Doxycycline hydrochloride capsules (100 mg Medochemie, USA) and chlortetracycline hydrochloride capsules (250 mg, F. E. Sillic, Thailand) were used in this study.
A mass powder of ten capsules of doxycycline hydrochloride (Medomycin, 100 mg) or chlortetracycline hydrochloride (Aureomycin, 250 mg) were transferred to each 1000 ml volumetric flask then dissolved in 0.1 M potassium dihydrogen phosphate solution (pH 2 for doxycycline and pH 2.5 for chlortetracycline). Both of sample solutions were filtrated through a 0.45