Thermal decomposition of tetracycline and chlortetracycline

• Tetracycline and chlortetracycline antibiotics were submitted to thermal analysis by TG and DSC.
• Evolved gas analysis for both were performed by TG-FTIR coupling.
• Tetracycline decomposed after melting in isocyanic acid, HCl and dimethylamine.
• Chlortetracycline decomposed in isocyanic acid, HCl and dimethylamine. Melting not detected.

The thermal behavior of tetracycline hydrochloride (TC) and chlortetracycline hydrochloride (CTC) antibiotics was evaluated using thermoanalytical techniques TGA-DTA, DSC and evolved gas analysis by TGA-FTIR, providing information regarding thermal stability, decomposition steps and melting. TGA-FTIR revealed hydrochloric acid, water, isocyanic acid, dimethylamine, methane as main decomposition products for both antibiotics. Ammonia and carbon dioxide from the decomposition of the isocyanic acid have also been detected. According to DSC data the melting process occurred just before decomposition at 220.9 °C for TC. Tentative mechanisms for the thermal behavior of TC and CTC are presented.