Evaluation of the stability of chlortetracycline in granular premixes by monitoring its conversion into degradation products

A methodology for the evaluation of the stability of chlortetracycline (CTC) in granular premixes is described. This methodology is based on the monitoring of the conversion of CTC into its degradation products by an improved gradient liquid chromatography (LC) method, based on one previously described by our laboratory. Sample preparation involves the extraction of CTC and its degradation products prior to LC analysis, using acidified methanol as extraction solvent. The gradient elution LC method proved to be very sensitive, especially towards the late eluting anhydro derivatives. The use of a Hypersil C8 BDS, 5 μm, 250 mm × 4.6 mm i.d. column is recommended since this column allowed a complete separation of the different impurities from each other and from the main component CTC. The applicability of this approach was demonstrated by the analysis of stability samples.