How to evaluate the separation efficiency of CZE methods?

In trying to estimate the separation efficiency of Capillary Electrophoresis (CE) methods, the resolution (RS), the number of theoretical plates (N) and the peak-to-valley ratio (p/v) are often used assessment criteria. This study demonstrates that these criteria are not as suitable to describe the separation efficiency in case of Capillary Zone Electrophoresis (CZE) methods as they are for Liquid Chromatography (LC) methods. The investigations were performed by means of a validated CZE method for the evaluation of tetracyclines and their related substances. Four impurities of tetracycline hydrochloride are described in the European Pharmacopoeia. Three were found in the sample used for our investigations, i.e. epi-tetracycline formed by keto-enol-tautomerism, anhydrotetracyclin and epi-anhydrotetracyline. It could be shown that higher values of these assessment criteria like RS do not necessarily represent better separation. Thus, a discussion on the usefulness of separation selectivity and efficiency as assessment criteria for capillary electrophoresis as well as on the introduction of additional parameters is needed.