Dissipation, residues and risk assessment of metaldehyde and niclosamide ethanolamine in pakchoi after field application

- A simple QuEChERS coupled with LC-MS/MS method was established.
- The method was applied to field samples determination.
- The half-lives of metaldehyde and niclosamide ethanolamine were 1.7-9.5 d.
- The associated risk of these molluscicides in pakchoi for Chinese people is low.

A method using LC-MS/MS after QuEChERS preparation for the simultaneous determination of metaldehyde and niclosamide ethanolamine residues in soil and pakchoi has been developed and validated. The mean recoveries were ranged from 90% to 101% with RSDs (relative standard deviations) less than 9.2%. The dissipation results showed that the half-lives of metaldehyde and niclosamide ethanolamine were 2.3-4.3 d and 1.7-9.5 d, respectively. The terminal residue results indicated that the residues of metaldehyde in pakchoi were lower than the temporary maximum residue limits (MRL) set by China on 1 d after last treatment and the maximum residue of niclosamide ethanolamine in pakchoi was 0.54 mg/kg. The risk quotients of metaldehyde and niclosamide ethanolamine were ranged from 0.015 to 0.033 and from 0.00064 to 0.0014, respectively. This work could provide guidance on reasonable use of these molluscicides and aid in the establishment of MRL in China.