Introduction to Pesticide Residue Testing

In Thailand a number of laboratories are involved in the detection of pesticide residues in food. Several of these laboratories are involved in procedures that lead to the marketing of food under various "safe" labels. Most of the analysis is done with a so-called GT-test kit, developed by Gobthong Thoophom from the Public Health Ministry, Division of food in Bangkok.

The principle of the GT-test kit is based on measuring the inhibition of the enzyme cholinesterase. Some pesticides e.g. organophosphorus and carbamates inhibit this enzyme. The GT-test kit is capable of detecting whether or not substances in the sample inhibit the enzyme.

The GT-test kit is very easy to use and can be carried out anywhere e.g. on markets. The results are produced very rapidly (few hours) and at a very low price. No expensive instruments are used.

Unfortunately, the method is very unspecific which means that the method doesn’t identify and quantify the specific pesticide in the sample. Comparing with Maximum Residue Levels (MRLs), is therefore not possible. Additionally, not all pesticides inhibit the enzyme and consequently the method gives no response for these. Furthermore, the GT method can give false positive results if used on unripe fruits, chilies, citrus fruits and onions, due to content of specific naturally compounds.

A more common way to analyze for pesticide residues is with so called standard methods. This is normally done by:

  1. Extracting the pesticide from the food with an organic solvent
  2. Cleaning up the extract to remove compounds that can interfere with the pesticide
  3. Measure the content of the pesticides by gas chromatography, GC (or liquid chromatography, LC).

A technician analyzing samples in a pesticide residue laboratory in Thailand
A technician analyzing samples in a pesticide residue laboratory in Thailand

These methods have the advantage of being qualitative (detects the individual pesticides) and quantitative (measure the amount of pesticide). Therefore it is possible to compare the result with MRLs. The disadvantages are that the methods are time consuming, expensive and requires laboratories and qualified staff.

Because of limitation of pesticides detected by the GT-test kit, the Department of Agriculture, Toxic Substances Division, were interested to implement other rapid testing techniques, especially to detect residues of pyrethroids, which are widely used in Thailand. A project under IPM DANIDA was initiated and a consultant from the Danish Veterinary and Food Administration was signed on the project. The project resulted in a recommendation of implementing a multi-residue method developed by Michelangelo Anastassiades, Steven J. Lehotay, and Darinka Stajnbaher from
U.S. Dept. of Agriculture, Agricultural Research Service, Eastern Regional Research Center; Wyndmoor, Pennsylvania, USA. This method, which is called QuEChERS method, has been published in Journal of AOAC International,Vol. 86. No. 2, 2003 p 412-431.

Pesticides residue testing laboratory
Pesticides residue testing laboratory

See also: GT test kit and QuEChERS method