Wavelengths, XRF

Wavelength XRF (WDXRF) is a sequential technique, more time-consuming than the other XRF methods and with worse sensitivity. Detection limits lie normally in the higher mg/kg range. WDXRF has been successfully applied for the determination of Al, Ca, Fe, K, Mg, Mn, Na, P, Si in plant samples and of Cr, Cu, Fe, Ni, Pb, and Zn also in vegetation (Flahn-Weinheimer et al., 1984). 

There are two further effects on K emission XRF which become more important with decreasing nuclear charge. One is the appearance of weak satellite transitions, to lower wavelengths of the main transitions, occurring in the small proportion of doubly ionized atoms which may be produced by the initial X-ray bombardment. The other is a tendency for some transitions to be broadened into bands, rather than the usual sharp lines, due to the... 

Wavelength dispersive x-ray fluorescence spectrometric (xrf) methods using the titanium line at 0.2570 nm may be employed for the determination of significant levels of titanium only by carefiil matrix-matching. However, xrf methods can also be used for semiquantitative determination of titanium in a variety of products, eg, plastics. Xrf is also widely used for the determination of minor components, such as those present in the surface coating, in titanium dioxide pigments. 

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). 

In X-Ray Fluorescence (XRF), an X-ray beam is used to irradiate a specimen, and the emitted fluorescent X rays are analyzed with a crystal spectrometer and scintillation or proportional counter. The fluorescent radiation normally is diffracted by a crystal at different angles to separate the X-ray wavelengths and therefore to identify the elements concentrations are determined from the peak intensities. For thin films XRF intensity-composition-thickness equations derived from first principles are used for the precision determination of composition and thickness. This can be done also for each individual layer of multiple-layer films. 

Measurements of the characteristic X-ray line spectra of a number of elements were first reported by H. G. J. Moseley in 1913. He found that the square root of the frequency of the various X-ray lines exhibited a linear relationship with the atomic number of the element emitting the lines. This fundamental Moseley law shows that each element has a characteristic X-ray spectrum and that the wavelengths vary in a regular fiishion form one element to another. The wavelengths decrease as the atomic numbers of the elements increase. In addition to the spectra of pure elements, Moseley obtained the spectrum of brass, which showed strong Cu and weak Zn X-ray lines this was the first XRF analysis. The use of XRF for routine spectrochemical analysis of materials was not carried out, however, until the introduction of modern X-ray equipment in the late 1940s. 

X-Ray Fluorescence analysis (XRF) is a well-established instrumental technique for quantitative analysis of the composition of solids. It is basically a bulk evaluation method, its analytical depth being determined by the penetration depth of the impinging X-ray radiation and the escape depth of the characteristic fluorescence quanta. Sensitivities in the ppma range are obtained, and the analysis of the emitted radiation is mosdy performed using crystal spectrometers, i.e., by wavelength-dispersive spectroscopy. XRF is applied to a wide range of materials, among them metals, alloys, minerals, and ceramics. 

XRF spectrometry typically uses a polychromatic beam of short-wavelength X-radiation to excite lines with longer wavelength characteristics from the sample... 

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

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)... 

In XRF, an X-ray beam or gamma rays are used to displace electrons from the inner orbitals of elements. When electrons fall into these orbitals, replacing the removed electrons, photons of specific wavelengths and energy are emitted, detected, and measured to determine which elements are present. The X-rays used in XRF do not penetrate deeply and so elements on the surface of the sample are measured, while those in the interior may not be detected [3],... 

Exposure of elements to a broad spectrum of X-rays results in the ejection of electrons from their inner shells. Electrons from outer shells falling into these vacancies emit radiation of specific wavelengths (see Figure 14.13). Analysis of this radiation, referred to as X-ray fluorescence (XRF), allows for the identification of the element from which the photon is emitted. Instruments for carrying out this analysis can be either laboratory sized or can be handheld... 

There are numerous types of instrumentation available for the measurement of XRF, but most of these are based either on wavelength dispersive methodology (typically referred to as WDX) or on the energy dispersive technique (typically known as EDX). For a detailed comparison of the two approaches for XRF measurement, the reader is referred to an excellent discussion by Jenkins [69]. 

The first XRF spectrometers employed the wavelength dispersive methodology, which is schematically illustrated in the upper half of Fig. 7.17. The x-rays emanating from the source are passed through a suitable filter or filters to remove any undesired wavelengths, and collimated into a beam that is used to irradiate the sample. For instance, one typically uses a thin layer of elemental nickel to isolate the Ka lines of a copper x-ray source from contamination by the Kp lines, since the K-edge absorption of nickel will serve to pass the Kq, radiation but not the Kp radiation. 

Fig. 7.17. Basic components of the apparatus used for the measurement of X-ray fluorescence by the wavelength and energy dispersive methods.WDX X-rays from the source (A) are allowed to impinge on the sample (B) the resulting XRF is discriminated by the crystal (C), and finally measured by the detector (D). EDX X-rays from the source (A) are allowed to impinge on the sample (B), and the resulting XRF is measured by the detector (D).

A and 12.094A. The presence of copper in a sample would be indicated by the presence of XRF peaks detected either at these wavelengths, or at their corresponding energies. 

This chapter discusses the range of analytical methods which use the properties of X-rays to identify composition. The methods fall into two distinct groups those which study X-rays produced by the atoms to chemically identify the elements present, and X-ray diffraction (XRD), which uses X-rays of known wavelengths to determine the spacing in crystalline structures and therefore identify chemical compounds. The first group includes a variety of methods to identify the elements present, all of which examine the X-rays produced when vacancies in the inner electron shells are filled. These methods vary in how the primary vacancies in the inner electron shell are created. X-ray fluorescence (XRF) uses an X-ray beam to create inner shell vacancies analytical electron microscopy uses electrons, and particle (or proton) induced X-ray emission (PIXE) uses a proton beam. More detailed information on the techniques described here can be found in Ewing (1985, 1997) and Fifield and Kealey (2000). 

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