[B._Beckhoff,_et_al.]_Handbook_of_Practical_X-Ray_(b-ok.org).pdf

83 will not compensate for this variability the

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of the internal standard element, but, the ratio of (5.83) will not compensate for this variability. The method of the added internal standard has also several drawbacks. The internal standard must be added to each of the standard specimens and all of the unknowns. Specimen preparation is thus made more com- plicated, and is more susceptible to errors; The addition of reagents and the fact that the specimens must be homo- geneous limit the practical application of the method to the analysis of liquids and fused specimens; although it sometimes finds application in the analysis of pressed powders. When dealing with liquids, care must be taken to ensure that the additive is in a stable solution over time, without precipitating; this is less of a concern when fused beads are used. Ideally, in this case, the added internal standard should be part of the flux; The rule ( Z + 1) or ( Z 1) can serve as a rule of thumb for simple cases. In samples where many elements are to be quantified, or in samples where only few elements need to be quantified but in a matrix whose composition varies from specimen to specimen it is quite clear that a suitable internal standard cannot be found for every analyte element. Sometimes, more elements are used in one internal standard solution to provide a suitable internal standard for more analytes. The internal standard method is most easily applied to liquids. However, even in this favorable case some problems persist. Elements with a higher atomic number (for example Mo) are more difficult to analyze using this method, because liquid specimens are generally not of infinite thickness for the K wavelengths of such elements. In these cases, the L line can be used with an appropriate internal standard. The method will also provide some compen- sation for non-infinite thickness, especially in cases where the wavelength of the internal standard selected is very similar to that of the analyte line. In principle, L lines of a given element can be used as internal standards for K lines of other elements and vice versa. The same requirements are to be satisfied as when dealing with K lines.
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5 Quantitative Analysis 361 The method allows the analysis of one or two elements in a specimen without requiring analysis or knowledge of the composition of the matrix. In favorable situations, the method can provide accurate results over a concen- tration range of up to 10%, and even 20% in some cases; it is most effective at lower concentrations and still finds widespread applications. For example, ISO prescribes the method for the determination of Ni and V in petroleum products [24]. In this case, Mn K α is used as an internal standard. Note that only one internal standard element is used for the two analyte elements. Accurate determination of the analytes is obtained over a much wider variety of petroleum products than a simple linear calibration would do.
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