4.1 Introduction and History
The separation of compounds based on their movement when exposed to an electric field was first observed in 1807 by Ferdinand Friedrich Reuß, who noticed the movement of clay particles in water when a constant electric field was applied. The theory of electrophoresis was refined in the early 1900s by Marian Smoluschowski, and further refined in 1937 by Arne Tiselius, who won the 1948 Nobel Prize in Chemistry for his work. Electrophoresis was initially conducted in polyacrylamide or agarose gels on a slab—a technique called slab gel electrophoresis—where charged molecules are applied to one end of the slab and an electric field is applied over the length of the slab. The molecules migrate down the slab at different rates according to the charge-to-size (m/z) ratio. Slab gel electrophoresis is still widely used in the fields of biology and biochemistry on large molecules such as nucleic acids and proteins to show relative concentrations of molecules, the purity of a sample, or, when used in conjunction with a standard, to identify compounds. Slab gel electrophoresis generally has long analysis times (often 20-40 minutes per sample), low efficiency, difficulties in analysis, and is unable to definitively identify compounds in a sample as migration time is not necessarily unique to each compound. Additionally, slab-gel electrophoresis is difficult to automate, making it very time-intensive to run multiple samples.
Capillary electrophoresis (CE) is widely used as an alternative to slab gel electrophoresis. Gel media are not necessary in capillary electrophoresis as capillary tubes are themselves anti-convective. Stellan Hjérten preformed the first work in capillary electrophoresis in 1967, using millimeter-diameter capillary tubes. By the early 1980s, the diameter of capillary tubes had been reduced to 75µm. Capillary electrophoresis generally runs faster than slab electrophoresis, provides better precision and accuracy, uses fewer reagents and is more easily automated. Capillary electrophoresis can also analyze smaller molecules than slab electrophoresis, thus expanding the range of possible analytes. By pairing capillary electrophoresis with mass spectroscopy, it is possible to obtain confirmatory results. This makes capillary electrophoresis an intensely useful and powerful instrument for many scientific disciplines. An overview of a CE system is shown in Figure 4.1.
Figure 4.1 Overview of a modern CE system.
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