Mosquito killer quantification achieved with negative chemical ionisation

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  • Published: Sep 18, 2017
  • Author: Ryan De Vooght-Johnson
  • Channels: Gas Chromatography
thumbnail image: Mosquito killer quantification achieved with negative chemical ionisation

Transfluthrin toxicology a concern

Transfluthrin is an insecticide often used indoors to combat mosquitoes, flies, moths and cockroaches, typically in the form of mosquito coils or as a liquid vaporiser. The compound has a relatively low acute toxicity towards mammals, including humans. Its oral LD50 value for rats is >5000 mg/kg, making its acute toxicity less than that of sodium chloride, which has a rat oral LD50 value of 3000 mg/kg. However, since the insecticide is so widely used, there are concerns about its long-term effects and the possibility of allergies. It has been suggested that the compound may affect the brain, since it can cross the blood–brain barrier.

There is therefore a desire to carry out animal experiments on the effects of transfluthrin, which require suitable analytical methods for the compound. Current methods tend to be cumbersome or require relatively large quantities of blood, such as 1 ml, which is too much if several samples are to be taken from a rat or mouse over a relatively short space of time. The University of Georgia workers devised a new GC-MS method, making use of negative chemical ionisation. The four electronegative fluorine atoms and two electronegative chlorine atoms in transfluthrin mean that strong signals can be obtained with this ionisation method.

GC-MS with negative chemical ionisation used for transfluthrin

The method used 100-μL samples of blood plasma or brain homogenate from rats fed with transfluthrin in corn oil. Internal standard (cis-permethrin, another pyrethroid) was added and the material was dissolved in diisopropyl ether. Sodium fluoride was added to prevent unwanted enzyme activity. Solids were removed by centrifugation and the solvent was evaporated. The residue was taken up in acetonitrile, the solids removed by centrifugation, and the solvent again evaporated. The subsequent residue was dissolved in toluene prior to GC-MS. This lengthy sample preparation procedure was necessary in order to minimise the amount of an impurity peak present in blank blood and brain samples that could not be readily separated from the transfluthrin peak.

GC was carried out using an Agilent 6890N instrument fitted with a Zebron ZB5-MS column, which is designed to give low bleed. The temperature was raised from 120 to 205 °C at 15 °C/min, then from 205 to 220 °C at 5 °C/min and finally from 220 to 300 °C at 15 °C/min. The column was held at 300 °C for 1 min, giving a total run time of 15 mins. An Agilent 5973 mass selective detector was used in negative chemical ionisation mode for detection. Selective ion monitoring was used for both transfluthrin and the internal standard.

The method was validated using standard solutions and was shown to give good linearity, reproducibility and precision. The lower limit of quantification (LLOQ) was only 1.0 ng/ml for plasma and 4.0 ng/ml for brain homogenate. Samples were shown to be reasonably stable over 24 hours in the autosampler.

Two hours after an oral dose of 10 mg/kg of transfluthrin, a mean plasma level of 11.7 ng/ml and a mean brain homogenate level of 12.1 ng/ml were detected in rats. These results confirm the ability of transfluthrin to cross the blood–brain barrier.

New method suitable for animal studies with transfluthrin

The new method is suitable for animal studies with transfluthrin since it requires much smaller samples then previous methods. It has a short GC run time, but a rather lengthy sample preparation procedure.

The advantage of using negative chemical ionisation (NCI) with fluorinated or chlorinated compounds is that lower limits of detection, stronger molecular ions and less interference from other compounds can be achieved compared to the standard electron ionisation (EI) GC-MS method. Labs that simply run all GC-MS with EI are liable to obtain inferior results in a number of cases.

Related Links

Rapid Communications in Mass Spectrometry, 2017, Early View paper. Hooshfar et al. Negative chemical ionization gas chromatography–mass spectrometry of transfluthrin in rat plasma and brain.

WHO Specifications and Evaluations for Public Health Pesticides — Transfluthrin

Wikipedia, Chemical Ionization

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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