Abstract: A novel spherical covalent organic framework (TPB-TP COF) was successfully prepared via a facile and room-temperature in-situ growth method and applied as stationary phase in open-tubular capillary electrochromatography (OT-CEC) for the separation of neutral analytes. The TPB-TP COF was synthesized through the Schiff-base condensation of 1,3,5-tris(4-aminophenyl)benzene (TPB) and terephthalaldehyde (TP). The inner wall of the fused-silica capillary was first pretreated with NaOH and HCl, followed by modification with 3-(2,3-epoxypropoxy)propyltrimethoxysilane (GLYMO) to provide active epoxy groups for the covalent attachment of COF nanocrystals. Comprehensive characterizations confirmed the successful construction of the coating. Powder X-ray diffraction (PXRD) patterns exhibited high crystallinity consistent with literature reports. Fourier transform infrared (FT-IR) spectra showed a characteristic stretching vibration of the imine bond (-C=N) at 1615 cm^-1, confirming the framework formation. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed a uniform spherical morphology. Nitrogen adsorption-desorption isotherms indicated a microporous structure with a pore size of 1.47 nm and a BET surface area of 20 m²/g. Thermogravimetric analysis (TGA) demonstrated excellent thermal stability up to 400 ℃. The electroosmotic flow (EOF) properties were systematically investigated, showing that the TPB-TP COF coating effectively masked the silanol groups on the capillary wall, resulting in a significantly lower and more stable EOF compared to the bare capillary. Notably, the unique structural features of TPB-TP COF provided significant kinetic advantages in OT-CEC. Unlike conventional COFs that pursue extremely high surface areas, the modest BET surface area (20 m²/g) of the spherical TPB-TP COF featured rapid mass transfer kinetics by minimizing solute entrapment in deep and complex pores. The dynamic mass transfer optimization strategy effectively suppressed peak broadening and enhanced separation efficiency. The separation performance was evaluated using four groups of neutral analytes, including substituted benzenes (toluene, p-xylene, 1,3,5-trimethylbenzene), halobenzenes (fluorobenzene, bromobenzene, o-dibromobenzene, 1-bromo-4-iodobenzene), and two sets of phthalate esters (PAEs). After optimizing parameters such as acetonitrile content, buffer pH, and borate concentration, the coated capillary achieved baseline separation for all analytes. The elution order followed the increasing order of their octanol-water partition coefficients (log K_ow), indicating a typical reversed-phase chromatographic mechanism dominated by strong hydrophobic interactions and π-π stacking interactions between the aromatic framework and the analytes. The stability and reproducibility of the TPB-TP COF coated capillary were rigorously validated. The relative standard deviations (RSDs) for intra-day, inter-day, and column-to-column migration times and peak areas were all below 10%. Furthermore, the coating maintained its structural integrity and separation efficiency even after 200 consecutive runs, demonstrating high mechanical and chemical robustness. Finally, the proposed OT-CEC method was applied to determine PAEs in Yellow River water samples. The method exhibited a wide linear range (2.0-100.0 μg/mL) with correlation coefficients (R²) higher than 0.9877. The limits of detection (LODs) and quantification (LOQs) were 0.14-0.37 μg/mL and 0.47-1.22 μg/mL, respectively. The spiked recoveries ranged from 94.9% to 108.9% with RSDs of 1.0%-11.3%, confirming the reliability and practical utility of the method. These results highlight the immense potential of TPB-TP COF as a high-performance stationary phase for advanced chromatographic separations.

Key words: covalent organic frameworks (COFs), in-situ growth, open-tubular capillary electrochromatography (OT-CEC), phthalate esters