Quantitative analysis of oxytetracycline and its impurities by LC-MS-MS

A liquid chromatographic-tandem mass spectrometric method using an Xterra MS C(18) chromatographic column ( 100 mm x 2.1 mm i.d., 3.5microm) that allows complete separation of oxytetracycline (OTC) and the impurities: 4-epi-oxytetracycline (EOTC), tetracycline (TC), 4-epi-tetracycline (ETC), 2-acetyl-2-decarboxamido-oxytetracycline (ADOTC), alpha-apo-oxytetracycline (alpha-AOTC) and beta-apo-oxytetracycline (beta-AOTC) was developed. Gradient elution was used and calibration curves were obtained using the scan mode selected reaction monitoring (SRM). Acceptable correlations were obtained for OTC, TC, EOTC and ADOTC whereas the correlations of alpha-AOTC and beta-AOTC were less accurate resulting in higher limits of quantification (LOQ) and limits of detection (LOD) relative to the other compounds. The intraday and interday accuracy varied for all the compounds from 90 to 112% and the intraday and interday precision were lower than 7.1%. The method was applied for analysis of commercial available ointments containing OTC resulting in an acceptable quantification of OTC and the impurities in the drug preparations. The advantage of this method compared to the other separation methods is an empty separation window right after the large peak corresponding to OTC in the chromatogram, which facilitates an accurate determination of ADOTC and the other impurities.