Selectivity enhancement for the separation of tocopherols and steroids by integration of highly ordered weak interaction sites along the polymer main chain

A novel alternating copolymer-based organic phase was synthesized using a new N-substituted maleimide monomer for the development of alternating copolymer-grafted silica for high-performance liquid chromatographic applications. This new monomer (DGMI) was copolymerized with octadecyl acrylate (ODA) from 3-mercaptopropyltrimethoxysilane-grafted silica to produce Sil-poly(ODA-alt-DGMI). The organic phase was characterized by the elemental analysis and the diffuse reflectance infrared Fourier transform spectroscopy. Tocopherol isomers and steroids were used as analytes for the evaluation of the chromatographic selectivity profiles of this novel stationary phase. The selectivity of this column was then compared with a polymeric ODS column and previously developed another alternating copolymer-grafted silica (without the glutamide-derived moiety) column, Sil-poly(ODA-alt-N-octadecylmaleimide). The complete baseline separation of tocopherol isomers in an isocratic mode has been achieved within 25 min with the Sil-poly(ODA-alt-DGMI). The separation of eight kinds of estrogenic steroids and corticoids has also been achieved in an isocratic mode with this column. Significant differences in separation selectivity between Sil-poly(ODA-alt-DGMI) and polymeric ODS columns were observed towards the steroids, and compared with the reference columns, a better separation profile for these analytes was obtained with the Sil-poly(ODA-alt-DGMI). The results of this investigation indicated that the enhancement of selectivity of Sil-poly(ODA-alt-DGMI) towards the test analytes arose from the multiple interaction mechanism such as hydrophobic effect, carbonyl-π and hydrogen-bonding interactions, and such integrated interactions originated from the addition of two amide groups in the N-substituted maleimide monomer.