Measurement of uncertainty in pesticide residue analysis: Implication in legal limits

The major sources of systematic and random errors influencing the reliabili ty of residue data are considered. In performing sampling and sample prepar ation the internationally agreed upon procedures and definitions should be carried out and followed if comparable results are sought. The field-to-fie ld variability of residues is the largest source of uncertainty (80-100%) o f residue data, generally followed by the within-field (30-40%) and analyti cal (10-20%) variability. The combined uncertainty of residue data, compris ing the uncertainty of sampling, sample processing and analysis, is estimat ed to be a minimum of 33-44% for plant samples, consisting of medium and la rge units, taken from a single lot. The combined uncertainty should be take n into account in deciding on the significance of differences of residue co ntent of replicate samples in dispute cases. The certification of the compl iance of a lot with Maximum Residue Limits (MRL) requires the analysis of a number of samples. Practical approaches are recommended for performing suc h tasks economically.