Deionized water was produced onsite with a Milli-Q Integral 3 unit from MilliporeSigma (Germany). Acetonitrile (gradient grade for liquid chromatography, LiChrosolv ® ) and ethanol (absolute for analysis, EMSURE ® ) were purchased from MilliporeSigma (Germany). Formic acid (≥99%, HiPerSolv CHROMANORM ® for LC-MS) was obtained from VWR Chemicals (Germany). The LC/MS Pesticide Comprehensive Test Mix Kit (PN 5190-0551), consisting of eight individual sub-mixtures of pesticides with a typical analyte concentration of 100 mg/L, was purchased from Agilent Technologies (USA). Furthermore, a mixture of X-ray contrast media (syn. Radiopaque agents, mix 5), antibiotics (mix 6), and pharmaceuticals (mix 17) were obtained from Neochema (Germany) and the concentration of analytes in these individual mixtures were also 100 mg/L.