Sensitive Simultaneous Determination of Synthetic Food Colorants in Preserved Fruit Samples by Capillary Electrophoresis with Contactless Conductivity Detection

A novel and sensitive analytical method, based on efficient sample clean-up and field-amplified sample injection (FASI) on-line preconcentration technique in capillary electrophoresis with contactless conductivity detection, for the simultaneous determination of synthetic food colorants in preserved fruit samples was developed. In order to remove the complex matrix interferences, i.e., large amount of inorganic salts (e.g., NaCl, KCl) and bio-molecules co-existed with trace amount of food colorants in the preserved fruit samples, a dispersive solid-phase extraction (dSPE) was utilized in the sample clean-up procedure. As a result, the complex matrix interferences were effectively removed by dSPE when dispersive powder polyamide was used as the dispersive solid-phase adsorbent and ethanol-ammonia solution was used as the eluting solvent. The sample clean-up, capillary electrophoresis separation, and FASI conditions were discussed in detail. Well separation of five synthetic food colorants (ponceau 4R, amaranth, tartrazine, sunset yellow, and brilliant blue) was achieved within 10 min under the following conditions: an uncoated fused-silica capillary, 25 mM acetic acid + 2 mM hydroxypropyl-beta-cyclodextrin (pH = 3.5) as running buffer, separation voltage of - 15.0 kV, and electrokinetic injection of - 12.0 kV x 10 s. The limits of detection for these five food colorants were in the range of 0.035 to 0.055 mg kg(-1). The accuracy was tested by spiking colorant standards in preserved fruit samples, and the recoveries ranged from 94.3 to 102%. The relative standard deviations of intra- and inter-day were below 5%. Finally, this newly developed method was applied successfully for the determination of these synthetic food colorants in commercial preserved fruit samples.