Fluorescence spectroscopy X-ray

The X-ray fluorescence (XRF) technique, already discussed in Chapter 1, has been applied extensively to the determination of macro- and micro-amounts of non-metallic elements in polymers. 

An interesting phenomenon has been observed in applying the XRF method to the determination of parts per million of chlorine in hot-pressed discs of low-pressure polyolefins. In these polymers the chlorine is present in two forms, organically bound and inorganic, with titanium chloride compounds resulting as residues from the polymerisation 

A Polymer not treated with alcoholic potassium hydroxide before analysis, ppm chlorine. X-ray fluorescence on polymer discs. Average of 2 discs (A) B Polymer treated with alcohoKc potassium hydroxide before analysis, ppm chlorine. X-ray fluorescence on polymer discs. Average of 2 discs (B) Difference between averj e chlorine contents obtained on potassium hydroxide-treated and untreated samples (B) - (A) 

A further example of the application of XRF spectroscopy is the determination of tris(2,3-dibromopropyl) phosphate on the surface of flame retardant polyester fabrics [211. The technique used involved extraction of the fabric with an organic solvent followed by analysis of the solvent by XRF for surface bromine and by high-pressure liquid chromatography for molecular tris(2,3-dibromopropyl) phosphate. The technique has been applied to the determination of hydroxy groups in polyesters [22, 231  

The XRF method of Wolska [24] discussed in Section 1.2.1 has been applied to the determination of bromine and phosphorus in polymers. Various other workers have applied this technique to the determination of chlorine and sulfur [25] and various other elements [26, 27]. 

Niino and Yahe [37] used XRF to determine the chlorine content of products obtained in the photoirradiation of polyvinylidene chloride film. 

The fluorescence process used in some x-ray sources, as described in Section 10.1, can also be used as an analytical tool. One can direct either high-energy electron beams or x-rays at an unknown sample and perform qualitative and quantitative analysis by making measurements on the lower-energy x-ray emissions that occur. Let us first briefly review what we have discussed to this point concerning the concept of fluorescence. 

X-ray fluorescence is a type of atomic spectroscopy since the energy transitions occur in atoms. However, it is distinguished from other atomic techniques in that it is nondestructive. Samples are not dissolved. They are analyzed as solids or liquids. If the sample is a solid material in the first place, it only needs to be polished well, or pressed into a pellet with a smooth surface. If it is a liquid or a solution, it is often cast on the surface of a solid substrate. If it is a gas, it is drawn through a filter that captures the solid particulates and the filter is then tested. In any case, the solid or liquid material is positioned in the fluorescence spectrometer in such a way that the x-rays impinge on a sample surface and the emissions are measured. The fluorescence occurs on the surface, and emissions originating from this surface are measured. 

Elemental qualitative analysis is a popular application of x-ray fluorescence spectroscopy. The values of the wavelengths reaching the detector are indicative of what elements are present in the sample. This is so because the inner-shell transitions giving rise to the wavelengths are specific to the element. Qualitative analysis 

FIGURE 10.7 The two instrument designs for x-ray fluorescence spectroscopy. Left, the energy-dispersive system. Right, the wavelength-dispersive system. 

Quantitative analysis is possible by measuring the intensity of the x-ray emissions. Thus, if quantitative analysis is important in all the above cited qualitative applications, for example, then this intensity is measured and compared to standards. 

The X-ray signal in STEM is usually collected by compact solid state detectors based on lithium-drifted silicon diodes. The X-ray photon is sorted by energy, henee the alternative names for X-ray fluorescence analysis Energy Dispersive X-ray Spectroseopy (EDS or EDX). The typical energy resolution for an X-ray photon is of on the order of 150 eV This is suffieient in most cases for resolving peaks of different elements, but is inadequate for deteeting 

The use of electron beam transparent samples facilitates the interpretation of the EDX result. In such thin foils, the self-absorption of the outgoing X-ray and secondary fluorescence can be ignored except for the very soft X-rays such as those produced by excited oxygen atoms. Hence the intensity of characteristic X-rays produced per incident electron is directly given by the formula 

The simplicity of the ratio method for thin films is to be contrasted with the complexity of microanalysis for bulk samples using EDX. Combined with the high spatial resolution achievable, it makes EDX a very attractive analytical technique. It has been extensively used in oxide superconductor research, for example, in phase identification of powdered samples, and in identifying the 

X-ray spectroscopy can be classified in the same manner as every other type of spectral analysis into absorption and emission spectroscopy. However, the most popular method of x-ray spectroscopy in crude oil chemistry is the emission spectroscopy, also called x-ray fluorescence spectroscopy. The effect used by this type of spectral analysis is the same as was described for fluorescence analysis. However, x-rays are used for this analysis instead of the ultraviolet radiation used for fluorescence analysis. 

