Soil sample preparation methods for the chromatographic determination of pesticide residues

Monitoring pesticide residues in the soil is not just an environmental formality, but a way to avoid the residual effects of products in crop rotation. When spraying vegetation, at least 30 % of active ingredients settle on the ground, and when applying soil herbicides, 100 % enters the soil. However, laboratory analysis is expensive: more than 60 % of its cost is accounted for by sample preparation.

• Amount entering soil during plant treatment — at least 30 %
• Amount entering soil during pre-emergence treatment — 100 %
• Share of sample preparation in analysis cost — more than 60 %

The process of preparing a soil sample for analysis consists of three consecutive stages:

1. Extraction of active ingredients from the soil.
2. Purification of the resulting extracts from foreign impurities.
3. Preparation of chemical derivatives of the analyte (if necessary).

Physicochemical properties of active ingredients

For the development of effective sample preparation schemes, it is important to consider the solubility of active ingredients in water and organic solvents. Below are the characteristics of herbicides of different chemical classes that were studied in the experiments. All samples were collected strictly according to GOST 17.4.4.02-84.

Solvent selection: extraction efficiency versus sample purity

In the studies of 2021–2023, the efficiency of three solvents of different polarity was evaluated: hexane, chloroform, and acetone. The tests were conducted on soil samples with an organic matter content of 2 %, 30 %, and 70 %. The quality of extraction was checked by the behavior of pendimethalin.

Purification of acetone extracts and selection of the optimal eluent

When determining residual quantities of herbicides in soils with different organic matter content, the key stage is the purification of the obtained acetone extracts. Without high-quality preparation, accompanying organic impurities quickly contaminate the chromatographic system and distort analysis results. To solve this problem, column chromatography using various brands of silica gel was tested. The studies were carried out using the examples of metribuzin, terbuthylazine, pendimethalin, chlorpropham, flurochloridone, and S-metolachlor.

To select an optimal purification scheme, the behavior of herbicides and impurities was first studied on Sorbifil thin-layer chromatography plates. The tests were performed in organic solvents with different polarity. Based on the evaluation results of chromatographic mobility (Rf values), the most effective eluent was determined. The obtained substance mobility coefficients are shown in the table.

Note: Rf = 0 — substance is not eluted; Rf = 1,0 — substance is completely eluted.

Experiments have shown that the optimal mixture for separation is hexane and acetone in a 5:1 volume ratio, and Silica Gel 60 is the most effective sorbent. This combination allows the accompanying organic impurities to be retained in the column, while the target herbicides are freely eluted. The quality of purification was evaluated by the change in the mass of the dry residue of impurities and the optical density of the extract.

• Mass of impurities before purification — 77,0 mg
• Mass of impurities after purification — 11,0 mg
• Optical density before purification — 0,702
• Optical density after purification — 0,103

Using a Silica Gel 60 column allows reducing the content of co-extractive substances in the sample by 7 times. At the same time, the overall degree of extract purification reaches 85,7 %. This guarantees reliable protection of chromatographic equipment from rapid contamination and extends the service life of the columns.

Step-by-step algorithm for sample preparation and extraction results

The developed methodology for soil sample preparation was tested on model samples of sod-podzolic and peaty soils with an organic matter content of 2 %, 30 %, and 70 %. It is suitable for soil analysis with a wide range of humus content. The sample preparation procedure consists of sequential steps and can be performed under standard analytical laboratory conditions.

1. Take a 5 g soil sample.
2. Perform extraction by shaking the sample with 50 ml of acetone twice for 30 minutes.
3. Filter the resulting extract through a "red ribbon" paper filter.
4. Dissolve the dry residue in a mixture of hexane and acetone (5:1) in two 2 ml portions.
5. Prepare a chromatographic column: fill a 5 ml medical syringe with Silica Gel 60 adsorbent (fraction 0.063–0.200 mm, weight 2 g), compacting the adsorbent by tapping lightly, and apply the prepared extract to the surface of the adsorbent.
6. Perform elution with a hexane and acetone mixture in a 5:1 ratio.
7. Collect 20 ml of eluate. When using packed columns SE-30, OV-101, OV-17 with ECD or NPD detectors, evaporate the eluate to a volume of 5 ml.

The efficiency of the developed scheme was confirmed by determining herbicide residues using gas-liquid chromatography. Analysis of samples with different levels of organic matter showed high completeness of compound extraction. Depending on the soil type and the chemical class of the active ingredient, the recovery rate ranges from 78.7 % to 97.3 %.

Kislusko, P. M. Determination of S-metolachlor residues in plant material, soil, and water by gas-liquid chromatography / P. M. Kislusko, S. A. Arashkovich // Plant Protection: coll. of sci. papers / Scientific and Practical Center of the NAS of Belarus for Agriculture, Institute of Plant Protection; editorial board: L. I. Trepashko (editor-in-chief) [et al.]. – Minsk,

2019. – Iss. 43. – P. 310–317.

Methodological guidelines for the determination of Gardoprim residues in water, soil, and plant material: MUK No. 1801, approved by the Ministry of Health of the USSR on 18.11.77 // Methodological guidelines for the determination of trace amounts of pesticides in food products, feed, and the environment: coll. – M., 1979. – No. 9. – P. 147–154.

Methodological guidelines for the determination of Chlorpropham residues in water, soil, and potatoes: MUK 4.1.1826-03, approved by the Chief State Sanitary Physician of the Russian Federation G. G. Onishchenko, Dec. 18, 2003 // Determination of pesticide residues in food products, agricultural raw materials, and environmental objects: coll. – M., 2007. – Iss. 5. – P. 226–234.

Methodological guidelines for the determination of pendimethalin (Stomp) in water, soil, cereals (wheat, barley, rye, rice, corn), legumes (peas, soy), industrial (sugar beet, sunflower), vegetable (cabbage, carrot, bulb onion, green onion, garlic, tomatoes), fruit (apples) by thin-layer and gas-liquid chromatography // Methodological guidelines for the determination of trace amounts of pesticides in food products, feed, and the environment: coll. – Kyiv, 2001. – No. 31. – P. 70 – 75.

Nature protection. Soils. Methods of sampling and preparation for chemical, bacteriological, and helminthological analysis: GOST 17.4.4.02-84. – Entered into force.

P. M. Kislusko, E. A. Myshkevich, S. A. Arashkovich, M. P. Loseva RUE «Institute of Plant Protection», Priluki, Minsk region

FEATURES OF SAMPLE PREPARATION IN

DETERMINING PESTICIDE RESIDUES IN SOILS

WITH DIFFERENT ORGANIC MATTER CONTENT

Annotation. Studies have been conducted on the development of methods for sample preparation of soil samples with different organic matter content when determining pesticide residues of chemical classes: triazinones, sym-triazines, nitro-xylidines, phenyl-carbamates, pyrrolidones, toluidines by gas-liquid chromatography. A scheme for sample preparation of soil samples with different organic matter content (2, 30, 70 %) is presented, the extraction of the studied active substances was 75.7–97.3 %.

Key words: gas-liquid chromatography, sample preparation, extraction, extract purification, pesticide residues, soil.