WDXRF Spectrometers

Schematics of sequential WDXRF spectrometers are shown in Figs. 8.14, 8.15, and 8.17. In the configurations shown, the source is placed under the sample the sample is presented surface-down to the X-ray beam. Some instruments have the tube above the sample, with the sample surface facing up. There are advantages and disadvantages to both designs, as we shall see. The sample fluoresces as a result of excitation by the source. The sample fluorescence is directed through the primary collimator to the analyzing crystal. Diffraction...

WDXRF spectrometer is plotted, for the case where either the (10 < Zj < 60) or L (40 < Z < 80) peak intensities are used as analytical signals. By selection of the excitation conditions (tube anode material, excitation voltage), the shape and location of the maximum in the sensitivity curve can be influenced to suit the needs of the appUcation at hand. 

Prior to impinging on the analyzer crystal, by means of a collimator or sUt, the spread in initial directions of the sample-to-crystal beam is limited. Since the maximum achievable angle on a typical WDXRF spectrometer is around 73°, the maximum wavelength that can be diffracted by a crystal of spacing d is equal to ca. 1.9d. 

Commercial instruments use multiple tube or multiple slit collimator arrangements, often both before the analyzing crystal (the primary collimator) and before the detector (the secondary collimator). The collimator positions in a sequential WDXRF spectrometer are shown schematically in Figure 8.30. In many wavelength-dispersive (WD) instruments, two detectors are used in tandem, and a third auxiliary collimator may be required. Such an arrangement is shown in Figure 8.31. 

The analyzing crystals are the heart of a WDXRF spectrometer. As we have discussed, a crystal is made up of layers of ions, atoms, or molecules arranged in a well-ordered system or lattice. If the spacing between the layers of atoms is about the same as the wavelength of the radiation, an impinging beam of X-rays is reflected at each layer in the crystal (Figure 8.8). 

The ceramics industry routinely measures 6-12 elements quantitatively in both pressed pellet and fusion bead form for quality control using either WD or ED XRF. The elements (reported as oxides) vary in concentration from 0.01 to 70 wt%. Using a modern WDXRF spectrometer, aluminum (reported as AI2O3) can be determined at the 1 wt% level in a magnesium oxide-based ceramic with a standard deviation of 0.006 and a DL of about 8 ppm. 

Since the resolution of a WDXRF spectrometer is relatively high, spectral overlap corrections are not required. However, with the EDXRF analyzer, some type of deconvolution method must be used to correct for spectral overlaps as it has poor resolution. The spectral deconvolution routines however, introduce error due to counting statistics for every overlap correction onto every other element being corrected for. This can double or triple the... 

The basic function of the spectrometer is to separate the polychromatic beam of radiation coming from the specimen in order that the intensities of each individual characteristic line can be measured. In principle, the wide variety of instruments (WDXRF and EDXRF types) differ only in the type of source used for excitation, the number of elements which they are able to measure at one time and the speed of data collection. Detectors commonly employed in X-ray spectrometers are usually either a gas-flow proportional counter for heavier elements/soft X-rays (useful range E < 6keV 1.5-50 A), a scintillation counter for lighter elements/hard X-rays (E > 6keV 0.2-2 A) or a solid-state detector (0.5-8 A). 

Table 8.40 compares the main characteristics of WDXRF and EDXRF. Multidispersive XRF combines the benefits of the WDXRF technique for routine elemental analysis with the complete flexibility offered by EDXRF for nonroutine analysis. Clearly, modem XRF instrumentation is rather varied, ranging from simple benchtop EDXRF equipped with a low-power X-ray tube and high-resolution proportional counter for some key elements, to 4 kW simultaneous multichannel spectrometers with 28 fixed element channels for... 

Wavelength dispersive X-ray fluorescence spectrometric (xrf) methods, 25 60 Wavelength dispersive spectrometer (WDS), 76 488, 26 433-434 Wavelength dispersive X-ray fluorescence (WDXRF)... 

CRMs Ni Cu Zn separation [N/MT] bis (dithiocarbamate), conduct X- WDXRF] ray analyses on a fluorescence model spectrometer [SEP/CONC-WDXRF] 1997... 

Various combinations of the instrument components discu.sscd in the previous section lead to two primary types of spectrometers wavelengih-dispersiie X-ray fluorescence (WDXRF) and energy-dispersive X-ray fluorescence (F.DXRF) instruments." ... 

ABSTRACT X-ray spectroinetty (XRF) is a versatile instrumental method for elemental analysis in a wide variety of materials. The performance of three different XRF systems will be con sared a high power wavelength dispersive x-ray spectrometer (WDXRF), a low-power WDXRF, and a bench-top energy dispersive instrument (EDXRF). 

FIG. 2—Calibration graphs for suljur obtained on two Afferent spectrometers. The upper curve (squares) is achieved on a high-power WDXRF instrument, while the lower curve is achieved on a benchtop EDXRF spectrometer. 

In Tab. 11.3, as an example, relative LD values for trace elements obtained by means of WDXRF in different matrices are listed. In Fig. 11.10, a plot of typical absolute LD values for TXRF spectrometers is shown. 

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