Sol–gel methyl coating in capillary microextraction hyphenated on-line with high-performance liquid chromatography: Counterintuitive extraction behavior for polar analytes

A sol–gel coating with anchored methyl groups was developed for capillary microextraction hyphenated on-line with high-performance liquid chromatography (HPLC). This was accomplished by using methyltrimethoxysilane as the sol–gel precursor. The methyl group on the sol–gel precursor ultimately turned into a pendant group on the created sol–gel coating and was primarily responsible for the extraction of nonpolar analytes. A 40-cm segment of 0.25 mm I.D. fused silica capillary containing the sol–gel methyl coating on the inner surface was installed as a sampling loop in an HPLC injection port. The analytes were extracted by the coating when an aqueous sample containing the analytes was passed through this capillary. The extracted analytes were then transferred to the HPLC column using isocratic elution with an acetonitrile/water mobile phase. This capillary demonstrated excellent extraction capability for polycyclic aromatic hydrocarbons and ketones. Unexpectedly, this coating also provided good extraction for polar analytes, including aromatic phenols, alcohols, and amines. Considering the fact that the methyl group is nonpolar in nature, such an extraction behavior of sol–gel methyl coating toward polar analytes is counterintuitive. Thus, sol–gel sorbents with short alkyl side chains have the potential to offer a polymer-free alternative to traditional sol–gel capillary microextraction (CME) media commonly prepared with the use of polymers in the sol solution. Elimination of polymers from the sol–gel coating solution is conducive to improving thermal stability and solvent tolerance of the created sol–gel extracting phase. This also makes the preparation of sol–gel coatings facile and cost-effective. Possessing excellent solvent stability, such sol–gel coatings offer the opportunity for effective on-line hyphenation of capillary microextraction with HPLC and other liquid-phase separation techniques that employ organo-aqueous mobile phases.