Characterization of MS/MS Product Ions for the Differentiation of Structurally Isomeric Pesticides by High-Resolution Mass Spectrometry

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.

Structural Elucidation of Agrochemicals and Related Derivatives Using Infrared Ion Spectroscopy

“…From an analytical perspective, this involves analyzing low-level (often micromolar or lower) compounds from complex biological/environmental sample matrices such as plants, human or animal body fluids/tissues, soil, and water samples. − Liquid chromatography (LC) and gas chromatography (GC) are commonly used to separate complex mixtures before analysis, typically by mass spectrometry (MS). − This approach works well for detecting and quantifying known compounds but faces many potential challenges when identifying unknowns and distinguishing closely related isomers. Tandem mass spectrometry (MS/MS) is commonly used to provide additional certainty in identifying molecular structures; fragmentation mass spectra of unassigned features of interest can be matched to MS/MS databases. − However, closely related isomeric compounds often have similar fragmentation spectra and retention times. Another challenge in this approach is that MS/MS fragmentation pathways remain challenging to predict in silico, and MS/MS spectra databases must be constructed based on measurements of reference standards. , Thus, identifying unknown compounds where standards are not commercially available often involves lengthy and costly syntheses of several candidate compounds to confirm a structural assignment.…”

Tandem mass spectrometry approaches for recognition of isomeric compounds mixtures

“…Generally, isomers show the same product ions and, even if different branching ratio are present, it is hardly to distinguish them without the support of chromatographic separations. Consequently, very few cases report a clear difference between MS/MS spectra of isomers, which limits this approach in the isomers mixtures analysis (Nunez et al, 2018).…”

Analysis of Chemical Contaminants in Food

“…Isomeric compounds share elemental composition and mass value and are not distinguishable by first generation mass spectrometry, even if high resolving power instruments are used. By means of tandem mass spectrometry (MS/MS), the studied isomer pairs are clearly differentiated on the basis of the different fragmentation pathways of the corresponding ions formed in MS ionization [ 4 ]. Raina-Fulton and Mohamad faced the problem of challenging food matrices as nutraceutical products.…”