Determination of Pesticide Residues in Soil Using a QuEChERS Approach

Soil is a complex matrix consisting of organic and inorganic material. It possesses many active sites that can retain pesticides and other residues. Compared to other matrices, soil samples are more difficult to extract and often require longer extraction times. Certain compounds may be covalently bound to the soil. These can only be removed using acid or base hydrolysis. If a hydrolysis step is employed, this may have a detrimental effect on pH sensitive analytes. This technique was not evaluated in the course of this study.

The aim of this study was to evaluate the effectiveness of the QuEChERS extraction and cleanup approach for pesticides in soil. A group of 21 LC–MS–MS amenable pesticides, comprising various chemical properties, were used for the study.

QuEChERS Procedure

Sample Extraction

1. Weigh 10 g soil sample (≥70% H₂O content) into a 50 mL centrifuge tube. Alternatively, weigh 3 g air-dried soil sample into a 50 mL tube and add 7 mL H₂O, vortex briefly, and allow to hydrate for 30 min.

2. Add 10 mL of acetonitrile to each sample.

3. Shake (manually or mechanically) or vortex samples for 5 min to extract pesticides. (In this study a Spex SamplePrep Geno/Grinder 2010 operated at 1500 rpm was used).

4. Add the contents of an ECQUEU750CT-MP (citrate salts) Mylar pouch to each centrifuge tube.

5. Immediately shake samples for at least 2 min.

6. Centrifuge for 5 min at ≥ 3000 rcf.

LC–MS–MS Conditions

Sample Cleanup

7. Transfer a 1 mL aliquot of supernatant to a 2 mL CUMPSC18CT (MgSO₄, PSA, C18) dSPE tube.

8. Vortex samples for 0.5–1 min.

9. Centrifuge for 2 min at high rcf (e.g. ≥ 5000).

10. Filter purified supernatant through a 0.2 μm syringe filter directly into a sample vial.

Table 1: Accuracy and Precision Data.

Results and Discussion

In the dSPE step, a combination of PSA/C18 gives cleaner extracts than PSA alone. In this study, no major variation in results was observed between PSA and PSA/C18. In fact, the PSA/C18 gave slightly better results overall.

Thiabendazole gave low, though reproducible recovery throughout the study. Thiabendazole is a planar pesticide and could potentially be retained by strong hydrophobic interactions on the soil. In addition, it is a basic compound that is positively charged at low pH and is capable of being retained on the soil through ionic interactions, particularly by humic/fulvic acids.

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