Separation of acidic solutes by nonaqueous capillary electrophoresis in acetonitrile-based media - Combined effects of deprotonation and heteroconjugation

Nonaqueous capillary electrophoresis (NACE) is a chemical separation techni que that has grown in popularity over the past few years. In this report, w e focus on the combination of heteroconjugation and deprotonation in the NA CE separation of phenols using acetonitrile (ACN) as the buffer solvent. By preparing various dilute buffers consisting of carboxylic acids and tetrab utylammonium hydroxide in ACN, selectivity may be manipulated based on a so lute's dissociation constant as well as its ability to form heterogeneous i ons with the buffer components. ACN's low viscosity, coupled with its abili ty to allow for heteroconjugation, often leads to rapid and efficient separ ations that are not possible in aqueous media. In this report, equations ar e derived showing the dependence of mobility on various factors, including the pK(a) of the analyte, the pH and concentration of the buffer, and the a nalyte-buffer heteroconjugation constant (K-f). The validity of these equat ions is tested as several nitrophenols are separated at different pH values and concentrations. Using nonlinear regression, the K-f values for the het eroconjugate formation between the nitrophenols and several carboxylate ani ons are calculated. Also presented in this report are the NACE separations of the 19 chlorophenol congeners and the 11 priority pollutant phenols (use d in US Environmental Protection Agency methods 604, 625/1625 and 8270B). ( C) 2000 Elsevier Science B.V. All rights reserved.