Rapid pKa estimation using vacuum‐assisted multiplexed capillary electrophoresis (VAMCE) with ultraviolet detection

A rapid approach for estimating the pKa value of small organic molecules was developed using vacuum‐assisted multiplexed capillary electrophoresis (VAMCE) with ultraviolet detection. The VAMCE method employed a 96‐capillary array, arranged in a standard 8 × 12 microtiter plate configuration, with each row of capillaries filled with 12 individual running buffers of equal ionic strength (I = 50 mM) covering a pH range from 2.2 to 10.7. A separate compound was injected hydrodynamically into each row of capillaries allowing the estimation of pKa values for eight compounds in a single run. The application of a vacuum during the separation generated a bulk fluid flow and allowed the electrophoretic separation to be completed within 5 min. The complete VAMCE method, conditioning, and electrophoretic separation was optimized to allow the pKa estimation for between 128 to 168 compounds in an 8‐h period. The VAMCE method provided a reliable approach for estimating pKa values both within‐ and between‐day. The pKa values for a series of 96 compounds estimated by VAMCE agreed well with some of literature pKa values with an average absolute difference of 0.22 log units. © 2005 Wiley‐Liss, Inc. and the American Pharmacists Association J Pharm Sci 94:576-589, 2005