RAPID HPLC DETERMINATION OF TETRACYCLINE ANTIBIOTICS IN MILK

A previously described method using on-line concentration and gradient elution was modified for more rapid isocratic analysis. Milk (5 mL) w as extracted/deproteinized with 1 mL of 1 N HCl and 15 mL of acetonitr ile. The resulting filtrate (12 mL) was evaporated directly, or the wa ter layer resulting from addition of hexane and methylene chloride was evaporated. The extract could be concentrated to about 1 mL without s ignificant degradation of tetracyclines. The concentrates were filtere d. For analysis, a Polymer Laboratories PLRP-S column was used with a mobile phase of 0.02 M H3PO4 and 0.01 M sodium decanesulfonate-acetoni trile, 72 + 28 for oxytetracycline and tetracycline and 68 + 32 for ch lortetracycline. The injection volume was 200 mu L with UV detection a t 380 nm. Average recoveries were greater than 80% with detection limi ts of 2-4 ppb.