Abstract
Structural isomeric pesticides are used in agriculture and may be challenging to differentiate for accurate identification in pesticide monitoring programs. Due to structural similarity, isomeric pesticides are difficult to separate chromatographically, and thus, their accurate identification may rely solely on mass spectrometric analysis (MS). In this study, we challenged the ability of high-resolution quadrupole-orbitrap (Q-Orbitrap) mass spectrometry to produce and evaluate the tandem mass spectrometry (MS/MS) product ions for the selected five pairs of isomeric pesticides from different classes: Pebulate and vernolate, methiocarb and ethiofencarb, uniconazole and cyproconazole, sebuthylazine and terbuthylazine, and orbencarb and thiobencarb. The use of Q-Orbitrap instrument with a mass error <3 ppm allowed proposed elucidation of the product ion structures with consideration of the ion formulae, data interpretation, and literature searches. Product ions unique to pebulate, vernolate, methiocarb, ethiofencarb, and uniconazole were observed. Elucidation of the observed MS/MS product ion structures was conducted, and the fragmentation pathways were proposed. This information is valuable to increase selectivity in MS/MS analysis and differentiate isomeric pesticides, and thereby reduce the rates of false positives in pesticide monitoring programs.
Highlights
Numerous pesticides are used in agriculture worldwide to protect crops and increase harvest yields, with over 1.1 billion pounds used annually in the USA, and nearly six billion pounds usage worldwide [1]
A very comprehensive compilation of drugs and pesticides mass spectrometric analysis (MS)/MS data interpretations has been published by Niessen and Correa [10], providing an excellent source for structural identification of MS/MS ions resulting from soft ionization mass spectrometry
Availability of commercial software for spectra interpretation and public compound databases can help in elucidation of structures of product ions, but this approach should be considered with caution and some of these limitations have been described by Wright et al [15]
Summary
Numerous pesticides are used in agriculture worldwide to protect crops and increase harvest yields, with over 1.1 billion pounds used annually in the USA, and nearly six billion pounds (three million tons) usage worldwide [1]. Thiobencarb (S-(4-chlorobenzyl)-N,N-diethylthiocarbamate) differs from orbencarb only by Cl positioning on benzene ring (Table 1) It is used for weed control in paddy fields, mostly on rice, and according to Pesticide Action Network of North America, has an active status in the US products [11], but it is not approved for use in the EU. As isomeric pesticides possess very similar properties, they are usually difficult or impossible to separate chromatographically Both orbencarb and thiobencarb had the same retention time of 19.38 min and the same ions at m/z 258, 125, and 100 when analyzed by LC-MS/MS [12]; they were not differentiated by either LC separation or MS identification. A Q-Orbitrap HRAM MS instrument with an error
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