Measurements for certification of chlortetracycline reference materials within the European Union Standards, Measurements and Testing programme

The experimental design of a material certification programme is described. The matrix reference materials (RMs) comprised chlortetracycline (CTC)-containing and CTC-free lyophilised porcine liver, kidney and muscle produced under the European Commission's Standards Measurements and Testing (SMT) programme. The aim of the certification programme was to determine accurately and precisely the concentration of CTC and 4-epi-chlortetracycline (epi-CTC) contained in the RMs. A multi-laboratory approach was used to certify analyte concentrations. Participants (n = 19) were instructed to strictly adhere to previously established guidelines. Following the examination of analytical performance criteria, statistical manipulation of results submitted by 13 laboratories, (6 withdrew) allowed an estimate to be made of the true value of the analyte content. The Nalimov test was used for detection of outlying results. The Cochran and Bartlett tests were employed for testing the homogeneity of variances. The normality of results distribution was tested according to the Kolmogorov-Smirnov-Lilliefors test. One-way analysis of variance (ANOVA) was employed to calculate the within and between-laboratory standard deviations, the overall mean and confidence interval for the CTC and epi-CTC content of each of the RMs. Certified values were within or very close to the target concentration ranges specified in the SMT contract. These studies have demonstrated the successful production and certification of CTC-containing and CTC-free porcine RMs.