Dynamic on-column pH monitoring in capillary electrophoresis: Application to volume-limited outlet vials

With capillary electrophoresis, buffer pH must be constant to achieve consistent migration times. Irreproducible separations have been attributed to pH changes due to water hydrolysis in the inlet/outlet vials. A method of measuring the pH of the electrolyte on-column is described that uses wavelength-resolved fluorescence detection. C.SNARF-1 is a fluorescent pH indicator that has a large change in fluorescence emission profile depending on pH. When it is incorporated into the running buffer, monitoring the pH-dependent emission spectra of the C.SNARF-1 allows column pH to be calculated. With reduced-volume outlet buffer vials in the nanoliter to low microliter range, significant changes in pH and column conductivity are measured during a single electrophoretic run, with pH fronts greater than 3 units passing a fixed point on the capillary over a several second period. These changes appear to be caused by reverse-migrating OH produced at the capillary outlet by the hydrolysis of water.