Sol-gel niobia sorbent with a positively charged octadecyl ligand providing enhanced enrichment of nucleotides and organophosphorus pesticides in capillary microextraction for online HPLC analysis

A niobia-based sol-gel organic-inorganic hybrid sorbent carrying a positively charged C-18 ligand (Nb2O5-C-18(+ve)) was synthesized to achieve enhanced enrichment capability in capillary microextraction of organophosphorus compounds (which include organophosphorus pesticides and nucleotides) before their online analysis by high-performance liquid chromatography. The sorbent was designed to simultaneously provide three different types of molecular level interactions: electrostatic, Lewis acid-base, and van der Waals interactions. To understand relative contributions of various molecular level analyte-sorbent interactions in the extraction process, two other sol-gel niobia sorbents were also created: (a) a purely inorganic sol-gel niobia sorbent (Nb2O5) and (b) an organic-inorganic hybrid sol-gel niobia sorbent carrying an electrically neutral-bonded octadecyl ligand (Nb2O5-C-18). The extraction efficiency of the created sol-gel niobia sorbent (Nb2O5-C-18 (+ve)) was compared with that of analogously designed and synthesized titania-based sol-gel sorbent (TiO2-C-18 (+ve)), taking into consideration that titania-based sorbents present state-of-the-art extraction media for organophosphorus compounds. In capillary microextraction with high-performance liquid chromatography analysis, Nb2O5-C-18 (+ve) had shown 40-50% higher specific extraction values (a measure of extraction efficiency) over that of TiO2-C-18 (+ve). Compared to TiO2-C-18(+ve), Nb2O5-C-18(+ve) also provided superior analyte desorption efficiency (96 vs. 90%) during the online release of the extracted organophosphorus pesticides from the sorbent coating in the capillary microextraction capillary to the chromatographic column using reversed-phase high-performance liquid chromatography mobile phase.