Analytical line

At X-ray fluorescence analysis (XRF) of samples of the limited weight is perspective to prepare for specimens as polymeric films on a basis of methylcellulose [1]. By the example of definition of heavy metals in film specimens have studied dependence of intensity of X-ray radiation from their chemical compound, surface density (P ) and the size (D) particles of the powder introduced to polymer. Have theoretically established, that the basic source of an error of results XRF is dependence of intensity (F) analytical lines of determined elements from a specimen. Thus the best account of variations P provides a method of the internal standard at change P from 2 up to 6 mg/sm the coefficient of variation describing an error of definition Mo, Zn, Cu, Co, Fe and Mn in a method of the direct external standard, reaches 40 %, and at use of a method of the internal standard (an element of comparison Ga) value does not exceed 2,2 %. Experiment within the limits of a casual error (V changes from 2,9 up to 7,4 %) has confirmed theoretical conclusions. 

The liquid was applied and dried on cellulose filter (diameter 25 mm). In the present work as an analytical signal we took the relative intensity of analytical lines. This approach reduces non-homogeneity and inequality of a probe. Influence of filter type and sample mass on features of the procedure was studied. The dependence of analytical lines intensity from probe mass was linear for most of above listed elements except Ca presented in most types of filter paper. The relative intensities (reduced to one of the analysis element) was constant or dependent from mass was weak in determined limits. This fact allows to exclude mass control in sample pretreatment. For Ca this dependence was non-linear, therefore, it is necessary to correct analytical signal. Analysis of thin layer is characterized by minimal influence of elements hence, the relative intensity explicitly determines the relative concentration. As reference sample we used solid synthetic samples with unlimited lifetime. 

Services - including programs for spectra processing and editing, manipulation with different types of data like samples compositions, terms of determination, analytical lines intensities. 

Other example of the application of described method may give analysis of copper in brass. It is well known that for the analytical line of copper CuKa the strong absorption takes place in Fe, Mn, Sn, Pb. These elements have the similar effect on ZnKa. It is possible to suppose that the ratio IcuKb IznKa less effected by the named elements. The analysis that was realized has confirmed that the variation of the named above ratio is about 25 less then variation of L. ... 

For the samples of high C concentrations, obtained by a chemical enrichment of coaly shales, the technique was developed, which uses in addition the CK analytical line intensity to correct interelernent effects. The application of this correction allowed to reduce errors in determining the studied element concentrations up to an acceptable level. The cai bon content was determined over the range 1 to 100 %. 

The existing models for emitting x-ray fluorescence intensity of elemental analytical lines from heterogeneous samples are limited in practical applications, because in most publications the relations between the fluorescence intensity of analytical lines elements and the properties of powder materials were not completely studied. For example, particles distribution of components within narrow layer of irradiator which emitted x-ray fluorescence intensity of elements might be in disagreement with particles distribution of components within whole sample. 

Ideally, the background absorption should be measured as near as possible to that of the analyte line. This approach has been achieved in the subsequent methods described below. 

It has always been difficult to do quantitative work with the characteristic x-ray lines of elements below titanium in atomic number. These spectra are not easy to obtain at high intensity (8.4), and the long wavelength of the lines makes attenuation by absorption a serious problem (Table 2-1). The use of helium in the optical path has been very helpful. The design of special proportional counters, called gas-flow proportional counters,20 has made further progress possible, and it is now possible to use aluminum Ka (wavelength near 8 A) as an analytical line (8.10). 

If the intensity of the x-ray line (the analytical line ) were being measured to establish the concentration of an element in a sample, then sc would indicate the highest precision to be expected in this determination. 

The analytical method to be discussed in this chapter consists in exciting a characteristic line (the analytical lined for each element sought in a sample in identifying each such element by measuring the wavelength of the analytical line and in drawing conclusions, from the measured intensity of the analytical line, about the amount of each such element present. This method is likely to become more important in analytical chemistry than all the other x-ray methods combined. It is presented after the absorptiometric methods and after the determination of film thickness because it is more easily understood on the basis of the earlier material. 

Suppose the weight-fraction Wes of element E in sample S is to be determined by measuring the intensity, Ies, of an analytical line. In the simplest case, we may assume that... 

In the region near zero thickness, there is no effect of matrix composition, and Equation 7-1 should be valid. The justification for this statement follows. The deviations under discussion rest ultimately upon x-ray absorption. Because x-ray absorption in this region is negligible, its only measurable result being the direct excitation of the analytical line, there should be no deviations from Equation 7-1. 

