Investigation of separation performance and mechanism of nanoscale hierarchical covalent organic frameworks with different surface areas in open-tubular capillary electrochromatography

Covalent organic frameworks (COFs) have attracted considerable attention as promising stationary phases in chromatographic separations, owing to their exceptional structural attributes. Nevertheless, a systematical methodology for correlating the specific surface area of COFs with their separation performance remains underdeveloped. In this study, four imine-based 1,3,5-tris(4-aminophenyl)benzene-2,5-dimethoxyterephthalaldehyde (TPB-DMTP) COFs exhibiting distinct specific surface area due to the differences in particle size, were employed as stationary phases in open-tubular capillary electrochromatography (OT-CEC). As the volume of acetic acid (HAc) was raised from 0.3 mL to 0.7 mL, the specific surface areas of the four TPB-DMTP COFs exhibited a corresponding increase from 1267 m²/g to 2226 m²/g. Four TPB-DMTP COFs-coated capillaries regulated by HAc amount were fabricated using an in-situ growth method at room temperature. TPB-DMTP-0.4 COF-coated capillaries (prepared by adding 0.4 mL HAc) were utilized as the model column, illustrating good separation performance for six representative groups of neutral, basic and acidic analytes. Moreover, TPB-DMTP-0.4 COF-coated capillaries showed good reproducibility and stability (relative standard deviations of retention time and peak area of <10 %) and long lifetime (>200 runs). Furthermore, it was found that the separation efficiency was significantly improved and the migration time was prolonged with the increasing specific surface area by comparing the four TPB-DMTP COFs-coated capillaries. Upon eliminating the influences of electroosmotic flow (EOF) and coating thickness, the specific surface area was identified as a key factor affecting separation performance. Notably, the results revealed that both excessively high and low specific surface areas were unfavorable for improving separation performance. These findings provide valuable insights for the rational design and optimization of COFs-based chromatographic stationary phases.