Achiral GC Analysis Using Alpha- and Beta-cyclodextrins as Chiral Selectors to Separate Diastereomers of Cypermethrin

Overview

Objectives/Goals Cypermethrin is a highly chiral synthetic pyrethroid pesticide. Despite having desired insecticidal effects, it is harmful to the environment because certain stereoisomers induce sharp levels of toxicity in aquatic wildlife and cause hyperactivity in voltage-sensitive sodium channels in the animal nervous system. In this study, cypermethrin was used in spiked water samples to determine whether chiral selectors, alpha- and beta-cyclodextrin would assist in isolation of insecticidal cis-cypermethrin diastereomers to prevent this environmental stress. Because of its larger cavity, beta-cyclodextrin should show greater resolution. Methods/Materials Emulsifiable CP-concentrate (100 mg) was immersed in 100mL hexane/acetone (1:1 v/v) stock solution with cyclodextrin to create spiked solutions with 1:1 guest-to-host molar ratios. Magnetic stirrers mechanically included analyte in cyclodextrin buckets; the hydrophobic guest binds to CD's chiral cavity through lateral interactions with hydroxyl groups. Unselected isomer formed in top layer, selected in the bottom, which was extracted using hexane/acetone to "pull" analyte from cavity. Using Thermo-Fisher's TR-5MS, each sample was injected in Autosystem XL GC at 2 microliters for 50 min at 260 C inlet temperature and FID detector at 290 C. Results Four peaks were resolved at the baseline with retention times similar to those in previous research journals. Two peaks represented two cis-diastereomer pairs, while two peaks represented two trans-diastereomer pairs. In cross-comparison with samples, little visible effect of cyclodextrin-assisted separation occured. However, elution order marked visible separation of diastereomers. Conclusions/Discussion Chemical reasoning seems to show that the H on the cyano-carbon H-bonded with carbonyl oxygen at one chiral center which created a 5-member ring structure reducing rotatability of the cyanocarbon. Rigidity increased, decreasing vapor pressure as retention time increases, such that molecules with greater linear stability adsorb onto stationary phase due to stronger IMFs while other more volatile diastereomers pass through mobile phase. Modifications with procedure like increasing host to guest ratio, using stronger non-polar extraction solvent like ethyl acetate, concentrating solution to near dryness, will help for future experimentation to ensure cyclodextrin participation in chiral separation.

Competition history

• CSEF 2013 Chemistry · Entry S0602

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