Capillary electrophoresis coupled with electrochemical detection using porous etched joint for determination of antioxidants

Capillary electrophoresis coupled with electrochemical detection (CE–EC) for determination of antioxidants, propyl gallate (PG) and tert-butylhydroquinone (TBHQ), in cosmetic samples was proposed in this work. A porous etched joint was used to isolate the electrochemical detection from the electrophoretic high voltage. Compared with the 25 μm i.d. capillary without a decoupler in a CE–EC system, a 75 μm i.d. capillary applied in the present system gave an improvement in both sample injection and sensitivity. Moreover, the carbon fiber working electrode could be directly in touch with the end of separation capillary due to the elimination of the effect of separation voltage on the EC detection, so the alignment of working electrode and capillary became easy and the dead volume was also decreased. Baseline separation of the two antioxidants was achieved by CE in a 50 cm long × 75 μm i.d. capillary at 20 kV using 5.0 mmol L−1 phosphate buffer (pH 8.00). 0.7 V (versus Ag/AgCl) was applied to the carbon fiber electrode for electrochemical detection. Under the optimal condition, the precisions (RSD, n = 4) of peak height and migration time of PG and TBHQ were 2.39–3.59% and 0.34–0.44%, respectively. The detection limits of PG and TBHQ were 2.51 × 10−6 and 4.88 × 10−6 mol L−1 for standard solution and 0.0751 and 0.0328 mg g−1 for the real cosmetic samples with consumption of 0.3 g sample. Analysis of TBHQ and PG in cosmetics samples was also achieved with the present system and the spiked recoveries of two analytes in cosmetics samples were in the range of 93.6–98.8%.