Ray Fluorescence XRF

When a sample is placed in abeam of X-rays, some of the X-rays are absorbed. The absorbing atoms become excited and emit X-rays of characteristic wavelength. This process is called X-ray fluorescence. Since the wavelength (energy) of the fluorescence is characteristic of the element being excited, measurement of this wavelength enables us to identify the fluorescing 

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 

Both solid and liquid samples can be analyzed by XRF as described earlier in the chapter. Very flat surfaces are required for quantitative analysis, as discussed subsequently. Liquids flow into flat surfaces, but cannot be run under vacuum. The best solvents are H2O, HNO3, hydrocarbons, and oxygenated carbon compounds, because these compounds contain only low atomic number elements. Solvents such as HCl, H2SO4, CS2, and CCLt are undesirable because they contain elements with higher atomic numbers they may reabsorb the fluorescence from lower-Z elements and will also give characteristic lines for Cl or S. This will preclude identification of these elements in the sample. Organic solvents must not dissolve or react with the film used to cover the sample. 

Each element fluoresces at its own characteristic wavelengths (energies) the fluorescing element can be identified from a table of wavelengths (energies) such as those in 

Qualitatively, it is easy to see that Cr, Fe, W, and Co are present. Cr and W have unobstructed lines that tell us they are in the alloy. Fe is clearly present, because if there is any Mn, it must be in low concentration compared to the Cr and Fe. The Mn K peak overlaps the Cr peak we know there is Cr present from its K peak, so some (at least) of the intensity of the small peak at 5.8 keV is due to Cr. If Mn is present but in low 

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Figure 8.21 shows schematically a set of lx, 2s, 2p and 3s core orbitals of an atom lower down the periodic table. The absorption of an X-ray photon produces a vacancy in, say, the lx orbital to give A and a resulting photoelectron which is of no further interest. The figure then shows that subsequent relaxation of A may be by either of two processes. X-ray fluorescence (XRF) involves an elecfron dropping down from, say, fhe 2p orbifal fo fill fhe lx... 

Figure 8.21 The competitive processes of X-ray fluorescence (XRF) and Auger electron emission...

Electron Microprobe A.na.Iysis, Electron microprobe analysis (ema) is a technique based on x-ray fluorescence from atoms in the near-surface region of a material stimulated by a focused beam of high energy electrons (7—9,30). Essentially, this method is based on electron-induced x-ray emission as opposed to x-ray-induced x-ray emission, which forms the basis of conventional x-ray fluorescence (xrf) spectroscopy (31). The microprobe form of this x-ray fluorescence spectroscopy was first developed by Castaing in 1951 (32), and today is a mature technique. Primary beam electrons with energies of 10—30 keV are used and sample the material to a depth on the order of 1 pm. X-rays from all elements with the exception of H, He, and Li can be detected. 

Electron Beam Techniques. One of the most powerful tools in VLSI technology is the scanning electron microscope (sem) (see Microscopy). A sem is typically used in three modes secondary electron detection, back-scattered electron detection, and x-ray fluorescence (xrf). AH three techniques can be used for nondestmctive analysis of a VLSI wafer, where the sample does not have to be destroyed for sample preparation or by analysis, if the sem is equipped to accept large wafer-sized samples and the electron beam is used at low (ca 1 keV) energy to preserve the functional integrity of the circuitry. Samples that do not diffuse the charge produced by the electron beam, such as insulators, require special sample preparation. 

Instrumental Methods for Bulk Samples. With bulk fiber samples, or samples of materials containing significant amounts of asbestos fibers, a number of other instmmental analytical methods can be used for the identification of asbestos fibers. In principle, any instmmental method that enables the elemental characterization of minerals can be used to identify a particular type of asbestos fiber. Among such methods, x-ray fluorescence (xrf) and x-ray photo-electron spectroscopy (xps) offer convenient identification methods, usually from the ratio of the various metal cations to the siUcon content. The x-ray diffraction technique (xrd) also offers a powerfiil means of identifying the various types of asbestos fibers, as well as the nature of other minerals associated with the fibers (9). 

Barium can also be deterruined by x-ray fluorescence (XRF) spectroscopy, atomic absorption spectroscopy, and flame emission spectroscopy. Prior separation is not necessary. XRF can be appHed directly to samples of ore or products to yield analysis for barium and contaminants. AH crystalline barium compounds can be analy2ed by x-ray diffraction. 

