X-Ray Fluorescence Spectroscopy for Laboratory Applications. Jörg Flock

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       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 to be investigated to the instrument geometry, for example, by detaching parts from a larger piece of the material, by filling loose powder into a sample cup, or by pressing it into tablets;to generate a sufficient representativity of the analyzed sample volume for the entire sample material, for example, by producing a planar sample surface or by cleaning the surface from contaminations; orto avoid or reduce the influence of inhomogeneities of the sample material on the analysis result, for example, by homogenization through grinding, by dilution, or by the manufacturing of fusion beads.

       However, the analyses can mostly be carried out without any changes in the aggregate state of the sample, i.e. the dissolution of solid samples is not required. Therefore, the effort for sample preparation compared to optical methods is relatively low and no or only slight dilution effects reduce the sensitivity of trace detection and therefore avoid analytical errors by contaminations.Nevertheless, it should be noted that even in the case of XRF, sample preparation must be carried out very carefully in order to achieve the desired analytical accuracy.

       Besides the analysis of homogeneous volume samples, the characterization of layered systems with XRF is also possible. Under certain conditions, both thickness and composition of layers can be determined. In this case, the mass per unit area can be determined by measurement, which then has to be converted into layer thicknesses and mass fractions by using the material density.The determination of layer thicknesses is a very common analytical problem in industrial process control of mechanical and electronic components or many other electroplated products.

       Another very important property of X-ray spectrometry is the possibility of automation, in particular, the automation of the measurement process, including data evaluation. In case there is no change in the sample type even sample preparation can be automated. This results in a fast analysis, but above all it provides for an analysis independent of subjective influences. Sample preparation and measuring operation can then be carried out under equivalent conditions, which reduces the uncertainty range of the measurement.Apart from this effect, the ongoing costs for analyses are reduced by means of automation.

       The analytical problem of X-ray spectrometry can be very different and can be classified into various “degrees of difficulty.”Qualitative analysis can be considered as a simple task. In this case, it is only necessary to determine whether certain elements are present in the sample or not.The next stage involves the monitoring of concentration ranges for selected elements. In this instance, it must be determined whether the mass fractions of the elements under consideration in the sample material are below or above a certain limit. Here, often no direct quantitative analyses are required but only a monitoring of the intensity level of the analyte. In this case, the matrix influence can be neglected because samples of similar qualities are investigated and their matrices do not change significantly.Without doubt, the most demanding analytical problem is the quantitative analysis. Here, the elements present in a sample have to be identified at first – for samples of same quality as in the case of quality control in the production process, this is not necessary – and then their mass fractions or their layer parameters have to be determined.

       The requirements regarding accuracy and sensitivity of the analysis can be very different, resulting in the selection ofthe sample preparation method (homogenization of the sample, elimination of influences of the surface roughness or of mineralogical effects)the measuring conditions (excitation conditions, measuring times, measuring medium)the evaluation model and, if available, of the reference samples to be used for the calibration to the accuracy requirements.

       Further, X-ray spectrometry can also determine element distributions of large sample areas by using specific excitation conditions. For this purpose, the incident beam has to be concentrated on a small sample area. The sample then needs to be moved under the fixed beam into the measuring position. This offers the possibility for the analysis of non-regular sample surfaces and additionally for the characterization of inhomogeneous materials.

       By using specific excitation geometries and conditions, it is possible to influence the sensitivity of the method. For example, in the case of a grazing incidence of the primary beam, the spectral background is greatly reduced and hence the sensitivity of the measurement is significantly increased.A similar effect can be obtained by using monoenergetic radiation for the excitation. Here also, the spectral background is reduced and an improvement in СКАЧАТЬ