Manipulation of separation selectivity for inorganic anions in capillary zone electrophoresis using control of electrolyte pH

The influence of the pH of chromate-tetradecyltrimethylammonium bromide-based electrolytes on the separation selectivity of anions using capillary zone electrophoresis was studied. Changes in migration order over the pH range 8–13 were marked for weak acid anions at or near their pKa due primarily to changes in effective charge. Precipitation problems encountered for electrolyte pH values less than 8 were prevented by the incorporation of small (less-than-or-equals, slant7.5% v/v) amounts of 1-butanol as additive. Using this approach the pH could be lowered to 7. Manipulation of electrolyte pH was used to increase the resolution between fluoride and phosphate, making possible the fully resolved separation of trace fluoride (1 μmg ml−1) in the presence of excess phosphate (> 800 μg ml−1). The separation of fluoride from phosphate in toothpaste is also shown.