Borate complexation-assisted field-enhanced sample injection for on-line preconcentration of cis-diol-containing compounds in capillary electrophoresis

A new on-line preconcentration technique called borate complexation-assisted field-enhanced sample injection (BCA-FESI) was proposed for preconcentrating cis-diol-containing compounds (CDCCs) in capillary electrophoresis (CE). The principle relies on amplification of the difference in the electrophoretic mobilities of CDCC in sample matrix and background electrolyte (BGE) through complexation of CDCC with borate in a sample matrix of basic pH and dissociation of the complex in a BGE of acidic pH. Meanwhile, CDCC and borate ions electro-injected into the capillary are finally in neutral state, which maintains the pre-filled low conductivity zone and thus allows for longer injection time. With catechol as a test compound, the principle and effectiveness of BCA-FESI was verified. As compared to conventional sample injection, BCA-FESI allowed for sensitivity enhancement of 1850-fold. The established method was further evaluated with three catechins, including (−)-epicatechin gallate (ECG), (−)-gallocatechin gallate (GCG), and (−)-epigallocatechin (EGC), in a standard mixture of trace content. The limit of detection (LOD) was found to be 1.4, 3.8, 17.5 nM (S/N = 3) for ECG, GCG, EGC, respectively. Finally, the BCA-FESI method was applied to a real sample of diluted tea beverage, in which the three catechins were detected.