New insights into ion migration time under nonuniform electric fields: Mechanism and application to designing fast exhaustive electroextraction

Electrokinetic techniques, such as electrophoresis, electroextraction, and electrodialysis, have found a wide range of applications in fields such as chemical engineering, chemistry, biology, and environmental science. A critical parameter in these techniques is the time required for ion migration, which is often driven by a nonuniform three-dimensional electric field. However, the influence of the inhomogeneity of an electric field on ion migration time remains unclear, impeding procedures for its control. Herein, for the first time, we propose a concise, universal equation that clearly describes the features of electrophoresis times in nonuniform electric fields and reveals a linear relationship between ion migration time and the degree of dispersion of the electric field (or ion velocity). This discovery should facilitate the rational design for development of electrokinetic techniques to replace previous trial-and-error approaches. We demonstrate the viability of our method using a test case involving the design of an electroextraction device with high efficiency and matrix tolerance. The device with the optimized electric field geometry enables the exhaustive enrichment of crystal violet from a milliliter-level donor phase into a microliter-level acceptor phase within 540 s and avoids agitation. The direct, rapid electroextraction of trace analytes from complex essential oils and honey samples within 300–480 s is realized using this device, which is further coupled with liquid chromatography-mass spectrometry. The resulting analytical method displays low limits of detection (0.01–0.06 ng·mL−1; 0.08–0.30 μg·kg−1), overcoming the issues of time- and labor-intensive and error-prone sample preparation.