Zwitterionic polymeric ionic liquid-based sorbent coatings in solid phase microextraction for the determination of short chain free fatty acids

Highlights

• Five zwitterionic polymeric ionic liquid sorbent coatings have been developed.

• Coatings were used in solid-phase microextraction for free fatty acid determination.

• Their analytical performance was compared with commercial PA and CAR/PDMS fibers.

• Zwitterionic coatings were more reproducible and less affected by ethanol content.

• Wine was analyzed with similar results for zwitterionic and CAR/PDMS fibers.

Abstract

Five different zwitterionic sorbent coatings based on polymeric ionic liquids (PILs) were developed by the on fiber UV co-polymerization of the zwitterionic monomers 1-vinyl-3-(alkylsulfonate)imidazolium or 1-vinyl-3-(alkylcarboxylate)imidazolium and different dicationic ionic liquid (IL) crosslinkers. The developed sorbent coatings were applied in headspace solid-phase microextraction in combination with gas chromatography-mass spectrometry for the determination of short chain free fatty acids in wine. The sorbent coatings were found to extract these analytes via a non-competitive extraction mechanism. The methodology was optimized for the two best zwitterionic PIL coatings and compared to the commercially-available carboxen/polydimethylsiloxane (CAR/PDMS) and polyacrylate (PA) fibers. The sorbent coating based on the 1-vinyl-3-(propanesulfonate)imidazolium IL (Fiber 1) was more sensitive than PA while providing similar limits of detection to CAR/PDMS for the determination of analytes in a diluted synthetic wine sample. At the same time, Fiber 1 required lower extraction times (only 20 min versus 60 min for CAR/PDMS and 40 min for PA), exhibited higher reproducibility (with relative standard deviation lower than 8.9% for a spiked level of 7 µM) and was more tolerant to ethanol present within the sample. The zwitterionic PILs were also applied for the analysis of red wine, and the results were in agreement with those obtained for CAR/PDMS. The analytes were detected and quantified in the concentration range from 0.18 ± 0.03 mg L⁻¹ to 4.8 ± 0.9 mg L⁻¹, depending on the analyte and fiber.

Introduction

Polymeric ionic liquids (PILs) are a subclass of polyelectrolytes prepared by the polymerization of ionic liquid (IL) monomers [1]. PILs possess low-to-negligible vapor pressure at room temperature, high chemical and mechanical stability as well as impressive structural tunability, allowing for the preparation of task-specific materials by tailoring the chemical composition. Moreover, they often provide higher thermal stability and viscosity compared to ILs, properties that are derived from their polymeric nature. These aforementioned properties make PILs attractive materials for a number of applications within different scientific fields [2].

PILs are classified as polycations, polyanions or polyzwitterions based on the repeating electrolyte unit within the PIL backbone [1]. Zwitterionic PILs have garnered attention in the last several years [1], [3], [4]. These polymers are composed of both cations and anions covalently linked as a repeating unit within the polymer backbone. The presence of closely and oppositely charged moieties within the polymer structure has led to interesting materials that exhibit high dipole moments while maintaining charge neutrality [3], [5]. In addition, they possess a wide chemical diversity due to the possibility of combining different cations (e.g., ammonium [6], imidazolium [5], [7] or phosphonium [5]) and anions (e.g., sulfonate [7] or carboxylate [7], [8]), and the feasibility of modulating the proximity of these positive and negative charges [3].

Within analytical chemistry, PILs have been particularly successful as sorbent coatings in solid-phase microextraction (SPME) [9], [10]. SPME is a non-exhaustive extraction and preconcentration method based on the partitioning of analytes from the sample to a small amount of sorbent phase typically coated on a solid fiber support [12]. The success of this technique in environmental, biological and food analysis lies with its simplicity, environmental-friendliness, ease of automation, and high enrichment factors. However, for a number of analytes and sample matrices current commercially-available coatings lack selectivity [11]. PIL-based sorbent coatings, due to their wide solvation capabilities, can overcome some limitations of commercial fibers [9], [13].

A high number of PIL-based SPME sorbent coatings are based on the co-polymerization of two different ILs, generally a monocationic IL monomer and a dicationic IL crosslinker [14], [15], [16]. Moreover, taking advantage of the synthetic versatility of these materials, a large variety of PIL coatings have been developed [13]. Double-confined PILs have been prepared by employing ILs containing polymerizable moieties in both the cation and anion with the objective of enhancing the mechanical stability of the coatings to yield matrix-compatible coatings [17]. IL monomers have also been functionalized to include specific groups in their chemical structures. PIL-based coatings containing halide anions or polar substituents in the cationic moiety have been demonstrated to be more efficient in the extraction of polar compounds [14], [18], while those with vinylbenzyl groups within the cation provided better results for aromatic compounds [19], [20]. Crosslinked PIL-based sorbent coatings with carboxylic groups in the cationic moiety have also been developed for the selective extraction of DNA [21], while PILs containing cations with unique structural motifs have demonstrated increased sensitivity for the extraction of acrylamide [22]. More recently, the incorporation of silver ions within the IL monomer was reported, leading to the preparation of coatings with unique selectivity towards alkenes and alkynes [23]. However, to the best of our knowledge, zwitterionic PILs have never been applied in SPME.

The determination of volatile compounds in foods and beverages is important since their concentration affects sensory properties and often determines the quality of the product [24]. Volatile short chain free fatty acids (SCFFAs) are among the main compounds that contribute to the aroma of fermented products, while also being responsible for various characteristic flavors and off-flavors in dairy products and alcoholic fermented beverages [25], [26], [27]. Headspace (HS-) SPME has been explored in these applications [25], [28], [29], [30]. Adsorption-type commercial fibers such as carboxen/polydimethylsiloxane (CAR/PDMS) and divinylbenzene/carboxen/polydimethylsiloxane (DVB/CAR/PDMS) have generally been used over absorption-type coatings given their higher sensitivity. However, a large number of interferences are generally extracted from samples together with target analytes, which makes the analysis of real samples very challenging [30]. Therefore, the development of sorbent coatings able to selectively extract specific groups of compounds without being affected by the presence of interfering substances is of particular interest when analyzing complex matrixes with SPME. In the case of alcoholic beverage analysis, the use of sorbent coatings that are not affected by the alcohol content of the sample is of special interest [30].

In this study, PIL-based sorbent coatings comprised of zwitterionic IL monomers and dicationic IL crosslinkers were developed for the first time and their unique selectivity examined. The prepared fibers were assessed in HS-SPME for the extraction of SCFFAs and their primary extraction mechanism was determined. The analytical performance of the zwitterionic PIL fibers, in combination with gas chromatography-mass spectrometry (GC-MS), was evaluated in synthetic and real wine samples. The influence of ethanol content within the sample on the extraction efficiency of the method was studied for these sorbent coatings. Moreover, the proposed methodology was compared with suitable commercial fibers for the application.