Fluorescence Correction

Fluorescence Corrections. Fluorescence occurs when the characteristic radiation of a given element (A) is excited by X-ray photons of higher energy than 

In order to apply fluorescence corrections it is necessary to calculate the intensity of the fluorescence radiation of element A (7f) excited by the characteristic radiation of element B, and to obtain the ratio /f//A, where IA is the intensity of A radiation produced directly by electron bombardment. The fluorescence factor F is given by Ff=l/[l+(/f//A)]. 

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For an electron-transparent specimen the absorption and fluorescence correction parts can often be neglected, this is the so-called thin-film criterion introduced by Cliff and Lorimer [4.118]. Thus, for a thin specimen containing two elements A and B yielding the net X-ray intensities I a and 1b, the concentration ratio reduces to ... 

The Cliff-I,primer and the f methods break down when the thin foil assumptions are invalid. Absorption and fluorescence corrections may have to be made if thicker foils are employed. 

Fluorescence Correction. Fluorescence is usually a minor effect, and is often ignored. The classic case is that of Cr in stainless steels, where the Cr K line us fluoresced by the Fe K line, giving rise to an apparent increase in Cr content as the foil gets thicker. 

ZAF Z (stands for atomic number) Absorption Fluorescence (Correction) (XFA)... 

Characteristic fluorescence of x-rays of one element by the x-rays from another element in the specimen also can lead to errors in certain cases, e.g., CrK in an iron alloy. Although at times significant, this correction is usually much smaller than the absorption correction. The general formulation of the characteristic fluorescence correction factor is[12] ... 

For samples thicker than the depth of field, the images are blurred by out-of-focus fluorescence. Corrections using a computer are possible, but other techniques are generally preferred such as confocal microscopy and two-photon excitation microscopy. It is possible to overcome the optical diffraction limit in near-field scanning optical microscopy (NSOM). 

In aerosol samples 12 elements (K, Ca, Ti, V, Mn, Fe, Cu, Zn, Br, Rb, Sr and Pb) have been determined in concentration range 10-700 ng/m (Bandhu et al., 1996). In fly ash where the concentrations are higher the elements Al, Si, K, Ca, Ti, Mn, Fe, Ni, Cu, Zn, As, Rb, Sr, Y, Pb and Th are determined with precision below 10% in an concentration interval 15 mg/kg (Th) - 13% (Si). The accuracy evaluated by analysis of NIST-SRM 1633a is very good for all elements but As (Van Dyck et al., 1986). The good analytical parameters have been achieved after introduction of a secondary fluorescence correction for medium thickness samples. The same approach has lead to the successful determination of 20 elements in soils (IAEA Soil 5) with excellent accuracy and precision, and of 12 elements in plant matrices (NIST-SRM 1571). 

Chrost, R.J. and Krambeck, H.J. (1986) Fluorescence correction for measurements of enzymatic activity in natural waters using methylumbel-liferyl-substrates. Archiv fur Hydrobiologie 1 05, 70-90. 

If thin specimens are used in the AEM, high magnification images and diffraction information are accompanied by EDS of resolution about 10-100 nm. EDS of solid specimens in the SEM has micrometer resolution. Just as for imaging, this difference is due to the small interaction volume in thin films, where the beam does not spread out. Thin specimens also limit the need for absorption or fluorescence corrections, permitting the application of quantitative analysis techniques. 

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