Finding disguised pesticide?

Title: Can Occurrence of Pesticide Metabolites Detected in Crops Provide the Evidence on Illegal Practices in Organic Farming?

Authors: Dana Schusterova, Marie Suchanova, Jana Pulkrabova, Vladimir Kocourek, Jiri Urban, and Jana Hajslova

Year: 2019

Journal: Journal of Agricultural Food Chemistry
https://pubs.acs.org/doi/10.1021/acs.jafc.8b06999

 

The word “organic” has often been associated with the reputation of “natural” and “healthy”. And most people consider organic food as pesticide-free. In fact, organic farming forbids the use of synthetic pesticides. Even though organic farmers are cautious and moral in their farming practice, organic crops, however, may still possess traces of pesticides due to contaminated soil and water, wind carrying pesticides from neighbouring farms or contamination due to storage and transportation.

Testing for pesticide residues in crops may be easier said than done. Not only there are thousands of pesticides existing in the market, but some pesticides also degrade easily and increase the difficulty in pesticide monitoring in crops. The dissipation of pesticides into metabolites could provide insight into unauthorized pesticide practice in farming. Knowledge of pesticide metabolites could also provide insight on the transformation of pesticides in plants and benefits in pesticide registration. Therefore, the authors in this paper propose to identify pesticide metabolites in addition to pesticide compounds in conventional farming to characterize pesticide degradation and monitor pesticide use.

 

The current challenge in pesticide metabolites detection lies in the trace amount of pesticide use as well as their physical and chemical properties. Detection of trace content of compounds is highly dependent on the sensitivity of instrumentation. In addition, one pesticide compound produces several metabolites. Therefore, multiple metabolites need to be targeted in detection. These pesticide metabolites are usually more polar than their original pesticide (Figure 1). As a result, the same instrumentation method with liquid chromatography – mass spectrometry (LCMS) may not be suitable for the detection of these metabolites. Increasing the challenge of the detection method, analytical standards for these metabolites may not be commercially available, making quantification more difficult. Since the only fungicides allowed in organic grape production are sulfur, lime sulfur, or some copper compounds researchers use grape and wines to investigate the transfer of synthetic pesticides and optimize analytical strategy for the extraction of metabolites.

Figure 1                                The chromatogram of Fenhexamid and its metabolites. The polar metabolites elute earlier than the original pesticide compounds with LCMS. The early elution of targeted compounds could bring along interference from food samples and pose difficulty in the detection and quantification.

Grape and vine leave samples are fortified with a known amount of pesticides. Researchers then useacommon sample extraction method – Quick, Easy, Cheap, Efficient, Robust, Simple (QuEChERS) – to extract major pesticides from samples. By comparing the retention time, accurate mass and isotopic pattern, and the mass of MS/MS fragments in the extract with pesticide reference standards, pesticide compounds are identified. Through comparison of the pesticide compound with a calibration curve constructed by reference standard, if readily available, the amount of pesticide residue is then quantified.

In order to identify pesticide metabolites, researchers have also modified the extraction solvent for these metabolites using water-methanol mixture for the polar metabolite compounds. They need to find the MS/MS fragments corresponding to these compounds. They searched for diagnostic ions known for fungicides, common fragments detected in the MS/MS spectrum of the parent pesticide and metabolites as well as the fragments characterizing the part of the molecule undergone metabolic modification. Since standards of pesticide metabolites were not available, researchers tested the precision of their results with six replicates using QuEChERS method.

Researchers have also investigated the possibility of pesticide screening with their metabolites. Assuming the elemental formula of pesticide metabolites, they have used high-resolution mass spectrometry for detection. Based on the available information of existing pesticide metabolites, researchers have established a database of exact masses of the calculated metabolites’ molecular ions. They have confirmed the identity of the metabolites using isotopic pattern and critical analysis of their fragmentation spectra.

Overall, concentrations of pesticide residues in vine leaves significantly dropped in the first week after specific treatment, such as crushing and juicing of wine grapes. In subsequent weeks, the decline was slower. Most pesticides were still detectable in the final product, white wine.

In short, the detection of pesticide metabolites can indicate illegal practices in organic farming. The study of pesticide metabolites could be helpful in future biomonitoring studies and consideration as exposure markers to respective pesticides.

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