Highly water-dispersible hydroxyl functionalized covalent organic frameworks as matrix for enhanced MALDI-TOF MS identification and quantification of quaternary ammonium salts in water and fruits

A composite matrix of sodium alginate (SA) and COF (Tp-DHBD) was designed and prepared for the detection of quaternary ammonium salts (QAs) containing heteroatoms. SA@Tp-DHBD self-assembled by the hydrogen bonding interaction between SA and COF has abundant hydroxyl and carboxylate anion sites, which makes for the excellent enrichment of the QAs through multiple interactions. Besides, the abundant hydroxyl groups can enhance laser absorption and energy transfer to achieve sensitive detection. The introduction of hydrophilic SA greatly promotes the dispersion of COFs to obtain good detection repeatability. Under the optimized experimental conditions, SA@Tp-DHBD was added to the solution of analytes and vortexed evenly, then directly analyzed by MALDI-TOF-MS. The detection limit of choline chloride, chlormequat chloride, and mepiquat chloride is 0.001 mg L-1; the detection limit of acetylcholine is 0.005 mg L-1. The relative standard deviations of target-target and sample-sample are below 6.0% and 7.0%, respectively. Moreover, the satisfactory recoveries in complex lake water and fruits meet the needs of practical applications. Comparative experiments were performed with the reported representative matrices, including ZIF-8 (MOF), TpBD (COF), UiO@TapbTp (MOF@COF), and α-cyano-4-hydroxycinnamic acid (organic matrix, CHCA). SA@Tp-DHBD exhibits the best matrix performance for QAs.