Capillary Electrophoresis: Instrumentation

Capillary electrophoresis instrumentation consists of a high-voltage power supply connected to an anode and cathode in buffer reservoirs, a fused silica capillary tube coated with polyimide for mechanical strength, a sample vial, and a detector. The capillary tube serves as the separation channel where sample components migrate during analysis. These components work together to generate the electric field and detect separated analytes.

Sample introduction begins by detaching one end of the capillary and its electrode from the buffer reservoir and placing them in the sample vial. Two methods can inject the sample: hydrodynamic injection applies pressure to the sample vial, while electrokinetic injection uses an electric field to drive the sample into the capillary. Both methods load the sample into the separation channel for analysis.

The stacking technique enhances detection sensitivity when sample concentration is low. The sample is injected into a solution with lower ionic strength than the buffering solution, causing sample components to concentrate at the interface between the two solutions. This concentration effect improves signal detection and allows analysis of dilute samples.

Joule heating occurs when electric current passes through the conductive buffer solution, generating heat according to Q = I²Rt. In narrow capillaries, this heat changes buffer viscosity, causing solutes in the capillary center to migrate faster than those near the walls. This results in band broadening and degraded separation quality. Smaller inner diameters and larger outer diameters help minimize this effect.

When high voltage is applied across the buffer system, charged species migrate toward the cathode through electroosmotic flow. Component separation occurs based on their electrophoretic mobility within the electric field. This differential mobility allows ions and molecules with different charge-to-mass ratios to separate as they travel through the capillary.

Capillary electrophoresis employs several detector types to analyze separated components. Absorption detectors measure light absorption, fluorescence detectors detect emitted light, conductivity detectors measure ionic conductivity, and mass spectrometry detectors identify molecular mass. Selection depends on analyte properties and required sensitivity for the application.

The polyimide coating provides mechanical strength to the fused silica capillary tube, protecting it from breakage during handling and operation. This protective layer allows the delicate fused silica capillary to withstand the physical stresses of sample introduction, high-voltage application, and detector integration while maintaining the tube's optical and electrical properties.