Improved accuracy and robustness of NAA results in a large throughput laboratory by systematic evaluation of internal quality control data

The laboratory for INAA in Delft processes several thousands of samples per year for multielement determinations in a variety of matrices. Samples are measured on different spectrometers (using well-type and coaxial detectors), by different persons and using many different analytical protocols. All these results should be consistent with each other with respect to the degree of accuracy, i.e., the combination of trueness and precision. A rigorous internal quality control program has been implemented with automatic on-line evaluation. Annually an additional evaluation of the internal quality control results is carried out using statistical techniques. The Naji-plot approach has proven to be an important graphical tool since it provides direct insight in both trueness and precision. It is demonstrated that the degree of accuracy of the results obtained under the large variety of operational parameters is under constant improvement.