Combination of micelle collapse and field-amplified sample stacking in capillary electrophoresis for determination of trimethoprim and sulfamethoxazole in animal-originated foodstuffs

•A combination of two-state mechanism for trimethoprim and sulfamethoxazole determination was proposed.•The sensitivity could be improved 301 and 329 times for SMZ and TMP, respectively.•The limits of detection (LODs) were 8.5 ng/mL for TMP and 7.7 ng/mL for SMZ.•The LODs were lower than the maximum residue limits in foods of animal origin.

An on-line preconcentration method combining micelle to solvent stacking (MSS) with field-amplified sample stacking (FASS) was employed for the analysis of trimethoprim (TMP) and sulfamethoxazole (SMZ) by capillary zone electrophoresis (CZE). The optimized experimental conditions were as followings: (1) sample matrix, 10.0 mM SDS-5% (v/v) methanol; (2) trapping solution (TS), 35 mM H3PO4-60% acetonitrile (CH3CN); (3) running buffer, 30 mM Na2HPO4 (pH = 7.3); (4) sample solution volume, 168 nL; TS volume, 168 nL; and (5) 9 kV voltage, 214 nm UV detection. Under the optimized conditions, the limits of detection (LODs) for SMZ and TMP were 7.7 and 8.5 ng/mL, and they were 301 and 329 times better compared to a typical injection, respectively. The contents of TMP and SMZ in animal foodstuffs such as dairy products, eggs and honey were analyzed, too. Recoveries of 80-104% were acquired with relative standard deviations of 0.5-5.4%.