Quantitative Analysis by XRF

Quantitative analysis can be carried out by measuring the intensity of fluorescence at the wavelength characteristic of the element being determined. The method has wide application to most of the elements in the periodic table, both metals and nonmetals and many types of sample matrices. It is comparable in precision and accuracy to most atomic spectroscopic instrumental techniques. The sensitivity limits are on the order of 1-10 ppm, although sub-ppm DLs can be obtained for the 

X-rays only penetrate a certain distance into a sample, and the fluorescing X-rays can only escape from a relatively shallow depth (otherwise, they would be reabsorbed by the sample) as we have seen. While XRF is considered a bulk analysis technique, the X-rays measured are usually from no deeper than 1000 A from the surface. True surface analysis techniques such as Auger spectroscopy only measure a few angstroms into the sample, so in that respect, XRF does measure the bulk sample, but only if the surface is homogeneous and representative of the entire sample composition. 

The relationship between the measured X-ray intensity for a given peak and the concentration of the element can be written 

A variety of mathematical approaches to the correction of absorption-enhancement effects and calibration is now in use, due to the availability of inexpensive, powerful computers and powerful software programs. One approach is the fundamental parameters (FP) method. 

Most commercial XRF systems have a fundamentals parameters software program as part of their data processing package. The advantage to the FP approach is that many industrial materials do not have readily available matrix-matched calibration standards commercially available. Preparation of good, stable calibration standards takes a lot of time and money even when it is possible to make 

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The following sections will briefly introduce the methods of quantitative analysis by XRF and EDS in EM, separately. There are differences in the factors affecting the analysis because EDS in EM uses an electron beam rather than X-rays as the primary source. 

In general, quantitative analysis by EDS in EM is similar to that of XRF. The analytical methods, however, are different for two main reasons. First, the interactions between the electron beam and specimen are different from those of primary X-ray radiation. Second, an EM specimen for chemical analysis cannot be modified as in the internal standard method. For accurate quantitative analysis of EDS in EMs, separate standard samples containing the elements in the specimen to be analyzed are necessary. The standards should be measured at identical instrumental conditions to the specimen. It means that the spectra of specimen and standard should be collected under the same conditions with regard to the following parameters ... 

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

Matrix effects make quantitative analysis with XRF quite complicated. Many methodological processes are developed for calibration of the measuring arrangement, which may be performed by two major approaches empirical and fundamental parameters (FP) calibration. 

Care must be taken over the quantification of inorganic flame retardants, since some of them (e.g., antimony trioxide) can react with the organic flame retardant present, or break down to produce volatile products, under quantitative ashing conditions, and during analysis by thermal techniques such as TGA. A good initial approach is a semi-quantitative elemental analysis by XRF, to see which types are present. Accurate quantifications can then be obtained by precise elemental determinations. 

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. 

Conventional XRF analysis uses calibration by regression, which is quite feasible for known matrices. Both single and multi-element standards are in use, prepared for example by vacuum evaporation of elements or compounds on a thin Mylar film. Comparing the X-ray intensities of the sample with those of a standard, allows quantitative analysis. Depending on the degree of similarity between sample and standard, a small or large correction for matrix effects is required. Calibration standards and samples must be carefully prepared standards must be checked frequently because of polymer degradation from continued exposure to X-rays. For trace-element determination, a standard very close in composition to the sample is required. This may be a certified reference material or a sample analysed by a primary technique (e.g. NAA). Standard reference material for rubber samples is not commercially available. Use can also be made of an internal standard,... 

As with XRF, electron microscope-based microanalysis is relatively-insensitive to light elements (below Na in the periodic table), although this can be improved upon with developments in thin-window or windowless detectors which allow analysis down to C. It is better than XRF because of the high vacuum used ( 10-8 torr), but a fundamental limitation is the low fluorescent yield of the light elements. As with XRF analysis it is surface sensitive, since the maximum depth of information obtained is limited not by the penetration of the electron beam but by the escape depth of the fluorescent X-rays, which is only a few microns for light elements. In quantitative analysis concentrations may not add up to 100% because, if the surface is not smooth, some X-rays from the sample may be deflected away from the detector. It may be possible in such cases to normalize the concentration data to 100% if the analyst is certain that all significant elements have been measured, but it is probably better to repeat the analysis on a reprepared sample. 

