UHPLC-MS/MS Determination of Paraquat in Tea

UHPLC-MS/MS was applied to the determination of paraquat in tea. (2±0.02) g of homogenized sample of tea was extracted with H₂SO₄ (7+13) solution. The extract (5 mL) was purified by passing through SPE column, and then 15 mL of 5 mol·L^-1 NH₄Cl solution was used for elution. 12 mL of 12 mol·L^-1 NaOH solution and 1 mL of 10 g·L^-1 potassium ferricyanide solution were added in the eluate. The solution was extracted twice with dichloromethane (20 mL each). The combined dichloromethane layer was evaporated to dryness. The residue was taken up with 2 mL of acetonitrile (1+9) solution, and then the solution was filtered on 0.22 μm organic filtering membrane. Waters BEH UPLC C₁₈ chromatographic column was used as stationary phase, and the mixture of acetonitrile and acetonitrile (5+95) solution mixed in different ratios was used as mobile phase in gradient elution. ESI⁺ and multi-reactions monitor mode were adopted in MS/MS. Linear relationship between values of peak area and mass concentration of paraquat was kept in the range of 0.001-0.1 mg·L^-1, with detection limit (3S/N) of 0.5 μg·L^-1. Tests for recovery were made by standard addition method at the concentration levels of 10, 100 μg·kg^-1, giving values of recovery of 71.0%, 89.2% and RSD's (n=6) of 11%, 1.1%, respectively.