Preparation, characterization and application of polymethacrylate-based monolithic columns for fast and efficient separation of alkanes, alcohols, alkylbenzenes and isomeric mixtures by gas chromatography

Abstract

Application of monolithic columns in gas chromatography is still considered very limited. In this work, several polymethacrylate-based monolithic capillary columns were fabricated, characterized and used in gas chromatography. The five monomers used were: methyl methacrylate, (MMA), hexyl methacrylate (HMA), glycidyl methacrylate (GMA), 2-butoxyethyl methacrylate (BEMA) and isobornyl methacrylate (IBMA), while ethylene dimethacrylate was the crosslinker. The monoliths were synthesized in 30 cm length capillaries possessing inner diameters (i.d.) of 0.25 mm. The prepared monolithic columns were applied for separation of 3 series of homologous alkanes, alcohols and alkylbenzenes, as well as some isomeric mixtures. Van Deemter plots were used to optimize and compare the columns performance. The smaller methacrylates (MMA and GMA) exhibited higher porosity and permeability with low column backpressure values and poorer efficiency than the larger methacrylate monomers (HMA and BEMA). The columns prepared from IBMA monomer showed the highest pressure and the least separation efficiency. The fastest full separation of alkanes was achieved on HMA-co-EDMA column in about 3.0 min with resolution better than 2.73, while the fastest full separation of alcohols and alkylbenzenes was carried out using BEMA-co-EDMA column in less than 0.8 and 1.75 min with chromatographic resolution better than 1.27 and 1.85, respectively. Again, BEMA-co-EDMA column gave the best performance with the fastest and complete separation of all studied isomeric mixtures. For all tested series of solutes, the better separation efficiency was reached with tridecane, which gave 25,200 plates/m on the HMA-co-EDMA column. Another application was carried out using HMA-co-EDMA column for determination of myrcene and limonene, two monoterpenic isomers, in some fruit peels. Under the optimum GC conditions, a rapid separation of myrcene and limonene was achieved in less than 1.0 min with chromatographic resolution of 2.56. The highest contents of myrcene (0.131 mg/g) and limonene (1.225 mg/g) were measured in the hexane extracts of grapefruit and Egyptian orange, respectively. Finally, a comparison between the prepared columns and a commercial capillary column was performed. Based on the measured run time and HETP values, HMA-co-EDMA and BEMA-co-EDMA monolithic columns exhibited faster separation and higher efficiency for n-alkanes and alkylbenzenes than the TR-5 open tubular column, although they are 100 times shorter.