Electrophoresis is based on the differential migration of charged components in an electric field. Until recently, gel electrophoresis and particularly sodium dodecyl sulfate (SDS) polyacrylamide gel electrophoresis (PAGE) were the main electrophoretic techniques used in food analysis. Recently, capillary electrophoresis (CE) has become widely adopted for the monitoring of many food components.
Such rapid development is due to several factors, but especially to speed and versatility. Most often separation can be performed in a few minutes with minimal sample preparation. A new method can be started by filling the capillary with the proper buffer, and new modular instruments can be rapidly fitted with the appropriate detector. Various CE procedures have been developed since the introduction of this technique. Capillary zone electrophoresis (CZE), the basic CE method, consists of a capillary tubing of fused silica with a diameter of 50–150 µm. The tube is filled with a buffer and subjected to an electric field, and under these conditions the positive charged species migrate toward the cathode at a rate determined by their mass‐to‐charge ratios. However, a net movement of the buffer toward the cathode, electroosmotic flow (EOF), is generated within the tube by the ionized capillary wall silanol groups. As a result, all the molecules present in the buffer proceed toward the cathode, the neutral molecules migrate as a single group, and each negative species moves toward the cathode at a rate determined by the difference between the EOF and its anodic attraction. The other main CE techniques are micellar electrokinetic chromatography (MEKC), where an anionic surfactant is added to the buffer to form negatively charged micelles, isotachophoresis (ITP), in which the sample is sandwiched between electrolytes of high and low mobility and the analytes migrate in order of decreasing mobility, and isoelectric focusing (IEF), which is one of the main techniques used in protein gel electrophoresis: amphoteric electrolytes are allowed to migrate in a capillary where they form a pH gradient. In such a gradient, proteins move until they reach their isoelectric point (pI). Many detection techniques have been applied to CE, including direct and indirect ultraviolet (UV) absorption, fluorescence, electrochemical detection and even mass spectrometry (MS).
Most food components can be examined with these techniques, particularly proteins, ions, organic acids, sugars and various plant chemicals.
