Behavior of cation-exchange materials in capillary electrochromatography

The behavior of a strong, cation-exchange material (propanesulfonic acid, SCX) has been studied in capillary electrophoresis (CE) and capillary electrochromatography (CEC) by the use of coated and packed capillaries. In aqueous electrolytes, the SCX-coated capillary showed a far more consistent electroosmotic flow over the pH range 3.6-10.5, compared to untreated fused silica. However, in similar electrolytes containing 80% (v/v) acetonitrile, both coated and untreated capillaries performed similarly, casting doubts upon the stability of the SCX coating. The effect of voltage and mobile-phase parameters such as pH, ionic strength, and organic content was studied in CEC for both 3-[[micro]meter] SCX and [C.sub.18] packing materials, and the results were compared in terms of linear velocities, currents, and conductivities. Only at pH 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 decreasing ionic strength for the [C.sub.18] column. However, for the SCX 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 acetonitrile composition: the EOF showed a consistent decline with increasing organic, whereas the EOF in both the open capillary and [C.sub.18] column decreased and then started to rise with acetonitrile contents above 70% (v/v).