Microchip Capillary Electrophoresis

Let’s do a small introduction to this very interesting technique in Analytical Chemistry. In this post, I will explain the basics, for a better and more complete information, please read the references.

A microchip or integrated circuit is basically a very small device on which electronic circuits are fabricated.

In the case of the microchips for Capillary Electrophoresis, our electronic circuit is a conductive liquid solution, so the microchip instead of a printed electronic circuit, will have small channels where the liquid solution is placed.

Capillary electrophoresis is a separation technique based on the differential movement of charged molecules in a conductive medium to which we apply an electric field. Each specie will have a different migration velocity depending on their  charge/radius ratio, hence the different species will be separated. The Electroosmotic flow also influences the separation.

Therefore, Microchip Capillary Electrophoresis will be the implementation of the Capillary Electrophoresis technique inside one of these microchips.

The main benefits of this technique are several, such as: low cost, small size (it’s a microchip!), fast analysis times (seconds or minutes), microchips consume only picoliters of samples, can be added other techniques in the pre-sampling phase or after it (for example, PCR (Polymerase Chain Reaction) can be coupled, or even solid phase extraction, etc.).

As the most important problem is the control of the surface chemistry of microchips CE devices, since this surface chemistry is not very well known.

These microchips are highly related to lab-on-a-chip devices, but we’ll talk about it in another post.

Materials

Most used materials for manufacturing these microchips are glass and different polymers. Polymers are cheaper.

Injection

The injection of the sample and the mobile phase is integrated into the system. There are different ways of injecting the sample into the separation channel, but we will not comment about it in this short summary. Check the references for more information.

Separation

The separation of the different species occurs in the same way as in capillary electrophoresis, being able to apply one of these techniques (adapted to the microchip format):

• Micellar electrokinetic chromatography (MEKC).
• Capillary electrochromatography (CEC).
• Capillary gel electrophoresis (CGE).
• Capillary isoelectric focusing (CIEF).

Detection

The most common detection modes are the following ones:

• Laser-induced fluorescence (LIF): very sensitive but not every analyte is fluorescent. Also, the equipment is expensive.
• Mass spectrometry (MS): Expensive, and large size equipment. But, well, everyone knows the MS advantages.
• Electrochemical detection: in my humble opinion is, generally, the best option (I’m biased for Electroanalysis). It’s cheap, easy to miniaturize and can be integrated in the microchip. Moreover, many molecules are electrochemically active. Of course, not everything is perfect, and sensitivity can be a problem for some applications.

Applications

There are many applications for the MCE, and each day new applications are being researched, with an increasing use of these microchips and the related miniaturized techniques.

Among the most important applications, but not the only ones are:

• DNA separation.
• Protein analysis, aka Proteomics.
• Many biological, environmental, and industrial application.

References

• Microchip capillary electrophoresis: methods and protocols By Charles Sherman Henry.
• Microchip-based capillary electrophoresis: sequencing and beyond by Anthea Hammond.
• Microchip Capillary Electrophoresis: Progress Toward an Integrated Forensic Analysis System by Susan A. Greenspoon, Stephanie H.I. Yeung, Jeffrey D. Ban and Richard A. Mathies.
• Polymer Microchips for Capillary Electrophoresis and Electric Field Gradient Focusing of Biomolecules by Ryan Thomas Kelly.
• Capillary electrophoresis on microchip by Vladislav Dolník, Shaorong Liu, Stevan Jovanovich. Electrophoresis, 2000, Volume 21, Issue 1.
• Capillary Electrophoresis in Wikipedia.

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