X-ray pipes are used as the light source for x-ray fluorescence spectroscopy. There are very many types of x-ray pipes in the modern market. The functioning principle of the x-ray pipe is the same as for cathode lamps described in an earlier section of this chapter. The x-ray pipe contains an electrical heated cathode, anode and radiation output window. This window is made from beryllium because this material is transparent to x-rays. The x-rays pipes offered in the market differ because they have a different anode material, and consequently the spectral characteristics of the emitting radiation are different. 

The monochromator for x-ray fluorescence spectroscopy is called the analyzing crystal. It differs from all the monochromators described earlier for all the other optical analytical instruments. The effect used in this type of monochromator is not diffraction, but interference. The wavelength of the analyzing light is changed by rotation of the analyzing crystal by certain angle. 

The homogeneity of the sample is very important for successful x-ray fluorescence spectrometry. Hence, the preparation of solid samples for this analysis by melting. The samples analyzed by crude oil chemist are, in most cases, liquids or 

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The relative simplicity of x-ray spectra (compared to the complexity of optical spectra) makes x-ray spectroscopy an extremely important analytical and investigative tool. Three methods will be described x-ray fluorescence spectroscopy, electron probe microanalysis, and x-ray photoelectron spectroscopy. 

In scanning the wavelength range of the fluorescence the crystal must be smoothly rotated to vary the angle 6, and the detector must also be rotated, but at twice the angular speed since it is at an angle of 26 to the direction of the X-ray fluorescence beam. 

An alternative type of spectrometer is the energy dispersive spectrometer which dispenses with a crystal dispersion element. Instead, a type of detector is used which receives the undispersed X-ray fluorescence and outputs a series of pulses of different voltages that correspond to the different wavelengths (energies) that it has received. These energies are then separated with a multichannel analyser. 

An energy dispersive spectrometer is cheaper and faster for multielement analytical purposes but has poorer detection limits and resolution. 

In contrast to most spectroscopic techniques, which generally require the sample 

X-ray spectroscopy is also a useful tool when used in combination with a SEM. The SEM uses very short-wavelength, high-energy electrons, commonly exceeding 15,000 V, to stimulate X-ray emission much as an incident X-ray beam would. When an X-ray detector is coupled to an electron microscope, a compositional analysis of very small areas, only a few microns wide, is possible. One application is the identification of the mineral grains that are found in complex materials such as prehistoric pottery, which is particularly useful for identifying small mineral grains in complex matrices such as tempered pottery. 

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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. 

Lithium tetraborate [1303-94-2], is used as a flux in ceramics and in x-ray fluorescence spectroscopy. The salt has also been proposed for... 

Analytical deterrnination of nickel in solution is usually made by atomic absorption spectrophotometry and, often, by x-ray fluorescence spectroscopy. 

Zirconium is often deterniined gravimetrically. The most common procedure utilizes mandelic acid (81) which is fairly specific for zirconium plus hafnium. Other precipitants, including nine inorganic and 42 organic reagents, are Hsted in Reference 82. Volumetric procedures for zirconium, which also include hafnium as zirconium, are limited to either EDTA titrations (83) or indirect procedures (84). X-ray fluorescence spectroscopy gives quantitative results for zirconium, without including hafnium, for concentrations from 0.1 to 50% (85). Atomic absorption determines zirconium in aluminum in the presence of hafnium at concentrations of 0.1—3% (86). 

The chemical composition of particulate pollutants is determined in two forms specific elements, or specific compounds or ions. Knowledge of their chemical composition is useful in determining the sources of airborne particles and in understanding the fate of particles in the atmosphere. Elemental analysis yields results in terms of the individual elements present in a sample such as a given quantity of sulfur, S. From elemental analysis techniques we do not obtain direct information about the chemical form of S in a sample such as sulfate (SO/ ) or sulfide. Two nondestructive techniques used for direct elemental analysis of particulate samples are X-ray fluorescence spectroscopy (XRF) and neutron activation analysis (NAA). 

Cadmium and inorganic compounds of cadmium in air (X-ray fluorescence spectroscopy) Chromium and inorganic compounds of chromium m air (atomic absorption spectrometry) Chromium and inorganic compounds of chromium m air (X-ray fluorescence spectroscopy) General methods for sampling and gravimetnc analysis of respirable and mhalable dust Carbon disulphide in air... 

For the preparation of samples for X-ray fluorescence spectroscopy, lithium metaborate is the preferred flux because lithium does not give rise to interfering X-ray emissions. The fusion may be carried out in platinum crucibles or in crucibles made from specially prepared graphite these graphite crucibles can also be used for the vacuum fusion of metal samples for the analysis of occluded gases. 

X-ray fluorescent spectroscopy, see X-ray emission spectrography X-ray gaging of thickness, see Thickness gaging... 

MDHS7 Lead and inorganic compounds of lead in air (X-ray fluorescence spectroscopy). 

Hackens, T., H. McKerrell, and M. Hours (eds.) (1977), X-Ray Fluorescence Spectroscopy Applied to Archaeology, Vol. 1, PACT Rixensart. 

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