Analysis of DDT residues in soil by ELISA: An international interlaboratory study

An international interlaboratory study was conducted to determine the perfo rmance of a group of laboratories from developing and developed countries. The study used a commercial microwell ELISA on unknown samples spiked with different levels of DDT, The study design was based on Youden pairs and bal anced replicates, Two soils, differing in particle size distributions, orga nic matter content, and cation-exchange capacities and thought to be DDT-fr ee, were spiked at 5 DDT levels between 0.025 and 2 mg/kg, Nineteen laborat ories in 17 countries took part in the collaborative trial; of these, the m ajority were modestly equipped laboratories in developing countries. Sample s were analyzed without filtration or cleanup and using standards of pure D DT in methanal. Data were analyzed for repeatability and reproducibility, a nd average recoveries at the spike levels were calculated, Mean real recove ries for both soils were similar (103% for soil A and 100% for soil B), wit h values between 0.1 and 2 mg/kg DDT, Precision estimates were best in the linear working range of the assay (0.1-0.5 mg/kg DDT), with reproducibility relative standard deviations (RSDR) typically averaging about 38 and 46% n ear the upper and lower detection limits, respectively. Corresponding repea tability relative standard deviation (RSDr) values were 20-36% and 36-57%. Thus, even though much of the trial was performed under developing country conditions, performance statistics were similar to other reported results o btained with ELISAs on small molecules of agricultural importance, such as mycotoxins and pesticide and antibiotic residues.