Abstract:

Tea is an important cash crop in China， and pesticide residues in tea products are among the main issues affecting tea exports. Glufosinate （GLU） is a widely used herbicide in tea gardens and its residues in tea samples have raised health concerns. However， there are few reports on analytical methods for the simultaneous determination of GLU and its metabolite residues in tea. Therefore， the development of suitable analytical methods to monitor the content of GLU and its metabolite residues in tea is essential to ensure compliance with the export quality standards of tea products. In this study， a sensitive analytical method was developed for the determination of GLU and its two main metabolites， N-acetyl-glufosinate （NAG） and 3-［hydroxy（methyl）phosphinoyl］propionic acid （MPP）， in tea. The method is based on ultra-high performance liquid chromatography-tandem mass spectrometry （UHPLC-MS/MS） coupled with precolumn derivatization. Tea samples （2.00 g） were extracted with 20 mL of ultrapure water by shaking， and followed by centrifugation. After freezing treatment， aliquots of the extract were centrifuged at 15 000 r/min. Then 200 μL of the supernatant was evaporated to complete dryness using nitrogen blowdown. The resulting residue was derivatized with triethyl orthoacetate （TEOA） at 80 ℃ for 1.5 h， in the presence of acetic acid as the catalyst. The derivatization mixture was evaporated to dryness and reconstituted with methanol. NAG was transformed into the same reaction product as GLU， and therefore quantified together with GLU. The derivatization products were separated on an Agilent ZORBAX Eclipse Plus C18 column （100 mm×2.1 mm， 1.8 μm）. The mobile phase consisted of an aqueous solution of 0.1% （V/V） formic acid （containing 5 mmol/L ammonium formate） as phase A and acetonitrile as phase B. The gradient elution was carried out as follows： 0–0.5 min， 90%A； 0.5–7.5 min， 90%A–80%A； 7.5–8.0 min， 80%A–10%A； 8.0–10.0 min， 10%A； 10.0–10.1 min， 10%A–90%A； 10.1–12.0 min， 90%A. The separated compounds were detected by tandem mass spectrometry with positive electrospray ionization （ESI⁺） in multiple reaction monitoring （MRM） mode， and stable isotope-labeled internal standards were used for quantitative analysis. Good linearity was obtained in the 0.5–20 μg/L range； the correlation coefficients （r²） were higher than 0.995， the limits of detection were 2.2–5.3 µg/kg and the limits of quantification were 25 µg/kg. For two representative matrices of green tea and black tea， the average recoveries of GLU and MPP spiked at four levels of 0.025， 0.05， 0.1， and 0.5 mg/kg were 83.9%–105.0%. The intra-day precisions （n=5） ranged from 1.9% to 6.9% and the inter-day precisions （n=10） ranged from 2.6% to 10.3%. Furthermore， the developed method was used to detect GLU and its metabolite residues in 34 tea samples， and the positive detection ratio was 26.5%. In summary， the proposed method is simple， reliable， practical， and suitable for determining GLU and its metabolite residues in various types of tea.

Key words: precolumn derivatization, ultra-high performance liquid chromatography-tandem mass spectrometry （UHPLC-MS/MS）, glufosinate, N-acetyl-glufosinate, 3-［hydroxy（methyl）phosphinoyl］propionic acid, triethyl orthoacetate, tea