specific problem is the analysis of archeological objects, because they cannot be modified by preparation due to their uniqueness. Examples for such analytical tasks are discussed in Section 16.5.
In Chapter 17, special applications of the XRF analysis are described. This implies the high-throughput analysis (HTS) for the characterization of small sample quantities, chemometric spectral evaluation with the resulting possibilities for material characterization, as well as speciation analysis.
Chapter 18 presents the requirements and conditions for the use of the XRF in process analysis, with particular attention to the automation of sample preparation. Requirements and possibilities of automated analyses are given, but the associated problems are pointed out as well.
Finally, in a brief discussion in Chapter 19 the assurance of the quality of analyses by means of a corresponding quality management system in test laboratories and also the requirements for the validation of test methods are mentioned.
All these applications are intended to demonstrate the wide range of possible measurement applications of XRF as well as the analytical performance that can be achieved.
At the end of the book, in Appendix A numerical data required for X-ray spectrometry is compiled in a comprehensive set of tables, and in Appendix B important references with information on instrument manufacturers, basic literature for the field of XRF spectrometry, important websites, as well as magazines, standards, and laws that help readers quickly find the right information and contacts for solving their analytical tasks can be found.
2 Principles of X-ray Spectrometry
2.1 Analytical Performance
X-ray analysis has been established as an important method for element analysis. Already since Moseley's discovery in 1913 that the energies of the X-ray lines of individual elements differ from each other and depend on the square of the atomic number of the emitting atoms, preconditions for using this method for element analyses were given. However, it took several years until the first usable equipment for routine analyses was available. In the 1930s the first laboratory instruments were available, but they were not yet suited for routine analyses.
To this purpose, various instrumental prerequisites had to be developed, such as an effective excitation source with sufficient intensity and high stability, the supply of dispersive elements, i.e. crystals with high reflectivity and sufficient size, and then also synthetic multilayers with larger d-spacings, detectors with sufficient counting capacity, instruments that allow for simple and safe operation, in particular for sample positioning, and later also for radiation protection, possibilities for measurement in vacuum, as well as the effective recording of the measurement data and their evaluation. The first applications focused on the identification of the elements present in a sample, i.e. a purely qualitative analysis. In this way, some elements could even be discovered, such as hafnium in 1923 (Coster, v.Hevesy), rhenium in 1925 (Tacke 1925), and technetium in 1947 (Perrier and Segrè 1947).
However, very soon the mass fractions of the different elements in the examined materials became interesting, mainly for the quantitative assessment of the investigated materials.
After the availability of the first commercial equipment, very fast development and distribution of X-ray spectrometry began. The following characteristics of X-ray spectrometry have undoubtedly been of assistance in this process:
X-ray spectra have much less lines per element than optical spectra. This means that the lines in the spectrum are easy to identify and due to the corresponding small number of line interferences the requirements for the spectrometer resolution are not very high.
All important parameters for the spectrometry depend on the atomic number of the considered element, which significantly supports the interpretation and evaluation of the spectra.
The analysis can be carried out on very different sample qualities. Both compact solid samples and powder samples as also liquids and layered materials can be directly examined.
The analysis is nondestructive, i.e. the material to be examined is not consumed or changed by the analysis. Therefore, the sample is available for further or repeated examinations.
A large element and concentration range is covered. All elements except very light elements can be analyzed. The detectable element contents range from a few milligrams per kilogram to pure elements, i.e. at least 5 orders of magnitude. In cases of specific excitation geometries, instrument designs, or preparation methods, the detection limits can even be lowered to the sub-milligram per kilogram range.
The development of novel components for X-ray analysis, such as X-ray optics and energy-dispersive (ED) detectors, initialized a strong dynamic in the development of new methodical possibilities. In recent years, therefore, a clear extension of the application range of X-ray spectrometry could be observed.
The analytical performance of X-ray fluorescence spectrometry (XRF), however, is characterized by further properties, which, in some cases, have a limiting character.
The analysis can be carried out with very high precision because the statistical error can be kept very small due to the high measurable intensities. Typical analytical errors for the analysis of homogeneous samples are between 0.3 and 0.5 rel. wt%. With corresponding methods, these limits can be even further reduced.
The analytical accuracy can be influenced by the type of sample preparation, the selection of measurement conditions, the measurement sequences, and the effort on data processing.
The abovementioned high accuracies can be achieved only by comparative measurements with samples of exactly known composition, i.e. by calibrations using reference samples or primary substances (pure substances).
The strong matrix dependence of the method can be considered as a limiting factor. This means that the element intensities have a nonlinear dependence on the sample composition. This makes quantifications more difficult and complex.During the development of X-ray spectrometry, it has been found that most of the interactions of both the incident radiation and the fluorescence radiation with the sample are physically very well understood and mathematically describable.This means that X-ray spectrometry is a very well understood analytical method, which now can be used even standard-less, i.e. quantifications are possible without the use of reference samples, only based on fundamental parameters such as absorption cross sections, transition probabilities, fluorescence yields, and others. This widely reduces the effort for the analyses of unknown samples, but it can also reduce the accuracy of an analysis. In particular, exact knowledge of the fundamental parameters and of the measuring geometry is required for high accuracy. On the other hand, in the case of inaccurate knowledge of these parameters, the analysis accuracy is limited.
Typically, the analyzed sample volume is not very large. It is determined by the size of the area under investigation and the information depth, i.e. the thickness of the material that can be penetrated by the excited fluorescence radiation and contributes therefore to the measurement signal. For correct analysis, this volume should be representative of the material to be characterized.The size of the excited area can be easily adjusted and depends substantially on the homogeneity of the sample. The depth of penetration depends on the energy of the fluorescence radiation of the investigated element as well as on its absorption in the sample, i.e. from the composition of the matrix of the sample.
X-ray fluorescence is known as a nondestructive analysis method that is capable of analyzing materials in various aggregate states, i.e. liquids, solid samples, or powders. Nevertheless, in many cases modifications of the material to be examined may be necessary for the analysis. These preparation procedures may be necessary, for example,to adjust the material