Exploring Chemoselective Behavior and Interplay Between Enantiorecognition of Organophosphorus Pesticides Under Multimodal Elution

A recent study published by Journal of Chromatography Open (1) investigated the chemoselective behavior and the interplay between enantiorecognition of organophosphorus pesticides and their metabolites and eight chiral stationary phases under multimodal elution using liquid chromatography (LC). The authors state that their conclusions provide valuable insights into the optimization of chiral separation strategies for a variety of organophosphorus pesticides.

Enantiorecognition is defined as the physical or chemical sensing and recognition of enantiomers (2), which are either of a pair of molecules related to each other as the right hand is to the left—that is, as mirror images that cannot be reoriented to appear identical. Molecular enantiomers have identical chemical properties, except in chemical reactions with both dissymmetric molecules and polarized light (3).

Organophosphates are chemical compounds formed through the esterification process involving phosphoric acid and alcohol. These chemicals serve as primary components in herbicides, pesticides, and insecticides; the most used organophosphate pesticides include parathion, chlorpyrifos, diazinon, dichlorvos, phosmet, fenitrothion, tetrachlorvinphos, azamethiphos, azinphos-methyl, malathion, and methyl parathion (4).

The study, a joint effort between the University of São Paulo, The Federal University of São Paulo, and the National Institute for Alternative Technologies of Detection, Toxicological Evaluation and Removal of Micropollutants and Radioactives (all in Brazil), systematically evaluated enantio- and chemoselective performance of eight polysaccharide-based chiral columns (Chiralcel OB-H, Chiralcel OD-H, Chiralcel OJ-R, Chiralpak AD-H, Chiralpak AS-H, Chiralpak IG-3, Chiralpak IK-3, and Lux-Amylose-2) and ten mobile phases across multimodal elution chemotographical techniques (normal-phase, polar organic, and reversed-phase conditions). Within the 80 combinations per analyte, the chiral screening highlighted specific enantioselectivity trends concerning hydrophilic and hydrophobic compounds when utilizing amylose- or cellulose-based polymers under different conditions. Notably, a set of columns including IK-3, OJ-R, AS-H, and AD-H demonstrated superior coverage, achieving at least a 62% success rate for enantioseparation. For the resolution between the pairs acephate and methamidophos and malathion and malaoxon, where no further efforts toward optimization were attempted, the success rate for enantio- and chemoselectivity was 10% and 30%, respectively.

Key findings underscored the efficacy of using isopropyl alcohol (iPrOH) as a polar modifier, allowing researchers to achieve superior enantioresolution, especially for hydrophilic compounds. Ethanol (EtOH) was shown to be an acceptable alternative. In both normal-phase and polar organic mode screening, iPrOH and acetonitrile (MeCN) emerged as effective solvents, which benefited the separation of more hydrophilic compounds. Reversed-phase chromatography exhibited notable success with iPrOH in aqueous mixtures for less hydrophilic pesticides, while amylose-based chiral stationary phases outperformed cellulose-based counterparts for certain compounds. Moreover, chemoselective analysis presented challenges, suggesting the potential of achiral-chiral or chiral-chiral column couplings for simultaneous chiral compound analysis.

The authors said that most evaluated chiral columns demonstrated good enantioselectivity under the chromatographic conditions they were operated under, but limited chemoselectivity, and while that may not present an issue for MS-based systems, it could possibly pose a challenge for analysts using spectrophotometric detectors. These conditions draw attention to the obstacles involved with the separation of two analytes on a single chiral column while preserving enantioresolution. Furthermore, the number of approaches established under normal-phase conditions presents a barrier for direct biological analyses and LC–MS methods, which triggers a swing towards polar organic or reversed-phase methods. However, on those occasions where chemoselectivity has been realized but not enantioselectivity, chiral-chiral column coupling, which leverages the enantioresolution capabilities of both columns, appears to be a possible resolution to the problem. Exploring column coupling, especially among those with demonstrated compatibility and complementary selectivity profiles, the authors believe that new possibilities for the refinement of analytical methodologies in chiral analysis are discovered.

References

(1) Oka-Duarte, L.; Cass, Q. B.: Moraes de Oliveira, A. R. Chemo- and Enantioselective Analysis of High Hydrophilic Organophosphate Pesticides by Liquid Chromatography Under Different Elution Modes. J. Chromatogr. Open 2024,100142. DOI:

10.1016/j.jcoa.2024.100142

(2) Wikipedia Enantiorecognition Home Page. https://en.wiktionary.org/wiki/enantiorecognition (accessed 2024-05-31)

(3) Britannica Enantiomer Home Page.https://www.britannica.com/science/enantiomorph (accessed 2024-05-31).

(4) Adeyinka, A.; Muco, E.; Regina, A. C.; Pierre, L. Organophosphates; StatPearls Publishing, 2024. https://www.ncbi.nlm.nih.gov/books/NBK499860/

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