Multivariate covalent organic frameworks guided carboxyl functionalized magnetic adsorbent for enrichment of fluoroquinolones in milk prior to high performance liquid chromatographic analysis

Herein, we prepared a carboxyl functionalized magnetic covalent organic framework (Fe3O4@iCOF-COOH) by combining multivariate synthetic strategy with post-synthetic modification. It was used as an adsorbent for magnetic solid phase extraction (MSPE) of six fluoroquinolones (FQs), and showed good absorption performance at neutral pH. Carboxyl groups are found to be crucial for the adsorption of fluoroquinolones. The adsorption mechanism was primarily attributed to strong hydrogen bonding, π-π interaction as well as potential hydrophobic effect. The optimal extraction conditions are sample pH at 6.0, adsorbent dosage of 3 mg, eluent of 1.0 mL methanol solution containing 7.5% ammonia, and extraction/desorption time of 30 min. Under the optimized conditions, the Fe3O4@iCOF-COOH was used as an adsorbent for MSPE of FQs in milk, an analytical method was established by combining with high-performance liquid chromatography-ultraviolet detection (HPLC-UV). The limits of detection (LODs) and limit of quantification (LOQs) were 1.24-4.58 ng⋅mL-1 and 4.12-15.3 ng⋅mL-1, respectively. The recoveries of target FQs in spiked milk were 68.4-105%. This work provides a new way to prepare covalent organic framework based adsorbents for solid phase extraction, and can be readily extended to other type of adsorbents.