In capillary electrophoresis (CE), chemical and dynamic modification of a fusedsilica surface can be used to control the electroosmotic flow (EOF), to minimize the interaction of analytes with the capillary surface and to modify the selectivity. Polyvalent, core-based hyperbranched polyethyleneimine (CHPEI) polymer was utilized as either a static or dynamic coating in this study. A CHPEI-coated capillary was simply constructed in a rinsing fashion or by adding CHPEI in a running buffer. Two generations of CHPEI, CHPEI5 and CHPEI25, were investigated with the concentration ranges from 5-20% (w/v) for the static-coating method and 0.6-1.25% (w/v) for the dynamic-coating method.

In CHPEI static-coated capillaries, several parameters were studied as follows: (1) EOF as a function of buffer pH; (2) effect of coating media (NaCl solution) concentration; (3) effect of buffer concentration; and (4) stability and reproducibility of the coating. The performance of a static-coated capillary was investigated in the separation of a wide variety of compounds, such as phenols, basic amino acids, B vitamins, aniline and its derivatives. From these studies, CHPEI25 was selected as a dynamic coating. Performance of CHPEI25 dynamic-coated capillaries were investigated by varying the polymer concentration. Test analytes were B vitamins, aniline and its derivatives. Migration-time repeatability can be problematic in static-coated capillaries due to a coating depletion. Therefore, utilization of a hybrid-coating technique was proposed and examined.

Further investigation of the dynamic-coating method was undertaken with a 1.25% CHPEI25 dynamic-coated capillary and a commercial eCAP™ capillary from Beckman Coulter. Although primary-amine surface groups are predominant in CHPEI coating and tertiary amines are the major surface groups in eCAP™, the separation performance of both capillaries is comparable. For basic drugs and related compounds, as well as, with B vitamins analysis, it is evident that CHPEI25 dynamic-coated capillary provides significantly improved peak resolution under identical separation conditions compared to that in eCAP™.