In order to accurately quantify the concentration of trace analytes within a complex mixture, appropriate sample preparation is imperative. Some of the most common pretreatment methods that are used prior to analysis by high-performance liquid chromatography (HPLC) or gas chromatography with mass spectrometry (GC-MS) include liquid-liquid extraction (LLE) and solid phase extraction (SPE).
Both LLE and SPE are widely utilized for this purpose because of their high recovery rates and salient extraction capabilities. Despite their usefulness, both LLE and SPE are associated with various limitations including excessive time requirements, tedious sample preparation and the requirement to use highly toxic organic reagents that subsequently result in excessive waste generation.
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In an effort to comply with Green Analytical Chemistry (GAC) technique developments, a number of microextraction techniques that are based on both LLE and SPE have arisen. Some of the GAC microextraction techniques based on LLE include hollow fiber liquid-phase microextraction (HF-LPME), dispersive liquid-liquid microextraction (DLLME) and single drop microextraction (SDME).
Similarly, many microextraction techniques have been developed based on SPE, some of which include solid phase microextraction (SPME), multiple monolithic fibers solid phase microextraction (MMF-SPME), in-tube solid phase microextraction (In-tube SPME), magnetic solid phase extraction (MSPE) and many more.
Ionic Liquid-based Sorbents
Regardless of whether a liquid or solid sorbent microextraction technique is employed, the extraction capabilities of these techniques are highly dependent upon the sorbent and extraction phase being used.
Some of the sorbents that are most commonly used for microextraction purposes include silica, carbon, carbon nanotubes, graphene, metal-organic frameworks (MOFs) and monoliths. More recently, researchers have looked to a unique green sorbent known as ionic liquids (ILs).
ILs are molten salts that can remain in the liquid state at temperatures as low as 100 °C below room temperature. The chemical composition of ILs includes large organic cations, such as imidazolium, pyridinium or quaternary ammonium, to name a few, as well as small inorganic or organic anions such as chlorine, bromine or trifluoromethylsulfonate.
Since various combinations of both cations and anions can be used to create ILs, researchers often refer to this sorbent as a designer solvent. For microextraction purposes, ILs is considered ideal due to their structural flexibility and ability to perform multiple different interactions with target molecules.
The Development of Poly Ionic Liquids (PILs)
Despite the clear advantages associated with ILs, their incorporation into microextraction techniques is often limited as a result of their instability and low utilization efficiency. As a result of these limitations, researchers have looked to poly ILs (PILs).
PILs are a subclass of polyelectrolytes that were originally polymerized from IL monomers. In addition to exhibiting some of the same useful properties as ILs, PILs are superior than ILs in terms of their thermal stability, durability, mechanical robustness, processability, spatial controllability and lifetime expectancies.
PILs in Forensics
PILs have successfully been applied as sorbents in a variety of microextraction procedures in forensics including SPME, MMF-SPME, IT-SPME, magnetic solid-phase extraction (MSPE), stir bar sorptive extraction (SBSE) and tip-based microextraction (TBME).
Researchers have found that the use of PILs for microextraction purposes greatly expands their analytical capabilities to various environmental pollutants including estrogen, drugs, heavy metals, pesticides and anions that are present in water, food, and biological samples.
As interest in these specialized microextraction techniques continues to rise, researchers are hopeful that more selective PIL-based sorbents will be developed in an effort to enhance the selectivity and specificity of these methods.
Mei, M., Huang, X., Chen, L. (2019). Recent development and applications of poly (ionic liquid)s in microextraction techniques. Trends in Analytical Chemistry 112; 123-134. DOI: 10.1016/j.trac.2019.01.003.