Energy-dispersive system

The samples were air-dried at room temperature, sieved to < 63 pm and analysed by x-ray diffraction (XRD) and scanning electron microscopy combined with an energy dispersive system (SEM-EDS). For chemical analysis, samples were submitted to an extraction with Aqua Regia and analysed by inductively coupled plasma-optical emission spectrometry (ICP/OES). Firing experiments were performed following the procedure described by Brindley Brown (1980). 

The pH, EC and Fe3+ were used as control parameters. The first two were measured with an Orion probe combined pH/ATC electrode Triode and a conductivity cell DuraProbe ref. 0133030. Fe3+ was determined by molecular absorption (thiocyanate method). Mineralogical composition of the precipitates was determined by X-ray powder diffraction (XRD). Scanning electron microscopy, combined with an energy dispersive system (SEM-EDS), allowed the observation of morphological and compositional aspects of the precipitates. 

Figure 5.5 Comparison of EDXRF and WDXRF detection systems. Fluorescent X-rays are emitted by the sample on the left. The upper line shows a wavelength dispersive XRF system the lower shows an energy dispersive system. (Reproduced from Pollard and Heron 1996 44, by permission of the Royal Society of Chemistry.)...

FIGURE 10.7 The two instrument designs for x-ray fluorescence spectroscopy. Left, the energy-dispersive system. Right, the wavelength-dispersive system. 

Photon counting is used in X-ray work because the power of available sources is often low. In addition, photon counting permits spectra to be acquired without using a monochromator. This property is considered in the section devoted to energy-dispersive systems. 

An obvious advantage ol energy-dispersive systems is the simplicity and lack of moving parts in the excitation and detection components of the spectrometer. Furthermore, the absence of collimators and a crystal... 

A further distinction in XRF instruments is the method of x-ray detection. Wavelength dispersive instruments (XRF-WD) are more precise, but more time consuming. Energy-dispersive instruments (XRF-ED) are more rapid, but less precise and with poorer detection limits. An isotope-source energy-dispersive system can process 70 samples per day for 14 elements (Boyle, 2000). 

Figure 3.5 Sensitivity curves for an energy-dispersive system with a molybdenum anode. Arrows mark the absorption-edge energies, dashed lines represent extrapolation. (Reprinted by courtesy of EG G ORTEC.)...

For both Eqs. (11.37) and (11.39), it can be shown that the percent standard deviation in the concentration at the detection threshold Cmdl is greater than 60.8% and approaches 60.8% as the background approaches infinity. To achieve a 10% standard deviation in the analyzed concentration, a concentration at least six times the Cmdl value is required. This result is identical to that given in Sec. 11.2.1 for the wavelength spectrometer. The general comments in Sec. 11.2.1 regarding the calculation and interpretation of minimum detection limits should be reviewed and applied to energy-dispersive systems at this point. 

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