ABSTRACT
The migration of charged species under the in uence of an externally applied electric eld is known as electrophoresis. Differences in the mobility of the analytes due to their average charge, size, shape, and properties of the used electrolyte solution form a basis of a valuable separation method in chemistry.
According to Ref. 1, a Russian physicist Reuss carried out the rst separations based on this principle already in 1809. He studied the migration of colloidal clay particles and discovered that the liquid adjacent to the negatively charged surface of the wall migrated toward the negative electrode under the in uence of an externally applied electric eld. Theoretical aspects of this electrokinetic phenomenon (electroosmosis by Reuss) were formulated in 1897 by Kohlrausch [2]. In the late 1800s and early 1900s, electrophoretic separations were carried out by several researchers in so-called U-shaped tubes. Starting in 1925 with his PhD thesis on the development of free moving-boundary electrophoresis, Tiselius advanced the analytical aspects of electrophoresis. This resulted in the separation of complex protein mixtures based on differences in electrophoretic mobilities [3]. In 1948, Tiselius was awarded the Nobel Prize for Chemistry for his work on electrophoresis. Possibilities of performing electrophoresis in capillaries were investigated by Hjertén [4], Everaerts [5], and Virtanen [6] but their work did not draw much attention to capillary electrophoresis (CE) until the studies of Jorgenson and Lucas appeared. They separated uorescent dansylated amino acids in a glass capillary with an i.d. of 75 µm. Applying voltages up to 30 kV, they provided the ef ciency of more than 400,000 theoretical plates within 25 min [7]. This ef ciency, not seen in separation science before, was mainly due to the fact that at diameters <100 µm the capillary wall dissipated the Joule heating generated in the buffer by electric current. Previously, that is, with capillaries having an i.d. over 100 µm, the analyte peaks were heavily broadened due to the unevenly distributed heat in capillaries. After the decrease of the capillary i.d., broadening was markedly diminished and the determining factor of band broadening has molecular diffusion that, in general, is very low in liquids. After the landmark work by Jorgenson, interest in CE started to grow rapidly [8]. Although CE was initially heralded for its speed and low sample volume, the technique was widely accepted because it is quantitative, can be automated, and will separate compounds that have been traditionally dif cult to handle by high performance liquid chromatography (HPLC). CE played a crucial role in determining the human genome sequence and CE is the basis for virtually all micro uidics for lab-on-a-chip devices. CE can separate polar substances, which are notoriously dif cult to analyze with HPLC. Chiral separations are another area in which the use of CE has expanded. The small sample volumes required for CE can be an advantage with a limited sample amount. An area closely related to CE that has not yet achieved its limits is CEC that combines features of CE with LC by using capillaries packed with chromatographic materials.
