Multi-channel contactless conductivity detection device for online detection of free-flow electrophoresis separation

Free-flow electrophoresis (FFE) is an all-liquid-phase electrophoresis technique without any supporting media, which has both analytical and preparative functions. Compared to other electrophoresis techniques, FFE has been used for the separation of peptides, proteins, cells, and microorganisms due to its advantages of mild separation environment, high recovery, and sustainable separation. Both the online detection of the characteristic parameters for each component solution and the real-time control of the progress of the separation experiment are of considerable importance for the study of FFE separation. Since the existing FFE devices do not have the online detection function, there are obvious deficiencies in their practicability. The absence of online detection function not only made it impossible to track the progress of the separation experiment in real time , but also made it difficult to detect the properties of the component solutions, which still require offline testing after separation. In this study, a multi-channel capacitively coupled contactless conductivity detection (MC-(CD)-D-4 ) device has been developed to solve this problem , and an automatic measurement software has also been developed. The MC-(CD)-D-4 device used a parallel time-sharing contactless conductivity detection technique. It consisted of several contactless conductivity detection modules arranged in parallel, which in turn consisted of a number of contactless conductivity cells that were switched on/off by analog multiplexers for detecting the conductivity of the solution flowing through the cells in real time. The number of cells was equal to the number of components of the FFE. The components were connected to each of the FFE flow channels, such that the MC-(CD)-D-4 device could be used to measure the conductivity of the solution flowing through each channel in parallel online. To verify the performance of the MC-(CD)-D-4 device, calibration was conducted by using potassium chloride (KCl) standard solutions on MC-(CD)-D-4 device. The experimental data showed that the detection range of MC-(CD)-D-4 was 0. 015-2. 5 mS/cm , and the limit of detection (LOD) was 0. 002 mS/cm. The intra-day relative standard deviation (RSD , n = 3) was 2. 31% , the measurement relative error (RE) was 3. 03% , and the measurement difference between channels was 1. 60%. All these data validated that the device had the advantages of wide detection range, low LOD, good repeatability, high accuracy, and low variation between channels. The MC-(CD)-D-4 device was also applied to reciprocating free-flow isoelectric focusing (RFFIEF) electrophoresis for real-time online detection of the conductivity of each component solution during protein focusing. At the start of isoelectric focusing, when the ions had not reached equilibrium loading in the electric field and the pH gradient had not yet been fully developed, there was little difference in conductivity between the different channels and the channel conductivity curve was relatively flat. As the experiment progressed, the proteins gradually started to enrich the anodic end. As the proteins accumulated towards the isoelectric point, their own net charge gradually decreased, and thus, the conductivity of the solution in the channels near the anodic region also decreased. Under sufficient isoelectric focusing, protein enrichment was evident. In the focusing region, the conductivity of the solution in the corresponding channel decreased further. There was also an increase in the conductivity of the solution in the corresponding channel due to the accumulation of ions near the electrode ends. These results showed that the MC-(CD)-D-4 device not only enabled real-time online detection of the conductivity of each component solution in FFE, but also aided in mastering the progress of separation experiment in RFFIEF , thus improving the practicality of the FFE device. Thus , the MC-(CD)-D-4 device, which had the advantages of good performance, small size, simple circuit system, easy installation and commissioning, and low cost, could play an important role in multi-channel measurement, online inspection, and process monitoring.

Automated summary

Free-flow electrophoresis (FFE) is an all-liquid-phase electrophoresis technique with analytical and preparative functions, used for the separation of peptides, proteins, cells, and microorganisms. The development of a multi-channel capacitively coupled contactless conductivity detection (MC-(CD)-D-4) device has enabled online detection of solution conductivity, improving practicality and real-time control.