CE of tricyclic antidepressant clomipramine and metabolites: electromigration and wall adsorption

CE of tricyclic antidepressants clomipramine and its metabolites demethylclomipramine, didemethylclomipramine and 8-hydroxyclomipramine resulted in partly extremely tailing peaks in bare fused-silica capillaries. Especially at high pH of the BGE this behavior was not unexpected as adsorption of the cationic analytes onto the negatively charged wall due to electrostatic attraction can be supposed. Less expected was the observation that peak tailing could not be overcome neither by using a capillary with dynamic coating with cationic CTAB added to the BGE, nor by the usage of a capillary permanently coated with polyvinyl alcohol (PVA), both operated at acidic pH. As this tailing was even more pronounced than with bare fused silica, and was suppressed upon addition ofMeCN to the BGE, nother source of adsorption than pure ion–ion interaction seems plausible. In the bare silica apillary the mobility, m, of the analytes followed roughly the pHdependence of a monoacidic base, but two deviations from the sigmoid theoretical curve were evident: (i) even at low pH the mobilities were not constant; they decreased in contrary withpHover the entire range; (ii) the apparent pKa values of two analytes, derived at the pH with halve the mobility at low pH, are significantly smaller than the thermodynamic pKa. Upon modifying the expression for m = f(pH), and considering thepHdependence of the negative charge density at the wall by an additional term which takes chromatographic retention into account, an equation was derived which enables the description of the observed electromigration of the analytes as function of pH, pKa of analytes and surface silanol groups, actual mobility of analytes, distribution coefficient (or retention factor) due to adsorption including itspHdependence. The interplay of electrophoretic movement and residual adsorptive retention allowed to resolve the analytes finally in an uncoated capillary, namely at pH 7.65 (30 mM ionic strength), whereas at the cost of the robustness of the separation system.