Mixtures of tetracycline and 4-epitetracycline were assayed utilizing the large difference in their circular dichroism spectra at 262 nm. The assay was evaluated using synthetic mixtures and applied to the analysis of commercial tetracycline hydrochloride capsules. The results obtained with the newly developed procedure compare favorably with a published spectrophotometric method. In comparison with other techniques, the proposed method has several advantages. It is specific in that it permits differentiation between tetracycline and its kpimer, permitting calculation of the actual level of each epimer. It is accurate and precise, having an average error of 2% for tetracycline detection and 3 . 3 x for 4epitetracycline. It involves a simple procedure that requires no prior separation of degradation products under usual use conditions. Furthermore, it is very rapid, requiring only the experimental measurements of ellipticity at 262 nm. and absorbance at 356 nm. The disadvantages of the assay are its inability to determine analytically the content of anhydrotetracyclines and the possible interference of these materials if they are present in large amounts. However, levels of 10% anhydrotetracyclines affect the assay for tetracycline by only 1 %. In the analysis of eight commercial products, the tetracycline content varied from 92 to 11 1 % of the labeled amount and 4-epitetracycline was present in amounts of 3-17z. When compared with a spectrophotometric assay that required prior separation of anhydro compounds, the method introduced here gave the same analytical results for tetracycline content in the various products. Key phrases 0 Tetracycline-epitetracycline mixtures-analysis by circular dichroism, determination in commercial products, compared with spectrophotometric method Epitetracycline-tetracycline mixtures-analysis by circular dichroism, determination in commercial products, compared with spectrophotometric method 0 Degradation products-circular dichroism analysis of tetracycline-epitetracycline mixtures, applied to commercial products 0
Circular dichroism-used to analyze tetracycline-epitetracycline mixtures, applied to commercial products ~~