Sorbent characterization and application of the in-tube microextraction technique

Extraction techniques are a key part of sample preparation. The miniaturization and automatization of these techniques provide the opportunity of a higher sample throughput with more precise results. Furthermore, solventless microextraction techniques are environmental friendly, in comparison to other extraction techniques. Nonetheless the selection of the sorbent for extraction is currently based on experience or via trial and error, which is time consuming and could lead to suboptimal choices. ITEX is a solventless microextraction where the headspace of samples is dynamically enriched on the chosen sorbent and thermodesorption is applied to proceed with the separation and detection of the sample. The analysis of the sorbent for the extraction of the sample needs a more detailed investigation to further understand and improve the ITEX technique. Among the material characterization techniques, inverse gas chromatography was applied to investigate three commercial sorbents for ITEX. Inverse gas chromatography provides the parameters which are needed to be determined for the purpose of this thesis, such as sorption enthalpy (ΔHs), partitioning constant between the solid and gas phase (Kd) and kinetic parameters like the diffusion coefficient (D∞), dispersion coefficient (D*), and apparent permeability (Papp). PDMS, Tenax TA and Carbopack C were studied in this thesis. For the analyzed probes, Tenax TA proved to be the most suitable sorbent for extraction. Due to the 160 μL available in the body of the stainless steel ITEX cannula, a manifold of materials can be packed and tested. Carbon based-nanomaterials and polymeric ionic liquids are gaining popularity and both materials are not commonly applied as sorbent in ITEX. To evaluate their analytical performance, polycyclic aromatic hydrocarbons were chosen as analytes for carbon based-nanomaterials, while for the polymeric ionic liquids short chained alcohols were selected. A HS-ITEX-GC/MS method was applied for both materials. The optimal parameters for the two applications were selected in different fashions. On one hand with a Box-Behnken design and on the other hand with parameters reported in literature. Once the suitable extraction parameters were established, then the method was validated. Since five carbon based-nanomaterials and four extraction parameters at three levels were investigated, a response surface methodology was applied to determine the optimal parameters. A Box-Behnken design was set and from the results, fullerenes were the sorbent of choice to perform a full determination of the figures of merit. The linear range covered four orders of magnitude (from 0.25 to 250 μg L-1), the method detection limit was comparable or improved in comparison to other sorbents (0.01 to 0.3 μg L-1) and similar recovery values Summary IV were also obtained (45 to 103 %). For the analysis of larger polycyclic aromatic hydrocarbons with carbon based-nanomaterials, enough surface and active sites must be available to screen a large amount of them. Otherwise, only the small polycyclic aromatic hydrocarbons will be extracted. In comparison to Tenax GR as sorbent, carbon based-nanomaterials are a more suitable sorbent for extraction. For the polymeric ionic liquids, to some extent the same extraction parameters as for a commercial material were set. By this, direct literature comparison was performed. The results were comparable, but the expected improvement was not achieved. A linear range from 0.55 to 5,500 μg L-1 and limits of detection from 0.3 to 15.1 μg L-1 were achieved. The recovery values obtained from spiked beer yielded a poor response (0.1 to 45 %). Nonetheless, with polymeric ionic liquids a lower thermodesorption was set and the extraction yield was comparable. A hydrophobic behavior of the polymeric ionic liquid could improve its analytical performance. This can be modified during its synthesis, by changing the anion and increasing the alkyl chain. Material characterization via inverse gas chromatography could be applied to confirm that for the one type of polymeric ionic liquid as sorbent in ITEX, derivatized aldehydes are a more suitable group of probe compounds for analysis than short chained alcohols. As well as to define that for the second polymeric ionic liquid, short chained alcohols are appropriate probe compounds for investigation. Finally, via inverse gas chromatography, it would be possible to further explain the differences between the polymeric ionic liquids applied in this thesis.