Energy-dispersive XRF

In Fig. 11.19a, the most simple of ED-XRF instrumental configurations is shown. A low power X-ray tube (e. g., 50 W) and a Si(Li) detector are both placed at an angle of 45° with respect to the sample. Collimators are used to confine the excited and detected beam to a sample area between 0.5 and 2 cm. In such a direct-excitation configuration, the distance between the components can be fairly small (typically a few cm) and since both the tube anode lines and the bremsstrah- 

Jwng-component of the tube output spectram are used to irradiate the sample, only a limited tube power is required. Since the bremsstraMung continuum not only ensures a uniform excitation of many elements, but also causes a significant scatter background to be present in the recorded EDXRF spectra, most direct-excitation systems are equipped with a set of primary beam filters to alter the tube spectram. By selection of an appropriate filter, the excitation conditions for a particular range of elements can be optimized. In order to facilitate the determination of low-Z elements, commercial systems can be either evacuated or flushed with He, thus reducing the absorption of low energy radiation and scatter. 

The ability to simultaneously measure a wide range of elements is one of the greatest advantages of EDXRF. This advantage is strongly reduced when the count rate limitation of the ED detection electronics is taken into consideration. 

This is due to simultaneous recording of the entire primary source radiation scattered on the specimen and is especially true for examinations on samples with light matrices. 

The stationary arrangement of components used in EDXRF is ideally suited for geometrical configurations that exploit polarization phenomena to reduce background and thereby improve signal-to-noise ratios. 

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These samples were measured non-destructively by energy-dispersive XRF with synclirotron radiation excitation (SYXRS), by g-XRF, by wavelength-dispersive XRF (WDXRS), and by Rutherford back scattering (RBS), by X-ray reflectometry (XRR) and by destructive secondary ion mass spectrometry (SIMS) as well (both last methods were used for independant comparison). 

Wavelength-dispersive XRF is generally destructive not so energy-dispersive XRF... 

X-ray fluorescence spectroscopy (XRF) Wavelength-dispersive XRF Is generally destructive not so energy-dispersive XRF Giauque et al. (1993)... 

In 2006, a table-top energy-dispersive XRF (ED-XRF) spectrometer was acquired by the Archaeometry Lab to facilitate non-destructive analysis of obsidian and other types of artifacts. One of the first projects performed on the new XRF spectrometer was the re-analysis of the geological samples from sources in Peru. As a result, it is now possible for the Archaeometry Lab to use either XRF or NAA to successfully determine the provenance of obsidian artifacts from Peru. Due to its light weight, the spectrometer also has the potential to be transported from the laboratory to museums and to archaeological sites for in situ analysis. 

The light-weight Elva-X energy dispersive XRF spectrometer employed for this study has an air-cooled rhodium target anode X-ray tube with 140 micron Be window and a thermoelectrically cooled Si-PIN diode detector. The detector... 

The analyses were performed at The Henry Francis du Pont Winterthur Museum on a Kevex 4525P energy dispersive XRF spectrometer low-level radioactive isotopes were used for the incident radiation (4). The pigment area analyzed was successively irradiated by an iron-55 source, an americium-241 source, and a cadmium-109 source. A qualitative pigment analysis takes only 6 min. Neither the coloring matter nor the silk fabric is altered in any way by the measurement. Figure 8 shows our XRF system setup for the analysis of pigments on painted and printed silks. 

Energy dispersive XRF (EDXRF) has the advantage of being a typically multielement, quick method with sensitivities in the lower mg/kg range. It has been applied for analysis of a number of environmental materials aerosols and fly ash, soils, plants (Beitz et al., 1980 Hahn-Weinheimer et al., 1984 Van Dyck et al., 1986 Krumpen et al., 1989 Toelgyessy et al., 1990 Bandhu et al., 1996 Osan et al., 1996 Alfani et al., 1997). 

D4294 Energy Dispersive XRF All liquid petroleum products and oils Widely used in industry because of low cost instrumentation poor precision at low sulfur levels... 

Polarization EDXRF Polarized excitation geometry used with energy dispersive XRF employing low power tubes and X-ray end window tube with palladium or rhodium anode and beryllium window is claimed to lower the sulfur detection limit to 1 mg/kg. The precision of the method has not yet been established. 

EDXRF with proportional Counter A low background proportional counter is key in a new energy dispersive XRF based method to measure lower levels of sulfur. This low background proportional counter suppresses the noise generated when incident X-rays are absorbed near the wall with resulting incomplete charge collection. An electrode shield close to the wall detects incomplete charge collection, and associated electronic detection... 

