All chemicals were of analytical grade and all solutions were prepared by using deionized (DI) water. The stock solution of tetracycline (1000 lg mL1) was prepared by dissolving the appropriate amount of its hydrochloride salt in 5.0 M hydrochloric acid.
The solutions are stable for more than 1 month when kept at 4 C and protected from sunlight. The TC standard solutions were prepared daily from the previous solution by careful dilution with 5.0 M HCl.