Assay and purity control of tetracycline, chlortetracycline and
oxytetracycline in animal feeds and premixes by TLC
densitometry with fluorescence detection
Methods using TLC densitometry with fluorescence detection are described for the assay and purity control of
tetracycline (TC), chlortetracycline (CTC), and oxytetracycline (OTC) in animal feeds and premixes. With a silica gel
layer previously sprayed with 10% (m/v) sodium EDTA solution adjusted to pH 8.0 or 9.0, all the major impurities were
separated from the main components and from each other. The mobile phase consisted of dichloromethane, methanol,
and water. After development, the plate was dipped in a 30% (v/v) solution of liquid paraffin in hexane. Quantitation
was realized by fluorescence densitometry at 400 nm. The limit of quantitation (LOQ) for tetracycline impurities is 0.8
mg/g, corresponding to 0.2% of the label claimed tetracycline (400 mg/g). The LOQ for impurities of tetracycline and
chlortetracycline in premixes is 0.2% of the label-claimed TC (40 mg/g) and CTC (200 or 400 mg/g). The LOQ for
impurities of oxytetracycline in a premix is 0.1% of the label claimed OTC (100 mg/g).
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