Abstract

Concentration measurements made over long time periods, as obtained in quality control charts, provide a possibility of gaining supplementary information about an analytical method. Low frequency noise in the measurements has been considered in the present paper. Quality control charts for soil analysis by AAS with measurements lasting for more than 1000 days were studied, and Fourier spectra at frequencies as low as 10⁻⁸ Hz were obtained. When no distinguished uncertainty sources were present, a f^{−1.0 ± 0.1} noise power spectral density dependence on frequency was found for the low frequency region. The consequences of that dependence for both the analytical method improvement policy and the analytical quality management strategies are discussed.