Capillary electrophoresis of carbohydrates

Capillary electrophoresis has emerged as a highly promising technique for the analysis of mono- and oligosaccharides. The approaches developed for overcoming the lack of chromophoric and fluorophoric functions in most carbohydrates involve the use of indirect photometric detection, amperometry, mass spectrometry, and precolumn derivatization with various tags. The merits and drawbacks of the derivatizing agents, including 2-aminopyridine, 4-amino-benzoic acid and its analogues, which for the first time permitted the reproducible determination of aldoses, uronic acids and even ketoses in the low femtomole range by means of readily available UV detection, and other agents such as 8-aminonaphthalene-1,3,6-trisulphonic acid, 1-phenyl-3-methyl-5-pyrazolone and 3-(4-carboxybenzoyl)-2-quinoline-carboxaldehyde, are discussed in detail. Means to secure electromigration of the usually neutral carbohydrates are: (i) ionization of hydroxyl groups at high pH; (ii) complexation of vicinal or alternate hydroxyl groups with borate or other charged compounds such as alkaline earth metal ions; (iii) derivatization with a reagent possessing ionizable functions; and (iv) partitioning into a pseudostationary phase such as sodium dodecyl sulphate micelles. Each alternative has its own analytical rewards, and combinations of the above mechanisms allow the two-dimensional and perhaps even three-dimensional mapping of oligosaccharides. Pyridylaminated oligosaccharides, for instance, have been separated both according to size by exploiting differences in the charge-to-mass ratio, with the charge being identical for each oligomer under acidic conditions due to protonation of the imino group incorporated by precolumn derivatization, as well as on the basis of structural differences, as a consequence of differences in the ease of borate complexation of the peripheral monosaccharide residues. It is also shown that the 4-aminobenzonitrile derivatives of mono- and disaccharides can be separated by micellar electrokinetic chromatography with a resolving power superior to that achieved by capillary zone electrophoresis of sugar-borate complexes. Based on the progress made, it can be concluded that capillary electrophoresis represents a powerful alternative and complement to existing methodology in the area of carbohydrate analysis.