UDC 632.954:543.544

I. Sh. Kofman, Candidate of Chemical Sciences; V. I. Kofanov

Gas Chromatographic Analysis of Trace Amounts of Thiocarbamic Acid Herbicides in Environmental Samples

Institute of Plant Physiology, Academy of Sciences of the Ukrainian SSR; Institute of Colloid Chemistry and Water Chemistry, Academy of Sciences of the Ukrainian SSR, Kiev

Trace amounts of thiocarbamate herbicides, such as Eptam (S-ethyl-N,N-dipropylthiocarbamate), Sutan (S-ethyl-N,N-diisobutylthiocarbamate), Tillam (S-propyl-N-ethyl-N-butylthiocarbamate), Vernam (S,N,N-tripropylthiocarbamate), Yalan (S-ethyl-N-hexamethylenethiocarbamate), and Ronit (S,N-diethyl-N-cyclohexylthiocarbamate), are determined by colorimetry (G. A. Khokholkova and Z. V. Ivanova), thin-layer chromatography (L. G. Aleksandrova; L. S. Samosvat et al.), and gas chromatography with flame-ionization (A. L. Pertsovskii et al.; Hughes et al.) and flame-photometric detectors (materials from Stauffer Chemicals). It is of interest to investigate the feasibility of using a thermionic detector to analyze thiocarbamate herbicides. In view of the high steam volatility of these preparations and the trend toward standardizing methods for analyzing compounds of the same group, we used the techniques described below.

To isolate and purify thiocarbamates from water, soil, and plant samples, we used an extractor-distiller (Bleidner et al.). The apparatus operates as follows (Fig. 1). Water vapor generated by boiling the samples in the distillation flask passes into the condenser, where it condenses together with extractant vapor arriving from the receiving flask. Extraction takes place in the separating capillary

Fig. 1. Distillation–extraction apparatus.
1—Blaedner apparatus; 2—distillation flask; 3—receiving flask; 4—U-shaped tube; 5—separation capillary; 6—condenser; 7—air lock.

Fig. 2. Chromatographic separation of thiocarbamate herbicides.
1—solvent; 2—EPTAM; 3—Sutan; 4—Vernam; 5—Tillam; 6—Yalan; 7—Ronit. The dashed line represents the column temperature program.

…thiocarbamates from the aqueous phase into the organic phase, while the phases separate in the V-shaped tube. The aqueous and organic phases then enter their respective flasks, with the organic phase enriched in thiocarbamates. This method of isolating and purifying thiocarbamates is relatively simple and, with little expenditure of working time, permits maximum separation of the compounds being analyzed from co-extracted substances.

For chromatographic separation of the compounds, temperature programming is used with a thermionic detector. The temperature is controlled in stages (Fig. 2): programming from 140 to 150°C at 2°C/min, an isothermal hold at 150°C for 5 min, programming from 150 to 160°C, also at 2°C/min, and an isothermal hold at 160°C for 5 min. Only slight baseline deviations were observed. The injector temperature was 200°C.

Chromatography was performed on a coiled glass column measuring 1 m × 3 mm, containing Chromaton N-AWDMCS (0.16–0.20 mm) with 5% DC-550 silicone. Nitrogen was used as the carrier gas at a flow rate of 20 mL/min. The air flow rate was 400 mL/min. The hydrogen flow rate was initially set at 15 mL/min and, after ignition, adjusted to maintain a background current of 50 pA. Our procedures for using the thermionic detector have been described previously (V. I. Kofanov et al.). Under the conditions given above, the absolute retention time of EPTAM, the first thiocarbamate to elute, was 4½ min. The relative retention times were 1.0 for EPTAM, 1.3 for Sutan, 1.5 for Vernam, 1.6 for Tillam, 2.6 for Yalan, and 3.5 for Ronit. The minimum detectable amounts of the compounds differed somewhat because of variations in the physicochemical properties of the thiocarbamates, but were generally around 2.0 ng. The linear dynamic range was at least \(10^3\).