The complex of the following destmctive and nondestmctive analytical methods was used for studying the composition of sponges inductively coupled plasma mass-spectrometry (ICP-MS), X-ray fluorescence (XRF), electron probe microanalysis (EPMA), and atomic absorption spectrometry (AAS). Techniques of sample preparation were developed for each method and their metrological characteristics were defined. Relative standard deviations for all the elements did not exceed 0.25 within detection limit. The accuracy of techniques elaborated was checked with the method of additions and control methods of analysis. 

X-ray fluorescence (XRF) analysis is successfully used to determine chemical composition of various geological and ecological materials. It is known that XRF analysis has a high productivity, acceptable accuracy of results, developed theory and industrial analytical equipment sets. Therefore the complex methods of XRF analysis have to be constituent part of basis data used in ecological and geochemical investigations... 

X-Ray fluorescence (XRF) analysis favorably differs from other instmmental techniques by rapidity, automation ability, selectivity, accuracy. Quality of specimen obtained from sample to be analyzed has influence to the accuracy. 

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. 

Three techniques involving the use of X-ray emission to obtain quantitative elemental analysis of materials are described in this chapter. They are X-Ray Fluorescence, XRF, Total Reflection X-Ray Fluorescence, TXRF, and Particle-Induced X-Ray Emission, PIXE. XRF and TXRF use laboratory X-ray tubes to excite the emission. PIXE uses high-energy ions from a particle accelerator. 

X-Ray Fluorescence (XRF) is a nondestructive method used for elemental analysis of materials. An X-ray source is used to irradiate the specimen and to cause the elements in the specimen to emit (or fluoresce) their characteristic X rays. A detector s)rstem is used to measure the positions of the fluorescent X-ray peaks for qualitative identiflcation of the elements present, and to measure the intensities of the peaks for quantitative determination of the composition. All elements but low-Z elements—H, He, and Li—can be routinely analyzed by XRF. 

The emission spectmm of Co, as recorded with an ideal detector with energy-independent efficiency and constant resolution (line width), is shown in Fig. 3.6b. In addition to the expected three y-lines of Fe at 14.4, 122, and 136 keV, there is also a strong X-ray line at 6.4 keV. This is due to an after-effect of K-capture, arising from electron-hole recombination in the K-shell of the atom. The spontaneous transition of an L-electron filling up the hole in the K-shell yields Fe-X X-radiation. However, in a practical Mossbauer experiment, this and other soft X-rays rarely reach the y-detector because of the strong mass absorption in the Mossbauer sample. On the other hand, the sample itself may also emit substantial X-ray fluorescence (XRF) radiation, resulting from photo absorption of y-rays (not shown here). Another X-ray line is expected to appear in the y-spectrum due to XRF of the carrier material of the source. For rhodium metal, which is commonly used as the source matrix for Co, the corresponding line is found at 22 keV. 

Rosen JF, Markowitz ME, Jenks ST, et al. 1987. L-X-ray fluorescence (XRF) A rapid assessment of cortical bone lead (Pb) in Pb-toxic children. Pedia Res 21 287A. 

Now, we are not particular experts in X-ray and gamma-ray spectroscopy (nor mass spectroscopy, for that matter), but our understanding of those technologies is that they are used mainly in emission mode. Even when the exciting source is a continuum source, such as is found when an X-ray tube is used to produce the exciting X-rays for an X-ray Fluorescence (XRF) measurement, the measurement itself consists of counting the X-rays emitted from the sample after the sample absorbs an X-ray from the source. These measurements are themselves the equivalent of single-beam measurements and will thus also be Poisson-distributed in accordance with the basic physics of the phenomenon. 

The capability of simultaneous acquisition of X-ray Fluorescence (XRF) spectra has been added. This new instrument, capable of both mineralogical and chemical applications (Klingelhofer et al. 2008) is briefly described and then a few examples of extraterrestrial and terrestrial applications are presented. 

Direct measurement of soil is most often carried out on air-dried soil and involves spectroscopic instruments and methods. For example, X-ray dispersion (XRD), X-ray fluorescence (XRF), infrared (IR) spectroscopy,... 

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