[B._Beckhoff,_et_al.]_Handbook_of_Practical_X-Ray_(b-ok.org).pdf

Methodological developments and applications 613 be

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Methodological Developments and Applications 613 be placed near to the specimen resulting in a large solid angle and thereby increasing the geometrical efficiency of the fluorescence detection. The TXRF method is a micro-technique in contrast to the conventional- XRF, since only a very small amount of sample is needed for analysis (typi- cally 1 ng–1 µ g of water residue). Itas a lot in common with ICP-AES rather than with the conventional-XRF. The range of elements from Z = 11 to Z = 92 can be analyzed and it can be further extended toward lower Z with some instrumental modifications [308]. The TXRF method is a multielement technique like ICP-AES, but the fluorescent signals are collected simultane- ously in the case of TXRF. Sample preparation is the most time-consuming step in the analysis, spectrum collection, and evaluation is straightforward and rapid. Fortunately, well-established sample preparation methods can be directly adapted from AAS or ICP-AES. Another similarity with the ICP-AES is the accessible DL. The instrumentation for the TXRF spectrometer is described in detail by Klockenk¨ amper [309] [see Fig. 7.117]. Table top laboratory instruments are available on the market, having the basic design very similar for all systems. The X-ray source is usually a fine-focus diffraction tube, with molybdenum anode for a wide energy range of excitation. The line-focus window is used for TXRF because a strip-like beam is needed for excitation. For the Mo-K α line (17.4 keV) total reflection condition can be set at a small angle of inci- dence ( α crit =0.1 for a quartz reflector as specimen holder). Almost all TXRF spectrometers are equipped with energy dispersive detectors, dominantly with Si(Li) demanding a regular liquid nitrogen supply, but the number of Peltier- cooled detectors is increasing. The whole TXRF instrument consists only of a few parts, this simplicity is an advantage in operation and maintenance. X-rays from the tube are passed through a thin Mo-foil before reaching the first reflector working as a low-pass filter. The first reflector allows the operation of the X-ray tube in its most effective mode, setting at 50 kV high voltage. The high energy bremsstrahlung will then be cut by the first reflector. X-ray tube Filter Slit Edge First reflector Second reflector Solid-state detector Fig. 7.117. The geometrical arrangement of TXRF instrumentation illustrates the simplicity of this method. Reproduced from: Klockenk¨amper [309] by permission of John Wiley & Sons
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614 S. Kurunczi et al Finally a small slit or edge is placed in front of the sample reflector to define the beam impinging on the specimen by blocking scattered rays. Although the beam-adapting unit is simple, the alignment requires some experience.
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