Objective of the study

Studies concerning storage stability of residues are conducted to evaluate the stability of residues in plants, plant products and products of animal origin during storage prior to analysis.

Ideally samples for metabolism studies and residue analysis should be stored at/or below 18°C. Storage stability studies are required because degradation and dissipation of the pesticide residue can occur, even under cold storage conditions. Storage stability studies are performed to demonstrate that pesticide residues are stable during frozen storage of the samples to be analysed or show the degree to which residues decline in that period of time.

If the storage stability study uses incurred residues (i.e. residue in a commodity resulting from specific use of a pesticide, consumption by an animal or environmental contamination in the field, as opposed to residues from laboratory fortification of samples) then it should be established that all components of the residue definitions are present in the samples and at sufficient levels to allow any decline to be observed. In this case it is important that the sample is analysed fresh, i.e. immediately after sampling, and at appropriate storage periods thereafter.

If test substances are added to untreated commodities (i.e. a fortification experiment in the laboratory) the active substance and/or relevant identified metabolites are normally added to the untreated substrate (also referred to as “spiking” of the substrate). Where the residue definitions contain more than one component, studies need to be designed to demonstrate stability of each component.

Further information on stability of residues during storage and sample processing can be found in Chapters 3.3 and 5.5 of the JMPR Procedure Manual.

Worked examples on this topic are provided in Chapter 5 of the JMPR Training Manual

Circumstances in which the study is required

Provided that samples are frozen within 24 hours after sampling and analysed within 30 days of their storage in frozen conditions, storage stability data are not required. In such cases, a justification should be given, e.g. data showing that residues are not volatile or labile.

Residue storage stability studies in livestock products should be provided if livestock is treated with or exposed to the pesticide.

Residue storage stability studies in crop commodities should be provided if food or feed crops are treated with or exposed to the pesticide, and for the non-food crop tobacco.

Test organism or substrate

Studies shall be carried out with representative substrates.

For livestock products, the study should be conducted in animal tissues, milk and eggs.

For crop commodities, the study should be conducted with one or more of the relevant commodity categories:

• commodities with high water content
• commodities with high acid content
• commodities with high oil content
• commodities with high protein content
• commodities with high starch content

If residues are shown to be stable in all commodities studied, a study on one commodity from each of the five commodity categories is acceptable. If the pesticide is to be registered for just one of the five commodity categories, the stability of the test substance in 2–3 diverse commodities within the desired category should best be tested.

Test substance

If the study is done with incurred residues, the test substance is a typical formulated product. All components of the residue definition should be present in the samples. at sufficient levels to allow any decline to be observed.
If a pesticide is added to untreated commodities in the laboratory, it is usually the active ingredient and/or relevant identified metabolites that are used for the test.

Typical endpoints of the study

Results shall be presented as absolute values in mg residue/kg substrate, not adjusted by recovery. In addition, stability may be presented as percentage of the nominal spiked residue concentration.

Testing guidelines

The following testing guidelines may be used for determination of storage stability of residues:

• OECD Guidelines for the Testing of Chemicals. Test No. 506: Stability of pesticide residues in stored commodities.
• USEPA-OCSPP Harmonized Test Guidelines. Test No. OPPTS 860.1380. Residue Chemistry Test Guidelines – Storage stability data.
• European Commission. Guidelines for Residue Data – Storage stability of residue samples. (Technical Guidelines - Appendix H)