Determination of sulfamethazine and trimethoprim in liquid feed premixes by HPLC and diode array detection, with an analysis of the uncertainty of the results

Sulfamethazine (SMZ) and trimethoprim (TMP) are antibacterials used in vete rinary practice. This paper describes a method for their determination in v eterinary liquid feed premixes that is based on liquid chromatography with diode array detection. Gradient elution with methanol and ammonium acetate achieved excellent separation of the two analytes within 15 min without any interference from the matrix. Absorbance of the column effluent was monito red at 264 nm for SMZ and at 230 nm for TMP. Detailed analyses of the uncer tainties of determinations afford estimated expanded uncertainties of, resp ectively, 0.2 and 0.1 w/v % for typical SMZ and TMP concentrations of 10.7 and 2.1 w/v %, respectively. At the lower end of the calibrated range of th e method, the dominant source of uncertainty is the preparation of standard s and the construction of the calibration line.