Development and utilization of ionic liquid-based materials in microextraction techniques
Ionic liquids (ILs) have attracted tremendous interest as extraction phases for a number of microextraction techniques in the past two decades. These compounds are a class of organic salts with melting points at or below 100 Ã ÂºC. They possess unique physicochemical properties such as negligible vapor pressure, a wide range of viscosity, high thermal stability, and can undergo multiple solvation capabilities. The biggest advantage for utilizing ILs as contemporary extraction phases in microextraction techniques lies in the ability to modify their cation/anion structures to exhibit desired selectivity towards analyte(s) of interest. The research work presented in this dissertation is focused on the development and exploitation of novel ILs as extraction phases for two popular microextraction techniques, including dispersive liquid-liquid microextraction (DLLME) and solid-phase microextraction (SPME), to address various analytical limitations in the analysis of real-world samples.
Polymeric ionic liquids (PILs) are polymers prepared from IL monomers. These ionic polymers were demonstrated to be considerably robust and selective as SPME sorbent coatings. A number of crosslinked PIL-based coatings were prepared and chemically bonded to derivatized silica or nitinol supports by UV-initiated polymerization. The PIL-based SPME system was coupled to gas chromatography-mass spectrometry (GC-MS) and HPLC for the analysis of various compounds including pharmaceutical drugs, phenolics, and insecticides. The ratios of the IL monomer, IL crosslinker, and photoinitiator, were optimized to provide the best extraction efficiency for the aforementioned analytes. The roles of various cations and anions comprised in different PILs were intimately studied to gain insight on their selectivity towards the chosen analytes. Extraction parameters were optimized by design of experiment (DOE) or as a single variable to achieve the best analytical results. Furthermore, the structural integrity and robustness of the coatings were closely monitored, in various matrix conditions, to determine the usefulness of PIL-based SPME coupled to different chromatographic systems for trace-level analysis. Additionally, a study using PILs as SPME sorbent coatings in a combined extraction mode (e.g., direct immersion-headspace SPME) was performed for the determination of compounds with varied volatilities. The combined extraction mode provided a simple and efficient method to study multiple classes of analytes.
An in situ DLLME method, using ILs as extraction solvents, coupled to LC-MS for the analysis of microcystins was also developed. Microdroplets of hydrophobic ILs were formed through the metathesis of ILs containing halide anions to their bis[(trifluoromethyl)sulfonyl]imide (NTf2-) anion counterparts in the sample matrix. Three structurally different ILs, namely, 1-butyl-3-methylimidazolium chloride ([BMIM][Cl]), 1-(6-hydroxyhexyl)-3-methylimidazolium chloride ([HeOHMIM][Cl]) and 1-benzyl-3-(2-hydroxyethyl)imidazolium bromide ([BeEOHIM][Br]), were applied as extraction solvents. The [BeEOHIM][Br] IL, consisting of both an aromatic moiety and a hydroxyl group, exhibited the highest extraction efficiency and demonstrated good recoveries of microcystins in different environmental waters.
Magnetic ionic liquids (MILs) are a subclass of ILs which are comprised of either magnetic cations or anions. Three hydrophobic MILs, namely, aliquat tetrachloromanganate(II) ([Aliquat]2[MnCl4]), methyltrioctylammonium [MnCl4] ([N1,8,8,8]2[MnCl4]), and trihexyltetradecylphosphonium [MnCl4] ([P6,6,6,14]2[MnCl4]), were developed as extraction solvents in the DLLME of pharmaceutical drugs, phenolics, insecticides, and polycyclic aromatic hydrocarbons. In the DLLME procedure, a disperser solvent, either acetonitrile or methanol, was homogenized with the MIL in order to disperse them into fine droplets. This increased the contact surface area of the MIL during extraction and improved the extraction efficiency. Afterwards, the analyte-enriched MIL was retrieved using a neodymium magnetic rod and directly injected into a high-performance liquid chromatograph (HPLC) for analysis. The aforementioned approach was faster and simpler compared to traditional phase separation methods in DLLME (e.g., centrifugation, lowering temperature). Using MILs containing the MnCl42- anion provided a significant improvement over analogous compounds containing the FeCl4- anion, since they are less prone to undergo hydrolysis in aqueous sample. Furthermore, the MnCl42--based MILs exhibited significantly less absorbance in the UV wavelength region (with respect to the MILs comprised of the FeCl4- anion), which is highly beneficial for chromatographic analysis.