X-ray Absorption and Fluorescence

XAS is the measurement of the X-ray absorption coefficient of a material. X-rays of a narrow energy resolution are shone on the sample and the incident and transmitted X-ray intensity is recorded as the incident X-ray energy is incremented. When a beam of monochromatic X-rays passes through matter, it loses its intensity due to interaction with the atoms in the material. The intensity decreases exponentially with incident distance if the material is homogeneous, and after transmission, according to Beer s law, the intensity is  

At most X-ray energies, the absorption coefficient pis a. smooth function of energy, with a value that depends on the sample density p, the atomic number Z, atomic mass A, and the X-ray energy E roughly as  

The second process for de-excitation of the core hole is the Auger effect, in which an electron drops from a higher electron level and a second electron is emitted into the continuum (and possibly even out of the sample). In the hard X-ray regime ( 2keV), X-ray fluorescence is more likely to occur than Auger emission, but for lower energy X-ray absorption. Auger processes dominate. Both processes are proportional to the rate of absorption, therefore, either of the two processes can be used to measure the absorption coefficient p however, fluorescence is somewhat more commonly used. 

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For the determination of sulfur contents of residual fuels a variety of procedures are available. The bomb (ASTM D-129, IP 61) and quartz tube (ASTM D-155, IP 63) combustion methods have long been established. Other, more rapid techniques are becoming increasingly available, including high-temperature combustion (ASTM D-1552), X-ray absorption and fluorescence methods, and the Schoniger oxygen flask procedure. 

Neal RH, Sposito G (1989) Selenate adsorption on alluvial soils. Soil Sci Soc Am J 53 70-74 Newville M, Sutton S, Rivers M, Eng P (1999) Micro-beam X-ray absorption and fluorescence spectroscopies at GSECARS APS beamline 13 . J Synchrotron Rad 6 353-355 Nier AO, Schlutter DJ (1993) The thermal history of interplanetary dust particles collected in the Earth s stratosphere. Meteoritics 28 675-681... 

In WDS, or in the EPMA, quantitative analysis is possible if the specimen is flat and standards are used for calibration. Computer software is available to permit reliable analyses. It takes into account such critical complicating features as atomic number, x-ray absorption and fluorescence, which depend on the path length and depth in the specimen. These techniques are used with polymeric materials, but normally to... 

X-rays This region encompasses both hard X-rays (wavelengths down to 10 pm) and less penetrating soft X-rays (wavelengths up to 10 nm) with optical frequencies lying between 3 x 10 and 3 x 10 Hz. With the capacity to produce ionization by electron detachment, photon energies in this region are often reported in electronvolts (eV), and run from around 10 eV down to 100 eV (1 eV = 1.602 x 10 J). X-ray absorption and fluorescence spectra as such mostly relate to atomic core electronic transitions. 

Further particle size measurement techniques are light scattering, cascade impaction, X-ray absorption and fluorescence, permeability, and adsorption. The last two are discussed further in the next section. 

A - correction for differences between X-ray absorption and F - correction for corresponding X-ray fluorescence differences. 

In bulk samples, X-ray yields need to be adjusted by the so-called "ZAF" correction. Z stands for the element number (heavier elements reduce the electron beam intensity more than lighter elements, because they are more efficient back-scatterers), A for absorption (different elements have different cross sections for X-ray absorption), and F for secondary fluorescence (the effect described above). Corrections are much less important when the sample is a film with a thickness of 1 pm or less, because secondary effects are largely reduced. The detection limit is set by the accuracy with which a signal can be distinguished from the bremsstrahlung background. In practice, this corresponds to about 100 ppm for elements heavier than Mg. 

X-ray methods include x-ray diffraction, x-ray absorption, and x-ray fluorescence. X-ray diffraction is a technique for determining ultrasmall spacings in materials, such as the spacings between the atoms or ions in a crystal structure, or the thickness of a thin electroplated material. An example of the former is in soil laboratories in which the minerals in various soils need to be characterized. X-ray absorption is limited in application, but has been used to determine heavy elements in a matrix of lighter elements, such as determining lead in gasoline. X-ray fluorescence is much more popular and is used to determine elements in a wide variety of solid materials. 

Inner electrons are usually excited by X-rays. Atoms give characteristic X-ray absorption and emission spectra, due to a variety of ionization and possible inter-shell transitions. Two relevant refined X-ray absorption techniques, that use synchrotron radiation, are the so-called Absorption Edge Fine Structure (AEFS) and Extended X-ray Absorption Fine Structure (EXAFS). These techniques are very useful in the investigation of local structures in solids. On the other hand, X-Ray Fluorescence (XRF) is an important analytical technique. 

Yttrium tantalate and yttrium niobate-tantalate have good X-ray absorption and are used in X-ray intensifying screens because of their high conversion factor (Section 5.5.4.2.). Substitution of tantalum by small quantities of niobium considerably increases the blue fluorescence when excited by X rays. 

Hunter, D.B. and Bertsch, P.M., In situ examination of uranium contaminated soil particles by micro-x-ray absorption and micro-fluorescence spectroscopies, J. Radio-anal. Nucl. Chem., 234, 237, 1998. 

Noninvasive surface spectroscopies can be applied in the presence of liquid water most of them involve the input and detection of photons. The best known examples are nuclear magnetic resonance, electron spin resonance, Raman, Fourier transform infrared, UV-visible fluorescence, X-ray absorption, and Mossbauer spectroscopies, although Brown (28) enumerated many others that are available to detect adsorbed ions. These methods, some of which are listed in Table II along with citations of illustrative applications, can be used both noninvasively and in conjunction with in situ probes. 

Hunter DB, Bertsch PM (1998) In situ examination of uranium contaminated soil particles by micro-X-ray absorption and micro-fluorescence spectroscopies. J Radioanal Nucl Chem 234 237-242 Hunter DB, Bertsch PM, Kemner KM, Clark SB (1997) Distribution and chemical speciation of metals and metalloids in biota collected from contaminated environments by spatially resolved XRF, XANES, and EXAFS. JPhys IV 7 (Colloque C2, X-Ray Absorption Fine Structure, Vol. 2) 767-771 Hunter KA (1980) Microelectrophoretic properties of natural surface-active organic matter in coastal seawater. Limnology and Oceanography 25 807-822... 

In contrast to optical spectroscopy, where absorption methods are most important. X-ray absorption applications are limited when compared with X-ray emission and fluorescence procedures. Although absorption measurements can be made relatively free of matrix effects, the required techniques are somewhat cumbersome and time-consuming when compared with fluorescence methods. Thus, most applications arc con-ftned to samples in which matrix effects are niiniinal. 

X-ray fluorescence spectroscopy is the most widely used x-ray technique for quantitative analysis this chapter will be primarily concerned with this method of analysis. X-ray absorption and x-ray diffraction analysis are treated briefly at the end of the chapter. 

Describe the principles of x-ray emission, x-ray absorption, and x-ray fluorescence analysis. Distinguish between each technique with respect to instrumentation requirements. 

Inductively Coupled Plasma. Mass Spectrometry Archaeological Applications. Microscopy Techniques Scanning Electron Microscopy. Surface Analysis X-Ray Photoelectron Spectroscopy Particle-Induced X-Ray Emission Auger Electron Spectroscopy. X-Ray Absorption and Diffraction X-Ray Diffraction - Powder. X-Ray Fluorescence and Emission X-Ray Fluorescence Theory. 

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