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Тезисы VII конференции "Аналитика Сибири и Дальнего Востока - 2004" » Пленарные лекции » ...


P.S. Hoffmann

Darmstadt Technical University, Materials and Earth Sciences, Chemical Analytics,
Petersenstr. 23, D-64287 Darmstadt, Germany dg7jhrzpub.tu-darmstadt.de

    Each analytical result has to be characterized by an uncertainty. This uncertainty (or error of the result) has to be distinguished into “random errors“ and into “systematic errors“ (1). To determine a random error usually the arithmetic or geometrical mean of the results of repeated measurements is used for calculation of the “relative standard deviation (RSD)“. This RSD (or the multiple) is a measure for the width of the scatter of all data produced at identical conditions and therefore for the precision (repeatability) of the result. The determination of the systematic error and of the accuracy (difference to the true value) will be performed by very different methods:

Use of two, or better more different, independant analytical methods,

Use of Certified Reference Materials as test samples,

Spiking of samples with analytes,

Participation in round robins,

Use of control charts and duplicate plots, use of Identity Line plots.

    For the X-ray methods in discussion all these methods can be used. But in practice not all can be recommended. The reason for the selection of the method of quality assurance (QA) is in this case mainly dominated by the advantage of X-ray fluorescence analysis (XRF) of non-destructive characterization of solid material. For comparison of results for different samples absorption and scattering effects (matrix effects) and enhancement effects (secondary excitation) have to be considered. The method of quantification and of QA to be used in XRF is in practice a function only of the sample thickness: infinitely thick specimen, intermediate-thick samples, and practically infinitely thin samples (2).

    In XRF-methods the laboratory standards, the certified standards and the samples for examination need not only a similar chemical composition but additionally similar physical properties (e.g., particle size distribution). For infinitely thick samples the total matrix effect can be compensated by application of the Compton scatter intensity, by internal standardization, standard addition, dilution methods, and can be calculated by mathematical methods (fundamental parameter, influence coefficient algorithms, algorithms with constant coefficients, algorithms with variable coefficients, specimen with more than two compounds, algorithms with empirical coefficients). For intermediate-thick samples the Compton and the Rayleigh scattering intensities ratio is a measure for the total mass absorption coefficient. Additionally, the particle effect has to be considered in intermediate-thick and thin specimen. Infinitely thin samples can be analysed quantitatively by internal standardization, as matrix effects and secondary excitation do not play any role.


(1) Wenclawiak B.W., Koch M., Hadjicostas E. (Eds.) Quality Assurance in Analytical Chemistry, Springer-Verlag, Berlin, 2004

(2) Van Grieken R.E., Markowicz A.A. (Eds.) Handbook of X-Ray Spectrometry, Marcel Dekker, New York – Basel, 2002

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