Sol–gel coatings with covalently attached methyl, octyl, and octadecyl ligands for capillary microextraction. Effects of alkyl chain length and sol–gel precursor concentration on extraction behavior

Fused silica capillaries with surface-bonded sol–gel coatings containing covalently attached octadecyl, octyl, and methyl groups were prepared for capillary microextraction (CME) hyphenated on-line with high-performance liquid chromatography (HPLC). For this, octadecyltrimethoxysilane (C18TMS), octyltrimethoxysilane (C8TMS), or methyltrimethoxysilane (MTMS) was used as the respective sol–gel precursor. Hydrolytic polycondensation of these precursors led to the formation of surface-bonded sol–gel sorbents with pendant alkyl groups ready to serve as the extraction medium; no additional surface derivatization reactions were needed to anchor these ligands to the surface. Extraction behaviors of two sets of microextraction capillaries with alkyl-bonded sol–gel coatings were investigated: (a) capillaries prepared with a constant molar concentration of these precursors in the sol solution, and (b) capillaries prepared with varied molar concentrations of C8TMS in the sol solution. Among the capillaries prepared using sol solutions with the same molar concentration of sol–gel precursor, the detection limits for nonpolar and polar analytes ranged from 0.3 ng/L to 213.9 ng/L. The sol–gel octadecyl-coated capillaries were found to be the most efficient at extracting these analytes, followed by the sol–gel octyl-coated capillaries, followed by the sol–gel methyl-coated capillaries. The results of this study point to the possibility that polar analytes are extracted through synergistic molecular level interactions of the polar and nonpolar parts of the analyte molecules with the alkyl chains and silanol groups within the sol–gel coatings. These coatings also demonstrated run-to-run and capillary-to-capillary reproducibility, with HPLC peak area RSD values ranging from 1.1% to 9.6% and 1.3% to 10.0%, respectively. In the set of sol–gel octyl capillaries with varied molar concentrations, the capillaries prepared with 0.514 M concentration of C8TMS in the sol solution were most efficient in extracting nonpolar and polar analytes. When higher or lower concentrations of C8TMS were used in the sol solution, the resulting sol–gel coated capillaries were less efficient in extracting nonpolar and polar analytes.