Quantitative analysis by X-ray fluorescence

The relationship between the weight concentration of the element to be analysed and the intensity measured from one of its characteristic spectral lines is a complex one. For trace analysis several mathematical models have been developed to correlate fluorescence to the atomic concentration. A series of corrections must be introduced to account for inter-element interactions, preferential excitation, self-absorption and the fluorescence yield (the heavier atoms relax by internal conversion without photon emission). All of these factors require the reference samples to be practically the same structure and atomic composition than the sample under investigation, for all of the elements present. It is mostly because of these reasons that quantitative analysis by X-ray fluorescence is difficult to obtain. When operating upon a solid sample, a perfectly clean surface is important, preferably polished, since the analysis concerns the composition immediately close to the surface. 

Interelement (matrix) effects often complicate quantitative analysis by X-ray fluorescence. However, a wide selection of methods is now available for minimizing these effects, allowing excellent accuracy to be obtained in many cases. Detection limits are achievable down to the low parts per million (ppm) range and it is possible to obtain reasonable responses from as httle as a few milligrams of material. 

It is possible for atoms to absorb higher-energy radialion, in the X-ray region such absorption may result in the inner shell (core) electrons being promoted to an excited state, with the subsequent anis-sion of X-ray radiation. This process forms the basics for quaUtalive and quantitative elemental analysis by X-ray fluorescence (XRF) spectroscopy, as well as other X-ray techniques, discussed in Chapter 8. 

Metallic Substance Discovered in P-500. A small amount of a metallic substance was found in a sample of P-500 that was used in most of our laboratory and field tests. This substance could be observed clinging to a magnetic stirring bar which had been dipped in the dry polymer powder. Analysis by X-ray fluorescence indicated that the substance was predominantly iron. Quantitative analysis indicated that the P-500 sample contained 6.5 ppm iron. This level of iron contamination was not serious, since it was reduced eventually to about 6.5 ppb in a polymer solution containing 1,000 ppm P-500. 

The iron content of the produced samples was determined by X-ray fluorescence spectroscopy. Quantitative analysis was performed with the help of analytictJ standard and the results are listed in the second column of Table 1. 

Table 11.3 Comparison of errors generated during the analysis of XRD data (Cu Kot) from three sub-samples of Sample 4 from the lUCr CPD round robin on quantitative phase analysis. The bias values are (measured - weighed) while the values denoted XRF are the phase abundances generated from elemental concentrations measured by X-ray fluorescence methods.

The composition of a specimen is often determined by X-ray fluorescence (XRF) spectrometry, which performs rapid, qualitative, and semiquantitative determination of major and minor surface elements. Although both wavelength- and energy-dispersive (ED) analyzers can be used to detect the secondary X-rays, ED-XRE instruments are more common for the compositional determination of archaeological and conservation samples. Detection limits of 0.1% are expected therefore, the analysis is difficult for trace elements. A laboratory XRE system, commonly used to quantify elements in metal and ceramic samples (noninsulating materials need to be coated), is considered to be an indispensable tool. As with all these surface analytical techniques, care has to be taken that weathering products (thick patinas or corrosion crusts) do not obscure bulk analysis results. Thus, samples are normally prepared to provide a flat polished surface to produce quantitative results. 

The analysis of metals by X-ray fluorescence has been widely used on geological and sediment samples, either deposited on filters or as thin films. The method can be made quantitative by using geological standards and transition metals can be determined in the 1-5 tg per g range. The surfaces of sediment particles can be examined by the direct use of electron microprobe X-ray emission spectrometry and Auger electron spectroscopy. Although these methods are not particularly sensitive, they can allow the determination of a depth-profile of trace metals within a sediment particle. 

With regard to chemical reactions, target aquifer samples have been determined to give information on grain size distribution and porosity, and quantitative chemical analysis for total element content (by X-ray fluorescence (XRF)), iron sulfides, calcium carbonate, exchangeable cations, organic matter, and organic carbon. 

