How to take a sample for pesticide residue analysis

Introduction

Pesticides used in agriculture, public health or general pest control can enter the environment in a number of ways.  Pesticide residues are the deposits of pesticide active ingredients, metabolites or breakdown products present in some component in the environment after its application, spillage or dumping. Residue analysis provides a measure of the nature and level of any chemical contamination within the environment and of its persistence.

Selected sampling programs and analysis can be used to:-

  • investigate residual levels of petsicide in the environment, their movement and their residual rates of degradation
  • identify contaminated areas and/or sources of contamination
  • examine the uptake of pesticides by agricultural components (food, vegetables, flowers, seeds).

All pesticides are subject to degradation and/or metabolism once released into the environment.  The rates of degradation and disspiation vary considerably according to the pesticide and the conditions. The object of a residue analysis is to indicate which residues are present at the time of sampling, and every precaution must be taken to ensure that the sample arriving at the laboratory has not been allowed to deteriorate in such a way that the results are meaningless.

Personal Protection

There may be risk of personal contamination when entering a heavily treated or contaminated area and proper care should be taken to wear the appropraite personal protective equipment. Cover bare skin, wear a mask and gloves and appropriate foot wear.

Gloves and clothing can become contaminated which can in turn contaminate the samples being collected.  Wear clean clothing, and clean disposable gloves.  Change your gloves before taking each sample.  If going from a pesicide contaminated area to an area contaminated with a different pesticide or an unsprayed or uncontaminated area, it is advisable to change your clothing.

Prevention of contamination

Pesticide residue analysis is a very sensitive science with extremely low limits of detection.  It is of important to take great care not to contaminate the samples in any way:-

  • Wear disposable gloves and change between samples
  • Change personal protective gear between samples and do not use any PPE that has been used for previously for pesticide application.  Individuals themselves should be clean and not recently involved in spray operations.
  • Tools used in sampling procedures should be clean, washed in hot water and detergent between sampling procedures, and rinsed with clean water or acteone.
  • Use clean unused sample bottles or sample bags supplied by Crop Nutrition Laboratory.  Do not use PVC materials - these can be a source of sample contamination. 
  • Samples should be sealed, stored and transported in such a way that they cannot become contaminated by spills, leaks, gases.

Sample Storage and transport

Pesticides can and do degrade during transport and in order to maximise on the correlation between the residues at the sample site and the residues in the sample reaching the laboratory it is important to transport the samples to the laboratory as soon as possible and to keep the residues as cool as possible, and out of direct sunlight. It is better for the samples to arrive at the laboratory at the beginning of the working week.

Sample labelling and dispatch

All samples should be clearly labelled and accompanied by a completed  Sample Submission Form.  Please also let the laboratory know that the samples are on the way at support@cropnuts.com, so that the samples can be processed as quickly as possible.  Samples should arrive at the laboratory at the beginning of the week.

Samples should be sent to Crop Nutrition Laboratory Services Limited, Coopers Center, Kaptagat Road, (nr Kangemi Flyover,off  Waiyaki Way), Loresho, Nairobi, Kenya Tel 0733 839933, 0723639933.

Sample selection

Each part of collecting samples for residue analysis is critical because the analysis can become meaningless if it is not reprsentative or if it has been compromised in any way.  eg if it has become contaminated during or after sampling or is has been allowed to deteriorate through exposure to high temperatures or light.

The nature of the sampling is determined by the objectives of the sampling excercise.  What do you hope to achieve? and does the sample represent this?  The sampling points should be established and marked in such a way that they can be re-visited (map, GPS field number, batch number etc).

Soil Sampling

Pesticide residues due to agricultural pest control are normally confined to the top layers of the soil.  Take approximate 15 X core samples using a clean soil auger, in a W pattern as below, down to a depth of 15 cm.  Mix these to form a composite sample, put 500g of this sample in the sample bag provided.  Complete the details of the sample on the sample bag and the  Sample Submission Form.  Store in a clean cool dark place until you can get it to the laboratory.

Vegetative Tissue sampling

The sample needs to be representative of the area/product that is being tested for.  If for example you require to test the residues of beans, in a bean field, the best protocol would be to take bean samples from the four corners of the field and in the centre, mix the sample and take a sub-sample out of this sample.  Put the sub-sample in a clean Crop Nutrition sample bag (more than one sample bag may be required), label clearly, chill and send to the laboratory.  Vegetative tissue can 'sweat' during transport and create conditions that lead to the rapid development of moulds, which can lead to microbial degradation of the pesticide residues.  It is therefore important to keep the sample cool, and transport to the laboratory as quickly as possible.

For pesticide residue analysis of produce ideally we require approximatelly 1 kg of tissue.  This should be at least 6-8 units of larger commodities, (avocados etc), 500 g of berries, or 200-500 g of herbs.

Water Sampling

When taking a water sample - it is important to think about why you are doing it and what you hope to achieve.  Water will tend to show pesticides for only a short time.  Pesticides are often absorbed onto sediment or other organic matter and removed from aqueous solution.  Some pesticide residues form a surface film, rather than being dispersed, in this case you need to take a surface water sample.  Water samples often contain suspended matter, these can contain significantly higher pesticide residue than the water itself.  For many purposes the water and suspended matter are considered together.  For other, specialised circumstances there is merit in filtering the sample and analysing the water and suspended matter separately.

Important considerations include:-

  • is the sample to be taken close to the shore or further out - in which case a boat may be needed
  • at what depth the sample is to be taken - surface, sub-surface or at depth - there can be differences depending on temperature, and wether there are surface films from pollutants or decaying vegetation, or whether sediment is present at different depths.

For water coming out of a tap, or irrigation system - allow to water to run for a few minutes to flush the pipes before taking the sample.

For pesticide residue analysis of water 2 litres (2X sample bottles) of sample are required.  Please label the samples properly, keep chilled and transport to the laboratory as soon as possible.

Further information can be found on the following link,   How much sample is required for pesticide residue analysis

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