Crop protection chemicals undergo change after application. The nature and extent of the change must be determined before appropriate methods of analysis can be developed and maximum residue limits (MRLs) for commodities can be established. The method might measure the chemical or a derivative of it and can include metabolites. In some cases, the nominal concentration of the parent compound is calculated from the measured concentration of a metabolite; in other cases, a derivative or metabolite is used as a measure of the residue.
1. Residues and metabolites
A residue is defined as the chemical, its metabolites and related compounds to which the MRL applies.
The inclusion of specific metabolites or degradation products in the expression of a residue depends on their toxicology profile and the extent to which they occur.
Metabolites that occur at levels greater than 0.1 milligram per kilogram or that comprise more than 10% of the total radioactive residues should be identified.
Analytical methods must be available to determine the residue levels in a range of substrates.
Metabolism studies are particularly important if the commodity is to be used as food.
2. Metabolites as components of the total residue
A number of principles and specific options may be used to decide which metabolites or degradation products should be included in the definition of a residue. They are based largely on discussion in the report of the 1979 Joint FAO/WHO Meeting on Pesticide Residues. Also, note the relevant OECD Guidance Documents and Test Guidelines.
Residues expressed as the parent compound
Residues may be expressed as the parent compound if:
- there are no metabolites
- metabolites are known to be insignificant
- metabolites of toxicological concern are known to be present in significant amounts and the analytical method measures the total residue as a single compound that may be numerically expressed as the parent compound (in this case, the metabolites included in the residue are listed), or
- metabolites of toxicological concern are known to be present in significant amounts and the analytical method measures the parent compound and metabolites separately. In such cases, the compounds of the total residue may be expressed additively as parent compound, with recalculation for differences in molecular weight only when the differences are substantial (for example, greater than 20%).
Residues expressed as a single metabolite or alteration product
Residues may be expressed as a single metabolite or alteration product if:
- the parent is quantitatively converted to another chemical entity (for example, aluminium phosphide to phosphine)
- metabolites of toxicological concern are known to be present in significant amounts and the analytical method measures the total residue as a single metabolite (in this case, the result may be expressed as that metabolite but the compounds included in the residue should be listed), or
- metabolites of toxicological concern are known to be present in significant amounts and the analytical method measures the residue components, including parent compound if present, separately. The result may be expressed additively in terms of a metabolite with recalculation for differences in molecular weight only when differences are substantial (for example, greater than 20%).
Residues expressed as parent and metabolites separately
Residues may be expressed as parent and metabolites separately if metabolites are known to be present in significant amounts and the analytical method measures each component of the total residue separately. All metabolites and degradation products included in the definition of the residue should be listed regardless of the method of determination.
Metabolites of some pesticides are also registered pesticides. Analysing food commodities in trade for the metabolite may provide little or no information on which compound was used. Whenever possible, the parent pesticide and its metabolite(s) used as pesticides should be subject to separate MRLs. If it is not possible to set separate MRLs (for example, when the parent pesticide is degraded rapidly), the MRLs applying should be expressed only in terms of the metabolites.
3. Extraction efficiency
Conventional recovery experiments do not necessarily reflect the efficiency with which aged residues can be extracted. Certain components of the residue may be bound with naturally occurring plant constituents and thus may not be recovered by extraction techniques that are adequate for the free components. There should be some determination of the completeness of extraction by the procedures. Radioisotope labelling provides the best evidence of the completeness of extraction. Fortification levels should reflect the proposed MRL and recoveries should be 70% to 120% and reasonably consistent between samples. Exaggerated rate studies may be necessary to obtain sufficient activity for identification.
4. Metabolism studies
Plant metabolism studies should be carried through to plant maturity to indicate terminal residues at harvest. All parts of the plant should be studied to identify any different metabolic paths. For the same reason, studies should be done on different types of plants on which the product might be used.
In the case of bound or conjugated residues, enzymatic, basic or acid hydrolysis may be necessary to release components of the residue.
5. Validation of methods
Methods must be validated by control sample data and recovery data for all components on an adequate representation of the commodities involved.
6. Storage stability
The issue of storage stability in analyses for residues in commodities for the setting of MRLs is an important factor to be considered – refer to the stability of residues during storage guideline for more information.
7. Processing studies
Processing studies are important when MRLs are to be set for commodities that undergo processing. For more information, refer to the processing studies guideline.