Ionic Liquids and Their Applications in Sample Preparation

This short review covers the use of ionic liquids (ILs) and polymeric ionic liquids (PILs) in solid-phase microextraction (SPME) and dispersive liquid–liquid microextraction (DLLME).

Over the past decade, ionic liquids (ILs) and polymeric ionic liquids (PILs) have been widely studied and applied in multiple areas of science and engineering. ILs are organic salts with melting points at or below 100 °C, and PILs are polymers synthesized from IL monomers. These "molten salts" are typically composed of an organic cation and an organic or inorganic anion. Given that the cations and anions can be readily interchangeable to form new compounds with different properties, it has been estimated that there can be as many as 1018 possible combinations of ILs (1). ILs and PILs have been previously applied in various extraction methods including single-drop microextraction (SDME), liquid-phase microextraction (LPME), solid-phase microextraction (SPME), dispersive liquid–liquid microextraction (DLLME), hollow fibre-supported liquid membrane extraction (HFSLME) and solid-phase extraction (SPE) (2–7). With the exception of sorbent-based extraction procedures, many of these sample preparation methods involve the use of organic solvents. Compared to traditional organic solvents, ILs can be engineered to exhibit low to negligible volatility, and sometimes lower toxicity, which makes their use in sample preparation highly attractive (3).

Although ILs and PILs have been applied in many sample preparation methods, this article will focus on the application of these compounds in two rapidly emerging techniques, namely, SPME and DLLME. ILs and PILs exhibit a number of unique properties that make them highly useful extraction media in SPME and DLLME because of their high thermal stability, variable viscosity and negligible vapour pressure. However, the main advantage of using ILs and PILs is their tunable selectivity for specific classes of analytes. By imparting functional groups to the chemical makeup of ILs and PILs, the selectivity of these compounds can be enhanced as a result of favourable analyte-to-IL or -PIL interactions. As a result, higher analyte enrichment factors and extraction efficiencies often can be achieved. In this column instalment, the thermal stability, viscosity and selectivity of ILs and PILs and their relevance to SPME and DLLME will be discussed. An overview of the basic principles of operation for both DLLME and SPME will be covered. Additionally, the current challenges of ILs and PILs in sample preparation will be highlighted and ideas for future research will be discussed.

Unique Characteristics of Ionic Liquids and Polymeric Ionic Liquids for Applications in DLLME and SPME

Figure 1: Chemical structures of IL and PIL cation–anion pairs previously used in IL- and PIL-based SPME and IL-based DLLME: (a) poly(1-vinyl-3-R-imidazolium), (b) 1-R-3-methylimidazolium, (c) poly(1-R-3-vinylbenzylimidazolium), (d) R-pyridinium, (e) N,N-methyl-R-D-glucaminium, (f) 1-R1-1-R2-pyrrolidinium, (g) bis[(trifluoromethyl)sulphonyl] imide, (h) hexafluorophosphate, (i) tris(pentafluoroethyl)trifluorophosphate, (j) tetrafluoroborate, (k) taurinate, (l) chloride, (m) bromide and (n) iodide.
Thermal Stability: The thermal stability of an IL or PIL is critical for applications in sample preparation that require the use of high temperatures. When SPME is coupled to gas chromatography (SPME-GC), the IL- or PIL-based sorbent materials must be capable of withstanding the high operating temperatures of the GC injector (250–280 °C) during analyte desorption (5). Figure 1 shows the structures and names of common ILs and PILs used in DLLME and SPME. Most ILs or PILs used in SPME and DLLME are composed of imidazolium-, pyrrolidinium- or phosphonium-based cations. Functionalizing the nitrogen atoms of the imidazolium ring with linear aliphatic hydrocarbons can further increase the thermal stability of the resulting IL (8). The anion, however, often plays a more significant role in the thermal stability of ILs and PILs (9). Typically, smaller and less polarizable anions, such as halides, decrease the overall thermal stability because of their susceptibility to undergoing nucleophilic substitution with the alkyl substituents of the IL cation (10). On the other hand, larger anions that exhibit higher electron delocalization, such as bis[(trifluoromethyl)sulphonyl] imide ([NTf2] ) or triflate ([TfO] ), produce compounds with higher thermal stability (8,10,11).

Viscosity: Similar to the thermal stability, the viscosity of an IL or PIL is an important consideration in sample preparation, particularly for SPME and DLLME. The viscosity of ILs is governed largely by their intermolecular interactions, such as hydrogen bonding, van der Waals forces and electrostatic interactions (12). ILs containing halide anions typically exhibit higher viscosities because of their ability to undergo stronger hydrogen bonding and electrostatic interactions with the cations while larger or more asymmetrical anions often possess lower viscosities (11,13,14). Substituting the cationic moiety of the IL with linear or branched aliphatic hydrocarbons will also increase the viscosity as a result of enhanced van der Waals interactions (9,12,14,15). In SPME, it is favourable to use ILs and PILs with relatively high viscosities because the viscosity of ILs tend to decrease at high temperature, such as during thermal desorption in the GC injector at high temperatures. Such thermal desorption may result in the IL coating flowing off the fibre support into the GC injector and jeopardizing both the fibre lifetime and its analytical performance. In addition to producing chromatographic ghost peaks, active sites and chromatographic background noise, IL buildup within the injector may require frequent GC system maintenance. In IL-based DLLME, the viscosity of an IL extraction solvent can be modified to a certain extent by using a disperser solvent, an organic modifier that is miscible in both the sample matrix and the extraction phase. In cases where a disperser solvent is not used, it is necessary to use an IL with a suitable viscosity to allow for homogenous mixing during extraction.

Variable Selectivity by Structural Tunability

A major advantage to using ILs and PILs as extraction phases in sample preparation is their tunable selectivity for specific analytes. For example, it is beneficial to take advantage of the electrostatic interaction capabilities of ILs in addition to functionalizing them with hydrogen-bond acidic hydroxyl groups to enhance the extraction efficiency of deoxyribonucleic acid (DNA) in DLLME (16). Likewise, the extraction efficiency of analytes containing aromatic moieties, such as polycyclic aromatic hydrocarbons (PAHs), can be enhanced by imparting aromatic character to the PIL-based sorbent coating to increase p-p interactions in SPME. In the same context, engineering PILs with large aliphatic hydrocarbon chains can increase the selectivity of longchained alkyl halides as a result of enhanced dispersion interactions (17). Therefore, to fully exploit ILs and PILs as selective extraction media, the intermolecular interactions between these compounds and the target analytes should be carefully considered.