LAUSR

A new liquid chromatography with tandem mass spectrometry method employing a mixed-mode zwitterionic stationary phase was developed for simultaneous determination of urease inhibitor (N-butylthiophosphoric triamide) and nitrification inhibitor (dicyandiamide) in urea fertilizer. Molecular modeling based on density functional theory calculations was employed to provide an insight into the interaction mechanism of urea, dicyandiamide, and N-butylthiophosphoric triamide with zwitterionic stationary phase in chromatographic separation system. The detection of analytes was performed on a triple quadrupole tandem mass spectrometer in multiple reaction monitoring mode using positive electrospray ionization. The ion transitions monitored were m/z 85→68 for dicyandiamide and m/z 168.2→74 for N-butylthiophosphoric triamide, respectively. The standard calibration curves of dicyandiamide and N-butylthiophosphoric triamide were linear over the range of 1.0 ̶ 15 ppm (coefficient of determination = 0.9984), 0.05 ̶ 1 ppm (coefficient of determination = 0.9995), with limit of detection of 25 and 5 ppb, respectively. The recoveries of low, middle, and high concentrations were from 96.7 to 105.8% for N-butylthiophosphoric triamide and 94.4 to 105.8% for dicyandiamide with accuracy (relative error %) of ≤5.8% and ≤5.8%, the precision (coefficients of variation) was ≤2.0% and ≤2.9%, respectively. The validated method was successfully applied on real urea samples to determine N-butylthiophosphoric triamide and dicyandiamide simultaneously.