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

Because of the high salt content possibly present in

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umes for thin sample preparation. Because of the high salt content possibly present in fresh waters and especially in sea water, evaporation can be done only for rain and some low salinity waters. An alternative way of evaporation can be designed when the dried residue is taken up into a small volume acidic solution. The possible contamination during a long evaporation process will impede this procedure. Freeze-drying with faster evaporation avoids the loss of volatile elements, and careful correction for matrix effects can be achieved by internal standards or by means of Compton scattered lines [300]. How- ever, freeze-drying as a simple preconcentration technique is recommended for waters with low matrix content. The development of micro-XRF methods, based on the efficient beam fo- cusing, opened the door to a novel approach [301]. Microfocus X-ray tubes or conventional diffraction tubes with polycapillary optics can be used as ex- citation sources for the analysis of small-area sample; see: Fig. 7.116. Sample preparation from water should meet the geometry requirement for the exci- tation; a special hydrophobic foil is used here to obtain a small spot of dried residue, which is then excited with the focused intensive beam. According to the standard procedure 50 µ l of liquid is dropped on the film (2- µ m-thick
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Methodological Developments and Applications 611 (a) (b) Exciting beam from the X-ray source Fluorescent beam from the specimen SPECIMEN Collimators Focused exciting beam Fluorescent beam from the specimen Collimator to the detector SPECIMEN ON A THIN FOIL Fig. 7.116. Excitation-detection geometry of XRF. ( a ) The conventional ED-XRF spectrometers are equipped with collimators of typical diameter of few millimeters. ( b ) A new approach of excitation delivers focused beam on the specimen surface. In this case a polycapillary or a microfocus X-ray tube produces a small-spot beam, typically with a diameter less than 100 µ m. There is a demand for specimen pre- sentation in the case of focused-beam excitation in order to exploit the X-ray flux available in a restricted area compared with the conventional case AP1 T M film), allowed to evaporate resulting in a small solid residue of less than 2 mm in diameter [302]. Instrumental DLs were obtained at 2–3 ppb in environmental water sample. By using a thinner sample support (0.15- µ m film) it was possible to reach DL below ppb concentration [303]. When membrane filters are used instead of thin foils, the preparation method may be much simpler. Chromium, arsenic, and lead were determined by EDXRF after adsorbing the species on hydrous ferric oxide [304]. In prac- tice Whatman filters were first impregnated with the hydrous ferric oxide and then were placed in 100 ml water and shaken for 24 h. The filter collects the analytes from the original water into the impregnated middle part. Although not extremely thin, this method can also be considered to be a variant of “thin-layer” technique, since there is no need for absorption correction. The DLs are attainable at the low-ppb level.
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612 S. Kurunczi et al
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  • Spring '14
  • MichaelDudley

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