An Overview of XRF Basics
2. Instrumentation
This chapter explains the instrumentation in Bruker AXS X-ray fluorescence spectrometers. The first three sections contain brief summaries of MRS multichannel X-ray spectrometers. The fourth section deals in detail with the technology of the SRS 3x00 and S4 sequential spectrometers.
2.1 Multichannel Spectrometers
The MRS multichannel spectrometer can measure up to 28 elements simultaneously. A multichannel spectrometer is always required when short measurement times are needed to analyze large numbers of elements, or when high sample throughput (e.g. 600 samples per day) is needed for industrial quality and process control.
An individual measuring channel, comprising crystal, detector and electronics module, must be installed for each element line. As there are limited possibilities for the geometric arrangement of 28 channels in close proximity to the sample, so-called monochromators with slit-optics are used. A monochromator is an arrangement of an entry slit, curved focusing crystal and an exit slit (Figs. 21 and 22). The crystals are curved in a logarithmic spiral and focus the desired wavelength of the beam passing through the entry slit onto the exit slit. The detector is located behind the exit slit. Scintillation counters or gas proportional counters are used depending on the element line. Flow counters and sealed proportional counters can be used as gas proportional counters. Sealed proportional counters can be equipped with a 25-µm Be or a Super-High Transmission (SHT) window. The 25-µm Be window is used for the elements Al to Fe. The SHT window is used for Be to Mg.
All monochromators are located in a large vacuum chamber. The beam is applied from above. The fixed channels are used exclusively for quantitative analyses. A scanner can be employed for qualitative analysis.
As all elements are measured simultaneously, a generator setting (kV/mA) must be selected that provides the best compromise for all the elements to be measured. The measurement time depends on statistical accuracy requirements of the element with the lowest intensity and is typically between 20 and 60 seconds. No background positions can be measured because the monochromators are at a fixed location in the angle for the corresponding line.
When measuring trace and major elements simultaneously, the generator is normally set so that the trace elements can be measured with the highest possible intensity. This means that the major elements are usually of such high intensity that the detectors cannot process them. For cases such as these, the MRS can be fitted with absorbers (attenuators) for the major elements. Absorbers sufficiently reduce the intensities of major elements so that they then lie in the operational range of the detectors.
Fig. 21: Beam path in multichannel spectrometers
Fig. 22: Monochromator with absorber and flow counter
2.1.1 Scanners for Multichannel Spectrometers
In addition to fixed channels, a scanner can be installed in the vacuum chamber of multichannel spectrometers. The scanner is a "moveable channel" enabling sequential coverage of a large element range. As only a single curved crystal (LiF(200) or PET) is fitted and the scanner's 2θ angular range is limited (30 - 120 degrees), several elements in the periodic table must be measured in the second reflection order. A flow counter or a sealed proportional counter serves as a detector.
The scanner works on the physical principle of the Rowland Circle, in which the crystal and detector move in such a way that the entry slit, crystal and exit slit lie on a fixed-radius circle that changes in position (Fig. 23).
The scanner can be used for qualitative as well as quantitative analyses.
Fig. 23: The Rowland Circle scanner principle