Determining ultraviolet absorbents in sunscreen products by combining direct injection with micelle collapse on-line preconcentration capillary electrophoresis

The on-line preconcentration technique of analyte focusing by micelle collapse-micellar electrokinetic chromatography (AFMC-MEKC) was combined with direct injection without extraction to determine ultraviolet absorbents in sunscreen products. The stacking mechanism is based on the transport, release, and accumulation of analytes bound to micelle carriers that are collapsed into the micelle dilution zone. The following optimized conditions were determined: the running buffer was 10 mM Tris buffer (pH 9.5) containing 60 mM SDS, 7 mM γ-CD and 20% ethanol; the SDS concentration was required to be slightly above the critical micelle concentration (cmc) value (7.4 mM) in the sample matrix, which allowed the micelle dilution zone to form when voltage was applied; and finally, the sample was prepared in 100 mM Tris buffer (pH 9.0) containing 7.5 mM SDS and 20% (v/v) ethanol to provide sufficient resolution and to improve the sensitivity. Samples were injected at 0.5 psi for 40 s, and the separation voltage was set at 15 kV for first 15 min and then increased to 23 kV to decrease the analysis time. The detection sensitivity for ultraviolet absorbents was enhanced by approximately 41-fold using AFMC-MEKC compared to conventional MEKC. The limit of detection (S/N = 3) was 98 nM for benzophenone-2 and benzophenone-4. The correlation of the regression curve was greater than 0.995. The relative error and relative standard deviation were lower than 9.94% with high precision and accuracy. The recoveries of nine ultraviolet absorbents in a homemade emulsion were between 95.08% and 104.57%. After optimization and validation, this AFMC-MEKC method combined with direct injection is considered to be established and successfully applicable to commercial sunscreen products.