HPLC-DAD and LC-ESI-MS analysis of doxycycline and related impurities in doxipan mix, a medicated premix for incorporation in medicated feedstuff

HPLC-DAD and LC-ESI-MS methods have been developed for the analysis of doxycycline (DOX), including the identification of the related impurities metacycline (MTC) and 6-epidoxycycline (EDOX) and its determination in a medicated premix. The chromatographic separations have been performed on Luna C18 stationary phase and on Synergi (4 μm) Polar-RP 80A, using both acidic (pH 2.5) and basic (pH 8.0) mobile phases. The Synergi Polar-RP column, in combination with a mobile phase of oxalic acid (0.02 M; pH 2.5)-acetonitrile 82:18 (v/v), allowed the complete separation of MTC, EDOX and DOX. The same separation was also obtained using Luna C18 stationary phase with a pH 8 mobile phase. Application of a LC-ESI-MS system and MS/MS analysis, using both positive and negative polarity, allowed the peak identity to be confirmed. A method based on Luna C18 column and UV detection at 346 nm was validated for the determination of DOX in a medicated premix for incorporation in medicated feedstuff.