X-ray Fluorescence (XRF) is a common technique for sulfur determination in hydrocarbon oils. At-line and laboratory XRF analysis is covered in the ASTM standard test methods for sulfur in petroleum products ASTM Standard Method D 4294 (for Energy Dispersive XRF, EDXRF) [1] and ASTM Standard Method D 2622 (for Wavelength Dispersive XRF, WDXRF) [2]. Polarized EDXRF [3] is also used at-line for the determination of very low sulfur content (<10 ppm sulfur) in diesel and gasoline fuels. 

Figure 8.29 A schematic energy dispersive XRF system with an X-ray tube source. There is no dispersion device between the sample and the detector. Photons of all energies are collected simultaneously. (From ElUs, used with permission.)...

Wavelength-dispersive XRF instramentation is almost exclusively used for (highly reliable and routine) bulk-analysis of materials, e. g., in industrial quality control laboratories. In the field of energy-dispersive XRF instrumentation, next to the equipment suitable for bulk analysis, several important variants have evolved in the last 20 years. Both total reflection XRF (TXRF) and micro-XRF are based on the spatial confinement of the primary X-ray beam so that only a Hmited part of the sample (+ support) is irradiated. This is realized in practice by the use of dedicated X-ray sources. X-ray optics, and irradiation geometries. 

Fig. 11.15 Energy-dispersive XRF spectrum of a multi-element standard, obtained in a TXRF spectrometer.

Qualitative analysis is, in principle, very simple with XRF and is based on the accurate measurement of the energy, or wavelength, of the fluorescent lines observed. Since many WD-XRF spectrometers operate sequentially, a 20 scan needs to be performed. The identification of trace constituents in a sample can sometimes be complicated by the presence of higher order reflections or satellite lines from major elements. With energy-dispersive XRF, the entire X-ray spectrum is acquired simultaneously. The identification of the peaks, however, is rendered difficult by the comparatively low resolution of the ED detector. In qualitative analysis programs, the process is simplified by overplotting so called KLM markers onto... 

There are also other techniques that are used frequently to characterize particulate matter collected from workplace air, for example. X-ray fluorescence (XRF) analysis - both lab-based wavelength-dispersive and hand-held energy-dispersive XRF spectrometry - are used for identification and quantitation of either bulk samples or air filters for numerous chemical elements. The latter is now available with both tube-excitation and radionuclide-excitation sources. 

All three types can be found in use in cement works control laboratories, but the preferred system is the multichannel type fitted with channels for the simultaneous determination of the eight elements Fe, Ca, K, S, Si, Al, Mg, and Na. Depending on the local geology, a chaimel for the determination of fluorine may be fitted if the limestone deposit is close to a source of fluorspar (CaF2). (The determination of fluorine is beyond the scope of an energy-dispersive XRF spectrometer.) At some works a channel is added to monitor the chlorine content of the clinker, which can be introduced from the fuel used to fire the kiln. 

Onsite total analysis by energy dispersive XRF spectrometry is increasingly popular. When linked with various contour-mapping software packages, the technique can give a rapid assessment of a suspected pollution incident. 

WDXRF 8 630 Energy-dispersive XRF (EDXRF) wavelength-dispersive XRF... 

The reason for the lack of RMs is the absence of reliable and accurate methods of analysis. Microanalysis is, hence, in need of at least one method that can be used as a reference tool for other techniques and to link RMs or round robin exercises to the international unit of mass. Micro-XRF can be used for this potentially, especially when used for analyzing microscopic samples, where matrix absorption effects are relatively unimportant. At present, XRF is considered to be a rather poor method for certification purposes due to intense matrix effects resulting from intense radiation absorption and enhancement by secondary fluorescence. In wavelength-dispersive XRF, reliable results can only be obtained through calibration with a set of reference samples of closely similar composition to the unknown sample. In the case of energy-dispersive XRF using monochromatic excitation, the correction for matrix effects is simpler but in this case the method suffers from a number of other drawbacks, such as spectral overlap and poor statistics in the spectra. 

Since the equipment used in XRF technique including radioisotope source is portable, the energy dispersive XRF spectrometers are used in various divergent fields like that in the metal industry, in gold mines, in oilfields for oil analysis (to determine sulfur in petroleum products and residual catalysts, monitor additives in lubricating oils, analyze regular wear metal in lubricants... 

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