TRACE-ELEMENTS IN A COMMERCIAL FREEZE-DRIED HUMAN URINE REFERENCE MATERIAL

A large batch of freeze-dried human urine reference material, Seronorm Trace Elements Urine, Lot 101021, was prepared commercially (Nycomed Pharma AS, Oslo, Norway) for quality control purposes in trace element analysis. Analytes are being determined by a voluntary, international co-operative effort so that the material will be available to the sci entific community at modest cost. The material is in stoppered glass v ials and is to be reconstituted with 5.00 ml of water prior to use. We have evaluated the trace element content for several elements, includ ing chromium and zinc, elements for which we have two independent meth ods available for the determinations, namely isotope dilution mass spe ctrometry (IDMS) and atomic absorption spectrometry (AAS). We also rep ort on other trace elements measured by IDMS alone, such as Se, for wh ich we have enriched stable isotopes available. Results for chromium i ndicate a mean +/- standard deviation (n = 10) of 1.2 +/- 0.3 (by IDMS ) and 1.4 +/- 0.3 (by AAS) ng Cr per ml of reconstituted urine, indica ting possible inhomogeneity and/or contamination (21-25% relative stan dard deviation, RSD). Approximately half of the observed chromium orig inates from the sample container. The values observed for zinc were 59 0 +/- 90 ng ml(-1) (15% RSD) for freshly reconstituted material, 760 /- 60 ng ml(-1) (8% RSD) for material reconstituted 4 d earlier, and 9 40 +/- 60 ng ml(-1) (6% RSD) 2 months after reconstitution. Selenium v alues by IDMS were very reproducible, with a mean concentration of 16 +/- 0.15 ng g(-1) (0.9% RSD), suggesting little or no contamination an d a high degree of sample homogeneity for this element. The source of potential contaminants has been evaluated by multielement determinatio ns of leachates of the vials and stoppers. Elements noted in significa nt amounts include B, Ba, Sr, Mo, Cu and Zn, with most of the zinc com ing from the rubber stopper.