Behavior of cation-exchange materials in capillary electrochromatography

The behavior of a strong, cation-exchange material (propanesulfonic acid, S CX) has been studied in capillary electrophoresis (CE) and capillary electr ochromatography (CEC) by the use of coated and packed capillaries. In aqueo us electrolytes, the SCX-coated capillary showed a far more consistent elec troosmotic now over the pH range 3.6-10.5, compared to untreated fused sili ca, However, in similar electrolytes containing 80% (v/v) acetonitrile, bot h coated and untreated capillaries performed similarly, casting doubts upon the stability of the SCX coating. The effect of voltage and mobile-phase p arameters such as pH, ionic strength, and organic content was studied in CE C for both 3-mu m SCX and C-18 packing materials, and the results were comp ared in terms of linear velocities, currents, and conductivities. Only at p H 5 and below was a higher EOF velocity than expected observed for the SCX column. In accordance with theory, the EOF was seen to increase with decrea sing ionic strength for the C-18 column. However, for the SOL column, this was not the case: the EOF showed a general reduction as the ionic strength was decreased. The greatest anomaly was observed on changing the acetonitri le composition: the EOF showed a consistent decline with increasing organic , whereas the EOF in both the open capillary and C-18 column decreased and then started to rise with acetonitrile contents above 70% (v/v).