In-house reference materials: 5-hydroxyflunixin and meloxicam in cow milk—preparation and evaluation

Reference materials are helpful to evaluate the performance of laboratories as well as being useful for the quality control of analytical procedures. Certified reference materials and other reference materials containing non-steroidal anti-inflammatory drugs in milk are however, not available. Therefore, production and evaluation of in-house reference materials with incurred residues of 5-hydroxyflunixin (5OHFLU) and meloxicam (MEL) in cow milk has been performed. The milk was collected 12 h after dosing from cows which received meloxicam (0.5 mg kg−1 b.w., i.v., single dose) or flunixin meglumine (2.2 mg kg−1 b.w., i.v. during three days). The concentrations of analytes were checked in the milk samples. The milk was diluted with milk free from NSAIDs residues, homogenised, put into sterile 20 mL vials, frozen and lyophilised. The vials were weighed before and after lyophilisation, in order to calculate the amount of water necessary for reconstitution, and were stored at a temperature of −20 ± 2 °C. For the homogeneity study, 10 random samples were analysed in duplicate and the results were interpreted using Cochran's test, Horwitz standard deviation and the test for a sufficient homogeneity. The assigned values, calculated from the results of the homogeneity test were 54.3 μg kg−1 for 5OHFLU and 46.4 μg kg−1 for MEL. The samples were tested for their stability every 14 days for 2 months and after 9 months. It has been confirmed that an appropriate homogeneity and stability of the produced in-house reference material has been obtained.