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

The ratio m i m s is thus less sensitive to variation

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The ratio M i /M s is thus less sensitive to variation in the matrix effect, and, in many cases, can be considered a constant. In practice, the internal stan- dard is added to all specimens (standards and unknowns) and the intensity of both the analyte and the internal standard is measured. Using linear regres- sion of the ratio of the intensities against the concentration of the analyte in the standard specimens, the value of the calibration constant K is in (5.83) is calculated. The difference between the internal standard method and the method using scattered radiation in their effectiveness to correct for matrix effects is due to the different origin of the “ratio” intensity (i.e., Compton scattered tube radiation or characteristic fluorescence from an element in the specimen). This is the reason why the internal standard method is capable of correcting for absorption as well as for enhancement. Variations in density of pressed specimens are also partially corrected for. To ensure proper compensation of the matrix effects, it is imperative that there are no absorption edges and no characteristic lines from other elements located between the two lines of interest. Also, in the ideal case, none of the characteristic lines of the analyte should enhance the internal standard or vice versa. This is difficult to realize. In practice, the requirement is reduced to avoid using an internal standard that contains elements, which change the matrix effects on the analyte signifi- cantly. For analytes with atomic number Z higher than 23, the elements with atomic numbers Z 1 and Z + 1 are very well suited to be used as internal standards. In those cases, there are no absorption edges and no K emission lines of other elements between the two elements considered. The element with atomic number Z is not enhanced by the K α radiation from element Z + 1, but only by the (much) weaker K β radiation. Some degree of enhancement between the internal standard element and the analyte is allowed as the con- centration of the internal standard is constant and the method is based on a
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360 B.A.R. Vrebos ratio of intensities. Please note that this guideline does not take into account the presence (or absence) of other elements with L absorption edges and L emission lines. The method will break down, or produce results of less than optimal qual- ity, when (1) a major emission line of a major matrix element with a variable concentration is present between the absorption edges of the analyte and the internal standard element. In this case, only one of the two elements (ana- lyte or internal standard) is enhanced in a variable way. Or (2) when a ma- jor absorption edge of a major element of variable concentration is situated between the energies of the measured characteristic lines of the analyte and the internal standard element. In this case, one of the two lines considered is absorbed in a variable way. In both cases, varying concentrations of the matrix element lead to different effects on the intensities of the analyte and of the internal standard element, but, the ratio of (5.83) will not compensate
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