To improve the accuracy of quantitative thiocarbamate determination, the internal standard method is used; one of the thiocarbamates may serve as the internal standard when analyzing the others. This is possible only if the history of the sample’s treatment is fully known. Usual—

but the use of all six thiocarbamates is ruled out, making it possible to select an internal standard, an accurately known amount of which is added to the sample before distillation. This accounts for losses at virtually all stages of the analysis, since all the compounds studied behave similarly during determination.

The fundamental differences from existing gas chromatographic methods described above formed the basis of a method for determining trace amounts of thiocarbamate herbicides. For analysis, 200 mL of water or 100 g of soil of known moisture content or finely ground plant material is taken. Soil or plant material samples are mixed with 200 mL of water; when plant material is analyzed, 1–2 g of paraffin is added to reduce foaming. The samples are placed in a distillation flask, and a known amount of internal standard in ethanol is added. The V-shaped tube is filled with distilled water to the level of the lower side arm, and 75–100 mL of hexane is added to the receiving flask. Both flasks are heated to boiling, and extraction is carried out for 1 h. The hexane is then dried over anhydrous sodium sulfate and distilled off on a rotary evaporator at a water-bath temperature not exceeding 40°C and a residual pressure of 0.2 kgf/cm², until the volume is reduced to 3–4 mL. One milliliter of ethanol is added to the residue, after which the solution is further evaporated in a stream of air. Neither the final volume of the purified extract nor the volume of the sample injected for chromatography enters into the calculation formula.

For quantitative calculations, the following procedure is recommended. First, calibration coefficients for the compounds being analyzed and the internal standard are determined using the general formula:

\[ K=\frac{1}{n}\left(\frac{H_1}{C_1V_1}+\frac{H_2}{C_2V_2}+\cdots+\frac{H_n}{C_nV_n}\right)\ \text{mm/ng}, \]

where \(n\) is the number of determinations; \(H\) is the peak height of the compound being calibrated (in mm); \(C\) is the concentration of the chromatographed solutions of the compound being calibrated (in µg/mL); and \(V\) is the volume of the chromatographed samples (in µL).

The content of a given herbicide in the sample is determined by the formula:

\[ X=\frac{K_{\text{с}}H_{\text{п}}CV}{K_{\text{п}}H_{\text{с}}A}\ \text{mg/kg}, \]

where \(K_{\text{с}}\) and \(K_{\text{п}}\) are the calibration coefficients of the internal standard and the compound being analyzed, respectively (in mm/ng); \(H_{\text{с}}\) and \(H_{\text{п}}\) are the peak heights of the internal standard and the compound being analyzed, respectively (in mm); \(C\) is the concentration of the internal standard solution added to the sample (in µg/mL); \(V\) is the volume of the internal standard solution added to the sample (in mL); and \(A\) is the sample mass taken for analysis (in g).

The developed gas chromatographic method for analyzing trace amounts of thiocarbamate herbicides has a detection limit of 0.02 mg/kg. Analytical recovery is \(85\pm5\%\). Use of the described method in routine sanitary and hygiene monitoring of thiocarbamate herbicide levels will make it possible to study the accumulation and movement of these compounds in environmental media and to take measures to prevent contamination and adverse effects on living organisms and humans.

References

Aleksandrova L. G. In: Methods for Analyzing Trace Amounts of Pesticides in Food, Feed, and the Environment. Moscow, 1977, p. 285.

Kofanov V. I., Kofman I. Sh., Barabanov Yu. S. Gigiena i Sanitariya, 1978, no. 2, p. 83.

Pertsovskii A. L., Botvin’eva A. M., Prismotrov Yu. A. In: Gas Chromatography of Pesticides. Tallinn, 1972, p. 267.

Samosvat L. S., Kofman I. Sh., Kunak V. D. Khimiya v Sel’skom Khozyaistve, 1976, no. 12, p. 52.

Khokhol’kova G. A., Ivanova Z. V. In: All-Union Conference on the Study of

residues of pesticides and the prevention of contamination of foodstuffs, animal feeds, and the environment by pesticides. Methods of Analysis. 2nd Proceedings. Tallinn, 1971, p. 324.

Bleidner W. E., Baker H. M., Lewitsky M. et al. — J. Agricult. Food Chem., 1954, vol. 2, p. 476.

Hughes R. E., Freed V. H. — Ibid., 1961, no. 9, p. 381.

Received December 11, 1978.