Super isomer separation succeeds with cellulose

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  • Published: Feb 15, 2018
  • Author: Ryan De Vooght-Johnson
  • Channels: HPLC
thumbnail image: Super isomer separation succeeds with cellulose

New methods needed for separating pyrethroid enantiomers

Pyrethroids are a group of commonly used insecticides; they are synthetic analogues of the naturally occurring pyrethrin insecticides. Although they have low toxicity to humans and other mammals, both pyrethroids and pyrethrins are toxic to many aquatic organisms and pollinating insects, such as bees, so their levels in the environment are a cause of concern. Commercial pyrethroids typically exist as a mixture of enantiomers, which usually differ in their biological and environmental effects. Analytical methods are needed to routinely separate the isomeric pairs; however, current HPLC methods mostly use normal-phase systems, which are not particularly convenient, especially for LC-MS.

The Southwest University researchers aimed to devise new reversed-phase HPLC methods for separating the enantiomers of bifenthrin and lambda-cyhalothrin. The former is a type 1 pyrethroid, while the latter is a type 2, meaning that it contains a cyanide group on the phenoxybenzyl portion of the molecule. Previous reversed-phase HPLC methods with chiral columns tended to give poor separation of the enantiomers of these two compounds so careful optimisation was carried out to give effective new methods suitable for environmental studies.

Chiral columns found to separate bifenthrin and lambda-cyhalothrin isomers

Samples of dried soil were spiked with either bifenthrin or lamda-cyhalothrin. Acetonitrile, sodium sulfate and sodium chloride were added, and extraction was carried out by sonication followed by centrifugation. A second acetonitrile extraction was carried out on the solids, the combined solutions being evaporated and then made up to 1 ml with acetonitrile. Spiked water samples were extracted with ethyl acetate and treated in a similar manner.

Reversed-phase HPLC was carried out on solutions of bifenthrin and lambda-cyhalothrin using an Agilent 1260 instrument fitted with a diode array detector (DAD). Three chiral columns were examined: Lux cellulose-1, Lux cellulose-3 and Chiralpak IC. The mobile phase consisted of water and either acetonitrile or methanol, with a flow rate of 0.8 mL/min. Runs were carried out at different column temperatures: 10, 15, 20, 25, 30, 35 and 40 °C.

Resolution factors, Rs, were calculated for the various conditions, with Rs > 1.5 taken as complete separation. Using methanol/water or acetonitrile/water as the solvent gave complete separation of the bifenthrin isomers on the cellulose-3 column but only partial separation on the cellulose-1 column. Both Lux columns separated the lambda-cyhalothrin isomers, both with aqueous methanol and aqueous acetonitrile. The Chiralpak IC column proved a disappointment, not clearly separating either pair of isomers regardless of the conditions.

The effects of column temperature were studied in detail but differed depending on the column and the solvent system. With a Lux cellulose-1 column, the resolution factor, Rs, tended to decrease as the temperature was raised from 10 to 40 °C. Lower temperatures gave better resolution, longer retention times and broader peaks. However, the results with the Lux cellulose-3 column were more mixed: for example, with bifenthrin and methanol/water (95:5) there was no great change, the maximum value of Rs being 8.60 at 30 °C compared to 8.02 at 10 °C and 8.25 at 40 °C. These results show that it is not possible to assume that the optimum temperature for one chiral column and solvent system will be the same as that for another.

The method using Lux cellulose-3 and methanol/water (95:5 for bifenthrin and 90:10 for lambda-cyhalothrin) was successfully validated, showing good linearity and high recoveries from the spiked soil and water samples. The limits of detection (LODs) were 0.01 and 0.015 mg/L for bifenthrin and lambda-cyhalothin, respectively, while the limit of quantification was 0.05 mg/L for both compounds.

New chiral HPLC method suitable for pyrethroid enantiomers

The new method has been shown to be applicable to the separation and quantification of bifenthrin and lambda-cyhalothin enantiomers. Careful optimisation by the researchers allowed clear separation to be achieved, giving a method that is applicable to both soil and water samples. It would be useful if the method could be extended to other pyrethroids and the natural pyrethrins in the future.

Related Links

Chirality, 2017, Early View article. Zhang et al.. Enantiomeric separation of type I and type II pyrethroid insecticides with different chiral stationary phases by reversed-phase high-performance liquid chromatography.

Chirality, 2005, 17, S127-S133. Liu et al.. Separation and aquatic toxicity of enantiomers of synthetic pyrethroid insecticides.

Chemical Society Reviews, 2008, 37, 2593-2608. Okamoto et al.. Chiral HPLC for efficient resolution of enantiomers.

Article by Ryan De Vooght-Johnson

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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