Separation and Identification of Impurity of Chlortetracycline Hydrochloride Not Included in the European Pharmacopoeia

Drug impurities can affect the clinical efficacy of medicines and may even present substantial health risks. Consequently, the regulation and investigation of such impurities are critically important. In this study, we observed that the chemical structure of impurity E in chlortetracycline hydrochloride, as described in the European Pharmacopoeia, was inconsistent with that of impurity E in the chlortetracycline for system suitability CRS (hereinafter referred to as Impurity 1), as provided by the European Directorate for the Quality of Medicines & HealthCare. To separate and identify the chemical structure of Impurity 1, which is not documented in the European Pharmacopoeia for chlortetracycline hydrochloride, 2D-LC-HRMS/MS and HPLC were employed to compare the discrepancies in the chromatographic behavior of Impurity 1 and impurity E, with their chemical compositions further confirmed by NMR and high-resolution mass spectrometry. Results showed that Impurity 1 and impurity E exhibited inconsistent chemical structures and chromatographic behaviors. Therefore, Impurity 1 and impurity E were identified as two distinct compounds, with impurity E representing the process impurity and Impurity 1 serving as the degradation impurity. These findings provide a comprehensive understanding of impurity in chlortetracycline hydrochloride, as this impurity is reported for the first time.