Retention and selectivity of monolithic silica stationary phase modified with octadecyl and amino moieties for mixed-mode liquid chromatography

Abstract

In this research, a monolithic capillary column was prepared for a mixed-mode liquid chromatography. A monolithic column provides high efficiency and high permeability and a mixed-mode mechanism offers a good separation efficiency for polar analytes, better stability and a symmetry peak for basic analytes. A column for mixed-mode liquid chromatography can be prepared with several modification methods. Methods. This work showed two different modification methods which were one-step and two-step modifications after silica monolithic synthesis. The one-step method modified a silica monolithic capillary column by a mixture of octadecylmethoxysilane and aminopropyltrimethoxysilane (C₁₈-AP column) and the two-step method modified a silica monolithic capillary column with glycidoxypropyltrimethoxysilane and followed by tetradecylamine (C₁₄-amine embedded column). The chromatographic behaviors of both columns were evaluated by retention and selectivity of test probes in Tanaka’s test. The lower hydrophobicity on C₁₄-amine embedded column comparing to C₁₈-AP column was observed with the low retention of all test probes. For shape selectivity, it was not significantly different between the columns. Lastly, the ion exchange behavior of C₁₈-AP column was different from that on C₁₄-amine embedded column column because of the difference of polar groups on the phase. In addition, the separation of alkyl benzenes, benzoic acid derivatives, aniline derivatives and polycyclic aromatic hydrocarbons were performed to evaluate the separation performance. Both columns showed the separation of analytes based on a mixed mode mechanism (reversed phase and anion exchange mechanism).