare the suitability of GC and HPLC for analyzing ionizable vs. neutral PFAS.
Both GC and HPLC can be used in the separation and quantification of PFAS. HPLC is suitable for analyzing ionizable PFAS since the analytical methodologies, including sample preparation and chromatographic separation, for ionizable PFAS has been well-established. However, GC is amenable for analyzing neutral and volatile PFAS since it is compatible with gas sample preparation techniques, such as headspace solid phase micro extraction (HS-SPME). HS-SPME is a promising sample preparation approach for neutral and volatile PFAS analysis that enables their effective isolation and preconcentration. In HS-SPMEfor neutral and volatile PFAS analysis, the selectivity of extractants for the PFAS is important in their downstream analysis to minimize a co-extraction and co-elution of non-targeted substances from samples.⁴

What role do stationary phase properties (such as polarity and fluorophilicity) play in the separation of PFAS in GC?
The overall stationary phase polarity and fluorophilicity plays a primary influence in the separation of PFAS in GC. For conventional non-fluorinated stationary phases, dispersive interactions predominate, leading to preferential retention of hydrocarbon analytes. In contrast, fluorinated ionic liquid stationary phases introduce fluorophilic interactions that enhance the retention of fluorinated compounds. The selectivity of the chromatographic system depends on the balance between multiple interaction mechanisms. The chemical structure features of the stationary phase, including fluorine content and molecular branching, further influence fluorophilic interactions. Consequently, tailoring the stationary phase chemistry enables the selective separation of fluorinated and non-fluorinated analytes in GC.

Explain the concept of “fluorophilic interactions” and how they influence retention behavior of fluorinated analytes in GC columns.
Fluorophilic interactions refer to intermolecular interactions between fluorocarbon moieties, arising from enhanced dispersion forces in fluorine-rich environments.⁵ In GC, these interactions are introduced by fluorinated stationary phases and selectively enhance the retention of fluorinated analytes, such as fluorotelomer alcohols and perfluoroalkenes. As the fluorine content of analytes increases, stronger fluorophilic interactions are observed, resulting in significantly increased retention and selectivity. In contrast, non-fluorinated stationary phases lack these interactions and therefore exhibit weaker retention for fluorinated compounds. Consequently, fluorophilic interactions play a critical role in enabling the selective separation of PFAS and other fluorinated analytes in GC.

Why do ionic liquid (IL) stationary phases offer advantages over conventional polysiloxane-based phases for PFAS analysis?
ILs have been widely studied as GC stationary phases due to their customizable physicochemical properties such as low vapor pressure, high thermal stability, and high viscosity. A key advantage of ILs as GC stationary phases is their structural tunability. The cation and anion components of ILs can be carefully designed for the chromatographic separation of targeted analytes by adjusting the various properties such as polarity, hydrophobicity, aromaticity, and fluorophilicity. In this study, fluorinated IL stationary phases were prepared to investigate the separation of volatile PFAS using fluorophilic interaction between the fluorinated stationary phase and volatile PFAS. Compared to the tested polysiloxane-based phases GC column, higher retention and selectivity of FTOHs on the fluorinated IL stationary phases were observed. The fluorinated IL stationary phases have some limitations in their applications, such as MS compatibility and analysis times due to their limited thermal stability. Therefore, ongoing efforts are focusing on developing fluorinated IL stationary phase with higher thermal stability.¹

How does two-dimensional gas chromatography (GC × GC) improve the resolution and analysis of complex PFAS mixtures compared to one-dimensional GC?
Two-dimensional gas chromatography (GC × GC) improves the separation of complex PFAS mixtures by combining two columns with different selectivity, thereby enabling orthogonal separation mechanisms. In this study, a nonpolar primary column separates analytes primarily based on volatility, while the secondary column, consisting of fluorinated ionic liquid stationary phases, introduces additional selectivity based on fluorophilicity or polarity. This dual separation approach increases peak capacity and resolves co-eluting compounds that cannot be separated using one-dimensional GC (1D GC). For example, weakly retained analytes, such as short-chain perfluoroalkenes, which exhibit minimal retention in 1D GC, can be effectively separated in GC × GC due to enhanced fluorophilic interactions in the second dimension. Consequently, GC × GC enables improved resolution and more comprehensive analysis of complex PFAS mixtures.

References

1. Ryoo, D.; Lee, S.-S.; Gionfriddo, E. et al. Fluorinated Ionic Liquids as Gas Chromatographic Stationary Phases for the Separation of Volatile Per- and Polyfluoroalkyl Substances. Anal. Chim. Acta 2026, 1402, 345373. DOI: 10.1016/j.aca.2026.345373
2. Ruyle, B. J.; Pennoyer, E. H.; Vojta S, et al. High Organofluorine Concentrations in Municipal Wastewater Affect Downstream Drinking Water Supplies for Millions of Americans. Proc Natl Acad Sci U S A. 2025, 122 (3), e2417156122. DOI: 10.1073/pnas.2417156122
3. M.L. Williams, M. L.; A. Fitzgerald, A.; D. Alonso, D. et al. Comparison of Solid Phase Microextraction Coatings for Headspace Extraction of Volatile Perfluoroalkyl Substances Using One-Dimensional and Comprehensive Two-Dimensional Gas Chromatography. Anal Chim Acta 2026, 1403, 345389. DOI: 10.1016/j.aca.2026.345389
4. Wang, J.; Lin, Z.; He, X. et al. Critical Review of Thermal Decomposition of Per- and Polyfluoroalkyl Substances: Mechanisms and Implications for Thermal Treatment Processes. Environ. Sci. Technol. 2022, 56, 9, 5355–5370. DOI: 10.1021/acs.est.2c02251
5. Román Santiago, A.; Yin, S.; Elbert, J. et al. Imparting Selective Fluorophilic Interactions in Redox Copolymers for the Electrochemically Mediated Capture of Short-Chain Perfluoroalkyl Substances. J. Am. Chem. Soc. 2023, 145, 9508−9519. DOI: 10.1021/jacs.2c10963