62 liquids 621 direct analysis of liquids and

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6.2 Liquids 6.2.1 Direct Analysis of Liquids and Solutions In many cases, liquids can be analysed as received. However, a serious prob- lem occurs when bubbles are formed in the liquid. Bubbles can cause problems such as incomplete filling, or expansion of the film covering the holder. In the latter case, the expansion changes the distance between the anode of the X-ray tube (source) and the specimen: resulting in a counting error. Liquid specimens require special holders made of stainless steel, polyethylene or poly- tetrafluorethylene (PTFE or Teflon). These holders have a thin (3–6 µ m) window made of a low-Z material such as Mylar or Kapton. Some windows contain micro-pores to allow gas to escape the holder, thereby eliminating bubble problems. Some spectrometers have the X-ray tube located below the specimen holder. This geometry requires that the liquid holder window be located at the bottom. The window material absorbs the secondary radiation,
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6 Specimen Preparation 413 especially that from low-Z elements, and also produces background radiation. Liquids are usually analysed at atmospheric pressure in the specimen chamber, and this air also absorbs the low-Z radiation. The result is that the analysis of low concentrations of very few parts per million of low-Z elements (such as B) is not possible. Liquids have an advantage, namely, minimum matrix effects, and it is easy to add internal standards to a liquid. However, in con- trast to standards prepared by fusion or pelletizing, liquid standards deteri- orate with time due to adsorption and precipitation effects. There are recent publications [18, 19] which discuss the use of on-line XRF, in which there is a continuous flow of the specimen through the specimen chamber. This sort of holder is also useful for the analysis of oils, thin greases, petroleum products such as used oils. Pitches and asphalt samples can also be analysed by special preparation techniques (see Sect. 6.2.2). Although the analysis of solutions is a common occurrence, it is more difficult than the analysis of solid specimens and errors are usually larger for liquids than for solids. 6.2.2 Conversion of Liquids into Quasi-Solid Specimens Most solid specimens are easier to analyse than liquids. Therefore, it is often advisable to convert a liquid into a quasi-solid by introducing an additive to increase the viscosity of the liquid, or by freezing it [20–22]. Alkaline solutions can be transformed into a quasi-solid state by adding a mixture of amylopec- tine and ethylene glycol. When eicosane is added to fuel oil, it produces a homogeneous mass which is solid at room temperature. Gelatinizing substances (such as gelatine or agar), when added to solutions, can produce a quasi-solid (jelly-like) specimen of suitable size and shape for use in different types of spectrometers [13]. For example, a 10% (mass) of gelatin or 3% (mass) of agar can be added, in powder form, to the solution and left to stand to expand. The mixture is then heated at 60–80 for a few minutes to obtain a homogeneous solution. The hot solution is transferred to
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  • Spring '14
  • MichaelDudley

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