Salt effects in capillary zone electrophoresis. IV. Resolution versus time and the effect of potassium phosphate and its concentration in the high ionic strength separation of sulphonamides.

The effects of potassium phosphate buffer and its concentration upon the capillary zone electrophoretic separation of 23 sulphonamides and a neutral marker were examined at pH 7. The resolution between the pairs was improved with the increased concentration of the buffer from 65 mM to 174 mM. Nineteen sulphonamides, a hydrolysis product and several unidentified minor components were baseline resolved in both 101 and 138 mM phosphate buffers. In 174 mM buffer all 21 ionised sulphonamides and the other compounds were separated. A simple relationship between the resolution of analyte pairs (Rs) and the square root of the mean analysis time for the pair (square root of tapp) was derived, but few of the pairs displayed this behaviour. For the majority of pairs of compounds, Joule heating appeared to cause a maximum in the Rs versus square root of tapp relationship, while non-ideality and shifts in ionisation with increasing salt concentration appeared dominant in other cases.