Ampholytes as background electrolytes in capillary zone electrophoresis: Sense or nonsense?: Histidine as a model ampholyte

A lot of phenomena, occurring in capillary zone electrophoresis (CZE), are linked with the ionic concn. of the background electrolyte (BGE). If weak bases and acids are used as BGEs in CZE, at a pH where they are scarcely ionized, the ionic concn. of the BGE is very low and this brings a strong peak broadening, limited sample stacking and low sample load. Because the electromigration dispersion increases extremely, moreover, the existence of low-cond. BGEs in CZE is a contradiction in terms. The behavior of ampholytes as BGE in CZE is examd., by means of histidine as a model ampholyte. For BGEs consisting of histidine, important parameters, including the ionic concns., buffer capacity, transfer ratio, and the indicator for electromigration dispersion E1m1/E2m2, are calcd. at various pH. Although the transfer ratio is fairly const. over the whole pH traject, the ionic concn. and buffer capacity decrease whereas the electromigration dispersion strongly increases near the pI of histidine. I.e., that ampholytes can be applied as BGEs in CZE, however, just not at pH near their pI value, except as the difference between the pK values of the basic and acidic group, the Delta pK value, is very small. For ampholytes with a low Delta pK value or at high concns., all the before-mentioned effects are less fatal, but in that case we can not speak of a real low-cond. BGE. If ampholytes are used at pH near their pK values, the use of ampholytes as BGE is not advantageously compared with simple weak bases and acids. This has been confirmed by calcns. and expts. [on SciFinder (R)]