Development of ionic liquid-based materials in gas chromatographic separations and microextraction techniques
Ionic liquids (ILs) are organic salts with melting points below 100 ˚C. These compounds exhibit a number of unique characteristics including high thermal stability, negligible vapor pressures, wide liquid range, and tunable viscosities. More importantly, they can be functionalized to process a broad range of solvation interactions and exhibit unique selectivities toward different classes of analytes. The research work presented in this dissertation is focused on the development of novel ILs and PILs in chromatographic separations (i.e., stationary phases for gas chromatography) and microextraction techniques (i.e., extraction phases for solid-phase microextraction and dispersive liquid-liquid microextraction).
A series of dicationic ILs containing different structural features were employed as secondary columns in comprehensive two-dimensional gas chromatography (GCÃ Â GC) for the separation of aliphatic hydrocarbons from kerosene. The solvation parameter model was applied to establish a quantitative structure-retention relationship to understand the role that the structural features of the IL play on the selectivity in GC separations. It was observed that long alkyl side chain substituents and long linkage chains between the two imidazolium cations are the most important structural features for the resolution of aliphatic hydrocarbons. However, it was also observed that the dicationic ILs did not exhibit good thermal stability at high operating temperatures (i.e., >250 ˚C). In order to address this issue, crosslinked PIL-based stationary phases were prepared using imidazolium-based IL monomers and crosslinkers possess similar structural features (i.e., alkyl side chain substituents and long linkage chains) via in-column free radical polymerization. The crosslinked PIL-based stationary phase containing 50% (w/w) of crosslinker exhibited excellent selectivity for the GCÃ Â GC separation of aliphatic hydrocarbons and showed a maximum allowable operating temperature of 325 ˚C, which is significantly higher than commercial available polar phases. Finally, the crosslinked PIL-based stationary phases were compared with SUPELCOWAX 10 and DB-17 columns for the separation of aliphatic hydrocarbons in diesel fuel. Better resolution of aliphatic hydrocarbons was obtained when employing the crosslinked PIL-based stationary phase as the second-dimension column.
Crosslinked PIL-based sorbent coatings were prepared and coupled to GC-FID/MS for the analysis of polycyclic aromatic hydrocarbons (PAHs) from aqueous samples. In this study, novel cross-linked polymeric ionic liquid (PIL) bucky gels were formed by free-radical polymerization of IL monomer gelled with multi-walled carbon nanotubes (MWCNT). The incorporation of MWCNTs to the PIL phases significantly enhanced the π-π interaction between the sorbent coatings and target analytes (i.e., PAHs). A partitioning extraction mechanism was observed for PIL bucky gel sorbent coatings, which allows the further determination of the analyte-to-coating partition coefficients (log Kfs). Recovery studies were also performed in different environmental water samples to validate the applicability of the PIL bucky sorbent coatings.
ILs were also employed as extraction solvents in in situ DLLME coupled to HS-GC-ECD/or MS for the analysis of polychlorinated biphenyl (PCBs) and acrylamide from complex food samples at trace levels. Five halide-based ILs containing varied functional groups were prepared to evaluate the effect of different structural features on the extraction efficiency of the target analytes. Extraction parameters including molar ratio of IL to metathesis reagent and IL mass were optimized. The effects of HS oven temperature and the HS volume of the sample vial on the analyte response were also evaluated. The matrix-compatibility of the developed method was proven by quantifying acrylamide in brewed coffee samples. This method is much simpler and provides higher sample throughput compared to the previously reported SPME GC-MS method and can be applied for the routine analysis of contaminants present in complex food samples.