Lyophilized and natural urine specimens in the quality control of analyses of toxic metals

A quality control program of analyses of toxic metals in urine has been carried out in the Nordic countries since 1978. In connection with these programs, the advantages and disadvantages of lyophilized compared to natural urine specimens as control materials have been investigated in three successive similar studies. Three parallel lyophilized and natural specimens were distributed to 12 participating laboratories. Two of the three specimen pools were spiked with known amounts of As, Cd, Cr, Hg, Ni, and Pb standard solutions. The data indicate no clear differences in the mean concentrations, coefficients of variation, or mean recoveries for the various metals between the two control materials used with the various types of analytes. However, rather wide random variations were observed, emphasizing the analytical difficulty of these analyses and the need for routine quality control.