Simultaneous Direct Determination of Glufosinate Ammonium and Its Metabolite Residues in Milk by Ultra-high Performance Liquid Chromatography-Tandem Mass Spectrometry

Using multi-walled carbon nanotubes (MWCNTs) as a novel sorbent and a new special anionic chromatographic column for pesticide residues, an ultra-high performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) method was developed for the simultaneous direct determination of glufosinate ammonium and its metabolites 3-methylphosphino-propionic acid and N-acetylglufosinate ammonium in milk. The samples were extracted with methanol, cleaned up using MWCNTs, and degreased by high-speed centrifugation at low temperature. The analytes were separated on an Anionic Polar Pesticide (2.1 mm × 100 mm, 5 μm) column using a mobile phase containing 0.9% formic acid solution (A) and 0.9% formic acid solution-acetonitrile (B). The instrument was operated in the negative ion scanning mode with multiple reaction monitoring. Glufosinate ammonium was quantified by an internal standard method, and its two metabolites by an external standard method. The calibration curves for glufosinate ammonium and its two metabolites were linear in the concentration range of 1–50, 2–50 and 2–50 μg/L with correlation coefficient (R²) greater than 0.99. For these compounds, the limits of detection were 2.5, 5.0 and 5.0 μg/kg, and the limits of quantification were 5.0, 10.0 and 10.0 μg/kg, respectively. The recoveries ranged from 79.35% to 101.80%, with relative standard deviation (RSD) between 1.15% and 8.63%. The proposed method is characterized by simple pretreatment, stable chromatographic conditions, low impurity interference, high sensitivity and good accuracy, and is suitable for the determination of glufosinate ammonium and its metabolite residues in milk.