Besides XRD, other important studies are elemental analysis, either by chemical or physical methods, such as neutron activation analysis (NAA), x-ray fluorescence (XRF), or x-ray energy dispersive spectroscopy (X-EDS), for example (see Sections 7.6.1, 7.3.3, and 7.5.2, respectively) the advantage of these methods is that they are non destructive, as oppossed to wet chemical analysis. Additionally, IR spectroscopy can bring useful complementary information. Sometimes, the chemical composition is required along XRD analysis to fully identify a mineral. Also, thermal analysis (Section 7.6.5) is a useful tool in the qualitative and, sometimes, quantitative determination of clay minerals. 

Figure 9.15 shows the diffractogram of the catalyst reduced at 500 °C and in that the entire CuO phase was reduced to metallic Cu , as verified by Rietveld quantitative phase analysis. The copper phase content in the reduced sample is 3.8 0.2 %, meaning that all the copper is reduced, since this catalyst has 4 % by weight of copper, according to the chemical analysis previously drnie by X-ray fluorescence. 

Chemical analysis of the metal can serve various purposes. For the determination of the metal-alloy composition, a variety of techniques has been used. In the past, wet-chemical analysis was often employed, but the significant size of the sample needed was a primary drawback. Nondestmctive, energy-dispersive x-ray fluorescence spectrometry is often used when no high precision is needed. However, this technique only allows a surface analysis, and significant surface phenomena such as preferential enrichments and depletions, which often occur in objects having a burial history, can cause serious errors. For more precise quantitative analyses samples have to be removed from below the surface to be analyzed by means of atomic absorption (82), spectrographic techniques (78,83), etc. 

X-Ray Methods. In x-ray fluorescence the sample containing mercury is exposed to a high iatensity x-ray beam which causes the mercury and other elements ia the sample to emit characteristic x-rays. The iatensity of the emitted beam is directly proportional to the elemental concentration ia the sample (22). Mercury content below 1 ppm can be detected by this method. X-ray diffraction analysis is ordinarily used for the quaUtative but not the quantitative determination of mercury. 

Over the last seventeen year s the Analytical center at our Institute amassed the actual material on the application of XRF method to the quantitative determination of some major (Mg, Al, P, S, Cl, K, Ti, Mn, Fe) and trace (V, Cr, Co, Ni, Zn, Rb, Sr, Y, Zr, Nb, Mo, Ba, La, Ce, Pb, Th, U) element contents [1, 2]. This paper presents the specific features of developed techniques for the determination of 25 element contents in different types of rocks using new Biaiker Pioneer automated spectrometer connected to Intel Pentium IV. The special features of X-ray fluorescence analysis application to the determination of analyzed elements in various types of rocks are presented. The softwai e of this new X-ray spectrometer allows to choose optimal calibration equations and the coefficients for accounting for line overlaps by Equant program and to make a mathematic processing of the calibration ai ray of CRMs measured by the Loader program. 

In Total Reflection X-Ray Fluorescence Analysis (TXRF), the sutface of a solid specimen is exposed to an X-ray beam in grazing geometry. The angle of incidence is kept below the critical angle for total reflection, which is determined by the electron density in the specimen surface layer, and is on the order of mrad. With total reflection, only a few nm of the surface layer are penetrated by the X rays, and the surface is excited to emit characteristic X-ray fluorescence radiation. The energy spectrum recorded by the detector contains quantitative information about the elemental composition and, especially, the trace impurity content of the surface, e.g., semiconductor wafers. TXRF requires a specular surface of the specimen with regard to the primary X-ray light. 

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. 

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. 

NAA is a quantitative method. Quantification can be performed by comparison to standards or by computation from basic principles (parametric analysis). A certified reference material specifically for trace impurities in silicon is not currently available. Since neutron and y rays are penetrating radiations (free from absorption problems, such as those found in X-ray fluorescence), matrix matching between the sample and the comparator standard is not critical. Biological trace impurities standards (e.g., the National Institute of Standards and Technology Standard Rference Material, SRM 1572 Citrus Leaves) can be used as reference materials. For the parametric analysis many instrumental fiictors, such as the neutron flux density and the efficiency of the detector, must be well known. The activation equation can be used to determine concentrations ... 

