Historical Development

The development of TLE occurred in tandem with that of paper electrophoresis (Table 1). This is not surprising, since both techniques require similar apparatus and reagents, and are generally used to achieve the same types of separation. TLE has always had an advantage over the paper technique in terms of chromatographic performance. The finer and more uniform surface structure achievable on thin layers results in considerably reduced band broadening when compared with fibrous media.

The earliest experiments were carried out in the 1940s using paper and layers of silica gel. A wide range of analytes was separated, largely employing aqueous systems. The high conductivity of the aqueous systems limited the applied potential to 10-50 V cm"1 but the electrophoretic separations achieved at these potentials were still a considerable improvement on those obtained by the equivalent paper chromatography/TLC separations. The run times were typically of the order of 1-3 h, but some experiments, particularly protein separations, were run for as long as 24 h.

The development of gas chromatography (GC) and high-pressure liquid chromatography (HPLC)

Table 1 Developments in thin-layer electrochromatography

1937 Earliest recorded use of paper electrophoresis, sep aration of snake venom proteins followed by UV detection (Konig) 1946 Earliest recorded use of 'thin-layer' electrophoresis, in a slab of silica jelly. Method used for the separation of amino acids and peptides (Consden etal.) 1954 First use of electroosmotic flow as driving force to effect a separation. Polysaccharides separated on collodion membranes (Mould and Synge) 1961 Separation of amines and amino acids by thin-layer electrophoresis (Honnegar) 1963 Investigation of the characteristics of solution flow in thin-layer electrophoresis on a range of thin-layer chromatography (TLC) media. (Kowalczyk) 1974 High-speed separation of organic compounds on silica TLC plates and in columns in electric fields (Pretorius et al.) 1994 Planar electrochromatography on non-wetted thin layers (Pukl etal.)

1998 TLE separation of non-polar dyes on commercial reversed-phase TLC plates using electroosmotic flow (Nurok et al.)

1999 Quantification of electroosmotic and separation of basic pyrimidines by thin-layer electrochromatography (Howard and Shafik)

enabled high-efficiency separations to be achieved and diverted attention away from planar techniques. This led to thin-layer electrophoresis/electro-chromatography being largely abandoned in the late 1960s in favour of the column techniques. Very few publications between 1970 and 1998 cite the use of TLE.

Following a period of active research into planar techniques between 1940 and 1960, interest in TLE has been sporadic at best. Long periods of inactivity have been punctuated by occasional reports of technical advances and/or applications of the technique. Possibly one of the most important of these is a paper by Pretorius et al., which described very high-speed separations both on TLC plates and in columns utilizing EOF to mobilize solvent. This paper set a precedent, by using nonaqueous and low aqueous solvents, potential gradients of around 1000 V cm"1 could be used - potentials at least five times greater than had been previously employed. The experiments carried out in columns were quickly followed up by several other research groups and are considered the direct predecessor of modern capillary electrophoresis (CE) and CEC.

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