Improving sensitivity by large-volume sample stacking combined with sweeping without polarity switching by capillary...

An easy, simple, and highly efficient on-line preconcentration method for polyphenolic compounds in CE was developed. It combined two on-line concentration techniques, large-volume sample stacking (LVSS) and sweeping. The analytes preconcentration technique was carried out by pressure injection of large-volume sample followed by the EOF as a pump pushing the bulk of low-conductivity sample matrix out of the outlet of the capillary without the electrode polarity switching technique using five polyphenols as the model analytes. Identification and quantification of the analytes were performed by photodiode array UV (PDA) detection. The optimal BGE used for separation and preconcentration was a solution composed of 10 mM borate-90 mM sodium cholate (SC)-40% v/v ethylene glycol, without pH adjustment, the applied voltage was 27.5 kV. Under optimal preconcentration conditions (sample injection 99 s at 0.5 psi), the enhancement in the detection sensitivities of the peak height and peak area of the analytes using the on-line concentration technique was in the range of 18-26- and 23-44-fold comparing with the conventional injection mode (3 s). The detection limits for (-)-epigallocatechin (EGC), (-)-epicatechin (EC), (+)-catechin (C), (-)-epigallocatechin gallate (EGCG), and (-)-epicatechin gallate (ECG) were 4.3, 2.4, 2.2, 2.0, and 1.6 ng/mL, respectively. The five analytes were baseline-separated under the optimum conditions and the experimental results showed that preconcentration was well achieved.