Development of potential candidate reference materials for drugs in bottom sediment, cod and herring tissues. 
Regular use of a reference material and participation in a proficiency testing program can improve the reliability of analytical data.  This paper presents the preparation of candidate reference materials for the drugs metoprolol, propranolol, carbamazepine, naproxen, and acenocoumarol in freshwater bottom sediment and cod and herring tissues.  These reference materials are not available commercially.  Drugs (between 7 ng/g and 32 ng/g) were added to the samples, and the spiked samples were freeze-dried, pulverized, sieved, homogenized, bottled, and sterilized by γ-irradiation to prepare the candidate materials.  Procedures for extraction and liquid chromatography coupled with tandem mass spectrometry were developed to determine the drugs of interest in the studied material.  Each target drug was quantified using two analytical procedures, and the results obtained from these two procedures were in good agreement with each other.  Stability and homogeneity assessments were performed, and the relative uncertainties due to instability (for an expiration date of 12 months) and inhomogeneity were 10-25% and 4.0-6.8%, respectively.  These procedures will be useful in the future production of reference materials.