Online preconcentration methodology that realizes over 2000-fold enhancement by integrating the free liquid membrane into electrokinetic supercharging in capillary electrophoresis for the determination of trace anionic analytes in complex samples

Abstract

The analysis of anionic compounds via the sensitive FLM-EKS method remains challenging due to their inverse migration behaviors to EOF in the capillary. A highly sensitive ionic liquid coating-based online preconcentration methodology via electrokinetic supercharging (EKS) coupled to a free liquid membrane (FLM) was first proposed and successfully used to determine vanillic acid (VA), salvianic acid A sodium (SAAS) and ferulic acid (FA) in a complex sample matrix. EKS was recognized as the first step preconcentration, and its separation conditions were optimized. Then, the second preconcentration step, namely FLM, which was performed by simple injection of a short water-immiscible solvent plug as a membrane interface prior to electrokinetic injection (EKI) of the sample, was further investigated to increase the stacking efficiency. The influences of multifarious parameters on FLM-EKS performance were examined, and the optimal FLM-EKS conditions were identified: a buffer electrolyte of 20 mM Tris-HOAc (pH = 6) with 5% (v/v) methanol; hydrodynamic injection (HI) of a leading electrolyte (50 mM NH4Cl, 30 s, 0.5 psi) followed by the HI of 2-nitrophenyl octyl ether (NPOE) as the FLM (5 s, 0.5 psi); EKI of the sample (120 s, −10 kV) and there with the HI of terminator (20 mM N-cyclohexyltaurine, CHES, 40 s, 0.5 psi). The newly developed method yielded analytes with 2006–2862-fold enhancement compared to the normal injection, there by producing detection limits of 1.5–5.0 ng mL−1 with satisfactory repeatability below 2.92%, which provides 1.5–2.0 times higher stacking efficiency than that obtained by EKS only. Feasibility in real sample analysis was demonstrated by using Naoxintong capsules and rat feces, which suggests that the presented methodology has a high potential for the reliable and sensitive determination of trace anionic components in complex sample matrices.