Performance evaluation of direct forced-air total solids and Kjeldahl total nitrogen methods: 1990 through 1995.

Results from collaborative studies of the performance of the direct forced-air oven-drying method for determination of milk total solids content (AOAC Method 990.20) and the Kjeldahl total nitrogen method for determination of milk total nitrogen content (AOAC Method 991.20) were published in 1989 and 1990, respectively. Method performance was characterized by using the harmonized ISO/IU-PAC/AOAC guidelines for method validation, and the methods now have final action status. During 1990 through 1995, the split sample collaborative study format was used to monitor the performance of these methods as part of a multilaboratory quality assurance program. Seven blind duplicate milk materials were sent from a central laboratory once every 2 months to participating laboratories. Data were analyzed with the same statistical procedures used in the original collaborative studies. Compared with the original collaborative study, the repeatability and reproducibility of the oven-drying method improved over time. For the Kjeldahl total nitrogen method, within-laboratory repeatability improved slightly, whereas between-laboratory reproducibility was similar to but not always as good as in the original study. The results demonstrate that the statistical protocol for collaborative studies can be used effectively as the basis for a multilaboratory quality assurance program and that the method performance achieved in a collaborative study can be maintained and even improved with time.