Relative Matrix Effect in the Quantification of Nitroimidazoles and Dyes in Meat, Eggs, Shrimp, and Fish Using an Ethyl Acetate/Salting-Out Extraction and Isotope Dilution Ultra-High Performance Liquid Chromatography–Tandem Mass Spectrometry

This paper presents the validation of a multiresidue method for quantifying seven nitroimidazoles and four dyes in poultry, pork, beef, eggs, shrimp, and fish. The average matrix factors of standard normalized to IS (0.82–1.12) indicated no significant matrix effect, and the CV % values (2.26–12.10%) complied with the 20% limit of the Commission Implementing Regulation 2021/808. However, medium suppression (50–80%) or enhancement (120–150%) was observed in 4.29% and 2.14% of the individual matrix extracts, respectively. The linearity ranges (in μg kg–1) were 0.50–3.0 for nitroimidazoles and 0.25–1.5 for dyes, and the determination coefficients (R2) were at least 0.9688. The limits of quantitation were the lowest calibration levels. Analyte recovery and reproducibility were satisfactory (fREC: 77.95–115.0%; CV: 5.09–27.24%). The decision limits (CCα) (in μg kg–1) were 0.59–1.00 for nitroimidazoles and 0.29–0.46 for dyes and complied with the EURLs minimum method performance requirements (1.0 and 0.5 μg kg–1, respectively).