X-Ray Fluorescence Spectroscopy for Laboratory Applications. Jörg Flock

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Figure 2.10 Analyst 0700 from KEVEX.

As a result of the different analytical tasks, but primarily due to the different equipment requirements for WDS and EDS instruments, the development of coating thickness analyzers was for a long time independent from that of conventional X-ray spectrometers. The main companies were UPA, TwinCity, and CMI in the United States, Helmut Fischer in Germany, and Hitachi and Seiko in Japan. In the meantime, companies that had previously concentrated on the development of WDSs have recognized the high demand for coating thickness analyzers and therefore have also focused on this application. This development is also supported by the increased use of EDSs for the analysis of bulk samples at these companies. On the other hand, the companies, which were once primarily focused on layer analysis, are increasingly turning to applications on compact samples, for example, the analysis of precious metal in jewelry, the determination of toxic elements in consumer products or toys, i.e. samples which are finished products and therefore are not homogeneous or do not have flat surfaces. As previously mentioned, these measurements require small sample areas. Fischer and Bruker in Europe, Hitachi in Japan, and Bowman in the United States are currently the leading companies for this type of equipment.

      A continuous trend in the development of X-ray spectrometers has always been the reduction of instrument size. The first commercially produced X-ray spectrometers were filling a complete room, in particular due to the elaborate electronics. Today, a table is often sufficient for setting up the instruments. Further, increase in the integration of the electronics has not only resulted in an increase in performance but also in a significant reduction of instrument footprint.

      A significant step in this direction is the use of instruments that can be held in one hand. This development began shortly after the turn of the millennium with the demand in the United States for the determination of lead pigments in wall paints. This task was important because lead as a toxic element was to be identified, in order to be replaced in wall paint. The requirement was that the analysis is done on site in order to avoid the logistical effort necessary for the analysis of pieces of paint in the laboratory. The first instruments still operated with radioactive sources for excitation; they were later replaced by low power tubes. Owing to the very small distances between radiation source, sample, and detector the tube power can even less than 5 W. This not only assures a higher safety of the instruments, but also a good analytical performance and higher flexibility. The integration of powerful computing technology has further also contributed significantly to the improvement of performance. In the meantime, handheld instruments have been used for a wide range of analytical tasks. The possibility of on-site analysis has played a decisive role in the selection of analytical tasks.

      Typical applications of handheld instruments are sorting of scrap, the determination of toxic elements in consumer products, the tracking of ores in mining explorations, or the investigation of art objects. Equipment is offered by companies such as Thermo, Bruker, Hitachi, Olympus, or Spectro; these companies usually also carry ED X-ray spectrometers in their portfolio. They all work with comparable components and the performance achieved in the meantime is remarkable as well as comparable for all companies. The available computing technology on the instruments is limited by the battery power supply but allows for quantification of predefined material classes by means of standard-based calibrations. The analytical accuracy however is mainly limited due to the lack of sample preparation, and due to sample contaminations on site. In special cases, the transfer of measurement data to an external computer is possible, on which more powerful evaluation algorithms and procedures for data management are available.

A further important influence on the development of XRF analysis was the availability of various X-ray optics. First, there were multilayer systems with large lattice constants for the detection of light elements in WD spectrometry. Increasingly, however, optics was used for beam shaping and influencing the properties of the primary radiation. The performance and application range of EDSs therefore could be continuously improved, for example, by the excitation of very small sample areas with high intensity or by reducing the scattering of the exciting radiation to improve the sensitivity.

      All these assemblies are now used in modern X-ray spectrometers. A more detailed description and compilation of the most important instrument classes is given in Section 4.3.

2.4 Carrying Out an Analysis

      2.4.1 Analysis Method

      In order to carry out an analysis various steps have to be taken starting with asking the right analytical questions – this applies not only to X-ray spectrometry.

      In the following discussions, unless otherwise stated, it is assumed that the sampling has already taken place and the material to be analyzed is available; this means that sampling here is not a part of the analysis. The sample material available in the laboratory is called laboratory sample (see DIN-51418-1 2008, Part 2, DIN-51418-2 2014).

      For carrying out an analysis it is required to determine the analytical strategy and the analytical method to be used. It is based on the material to be analyzed as well as the analytical task requested, specifically the definition of the elements to be analyzed, the estimation of the concentration range to be detected, and the desired uncertainties. Of course, one must take into account the available equipment and the required time. Therefore the following steps are necessary: