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This guideline provides information on the need for and conduct of storage stability trials. It does not give precise details on how such trials should be conducted.
Further information on the conduct of storage stability trials is available in OECD Test Guideline TG 506: Stability of Pesticide Residues in Stored Commodities.
1. Introduction
Samples taken for residue analysis should be analysed as quickly as possible after collection before physical and chemical changes take place. If samples are going to be stored for a significant period of time, either argument or data on the stability of residues during storage must be supplied.
It is acceptable to use overseas studies on the same or similar substrates. Once storage stability has been demonstrated in a few representative crops, and when necessary, their by-products, additional data will not be required as long as storage conditions and duration are similar.
Use of storage stability data from radiotracer studies is also acceptable provided the radioactivity is characterised.
2. Conducting trials
Studies on the stability of residues in samples, over the time and at the temperature of storage, should be carried out with representative substrates.
A stability study should be conducted with sample material subjected to similar sample preparation procedures and storage conditions as those used for the proposed magnitude of residue studies. However, samples being subjected to a storage stability study may be stored as homogenates rather than in a whole state. The homogenate represents a worst case as the homogenisation process can release enzymes, acids and other chemicals that can react with the pesticide or its metabolites. This may lead to unacceptable results if the residue degrades under these conditions.
It is also acceptable to follow the Food and Agriculture Organization’s guideline for residue trials, which recommends extraction of the sample followed by removal of most of the solvent prior to storage; however, in this case, both the storage stability study and the retained samples must be treated in the same manner. Different storage containers may be used for the stability study samples and the retained samples as long as the pesticide is not volatile.
Data is required for a representative range of crops, such as water crops (e.g. leafy vegetables, citrus apples), oil crops (e.g. canola), protein crops (e.g. legume vegetables) and starch (e.g. potato, cereals), to support crop residue trials, and animal tissues (e.g. milk, meat, eggs) to support animal transfer studies.
Experiments may be conducted on prepared samples with incurred residues. Alternatively, aliquots of prepared control samples may be spiked with a known amount of pesticide before undergoing normal storage conditions.
If degradation to a metabolite (contained in the residue definition) is likely during storage, stability studies should be conducted on samples spiked with the metabolite in addition to those for the parent compound.
The storage stability studies should be carried out at sufficiently high levels in the starting material to monitor a potential dissipation with significant precision. The residue values should be within the range of the expected residues, and up to at least 10 times the limit of quantitation of the analytical method with a minimum of at least 0.1 mg/kg. This will reduce the chance that the stability for the residues cannot be ascertained due to high variable recoveries. If typical residues observed in the magnitude of residue studies are much higher than 0.1 mg/kg, it is preferable (although not required) for a storage stability study to employ comparable residue levels.
Storage conditions should be used that allow the best stability of residue. Generally, samples should be stored at –20 °C. The actual temperature(s) and/or ranges should be stated in the report.
The intervals for sampling plus analysis depend on the available information on the potential stability of the residue. A minimum of 5 analytical intervals including zero time should be considered, regardless of the duration of the study.
The analytical method must be able to determine the parent compound and its relevant metabolites where applicable. For each sampling date, at least 2 samples along with procedural recoveries should be analysed.
The results of storage stability studies should be presented in tables, including mean and individual values. Procedural recoveries should be reported. In the case of a significant decline, a dissipation curve should be established based on the mean or individual values. This is achieved by plotting the time of storage (x-axis) versus percentage residues (y-axis). The percentage dissipation can then be read at any time point on the curve.