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

The hot solution is transferred to a cuvette made of

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minutes to obtain a homogeneous solution. The hot solution is transferred to a cuvette made of Teflon, and left until it starts to stiffen. Then the specimen to fit to the specimen chamber of the spectrometer is chapped: the cuvette with the specimen is turned upside down onto a smooth plate covered by a Teflon film. As a result of the contact with the hydrophobic surface, an elastic gelatinous specimen is formed with a polished surface suitable for XRF analy- sis. Such a specimen can be obtained from aqueous solutions having a pH of 1–10, as well as from organo-aqueous solutions containing a water-miscible solvent (e.g., ethanol, methanol or acetone). Gelatin specimens tend to melt if the X-ray beam is too intense. The analyst must experiment to find the proper working conditions to avoid melting or destruction of the specimen by the X-ray beam. We recommend the addition of a 0.25% (mass) of quinhy- drone to the specimens. This will cause oxidation of the gelatine heteroatoms and cause bonding of macromolecules together, thus favouring formation of a polymer with a higher thermal stability. Quinhydrone is a low-Z material
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414 J. Injuk et al. and it does not absorb much but does increase the viscosity of the liquid. A microscopic examination of the specimen showed that the formation of bub- bles is not a problem at an exposure of 10–100 s, current of 20 mA and voltage of 20–40 kV. Physical–chemical investigations of the process of obtaining quasi-solid specimens for X-ray fluorescence analysis were carried out [23]. To pre- pare specimens from organic solutions, in particular from organic extracts containing elements to be determined, it is proposed [24, 25] to make an organogel. In this procedure, 1 mL of a hot (60–80 C) aqueous 10% gelatine and 0.5 mL of surfactant solution (e.g., 5% aqueous solution of sodium oleate) are added to 1 mL of the extract; this is thoroughly mixed and the specimen is left until gelation begins. A flat specimen surface is formed when the above mixture contacts a Teflon film placed on a smooth, flat plate. The organogels produced look like an emulsion of oil in water. External standard, internal standard and background standard methods can be used to create calibra- tion curves. Jelly-like specimens have several advantages: a flat surface; the possibility of analysing unsaturated layers, suspensions, aqueous solutions of different acidity, aqueous-organic systems and organic solutions. Producing quasi-solid specimens is not very difficult. 6.2.3 Conversion of Liquids into Organic Glassy Polymer Specimens When only a small volume of aqueous solution is available, we recommend the use of the saccharose (sucrose) [26] procedure to produce an organic glassy polymer specimen. The procedure is: 1 g of saccharose is added to 2–3 cm 3 of solution having a pH of 2–9. The mixture is then heated to 130–150 C for 3–5 min. Caramelization takes place as the water evaporates. Specimens of a suitable form are made by cooling the caramelized mass on the smooth, flat surface of a Teflon plate, or on a film inside a Teflon mold, or on another hydrophobic material. The resulting mass is only about 1
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