Research progress of electrically-driven force based online rapid separation and enrichment techniques

Abstract

Sample preparation is a critical step in complex sample analysis, which enables analyte isolation and preconcentration from a complex matrix. Therefore, sample preparation is an effective approach to enhance the sensitivity, selectivity, and accuracy of the analytical method. However, the transfer of target analytes from a random state in the original sample matrix to a highly ordered pre-analysis state involves an entropy reduction process that cannot occur spontaneously. Therefore, sample preparation is always a time-consuming, labor-intensive, and error-prone process. Introducing additional energy or reducing the entropy of the system can enhance the separation and enrichment effects as well as accelerate sample preparation. The introduction of an electric field into an online sample preparation system can not only introduce additional energy into the system, but also drive the directional migration of the sample among the separation, enrichment, and detection processes, ensuring that the entropy reduction progresses smoothly. These advantages of electrically-driven force based online separation and enrichment techniques make them effective for accelerating sample preparation. Typically, there are four acceleration strategies in electrically-driven force based online separation and enrichment techniques: (1) the additional energy of the electric field is added into the system to accelerate mass transfer and energy exchange; (2) electrically-driven flows, including electroosmotic flow and electrophoretic flow, are applied to drive the directional migration of the sample among the separation, enrichment, and detection processes, ensuring that sample preparation and analysis are executed smoothly; (3) the online integration technique is applied to enhance the automaticity of the entire sample preparation and analysis processes, and reduce errors from manual operation; (4) device miniaturization or size reduction methods such as microextraction are applied to enhance the sample preparation efficiency and reduce the time consumed. This review summarizes the progress in electrically-driven force based online rapid separation and enrichment techniques in the last ten years. In this specific research area, more than one hundred research papers are published each year, and can be classified into three types based on the electrically-driven force based online rapid separation and enrichment techniques considered: capillary, microchip, and membrane extraction. Among these, over 50% of the studies focused on electrically-driven capillary force based online rapid separation and enrichment techniques. By applying a high-voltage electric field at the two ends of a capillary, charged species in the capillary can migrate along the direction of the medium. This makes the electrically-driven capillary technique not only fast, highly efficient, and low-cost, but also effective for rapid sample preparation. Typically, two modes of electrically-driven capillary force based online rapid separation and enrichment techniques are employed: online capillary electrophoretic separation and enrichment, and online microextraction-capillary electrophoresis. Device miniaturization from the capillary to microchip through microelectronic mechanical systems and microfluidics enables small-amount sample preparation and analysis, and is also advantageous due to being rapid and efficient, as well as low energy- and sample-consuming. The specific easy-integration trait of microchip devices enables the online integration of multi-step sample preparation and analysis. On the other hand, the controllable electrically-driven force can be used for both, directional flow transfer between different functional units in the microchip as well as for dynamic control of the electrically fluid pump and valve. Online microchip electrophoretic separation and enrichment, and online microextraction-microchip electrophoresis, are two common modes of microchip electrically-driven force based online rapid separation and enrichment techniques. In electrically-driven membrane extraction online separation and enrichment techniques, a supporting liquid membrane is used to eliminate the matrix interference, enabling large-scale real sample application. These techniques have received increasing attention in the research area of electrically-driven force based online separation and enrichment. Overall, rapid separation and enrichment techniques are highly desired in complex sample analysis, and electrically-driven force based online approaches offer significant application potential, especially in food, the environment, and medicine.