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In routine analysis it is often necessary to introduce minor changes in protocols or extend the applicability of previously validated methods to similar matrices. The SANTE guidelines contemplate on-going validation as a quality control process to verify analytical performance through a reduced number of experiments1.In this work, a validated QuEChERS method for the determination of pesticides and mycotoxins in corn and alfalfa was used for the analysis of a large number of different types of feed samples (cereals, pastures, silages). Briefly, 2 g of sample were hydrated with 10 ml H2O (2% CH2O2), followed by extraction and partition with 10 ml acetonitrile, MgSO4/NaCl (4:1). An aliquot of the extract was then cleaned-up using d-SPE with MgSO4, C18 and PSA, prior to analysis by UHPLC-MS/MS2.A modification of the clean-up step had to be introduced due to the large number of samples to be processed and availability of sorbent salts at the time of analysis. Thus, for cereal-based samples only MgSO4/C18 was used while MgSO4/PSA was used for pasture-based samples.Following the concepts of the on-going validation, the mean recovery (Rec%) was evaluated at the same concentration level from validation (100 µg/kg). Precision was evaluated as repeatability and intermediate precision, repeating the test 2 and 3 consecutive days for corn and alfalfa, respectively. The recovery results obtained after the modifications were compared with those from the initial validation through statistical t-tests. Finally, an additional quality control test was done by applying the methodology to a European Union Proficiency Test (EUPT CF18-2014) in wheat flour.As a result, 80% of the studied compounds (117 pesticides and 2 mycotoxins) showed recoveries in the 60-140% range accepted for routine analysis by the SANTE guide, with RSDs

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