How to take a sample for pesticide residue analysis
Pesticides are used in agriculture, public health or for general pest control. These can either be insecticides, fungicides, herbicides, rodenticides etc. Pesticide residues can be the deposits of pesticide active ingredients themselves or their metabolites present in the environment after application, spillage or dumping.
Selected sampling programs and analysis can be used to:-
Investigate residual levels of pesticide 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 dissipation vary considerably depending on the pesticide's chemical composition and the prevailing environmental conditions. The objective of a residue analysis is to indicate which active ingredient residues are present on a sample and to determine persistence of some active ingredients. Precautions must be taken to ensure that the sample arriving at the laboratory maintains the integrity as when sampled.
There is a risk of personnel contamination when entering a treated or contaminated area. Proper care should be taken, one needs to wear the appropriate personal protective equipment. Cover bare skin by wearing along sleeved clean clothing, a mask , gloves and appropriate foot wear.
Gloves and clothing can become contaminated during sampling, these can in turn contaminate the subsequent samples being collected. Therefore its important to change your clothing and gloves before taking each sample. If sampling both treated and untreated areas, always start with non treated areas or areas treated earlier and finish with recently treated portions.
Prevention of contamination
Pesticide residue analysis is a very sensitive science with extremely low limits of detection. It is important to take great care not to contaminate the samples in any way:-
Always wear disposable gloves and change between samples.
Change personal protective clothing between samples and do not use any PPE that has been used previously for pesticide application. The personnel themselves should be clean and not recently involved in spray operations.
Tools used in sampling should be clean, washed in hot water and detergent between sampling , and rinsed with clean water or acetone.
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. Zip lock bags are also convenient and safe for keeping the samples.
Samples should be sealed, stored and transported safely to avoid any form of contamination which can be either through spills, leakages or gases.
Sample Storage and transport
Pesticides can and do degrade during transport. In order to maximize 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 them as cool as possible. They should not be exposed to direct sunlight.
Sample labelling and dispatch
All samples should be clearly labelled and accompanied by a completed Sample Submission Form. Its advisable to communicate with the laboratory before sampling for more guidance at firstname.lastname@example.org, so that the samples can be processed as quickly as possible.
Samples should be sent to Crop Nutrition Laboratory Services Ltd.
Telephone + 254 (0) 720 839 933
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).
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.
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
For more information please contact us through:
Crop Nutrition Laboratory Services Ltd.
Telephone + 254 (0) 720 839 933