A woolly problem solved: 40 organo-phosphate pesticides measured by GC

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  • Published: Mar 28, 2011
  • Author: Steve Down
  • Channels: Gas Chromatography
thumbnail image: A woolly problem solved: 40 organo-phosphate pesticides measured by GC

Pesticides in wool: no official method prescribed

The International Wool Textile Organisation, based in Brussels, represents the interests of the world's wool-textile trade and industry. Its brief includes suggesting methods for the analysis of residual pesticides in wool. Although there are no official analytical protocols, several procedures are recommended.

The most popular method involves Soxhlet extraction of pesticides from the wool followed by gel permeation chromatography and analysis of the resulting fractions by GC. Other approved procedures involve the use of accelerated solvent extraction and supercritical fluid extraction.

Pesticide monitoring programs are required because many farms treat their sheep to rid them of lice, flies and scabies, which detract from the quality of the fleece. In addition, sheep that have not been treated might be put to graze in mixed agricultural land where crops have been sprayed with pesticides which transfer to the fleece by direct contact.

The main types of pesticide employed are the organo-phosphates and their lipophilic behaviour ensures that they are transferred to the wool lipids and can end up in wool by-products such as lanolin and wool grease, as well as on the wool.

A team of Uruguayan and Greek scientists has observed that many of the currently recommended procedures require a second clean up step following extraction, or fail to meet the required quality control standards by having high relative standard deviations.

So, this group have devised an improved method based on microwave-assisted extraction (MAE) followed by GC with flame photometric detection for the measurement of 40 organo-phosphate pesticides in wool.


Quicker procedure matches established method

Horacio Heinzen and co-researchers from the University of the Republic, Montevideo, Uruguay, and Aristotle University of Thessaloniki, Greece, had not seen any previous reports of the use of MAE followed by GC-FPD for organo-phosphates in wool.

In the first instance, they fortified pesticide-free wool with a mixture of 44 organo-phosphate pesticides and optimised the MAE step. The best solvent from four tested was acetonitrile, which gave good recoveries for the majority of the pesticides while co-extracting low amounts of the waxy wool grease. The magnetron power was set to 100% because no pesticide degradation was observed.

The extracts were frozen at -20°C for a few minutes to remove traces of the wool grease before evaporation to dryness under nitrogen. The residues were dissolved in ethyl acetate for GC analysis. Under optimised conditions, a 20-minute extraction gave recoveries of 70-120% for 40 of the pesticides, with relative standard deviations of less than 20%.

The four pesticides which were analysed ineffectively were dichlorvos, which was lost during the nitrogen evaporation step, and acephate, phosphamidon and methidathion, which were thermally degraded in the GC injection port despite the use of a programmed temperature vaporising (PTV) inlet. In this type of inlet, the temperature is raised gradually to evaporate the solvent to waste via a vent before a second temperature program transfers the analytes to the column.

A non-polar 100% dimethylpolysiloxane column was employed for organo-phosphate separation with a temperature gradient chosen to ensure the best separation of the 40 pesticides. The chromatograms were clear of interfering peaks, due largely to the clean MAE process, the cold precipitation step and the use of FPD.

The team observed a degree of signal enhancement following repetitive injections, which they attributed to a matrix effect. This was most prominent at lower fortification levels, especially for the more polar pesticides, but was absent at higher levels.

For quantitation purposes, matrix-matched calibration cures were prepared to counter the matrix effect and they were linear over the range 0.01-1.6 mg/kg. This range was narrower for some of the polar pesticides that suffered from the matrix effect. Under these conditions, the detection limits were 0.01-0.2 mg/kg (10-200 ppb).

Pesticides in raw wool

The GC-FPD method was applied to the analysis of raw wool samples from Uruguay that were taken for routine pesticide surveillance and the results were compared with those from the Soxhlet extraction-GPC GC method.

Ethion and diazinon were found in some of the samples, generally both at the same time, and the levels from both methods were in agreement. However, the research team championed their MAE/GC-FPD procedure because it requires no sample treatment following extraction and cooling, while providing good recoveries and detection limits for a host of organo-phosphate pesticides.

 


 


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

 
 A total of 40 organo-phosphorus pesticides have been measured in raw wool by gas chromatography with flame photometric detection following microwave-assisted extraction, to give detection limits down to ppb levels
 
A total of 40 organo-phosphorus pesticides have been measured in raw wool by gas chromatography with flame photometric detection following microwave-assisted extraction, to give detection limits down to ppb levels
 Horacio Heinzen
 

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