The elemental composition of unknown materials such as engine deposits can be determined qualitatively and the information used to develop dissolution methods prior to analysis by inductively coupled plasma atomic emission spectroscopy (ICPAES). Alternatively, a semi-quantitative analysis can be provided by XRF alone, especially important when only a limited quantity of sample is available and needed for subsequent tests. The deposit does not even have to be removed from the piston since large objects can be placed directly inside an EDXRF spectrometer. 

Aqueous and organic liquids, powders, polymers, papers, and fabricated solids can all be analyzed directly by XRF. The method is nondestructive, so unless dilution is required, the original sample is returned to the submitter. Although the method can be applied to the analysis of materials ranging in size from milligram quantities to bulk parts such as engine pistons, a minimum of 5 grams of sample is usually required for accurate quantitative analysis. 

The results of the quantitative analysis of the elements C, Si, S, Mo, Ni, Fe, and V by combustion and XRF analysis are given in Table 8 for six samples that were on stream for 10 months [the sample number is directly related to the proximity of the sample to the reactor inlet (1 = closest to inlet, etc)]. Comparison of these results to corresponding results in the absence of added PDMS (not shown here) suggests that silica alone is responsible for the rapid and irreversible deactivation." ... 

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. 

The quality of an XRF analysis depends on the counting statistics. Therefore, the reduction of the measuring time to achieve a very fast semi-quantitative analysis will be limited by the user s demand on the analytical quality. Concentration calculations in XRF analysis are based on fluorescence intensities from a layer at the surface of a sample, whose thickness may vary depending on the element and basic material, or from a layer of several centimeters down to a few layers of atoms. Thus, the limit of accuracy in Standardless XRF analysis also depends on the surface quality of the sample (Fig. 7). 

Interferences are physical or chemical processes that cause the signal from the analyte in the sample to be higher or lower than the signal from an equivalent standard. Interferences can therefore cause positive or negative errors in quantitative analysis. There are two major classes of interferences in AAS, spectral interferences and nonspectral interferences. Nonspectral interferences are those that affect the formation of analyte free atoms. Nonspectral interferences include chemical interference, ionization interference, and solvent effects (or matrix interference). Spectral interferences cause the amount of light absorbed to be erroneously high due to absorption by a species other than the analyte atom. While all techniques suffer from interferences to some extent, AAS is much less prone to spectral interferences and nonspectral interferences than atomic anission spectrometry and X-ray fluorescence (XRF), the other major optical atomic spectroscopic techniques. 

Quantitative analysis usually requires the use of standards and/or certified reference materials (CRMs), the selection of an appropriate elemental optical emission line, and, in most cases, the selection of a normalization line, used as an IS. Calibration curves are then constructed using the normalized peak area versus concentration, as previously described for calibration using an IS. When there is a dominant matrix component for which the concentration will remain approximately constant across the calibration set, it is best to use an emission line from that matrix element for normalization. This approach helps minimize effects due to changes in plasma conditions caused by shot-to-shot fluctuations in laser intensity. Alternatively, chemometric correlation analysis of the entire observed spectrum with the concentration of the analyte can be used to construct calibration curves automatically. In general, RSDs of 5%-10% are readily achievable. To improve quantitation, sample preparation methods such as pressing pellets may improve results for soils and sediments and fusion with salts to convert the sample into a glass bead can eliminate matrix effects. Fusion was discussed in Chapter 1 and is used extensively in XRF analysis (Chapter 8). 

Forensic samples are often very small and inhomogeneous. As discussed later, it is difficult to perform quantitative analysis on such samples. However, it is possible to use qualitative XRF to good advantage. Using the spectral fingerprint of a sample does not require exact concentrations to be determined. By proving that a spectrum of soil found in a shoe matches the spectrum of soil from the location of a crime scene, it is possible to place the object at that scene. 

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