The region of greater practical importance, the region of 4 infinite thickness, is characterized by a horizontal line in Figure 6-4. This line is the upper limit of a curve the form of which is determined in part by the mass absorption coefficients of the element for the incident, and for the emergent x-ray beam. If the sample contains elements other than E, the mass absorption coefficients of these other elements will similarly help determine the intensity I s of the analytical line at infinite thickness of sample. 

The n t effect of the presence of other elements is conveniently assessed by comparing the intensity of the analytical line in their presence with the intensity calculated from Equation 7-1. The net effect may be to increase the intensity over that calculated (positive), or to decrease it (negative). Individual effects may result from the following causes (1) Presence of an element with absorption coefficient smaller than that of E positive absorption effect). (2) The reverse of this situation negative absorption effect). (3) Presence of an element a characteristic line... 

Fig. 7—2. Spectral data to illustrate absorption and enhancement effects for three transition elements. (To avoid crowding, only part of the cobalt absorption curve is shown.) See Table 7-1. Case B. Substitution of A1 for Fe decreases absorption of incident beam and has little effect on analytical line. Net positive absorption effect. Case C. Substitution of Pb for Fe decreases absorption of primary beam but greatly increases absorption of analytical line. Net negative absorption effect. Case D. Note wavelength relationship indicated in figure. Enhancement impossible. Case E. Note wavelength relationship in figure. Enhancement occurs.

This qualitative discussion will be illustrated by a simple case. Consider the determination of iron in the five samples of Table 7-1, the analytical line being iron Ka, and all the samples being assumed uniform. Figure 7-2 gives some of the spectra significant for Table 7-1. 

Enhancement effects pose more difficult problems. Enhancement results when the analytical line Ae of element E is excited within the sample by the characteristic line Ap of some other element F also present in the sample thus, when enhancement occurs, the intensity of the analytical line exceeds the value given by Equation 7-6 (Table 7-1). Sherman s treatment15 of the enhancement effect problem is the most thorough and successful to date. 

The discussion preceding Equation 7-5 points to the weight-fraction as the logical unit for the x-ray emission spectrography of infinitely thick samples. Were all complications absent, one might expect proportionality between analytical-line intensity and weight-fraction such that... 

Adler and Axelrod,58 in their two-channel spectrograph, have taken the ultimate step in this direction by measuring the two intensities simultaneously. We may take for granted that the proper use of-an internal standard can eliminate the effect of different variations in equipment in different cases. It follows that care may be relaxed in connection with variations thus eliminated for example, approximate voltage regulation suffices for an x-ray source used to excite both analytical lines when these are measured simultaneously. 

On the basis of Section 7.3 the second statement above can be expressed as follows. If 7ee and IstSt are the intensities of the analytical lines for the two elements in question under the experimental conditions, then we may write for Sample 1 (denoted by )... 

To discuss the effectiveness of internal standards, it is helpful to recall the absorption and enhancement effects the standards are intended to compensate. In the present discussion, we shall ignore the incident beam even though this simplification is not always justified. We have indicated above that an internal standard will hot be completely effective if a change in composition influences the intensity ratio (Equation 7-12). Such influences could arise from differences in the extent to which the two analytical lines are absorbed or from differences in the extent to which the two lines are excited. Let us examine the way in... 

Interchanging the wavelengths of the analytical lines in Figure 7-5 will reverse the effect of the disturbing influence. 

An effect of the kind in Case I is exerted by the sample less E and St The effect is reduced as the difference in wavelength between the analytical lines decreases, because differential absorption of the two analytical lines is then reduced. 

Because the two analytical lines differ in wavelength, an internal standard can never compensate absorption and enhancement effects completely. If Cases II and IV %re avoided in selecting ah internal standard, the use of such a standard will usually prove satisfactory. Special cases may require special calibration curves run with the disturbing elements present. 

Fig. 7-6. Enhancement of the intensity of germanium radiation relative to arsenic radiation by selenium. The ordinate in this figure is, for the upper curve, the normalized Ge-As intensity ratio and, for the lower curves, the normalized absolute intensity. The abscissa is the composition of the diluent added to the base material. The relation of analytical lines and absorption edges is shown in IV, Fig. 7-5. Open circles = GeKar/AsKa closed circles = Ge crosses = As. (Courtesy of Adler and Axelrod, Spectrochim. Acta, 7, 91.)...

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