Towards the ultimate minimum particle diameter of silica packings in capillary electrochromatography

Porous silica beads with an average particle diameter between 0.2 and 3 mu m have been applied as packing material in capillary electrochromatography (CEC). The experiments were directed to investigate whether it is really fe asible and as promising as expected to use such small particles. In CEC, pl ate heights of H approximate to 1-2 d(p) can be achieved which is smaller t han the plate heights usually attained in high-performance liquid chromatog raphy. Using a capillary packed with 0.5 mu m silica beads we achieved a pl ate height of H = 3 d(p) indicating the presence of dispersive effects like Joule heating. Calculations demonstrate that at a field strength of about 800 V cm(-1) one third of the plate height can be lost by Joule heating eff ects if the heat is not removed by a cooling system. Additionally, the H(u) curve is still descending at the maximum electroosmotic how (EOF) velocity we generated with the modified capillary electrophoresis instrument. To fu lly exploit the potential of submicron size silicas higher held strengths, i.e., higher EOF velocities, must be attained. To study the influence of th e kind of packing on the EOF porous as well as nonporous silicas have been applied. The experiments clearly indicate that the EOF of porous and nonpor ous silicas is the same. Since the EOF is more or less exclusively generate d by the packing material the zeta potential of n-octyl bonded 0.5 mu m sil ica has been determined. The dependence of the zeta potential on the pH is identical to the dependence of the EOF on the pH in a packed capillary. The point of zero charge of the silica is at pH 2-3. (C) 2000 Elsevier Science B.V. All rights reserved.