Dissipation kinetics of spinosad in cabbage (Brassica oleracea L.var. capitata)

Abstract

An analytical method was standardized for the estimation of spinosad residues on cabbage. The samples were extracted with methanol, diluted with sodium chloride solution (5%), partitioned into dichloromethane, and dried over anhydrous sodium sulfate. The extracts were concentrated under vacuum and cleaned up with silica gel mixed with primary secondary amine and MgSO₄. The residues were estimated using a high-performance liquid chromatograph equipped with a photo diode array detector at 244 λ and a C₁₈ reversed-phase column. Consistent recoveries ranging from 85% to 99% were observed when samples were spiked at 0.01, 0.05, 0.10, and 0.20 mg kg⁻¹. The limit of quantification of the method was worked out to be 0.01 mg kg⁻¹. The residues were confirmed by high-performance thin-layer chromatography. A field study was carried out following three applications of spinosad at 15 and 30 g active ingredient ha⁻¹. The average initial deposits of spinosad were observed to be 0.33 and 0.56 mg kg⁻¹ at single and double dosages, respectively. The residue levels of spinosad dissipated below its limit of quantification of 0.01 mg kg⁻¹ after 5 and 7 days at single and double dosages, respectively. The half-life values of spinosad were determined to be 1.4 and 1.5 at single and double dosages, respectively.

Keywords:

• cabbage
• half-life
• HPLC
• HPTLC
• MRL
• spinosad

Acknowledgments

The authors are thankful to the Head of the Department of Entomology, Punjab Agricultural University, Ludhiana, for providing the research facilities. Financial assistance obtained from the Indian Council of Agricultural Research, New Delhi, is gratefully acknowledged.