Methacrylate bonded covalent organic framework monolithic column online coupling with high-performance liquid chromatography for analysis of trace estrogens in food

Covalent organic frameworks (COFs) are a burgeoning class of crystalline porous materials with unique properties and have been considered as a promising functional extraction medium in sample pretreatment. In this study, a new methacrylate-bonded COF (TpTh-MA) was well designed and synthesized via the aldehyde-amine condensation reaction, and the TpTh-MA was incorporated into poly (ethylene dimethacrylate) porous monolith by a facile polymerization reaction inside capillary to prepare a novel TpTh-MA monolithic column. The fabricated TpTh-MA monolithic column was characterized with scanning electron microscope, Fourier transform infrared spectrometer, X-ray diffraction, and N2 adsorption–desorption experiments. Then, the homogeneous porous structure, good permeability and high mechanical stability of TpTh-MA monolithic column was used as separation and enrichment media of capillary microextraction, which was coupled with high-performance liquid chromatography fluorescence detection for online enrichment and analysis of trace estrogens. The main experimental parameters influencing the extraction efficiency were systematically investigated. The adsorption mechanism for three estrogens was also explored and discussed based on hydrophobic effect, π-π affinity and hydrogen bonding interaction, which contributed to its strong recognition affinity to target compounds. The enrichment factors of the TpTh-MA monolithic column micro extraction method for the three estrogens were 107–114, indicating a significant preconcentration ability. Under optimal conditions, a new online analysis method was developed and exhibited good sensitivity and wide linearity range of 0.25–100.0 µg·L-1 with a coefficient of determination (R2) higher than 0.9990 and a low limit of detection with 0.05–0.07 µg·L-1. The method was successfully applied for online analysis of three estrogens of milk and shrimp samples and the recoveries obtained from spiking experiments were in range of 81.4–113% and 77.9–111%, with the relative standard deviations of 2.6–7.9% and 2.1–8.3% (n = 5), respectively. The results revealed the great potential for the application of the COFs-bonded monolithic column in the field of sample pretreatment.