Investigation of the applicability of polyHIPE materials in liquid chromatography
Choudhury, Sidratul (2016) Investigation of the applicability of polyHIPE materials in liquid chromatography. PhD thesis, Dublin City University.
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Polymer monoliths have gained significant interest as stationary phase materials over the last 25 years. More recently, enthusiasm for polymer high internal phase emulsions (polyHIPEs) in chromatographic separations has grown, primarily due to their large pore sizes (greater than 10 µm), which make polyHIPEs highly permeable and ideal for high flow rate separations. In this study, the applicability of polyHIPE materials as stationary phase materials was investigated and novel methods to increase their surface area explored. A range of different polyHIPEs were prepared using monomers such as Sty, GMA and VBC to impart varying surface functionalities. The first instance of an OTCEC polyHIPE column was demonstrated and applied in a modest separation of alkylbenzenes.
Upon development of various polyHIPE materials, it was found that styrenic polyHIPEs gave an ideal morphology and were most appropriate for HPLC separations compared with VBC and GMA polyHIPEs. Previously, polyHIPEs have predominantly been utilised in gradient separations of protein standards. However, the applicability of these materials as stationary phase materials have not yet been investigated in isocratic separations. PS-co-DVB polyHIPEs within silcosteel columns (100 mm x 1.02 mm in I.D) were fabricated and chromatographic performance characteristics of the columns produced were established. The separation efficiency for alkylbenzenes investigated was lower than traditional polymer monoliths. However, the polyHIPEs of low surface area demonstrated a high separation capacity in comparison to traditional polymer monoliths produced in previous studies and presented the novel use of PS-co-DVB polyHIPEs for the separation of alkylbenzenes in HPLC. In addition the resolution, peak asymmetry, and high batch-to-batch reproducibility of %RSD of up to 3% were established of the polyHIPEs as stationary phases.
While separation capacity of the PS-co-DVB polyHIPEs were exceptional for such low surface area materials of 20 m2 g-1, surface area would become a major limitation upon developing the materials for more complex analytes. Methods such as inclusion of NPs during emulsion fabrication, agglomeration of NPs after amination and inclusion of additional porogens were utilised to enhance the surface area of the polyHIPEs. The inclusion of additional porogen toluene and agglomeration of gold NPs were investigated; the polyHIPEs were unsuitable for downstream applications because of mechanical rigidity and permeability issues respectively. The inclusion of GONPs within the PS-co-DVB emulsion was found to decrease the surface area of the material by up to 40% (16 m2 g-1). Nonetheless, the GONP modified polyHIPEs were utilised in HPLC in their reduced and non-reduced states. The GONP modified polyHIPEs showed no difference in selectivity; however were found to give similar separation capacities when compared to the unmodified higher surface area polyHIPE. The significance of these results was that the GONP modified polyHIPEs demonstrated a superior method of adsorption for the RP-HPLC of alkylbenzenes.
Finally, the first instance of fabrication of a polyHIPE coated OTCEC column using a multiple layer polymerisation technique was investigated in an attempt to increase the separation efficiency of the polyHIPE materials. The successfully fabricated coated columns had an increased efficiency of up to 3460 plates. However, poor injection-to-injection reproducibility was observed with increasing injections resulting in co-eluted peaks and greater diffusion effects. In addition, effects on the polyimide coatings in OTCEC in general when using organic modifier ACN was also expected to be an attributing factor to the low injection-to-injection reproducibility. The latter, however, was deemed an issue in which OTCEC polyHIPE columns would not be able to overcome.
From this study, it was established that polyHIPE materials have potential as stationary phase materials; however, the issue of low surface area, mechanical rigidity and inherent concave void structure of polyHIPEs will ultimately need to be addressed to minimise low separation capacities and higher diffusion effects. Throughout this thesis, strategies have been explored to negate these issues, and additional strategies have been proposed for future work. It is certain that if these issues are addressed, polyHIPEs have the potential to not only be comparable to traditional polymer monoliths in terms of chromatographic performance, but to exceed these traditional formats in terms of fabrication reproducibility, ultimately increasing their application in real world applications.
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