1. Preparation of the stock solutions

NOTE: It is recommended to wear nitrile gloves, a lab coat, and safety glasses during the entire protocol.

1. Prepare a stock solution in acetone at 400 mg/L from a commercial mix of OCPs (see Table of Materials) at 2,000 mg/L in hexane:toluene (1:1) in a 25 mL volumetric flask. Table 1 shows each of the selected OCPs.
2. Prepare the subsequent stock solutions in acetone at concentrations of 50 mg/L, 1 mg/L, and 0.08 mg/L in 10 mL volumetric flasks, and store them in amber glass vials at −18 °C.
   NOTE: The same solutions can be used throughout the work, but it is important to store them under these conditions just after each use.
3. Prepare the stock solutions in acetone at concentrations of 20 mg/L and 0.4 mg/L from a commercial standard of 4,4'-DDE-d8 at 100 mg/L in acetone in 10 mL volumetric flasks, and store in amber glass vials at −18 °C. Use 4,4'-DDE-d8 as an internal standard (IS).

2. Sample collection

1. Collect approximately 0.5 kg of the upper 10 cm layer of an agricultural soil in a glass container. The soil object of this study was collected in a traditional agricultural zone of potato crops.
   NOTE: Surface sampling with a spatula was carried out. However, the depth of the soil could influence its physicochemical characteristics. Therefore, if the organic carbon content varies with the depth, it is necessary to take samples at different depths.
2. Take the soil sample to the laboratory, sift it with a 1 mm diameter sieve, and store it until analysis at 4 °C in an amber glass container.
   ​NOTE: The same soil sample can be used throughout the work, but it is important to store it under these conditions just after each use.

3. Sample preparation via the modified QuEChERS method using ammonium formate

NOTE: Figure 1 shows a schematic representation of the modified QuEChERS method.

1. Weigh 10 g of the soil sample in a 50 mL centrifuge tube, and add 50 µL of the IS solution at 20 mg/L to yield 100 µg/kg. For recovery purposes, also add the pesticide solutions prepared in step 1.2 to yield 10 µg/kg, 50 µg/kg, and 200 µg/kg (n = 3 each).
2. Shake the tube using a vortex for 30 s to better integrate the spike into the sample.
3. Add 10 mL of water. Shake the tube using an automated shaker at 10 x g for 5 min.
4. Add 10 mL of acetonitrile. Shake the tube again at 10 x g for 5 min.
5. Add 5 g of ammonium formate (see Table of Materials), shake the tube vigorously for 1 min by hand, and centrifuge at 1,800 x g for 5 min.
6. Transfer 1 mL of the acetonitrile extract to a 2 mL centrifuge tube containing 150 mg of anhydrous MgSO₄, 50 mg of primary-secondary amine (PSA), and 50 mg octadecylsilane (C₁₈) (see Table of Materials) for clean-up purposes by dispersive-solid phase extraction (d-SPE)⁸, vortex for 30 s, and centrifuge at 1,800 x g for 5 min.
7. Transfer 200 µL of the extract to an appropriately labeled autosampler vial with a 300 µL fused insert, and perform an instrumental analysis using a GC-MS system (step 4).
   ​NOTE: Matrix-matched calibration is carried out following the same steps as previously using blank extracts, but 5 mL of the supernatant is cleaned in 15 mL tubes in the d-SPE step (step 3.6) and the spike and IS solutions are not added until step 3.7. Add the calibration standard solutions in the autosampler vials to yield 5 µg/kg, 10 µg/kg, 50 µg/kg, 100 µg/kg, 200 µg/kg, and 400 µg/kg, evaporate to dryness, and add 200 µL of the matrix extracts.

4. Instrumental analysis by GC-MS

1. Perform the GC-MS analyses using a GC-MS system with a single quadrupole mass spectrometer and an electron ionization interface (−70 eV) (see Table of Materials).
2. Set the MS transfer line at 280 °C and the ion source at 230 °C.
3. Use a 5%-phenyl-methylpolysiloxane 30 m x 250 µm x 0.25 µm column (see Table of Materials) and ultrahigh purity He as the carrier gas at a 1.2 mL/min constant flow rate.
4. Maintain the GC oven at 60 °C initially for 2 min, then ramp up the temperature to 160 °C at 25 °C/min, and hold for 1 min. Then, increase the temperature to 175 °C at 15 °C/min, and hold for 3 min. Then, increase to 220 °C at 40 °C/min, and hold for 3 min. Again, increase to 250 °C at 30 °C/min, and hold for 2 min. Finally, take the temperature to 310 °C at 30 °C/min, and hold for 2 min. The total analysis time is 22.125 min.
5. Conduct a full autotune and an air and water check of the MS before each sequence.
   1. Open the MassHunter acquisition software that controls all the parameters of the GC-MS system.
      NOTE: The instrument system includes the MassHunter acquisition software by default.
   2. Open the "View" option on the toolbar, and click on Vacuum control, click on Tune, and click on Autotune. The autotune will end after a few minutes.
   3. Open the "View" option, and click on Instrument control.
   4. Click on Yes, and save the new tune file for the autotune.
   5. Open the "View" option on the toolbar, and click on Vacuum control, click on Tune again, and click on Air & Water check. The air and water check will end after a few seconds.
   6. Open the "View" option, and click on Instrument control.
   7. Click on Yes, and save the new tune file for the air and water check.
6. Perform the injection using an autosampler (see Table of Materials) at 280 °C in the splitless mode, keeping the injection volume 1.5 µL. After 0.75 min of the injection, open the split at a 40 mL/min flow rate.
   NOTE: Between injections, the 10 µL syringe must be washed three times with ethyl acetate and three times with cyclohexane. All the injections are in duplicate.
7. Analyze the analytes in selected ion monitoring (SIM) mode. This is the standard mode used in MS systems with a single quadrupole.
   ​NOTE: Table 1 shows the retention times (min) and the quantification parameters based on using one quantitation and two identification ions for the OCPs and the IS. The quantitative analysis is based on the ratio of the peak area of the quantitation ion to the ion of IS.

5. Data acquisition

1. Open the MassHunter acquisition software that controls all the parameters of the GC-MS system.
2. Open the "Sequence" option on the toolbar, and edit the sequence, including the sample name, the vial number, the number of injections, the instrumental method, and the name of the file to be generated. Add as many rows as necessary.
3. Click on OK, and save the new sequence.
4. Open the "Sequence" option on the toolbar again, and click on Run Sequence in the dropdown menu. A new window will open to confirm the injection method and the folder where the samples will be saved. Click on Run Sequence again, and the injection will begin.