Pyrethrins exposed: Flower extracts tested by LC/MS with direct electron ionisation
- Published: May 15, 2012
- Author: Steve Down
- Channels: Base Peak
Pyrethrins are a series of six natural compounds which have been employed as insecticides in a number of scenarios. They are used for insect control in agriculture, for the control of mosquitoes, and are used in the home to control animal fleas and lice and cockroaches. There are also some human applications like the treatment of scabies, which is spread by mites.
The popularity of pyrethrins is largely down to their low-to-moderate toxicity to mammals and humans and they are replacing the far more toxic organo-phosphate pesticides. The EPA reckons that there are at least 3500 registered pyrethrin products in the US.
Pyrethrins are extracted from the flowers of certain species of chrysanthemum and the commercial formulations have an active ingredient content of up to 30%. Unfortunately, they are somewhat unstable in sunlight, so antioxidants are also added to improve their active lifetimes. As an alternative, the closely related synthetic pyrethroids are often deployed due to their improved photostability.
The six chemicals which constitute the pyrethrins are pyrethrin I and pyrethrin II, which are the most prominent, cinerin I and II and jasmolin I and II. Their content in formulations is generally measured by LC procedures which are preferred to GC due to the inherent thermal instability of the compounds. The majority of them use HPLC with UV detection but there are some published LC/MS methods which use soft ionisation like electrospray.
Now, European scientists have developed the first LC/MS application which uses electron ionisation. Achille Cappiello and colleagues from the University of Urbino, Italy and Cornelia Flender from Merck KGaA, Darmstadt, Germany, wanted to take advantage of the ability of mass spectrometry to identify the individual pyrethrins as well as measure them.
Direct electron ionisation
They described the new method in Phytochemical Analysis, working with a commercial pyrethrum extract prepared from the flowers to establish the best conditions, then applying these to pyrethrins which they extracted from the flowers of Chrysanthemum cinerariaefolium of Ethiopian origin. The flowers were dried in the dark to prevent photodecomposition of the pyrethrins before extraction by a published method which minimises losses.
The commercial sample was analysed by LC/MS using a nano-C18 column and a mobile phase gradient of acetonitrile in water. The eluting compounds were directed towards a miniaturised direct electron ionisation interface developed in the researchers’ lab at Urbino that has "the same simplicity of a capillary GC column."
The liquid from the HPLC column was led directly into the heated ion source where it was converted rapidly to an aerosol which was desolvated quickly for vaporisation of the pyrethrin molecules and ionisation. The source was mounted on a single quadrupole mass spectrometer which was operated under full-scan mode for pyrethrin identification and selected ion monitoring mode for quantification, monitoring several characteristic ions for each compound.
Chrysanthemum flower extracts
The HPLC conditions gave good resolution of the six pyrethrins, which were all confirmed by comparing their mass spectra with those in the reference library. The detection and quantification limits determined from the commercial mixture were in the ranges 0.04-0.38 and 0.14-1.25 mg/g, respectively. The other analytical data were satisfactory, so the method was applied to the researchers’ own pyrethrin extracts.
Pyrethrin I and II were confirmed as the most abundant of the pyrethrins, varying between the samples in the ranges 1.25-5.60 and 2.00-10.38 mg/g, respectively. Of the remaining pyrethrins, cinerin I was detected in only two of the six samples and the others were slightly more abundant.
A statistical analysis of the compositions revealed that the samples fell into two clusters containing two and four samples. The main difference between the clusters was the levels of pyrethrin I and II which were lower in one set than the other. This variability could be useful when the ultimate application of the pyrethrin extract is taken into account, for instance, human versus agricultural use.
The LC/MS method with electron ionisation is a useful method for analysing multicomponent samples and provides a valid alternative to GC/MS methods and LC/MS methods using soft ionisation. The fragmentation that occurs helps to identify individual compounds from their mass spectra, as well as providing characteristic peaks for quantification.
Phytochemical Analysis 2012, 23, 191-196: "Determination of natural pyrethrins by liquid chromatography-electron ionisation-mass spectrometry"
Article by Steve Down
The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.