Investigation of the performance of the instrument by nonlinear calibration

In the search for high reliability in chemical analysis, a straightforward method of nonlinear calibration is presented. The corresponding standard deviation (SD) was calculated by the law-of-propagation of errors that allowed the determination of uncertainties as a function of concentration within the range of concentrations defined by the lower limit of analysis (LLA) and the upper limit of analysis (ULA). The advantage of using nonlinear calibrations was demonstrated by the correspondence of average values to the peak position of the normal distribution. The concentration of cobalt in two certified reference materials (CRMs) was determined by flame absorption spectrometry (FAAS) that is recognised as an experiment of very high precision. In order to ensure the determination of a reliable value for the SD, a high number of repetitions were required for the analysis. It was thus demonstrated that results that apparently deviated significantly from the certified values, actually belonged to the overall normal distribution of results. The data of experiments were grouped according to Scott’s method, and the distribution of experiments showed a particularly high frequency of results at the peak position that exceeded the expected value predicted by the normal distribution. It also deviated from the distribution peak of experiments, which demonstrates the importance of a full investigation of the distribution of results using more than 100 repetitions. The skewness of the distribution of results was eliminated by the nonlinear calibration. Correspondence: Jens E. T. Andersen, Department of Chemistry, Technical University of Denmark, Kemitorvet building 207, DK-2800 Kgs. Lyngby, Denmark