S TABILITY DURING S TORAGE The supervised trial samples are usually deep-frozen shortly after the sampling and

5.4.1 S TABILITY DURING S TORAGE The supervised trial samples are usually deep-frozen shortly after the sampling and

shipped deep-frozen to the laboratory within the shortest possible time, where they are kept deep-frozen until analysis. During this storage period, the concentration of residues of the pesticides and their metabolites may decline due to processes such as volati- lization or reaction with enzymes. Storage stability tests are carried out with represen- tative commodities to demonstrate the stability of residues during frozen storage before analysis. The storage stability studies are part of the data package submitted to support registration of a compound. The FAO=WHO Pesticide Residues

—Evaluations also include information on the stability of residues during storage.

Where it is foreseen that the samples have to be stored in the laboratory over

1 month, and appropriate information on the stability of residues is not available on representative sample matrices under similar conditions as the samples will

be stored, storage stability test should be carried out. The basic principles 20,33 to be considered for planning storage stability studies are briefly summarized later.

Stability data obtained on one commodity from a commodity group (see Section

5.5) can be extrapolated within the same group, provided that the storage conditions are comparable. The study can be performed with sample containing field- incurred residues, if the suitable homogeneity of the material had been verified

before (CV Sp < 0.25 –0.3 3 CV A ). Alternately, the test portions withdrawn from the homogenized untreated sample matrix should be spiked individually. Untreated test material should be prepared and stored under the same conditions. The treated and blank test material should be sufficient for a minimum of 8 3 4 treated as well as untreated test portions for analyses with some extra material as reserve. The total number of test portions should be larger, if the extension of the study period may be

Quality Assurance 139 necessary. The active substance and its metabolites or degradation products included

in the residue definition should be tested separately if spiked test portions are used. The initial residue concentration should be sufficiently high to enable the accurate determination of the residues if their concentration decreases during storage. Nor- mally, analyses at five time points are sufficient. The first test should be performed at day 0 to verify the initial concentration, and the others selected according to approximate geometrical progression (e.g., 0, 1, 3, 6, and 12 months or 0, 2, 4, 8, and 16 weeks if decline of residues are suspected). At each time, two treated test portions and at least one freshly spiked untreated sample should be analyzed.

The results should be reported in the form of individual residue concentration (milligram=kilogram) measured in the treated stored samples (survived residues), the concurrent recoveries expressed in percentage of the spiked amount, and the standard uncertainty of the measurement determined independently as part of the validation of the analytical method. The individual recoveries obtained should preferably be within the warning limits of the established method. If that is not the case, the analysis of residues in additional test portions of the stored samples should

be repeated together with additional recovery studies. Where the storage stability study carried out with samples belonging to the representative commodity groups indicates that the residue is stable, then it can be assumed that the residues would be stable in other matrices stored under similar conditions.