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-μm SCX and C(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(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(18) column decreased and then started to rise with acetonitrile contents above 70% (v/v).