Spectrograph x-ray

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. 

Highly sensitive iastmmental techniques, such as x-ray fluorescence, atomic absorption spectrometry, and iaductively coupled plasma optical emission spectrometry, have wide appHcation for the analysis of silver ia a multitude of materials. In order to minimize the effects of various matrices ia which silver may exist, samples are treated with perchloric or nitric acid. Direct-aspiration atomic absorption (25) and iaductively coupled plasma (26) have silver detection limits of 10 and 7 l-lg/L, respectively. The use of a graphic furnace ia an atomic absorption spectrograph lowers the silver detection limit to 0.2 l-ig/L. 

Solvent properties and dipoles, 313 Sorbitol, 423 Sprensen pH scale, 190 Space, interstellar, 448 Spectrograph mass, 242 simple, 247 Spectroscopy, 187 infrared, 249 microwave, 249 X-ray, 248 Spectrum... 

According to Equation 1-3, the x-ray power produced is 13 watts. The corresponding number of photons per second is 13/[ 1.2Af 10 l0)]7 or 10 5(1015). In an actual spectrograph, the geometrical relationship between source and sample will reduce the incident intensity to perhaps 1% of that at the source. Therefore,... 

Fig. 4-13. Bent arid ground crystal spectrograph. (Johansson). The x-rays make the same angle with the crystal planes as in Fig. 4 12, but the grinding has brought these planes to lie on the focal circle.

Fig. 4-15. A schematic view of the losses in an x-ray spectrograph. The Roman numerals refer to the steps in Table 4-4.

The idealized calculations of the efficiency of the parts of an x-ray spectrograph (4.3, 4.4) can be modified to apply to a laboratory instrument if account is taken of the pulsating character of the applied voltage, the polychromatic nature of the x-ray beam, and the absorption in the beam path. 

Table 4-4. Losses in a Flat-Crystal X-ray Spectrograph Used To Analyze a Monolayer of Cobalt...

The best precision attainable with present apparatus for reasonable counting intervals should correspond to a standard deviation near 0.02% for a major constituent in an ideal sample properly handled. In most x-ray emission spectrography, the standard deviation is 1% or greater. Much of this discrepancy must be traceable to the way in which samples are prepared, and handled in the spectrograph, manipulation of the... 

Fig. 7-4. Chart recording from x-ray spectrograph for high-temperature ailoy typical of Tables 7-4 and 7-5. The numbers above selected peaks indicate approximate line intensity expressed as counts per second. (Courtesy of Rrissey, Anal. Chem., 25, 190.)...

The precision attained in the x-ray work cap be assessed from Table 7-4, which gives results for ten samples from the same production lot. The only sources of error here should be in the sampling, in the counting, and in the spectrograph. 

Although the standard deviations in Table 7-4 are larger than those for the most painstaking analyses by conventional Wet methods, the x-ray emission spectrograph has the great advantage that an operator in one day has made twice as many x-ray determinations as are listed... 

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. 

It is obvious that a record of the kind in Figure 7-1 can yield valuable information about an unknown mineral in the minimum time at little cost. The x-ray emission spectrograph may well become the most valuable single tool for the qualitative analysis of minerals. Its advantages are obvious enough to make further discussion supererogatory. 

Table 7-1L Comparison of Results of Optical and X-ray Spectrographic Analyses of Ores for Niobium Oxide...

Ceramics and minerals present many common problems, but ceramics warrant special treatment because elements of low atomic number predominate in them and they consequently offer x-ray emission spectrog-raphy of the light elements an excellent opportunity to prove its usefulness. Scott,8 in making this clear, emphasized the absorption and enhancement effects to be expected, and pointed out the need for careful sample preparation. By use of a General Electric XRD-5 spectrograph and associated equipment, he set up working curves for alumina, silica, potash, lime, phosphate, titania, and iron oxide in clays, refractories, and other ceramic materials. 

Fig. 8-4. Histograms made from x-ray spectrographic chart recordings for one genuine and two counterfeit bank notes. (Bee also Fig. 7-1.) These counterfeit bills were obtained through the courtesy of the United States Secret Service Office at Syracuse, N.Y., with whose permission these data are presented.

In Section 8.13, it was pointed out that samples thin enough or small enough need not be quite uniformly distributed for acceptable results provided that all of the sample intercepts a uniform incident x-ray beam. How uniform is such a beam in an ordinary spectrograph ... 

Fig. 8-10. Contour maps showing spectrograph sensitivities for the iron Ka line for various positions of the sample, (a) At surface of sample holder, (b) 0.16 inch below surface of sample holder, (c) 0.32 inch below surface of sample holder. The sensitivity changes with the x-ray optical system, with the goniometer setting, and with the distance of the sample below the surface of the sample holder. The contour interval is 20 counts per second. (Authors unpublished results.)...

A large laboratory ought to have both kinds of spectrographic equipment. Any laboratory that now has equipment modifiable for x-ray emission spectrography should proceed with the modification. A small laboratory that needs diffraction equipment and can afford only one spectrograph should buy x-ray equipment. A laboratory similarly situated that haa no need of diffraction should select its spectrograph after a careful survey of its needs. We believe that all spectroscopists should become familiar with the x-ray methods described in this book. 

---