X-ray fluorescence analysis is a nondestructive method to analyze rubber materials qualitatively and quantitatively. It is used for the identification as well as for the determination of the concentration of all elements from fluorine through the remainder of the periodic table in their various combinations. X-rays of high intensity irradiate the solid, powder, or liquid specimen. Hence, the elements in the specimen emit X-ray fluorescence radiation of wavelengths characteristic to each element. By reflection from an analyzing crystal, this radiation is dispersed into characteristic spectral lines. The position and intensity of these lines are measured. 

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. 

Optical examination of etched polished surfaces or small particles can often identify compounds or different minerals hy shape, color, optical properties, and the response to various etching attempts. A semi-quantitative elemental analysis can he used for elements with atomic number greater than four by SEM equipped with X-ray fluorescence and various electron detectors. The electron probe microanalyzer and Auer microprobe also provide elemental analysis of small areas. The secondary ion mass spectroscope, laser microprobe mass analyzer, and Raman microprobe analyzer can identify elements, compounds, and molecules. Electron diffraction patterns can be obtained with the TEM to determine which crystalline compounds are present. Ferrography is used for the identification of wear particles in lubricating oils. 

Similar to the analytical procedure for trace analysis in high purity GaAs wafers after matrix separation, discussed previously,52 the volatilization of Ga and As has been performed via their chlorides in a stream of aqua regia vapours (at 210 °C) using nitrogen as the carrier gas for trace/matrix separation.58 The recoveries of Cr, Mn, Fe, Ni, Co, Cu, Zn, Ag, Cd, Ba and Pb determined after a nearly quantitative volatilization of matrix elements (99.8 %) were found to be between 94 and 101 % (except for Ag and Cr with 80 %). The concentrations of impurities measured by ICP-DRC-MS (Elan 6100 DRC, PerkinElmer Sciex) after matrix separation were compared with ICP-SFMS (Element 2, Thermo Fisher Scientific) and total reflection X-ray fluorescence analysis (TXRF Phillips). The limits of detection obtained for trace elements in GaAs were in the low ngg-1 range and below.58... 

X-ray fluorescence is a spectroscopic technique of analysis, based on the fluorescence of atoms in the X-ray domain, to provide qualitative or quantitative information on the elemental composition of a sample. Excitation of the atoms is achieved by an X-ray beam or by bombardment with particles such as electrons. The universality of this phenomenon, the speed with which the measurements can be obtained and the potential to examine most materials without preparation all contribute to the success of this analytical method, which does not destroy the sample. However, the calibration procedure for X-ray fluorescence is a delicate operation. 

Ultimately, all quantitative analytical methods rely upon standards, whose composition is determined by the classical techniques of wet chemical quantitative analysis. Obviously, the preferred techniques for analyzing art objects are nondestructive, such as x-ray fluorescence, neutron activation, electron microprobe (both dispersive and nondispersive techniques), and so forth. Emission spectrographic analysis is not suit-... 

Purity of the product was ascertained by quantitative X-ray fluorescence analysis for chlorine and mercury, which showed satisfactory agreement with calculated values. Compounds containing both mercury and chlorine are difficult to analyze by classical wet analytical procedures. 

The loaded filters were digested with nitric acid under pressure at 160 °C. The quantitative determination of the concentrations of 23 trace elements (As, Ba, Ca, Cd, Cr, Cu, Fe, K, Mn, Mo, Ni, Pb, Rb, S, Sb, Se, Sn, Sr, Ti, V, Y, Zn, and Zr) was performed by total-reflection X-ray fluorescence analysis (TXRF) after addition of an internal Co standard [STOSSEL, 1987 MICHAELIS, 1988]. Further details of the sampling method and the trace analysis were given in [MICHAELIS and PRANGE, 1988]. 

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