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

This is applicable only in situations for which the

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parameters. This is applicable only in situations for which the atomic numbers of these elements are not too different. Another possibility is the use of chemometric methods. A simple proce- dure is spectra matching for which the intensities of identified elements of an unknown sample are compared to the identified elements of a set of reference materials. The variation coefficient is calculated resulting in a ranking of the probability of agreement of the unknown with the reference sample. This is a fast and easy way to handle method for material identification without a high analytical effort. Another chemometric method in use for material identification is principal component analysis (PCA). Also for that method it is necessary to prepare a substantial library of particle spectra but for various compositions and also for various sizes and shapes. It requires considerable effort to “train” the instru- ment but the identification of the particle’s composition and size will, then, be possible. In PCA the complete spectrum measured is evaluated. Different components of the spectrum are used for the identification—the peak intensi- ties and the intensity of the scattered primary radiation. This facilitates the identification of different materials. Very often it is not necessary to know the
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448 B. Kanngießer and M. Haschke exact composition of a material but it is sufficient to identify its type. The identification of a material can be carried out with a higher confidence level than the analysis of the whole composition. PCA has been demonstrated for glass particles of various compositions and sizes [23]. Reference glasses were first measured as bulk material. Then the glasses were broken and the different pieces were sieved and classified according to their sizes. Measurements were performed on a wide range of glass particles of different sizes. Glass was used because of its relatively small matrix absorption. This, in turn, enables a significant influence of particle size and shape on the fluorescence intensity. Conventional analysis was not successful in this case. The evaluation of the spectra correcting for particle size resulted in a wide distribution of the composition for particles with the same size classification. This has been probably caused by the sample shape. However, the PCA plots displayed in Fig. 7.4a show that particles of the same composition but of different size are concentrated in clusters and in Fig. 7.4b that particle of the same composition but different size are arranged in a line according to the particle size with different lines for different composi- tions. The scattering of the values is relatively large but definite identification was possible. This example shows that especially for micro-XRF spectroscopy it is use- ful to consider new and alternative ways for quantification or identification.
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