Salt effects in capillary zone electrophoresis - III. Systematic and selective factors in the high ionic strength separation of sulphonamides in sodium phosphate buffers

The effect of high concentrations of sodium phosphate buffer (65-210 mM) on the capillary zone electrophoretic separation of 23 sulphonamides and the neutral marker have been examined at pH 7 and 18 kV. In 65 mM phosphate, all ionised analytes (21 of the 23 sulphonamides) are resolved sufficiently fcr screening purposes. At higher buffer concentrations, general resolution clearly improves due to the systematic salt effect, which provides increased separation space. This effect is progressively enhanced by Joule heating via increases in the electrophoretic mobilities of the analytes migrating away from the detector. However, one pair of analytes displays unfavourable selectivity changes and comigrates at higher buffer concentrations. This has been demonstrated to be due to selective but dissimilar salt effects upon the degrees of ionisation of the two analytes that leads to a convergence of their electrophoretic mobilities. Eighteen sulphonamides were baseline-resolved between 174 and 210 mM sodium phosphate buffer. (C) 1998 Elsevier Science B.V. All rights reserved.