Report on the techniques for the analysis of chlormequat and related pesticides.
Report No DWI0767

Feb 1994

SUMMARY

A review of the literature shows that currently there are no methods available for the simultaneous measurement of chlormequat (CQ), difenzoquat (DFQ), diquat (DQ) and paraquat (PQ) in waters or any other matrix. A variety of methods have been published for these individual compounds, although most publications have been concerned with mixtures of diquat and paraquat. This is probably due to the wide usage of these compounds for many years. The 1987 'MEWAM' method for paraquat and diquat employed reduction with alkaline sodium dithionite and determination of the reduced compounds by visible light spectroscopy or 2nd derivative measurement. Although this method is sensitive it requires up to 5L water samples, and cannot be used when both herbicides are present in the same sample. Gill et al [ 1 ] showed in 1983 that HPLC with UV detection could detect PQ and DQ to about 1g/ml urine in six minutes run time. Many authors have used this HPLC system for a wide range of matrices, and the method is probably very robust. The sensitivity obtained by Gill is equivalent to lmg/litre, which would require a sample concentration factor of 10,000 to be incorporated into any new method to achieve a target detection level of 0.1g/litre. Although it should be possible to improve the sensitivity of the Gill method by using 2mm i.d columns, and that the same method should be suitable for difenzoquat, it would not detect chlormequat. This is because chlormequat is almost undetectable by UV. This is the major problem facing the analysis of this compound.

There are few publications for chlormequat that could be considered suitable to replace the current MEWAM method, which requires a reagent that is no longer available. In 1970, Tafuri et al [ 2] showed that derivatisation of chlormequat with sodium benz enethiolate yielded a product that could be analysed by gas chromatography. This approach has been improved by Allender [3] in 1992 who used the related compound pentafluorothiophenol to yield a product that is highly detectable to GC-electron capture systems. However, the product is rather too volatile and elutes close to the solvent front, and the reaction produces other by-products that require the GC to be temperature programmed over a period of 35 minutes. It is not clear what effect this reaction would have upon diquat, paraquat and difenzoquat, but it is possible that after the initial dequaternisation step the reaction may stop. If that were the case, then it is possible that the products could be chromatographed by GC, although the conditions would require much higher temperatures than the chlormequat product, and detection may be complicated by the large number of unidentified peaks arising from the chemical derivatisation. The 'MEWAM' method for difenzoquat that utilises GC-NPD following conversion of DFQ to the monomethyl pyrazole derivative in the heated injector, is unlikely to be suitable for chlormequat, diquat and paraquat. It is , in any case, a method that depends upon the performance and suitability of a particular instrument, and is unlikely to produce a robust method. In general, quaternary ammonium compounds are considered unsuitable for analysis under GC conditions, and in this consultancy were given a low priority. No new data for the use of the pentafluorothiophenol reagent was generated.

The fact that chlormequat, difenzoquat, diquat and paraquat are all quaternary ammonium compounds means that it should be possible to exploit this common chemical property in order to develop a single analytical method for their measurement. This consultancy has investigated the HPLC method [4] that uses post-column ion-pair formation between organic amines with 9,10-dimethoxyanthracenesulphonic acid coupled with on-line extraction into an immiscible organic solvent followed by fluorescence detection. The equipment contains an aqueous-organic phase separator designed by Jefferies that is responsible for supplying a clean, droplet-free flow of organic extract to the detector [5]. This complete post-column unit is now commercially available, and can be used with any HPLC system. Although post-column HPLC is more complex than simple HPLC, the equipment is robust in operation and suitable for routine use.

Copies of this report may be available as an Acrobat pdf download under the 'Pre 2000 Reports' heading on the DWI website.