Analyzed Layer

Both solid and liquid samples can be analyzed by XRF as described earlier in the chapter. With the exception of micro-XRF, XRF is considered to be a bulk analysis technique. This means that the analysis represents the elemental composition of the entire sample, assuming the sample is homogeneous. The term bulk analysis is used to distinguish such techniques from surface analysis techniques (Chapter 14), which look at only a very thin layer at the sample surface. But there are conditions that must be considered in XRF in order to obtain accurate results. The limiting factor for direct XRF analysis or the analysis of prepared samples is that the signal of the characteristic radiation from the sample originates from different layers within the sample. 

XRF is a surface-sensitive technique because the depth of penetration and the thickness of the analyzed layer depend on the exciting radiation and the sample composition (atomic number). 

The analyzed layer is a function of the characteristic energy of the emission line being measured as well as the density and composition of the sample. The effect is based on absorption of the excited radiation. The effective analyzed layer is also dependent on the geometry of the instrument. The effective analyzed layer is usually defined as the layer from which either 50% or 90% of the signal originates. 

To better understand the implications for accurate XRF determinations of composition. Table 8.9 shows selected characteristic K emission lines and shows the analyzed layer depth (90% signal) for various matrices, which increase in density from the left to the right. Note that the table uses a typical shortcut for line notation KAl instead of K j and the energies of the lines are expressed in keV. Some examples of the use of this table are presented. 

Considering the S emission line, in a graphite matrix, the emission is detectable from a depth of 116 pm in the sample, but that depth is reduced to 14.8 pm in a silicate glass matrix and reduced even more in the dense matrices of iron and lead. This means that if the sulfur in the glass sample 

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Among the spectroscopic (EXAFS-like) methods of analysis of local atonuc structure, the SEFS method is a purely surface technique. The analyzed-layer depth in the SEFS method is determined by the mean free path of the secondary electrons and amounts to about 5-7 A. It is not feasible to obtain such a small depth of the analyzed layer by other EXAFS-like methods of surface structure analysis. It should be mentioned that from the standpoint of the physics of surfaces, information on the atonuc structure of superthin surface layers is of most interest, since it is precisely in these surface layers that one encounters the most considerable changes due to the presence of a free surface in a solid. The use of electron optics in the SEFS method makes it possible to obtain extrahigh resolution for the analyzed area (down to several nanometers), which presently is not attainable with X-ray optics used in the EXAFS method. 

Table 1. Thickness of analyzed layer and mass absorption coefficients for various elements in a typical boron oxide bearing glass...

Figure 8.48 Illustration of the analyzed layer from the sample The incoming primary radiation excites the lower and middle parts of the sample as shown but does not excite the upper part since the sample is infinitely thick for the primary radiation. The excited characteristic radiation from the element in the middle part is reabsorbed within the sample. Only the signal from the layer near the surface is able to be detected by the detector.

X-Ray Emission Lines and Depth of the Analyzed Layer in Various Matrices... 

Table 8.10 Analyzed Layer Thickness cf NIST 88B Dolcmite Sample for Various Elements...

A limitation for XRF in a metallurgical application is the measurement of carbon in steel. The problem with carbon in this application is due to the small analyzed layer and the inhomogeneous distribution of carbon in steel. To ID the correct steel grade for low carbon steels, OES or combustion analysis is required. 

The method XPS is the most informative technique in studies of the physicochemical surface structure of pol miers [12]. It allows obtaining of information on the qualitative and quantitative chemical composition of surfaces virtually without damaging the studied objects. The thickness of the analyzed layer for pol5miers is no larger than 10 nm. However, for state-of-the-art instruments, the area resolution is 10 pm (when synchrotron radiation is used, a value of 2.5 pm [13] can be reached). 

A large number of variously 2-, 4-, and 5-substituted thiazoles with alkyl, aryl, hydroxy, methylthio, mercapto, halo, and nitro groups have been analyzed by thin-layer chromatography on silica and alumina by the Stahl s technique (167, 170, 172). Among the many systems recommended for the elution of these compounds are the following ... 

The long-wavelength analyzers are prepared by dipping an optical flat into the film of the metal fatty acid about 50 times to produce a layer 180 molecules in thickness. 

Finally, the techniques of nmr, infrared spectroscopy, and thin-layer chromatography also can be used to assay maleic anhydride (172). The individual anhydrides may be analyzed by gas chromatography (173,174). The isomeric acids can be determined by polarography (175), thermal analysis (176), paper and thin-layer chromatographies (177), and nonaqueous titrations with an alkaU (178). Maleic and fumaric acids may be separated by both gel filtration (179) and ion-exchange techniques (180). 

A thorough description of the internal flow stmcture inside a swid atomizer requires information on velocity and pressure distributions. Unfortunately, this information is still not completely available as of this writing (1996). Useful iasights on the boundary layer flow through the swid chamber are available (9—11). Because of the existence of an air core, the flow stmcture iaside a swid atomizer is difficult to analyze because it iavolves the solution of a free-surface problem. If the location and surface pressure of the Hquid boundary are known, however, the equations of motion of the Hquid phase can be appHed to reveal the detailed distributions of the pressure and velocity. 

A wide variety of complex process cycles have been developed. Systems with many beds incorporating multiple sorbents, possibly in layered beds, are in use. Mathematical models constructed to analyze such cycles can be complex. With a large number of variables and nonlinear equilibria involved, it is usually not beneficial to make all... 

Electrode surfaces in elec trolytes generally possess a surface charge that is balanced by an ion accumulation in the adjacent solution, thus making the system electrically neutral. The first component is a double layer created by a charge difference between the electrode surface and the adjacent molecular layer in the flmd. Electrode surfaces may behave at any given frequency as a network of resistive and capacitive elements from which an elec trical impedance may be measured and analyzed. 

For water, organic and water-organic metal salts mixtures the dependence of integral and spectral intensities of coherent and non-coherent scattered radiation on the atomic number (Z), density, oscillator layer thickness, chemical composition, and the conditions of the registering of analytical signals (voltage and tube current, tube anode material, crystal-analyzer) was investigated. The dependence obtained was compared to that for the solid probes (metals, alloys, pressed powder probes). 

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