Fig. (4) Chromatogram of the separation of tetracycline and related antibiotics on HSS C18. Peaks obtained: 4-epioxytetracycline (1), oxytetracycline (2), tetracycline (3), alpha-apo-oxytetracycline (4), chlortetracycline (5), beta-apo-oxytetracycline (6), doxycycline hydrochloride (7)