Quantitative colorimetric method for sulfamethazine in swine feeds

Sulfamethazine (SM) in swine feeds is quantitated colorimetrically by using a relatively simple procedure. The drug is isolated from an extract of feed by trapping it on a small bed of anion-exchange resin buffered at pH 7.9 following the removal of possible interfering compounds by pre-columns. SM is then eluted, diazotized, and coupled using the Bratton-Marshall reagents. The intensity of color developed is determined spectrophotometrically at 540 nm and the concentration of SM is calculated using a molar absorptivity of 52 500. Feeds containing from 1 to 130 ppm can be analyzed with very good accuracy and precision. The method has satisfactory specificity, is inexpensive, and can be applied to a relatively large number of samples per day. A simple and safe method for preparing N1-methyl SM at the low microgram level is described together with a thin layer chromatographic system for tentatively verifying the presence of SM in the feed both as the underivatized drug and as N1-methyl SM.