XRF instrumentation

A good introduction to XRF instrumentation, qualitative and quantitative analyses, and chemical-bonding studies. 

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... 

This non-destructive technique is a very suitable tool for rapid in-line analysis of inorganic additives in food products (Price and Major, 1990 Anon, 1995). It can be readily used by non-skilled operators, and dry materials can be pressed into a pellet or simply poured into a sample cup. The principles of this technique related to food analysis are described by Pomeranz and Meloan (1994). A useful Internet site is http //www.xraysite.com, which includes information about different XRF instruments from various companies. Wavelength dispersive X-ray fluorescence (WD-XRF) or bench-top energy dispersive (ED-XRF) instruments are available. XRF is a comparative technique, thus a calibration curve needs to be established using food products of the same type as those to be... 

Acid-digestion is often used with composts derived from municipal wastes, sewage and slurry, where toxic amounts of heavy metals may cause problems on the land to which they are applied. It is probably more convenient to determine total elements in soils by a benchtop X-ray fluorescence spectroscopy (XRF) instrument. This only requires the soil to be ground, and several reference standards of a similar soil. A Reference Materials Catalogue, Issue 5, 1999, is available from LGCs Office of Reference Materials, Queens Road, Teddington, Middlesex TW11 OLY, UK. Tel. -i-44 (0)20 8943 7565 Fax h-44 (0)20 8943 7554. 

Until recently, there was no systematic survey, documentation, and chemical and physical analyses of western Mediterranean obsidian sources. Recently, Tykot completed an extensive survey and documentation of western Mediterranean obsidian sources on the islands of Sardinia, Palmarola, Lipari, and Pantelleria (24-27) for a more detailed discussion. Samples from these sources were analyzed at MURR by INAA and/or XRF and LA-ICP-MS. As expected, INAA (and XRF and LA-ICP-MS) of geologic samples from these sources demonstrated that obsidian from each island had a unique chemical signature(s). In the case of Sardinia, six compositional groups were identified. Because of the analytical cost and semi-destructive nature of INAA, artifacts were analyzed by LA-ICP-MS rather than INAA. XRF would have provided a viable analytical alternative, but many of the artifacts were smaller than the minimum size required for this analysis on a standard laboratory-based stationary XRF instrument... 

Source determination of all 70 Postclassic obsidian samples was possible using the field-portable XRF instrument. Although the majority of the samples (56%, n=39) represent the Ixtepeque source, 29% (n=20) of the sample comes from El Chayal, and 11% (n=8) can be sourced to San Martin Jilotepeque. In addition to these Guatemalan sources, two different Mexican sources were identified in the Postclassic sample Pachuca (3%, n=2) and Zaragoza (1%, n=l) (Figure 4). 

XRF analysis may be used in two modes in situ and ex situ. To take an in situ measurement, the window of a hand-held field portable XRF instrument is pressed... 

Readers should note that other analytical techniques also exist for investigating the elemental make up of samples, such as CHN analysers (especially for compositional analysis of pure organic chemicals) and X-ray fluorescence (XRF) instruments, and techniques such as X-ray photoelectron (XPS) spectroscopy are available for surface-specific analysis, but expensive. 

The analysis data of few selected catalyst samples, generated very carefully in triplicate by ICP, were used for calibration of XRF instrument (SRS 3000, Siemens Germany). After calibrating the instrument with quantitative software, the catalyst samples were analyzed as such without any sample preparation. For precision study, four catalyst samples were analyzed five times in different days. During each analysis, five consecutive measurements were made the results averaged. 

Monochromatic WDXRF Traditional wavelength dispersive XRF instrument uses polychromatic excitation. This new technology uses monochromatic focused excitation. Preliminary work has shown a reproducibility of about 2 mg/kg at a level of 10 mg/kg of sulfur in gasolines and diesel fuels. Earlier this year this method was issued as ASTM D 7039. 

An on-line analyzer must be packaged much more robustly than a laboratory instrument to withstand the process environment which, for example, may have an explosive atmosphere and significantly variable ambient temperature. It must also be capable of continuous, unattended operation over long periods of time. Clearly, the simpler the instrument the better. Of the methods listed in Table 1, WDXRF, polarized EDXRF, and Pyro-microcoidometry have not been adsqrted to on-line process instrumentation, whereas the other methods have. The relative simplicity of Pyro-EC makes it particularly suitable for adaptation to process instrumentation. The sulfur dioxide sensor is a small, plug-in, low cost electrochemical cell, easily replaceable and with an expected lifetime of over one year. The UV lamp, UV optics, and photomultiplier used in Pyro-UVF are not required. The X-ray tube (or radioactive source). X-ray detector, and X-ray optics used in all the XRF instruments are not required. 

Modern XRF instruments permit the determination of all elements from fluorine (Z = 9) to uranium (Z = 92). Some systems allow measurement of elements from Be to... 

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). 

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. 

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-... 

Fig. n.l9 Schematic drawings of (a) a direct-excitation XRF instrument, (b) a secondary target XRF instrument, (c) a polarized XRF instrument employing a cartesian (XYZ) irradiation geometry. 

XRF instrument parameters for ceramics differ from those used for some other materials because of the light element determinations. Rhodium, or less frequently scandium. X-ray tubes are used at voltages in the relatively low kilovolt range. Thallium acid phthalate or layered crystals are used for... 

ED-XRF instrumentation incorporating solid state semiconductor detectors offers different detection... 

Despite these differences in analytical performance and operation, WD- and ED-XRF instrumentation are based on common principles, which are considered further in this section. 

The X-ray tube is powered by an appropriate generator capable of being programmed by the operator to apply a potential of typically between 20 and 60-100 kV (depending on tube design) at up to 4 kW. The success of typically modern WD-XRF instrumentation depends in part on the very high degree of stability with which the X-ray output intensity of the tube can be maintained. 

In simultaneous XRF instruments, individual channels are normally fitted with a gas proportional counter appropriate to the energy of the fluorescence X-ray line to be measured. However, on sequential instruments the argon gas proportional counter is normally paired with a versatile scintillation counter. 

A combination of the standard argon gas flow proportional counter and the scintillation counter is often used on general-purpose XRF instruments, using tandem operation to compensate for the reduced detection efficiency of each, individually, in the 6-10 keV region. 

Although the principles and mechanism of excitation for energy dispersive X-ray fluorescence (ED-XRF) are exactly the same as for wavelength dispersive (WD)-XRF, the analytical characteristics of ED-XRF instrumentation demonstrate some significant... 

This mode of operation offers unparalleled opportunities for qualitative analysis (from a visual appraisal of spectra, or even quantification in real time whilst spectra are accumulating). However, there are also additional criteria in optimizing excitation conditions to maximize the emission of XRF intensities in the region of analytical interest. The reason for this is that whereas the WD X-ray spectrometer is designed as a monochromator, the entire spectrum emitted from an excited sample is available for detection in an ED-XRF instrument. 

ED spectrometers have no moving parts and have benefited extensively from the miniaturization that has taken place in integrated circuit technology. To an increasing extent, ED-XRF instruments can be adapted to XRF applications in a much more flexible manner than WD spectrometers. There are a number of reasons for this flexibility, which may be summarized as follows ... 

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