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

To be the best when preconcentration is performed in

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to be the best when preconcentration is performed in the dynamic mode on a filter (membrane) using a peristaltic pump [13]. An examination of the literature reveals that there are several XRF papers which refer to methods using a preliminary extraction and concentration of the elements to be determined. These papers can be divided into several groups: Liquid extraction of the elements to be determined; specimens can be obtained by evaporation of an organic solvent or aqueous phase on a suit- able solid support [55], or formation of a thin film after extraction onto polymeric films such as Chlorin [56] or Carbosil-70 followed by solvent evaporation [57]; Preparation of a gel-like specimen (organogel) from an organic extract [22, 58] by the addition of a surfactant and aqueous gelatin solution. According to the papers of the latter group [59–61], elements to be deter- mined are extracted from solutions by using a low-melting organic compound which solidifies at room temperature to permit the extracts to be used with- out additional treatment. The low-melting extractants used include: C17–C20 aliphatic monocarboxylic acids; and long-chain alcohols with admixtures of various reagents (2,2 -dipyridyl, 1,10-phenantroline, trialkyl amine, etc.). Another preconcentration method which has been proposed to combine with XRF is crystallization preconcentration. It implies the segregation of mi- cro components by directed crystallization of the solution or melt, that is, crys- tallization of liquid into a given direction which is perpendicular to the plate interphase boundary (the crystallization front) [62, 63]. Low-temperature
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6 Specimen Preparation 419 directed crystallization by the Bridgman–Stockbarger method may be auto- mated [62]. The method is a directed crystallization of a liquid from down to top. This method consists in lowering an ampoule which contains the solution at a pre-set constant rate, from the zone of a circular heater to a refriger- ating thermostat chamber filled with a liquid antifreeze agent [62]. Different variants of preconcentration techniques with XRF analysis of the concen- trates were applied for impurity determination in wastewater [25]. At that a low-temperature directed crystallization and a liquid–liquid extraction as preconcentration methods were used. Humic substances presenting in natural water often prevent preconcentra- tion of metal ions. It was proposed [64] to use the ultrasonic pretreatment before analysis of water to eliminate this drawback. The analysis of solid materials in aqueous samples requires that the solution be filtered through Nuclepore or Millipore membranes with a 0.4 or 0.45 µ m pore-size (this is the conventional limit between “dissolved” and “particulate” matter in environmental waters) and dried; the filter membrane is then analysed as a solid. The thin films prepared by chemofiltering the com- plexes of rare earth elements with Arsenaso III or Thorin through membrane polyamide filter papers were applied for XRF analysis using a conventional tube system or synchrotron radiation source [65].
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