Analysis of Soil-Bound Residues of ¹³C-Labeled Fungicide Cyprodinil by NMR Spectroscopy

Abstract

13C-NMR spectroscopy was applied to the evaluation of soil-bound residues of the fungicide cyprodinil (4-cyclopropyl- 6-methyl-2-phenylaminopyrimidine). A mixture of the 13C- and 14C-labeled fungicide was used to obtain structural information as well as information on the quantitative distribution in the various fractions. Bound residues were accumulated by a 6-month incubation of the labeled compound with a clay loamy soil. Depending on the concentration of [13C]cyprodinil (500, 250, 80, and 3 mg/kg), binding ranged from 18% to 54% of the initial radioactivity. After methanol extraction of soil (10 g dry weight) treated with 500 mg/kg (5.0 mg) of the fungicide, the amount of unextracted bound material was equivalent to 0.9 mg of 13C-labeled cyprodinil. Upon fractionation, 0.21 mg of the bound fungicide was found in the dialyzed humic acid, 0.13 mg in fulvic acid (after extraction with CH2Cl2), and 0.24 mg in humin. The methylene chloride extract from fulvic acid mainly contained unchanged cyprodinil (0.21 mg) that was apparently sequestered in soil by physical forces. The humic acid fraction was dissolved in a 1% solution of NaOD and examined by 13C-NMR. The NMR spectrum of the material from the control sample exhibited all the characteristic features of a typical humic acid. When the control humic acid was spiked with cyprodinil labeled uniformly with 13C at the phenyl ring, four additional signals at 121.9, 124.4, 131.8, and 143.4 ppm could be distinguished in the NMR spectrum. However, when humic acid originated from the soil that was incubated with the phenyl-labeled fungicide, only two strong NMR signals, at 122.5 and 131.8 ppm, and two less significant signals around 142 and 162 ppm were observed. The difference in the signal pattern indicated cleavage of the cyprodinil molecule between the aromatic rings and independent binding of the phenyl and pyrimidyl moieties to humic acid.

Carbon-13 solid-state NMR of soil organic matter - using the technique effectively

“…In addition to incorporation of highly available substrates, labeling has also been used to study the fate of xenobiotics -organic pollutants and pesticides. This has mostly been done by solution 13 C NMR, including incubation with organic matter prepared from 13 C-depleted precursors followed by silylation to increase solubility and visibility of the 13 C-labeled substrate (Haider et al 1992(Haider et al , 1993Hatcher et al 1993;Wais et al 1995Wais et al , 1996Dec and Bollag 1997;Dec et al 1997aDec et al , 1997b. There have also been a few studies using solid-state 13 C NMR to detect the bound substrate (Jurkiewicz and Maciel 1995b;Weissmahr et al 1997;Witte et al 1998;Guthrie et al 1999;Benoit and Preston 2000).…”

Reaction kinetics and transformation products of 1-naphthol by Mn oxide-mediated oxidative-coupling reaction

“…These complexes are often referred to as 'bound residues', which are difficult to decompose by microbial activity or chemical treatments [9]. Many studies have provided direct evidence for the formation of covalent bonds using 13 C-or 15 N-labeled chemicals in combination with 13 C-or 15 N-NMR spectroscopy [10,11]. This immobilization of contaminants in soil is of considerable importance because it can lead to a substantial decrease in the bioavailability of contaminants and restrict their leaching into the groundwater [5,11].…”

“…Many studies have provided direct evidence for the formation of covalent bonds using 13 C-or 15 N-labeled chemicals in combination with 13 C-or 15 N-NMR spectroscopy [10,11]. This immobilization of contaminants in soil is of considerable importance because it can lead to a substantial decrease in the bioavailability of contaminants and restrict their leaching into the groundwater [5,11]. The oxidative-coupling reaction can also be considered to be a natural detoxification process because a decrease in toxicity has been confirmed by several research groups [12,13].…”

Incorporation of pesticides by soil micro-organisms as a way of bound residues formation

“…The main mechanisms proposed to explain the formation of bound residues imply usually binding with soil organic matter, either by covalent binding with pesticides and degradation products (Bollag et al 1992;Dec et al 1997), or by sequestration into the three dimensional structure of humic substances (Guthrie et al 1999). A third mechanism of pesticide stabilisation involves the incorporation of residues into microbial biomass.…”