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

Grinding in liquid nitrogen with a spex freezermill

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Grinding in liquid nitrogen with a SPEX freezer/mill can also be effective for some materials, in particular, polymers and plastics. Mechanical grinders can also be used under dry and wet conditions. Grind- ing samples which contain a mixture of particles with different physical prop- erties (such as different phases) may lead to significantly different grinding of the different phases. Experience has shown that all grinding tools are a potential source of contamination. Therefore, the composition of the grinding media should be selected with care to avoid materials which contain elements of interest in the analysis. Agate, for instance, introduces traces of SiO 2 , Mg and Ca to the specimen. This might be of less importance for geological mate- rials, but for biological materials these blank values represent a major source of errors. The effect of surface roughness of particles on XRF analysis of powdered samples is discussed in [72]. The authors show that errors due to surface roughness can be minimized if measurements are performed at a steep take- off angle 70 . Unfortunately, most wavelength-dispersive spectrometers now in production do not allow the take-off angle to be varied. Pelletizing Pelletizing a powder sample is necessary to reduce surface effects and to yield a better precision than that obtained by the analysis of loose powders. In general, provided that the powder particles are of a uniform size distribution, and are less than 50 µ m in diameter (300 mesh), the sample should be pressed into a disc (pelletized) at 600–800 MPa (6–8 tons per cm 2 ). If the self-bonding properties of the powder are good, low pressures of perhaps up to 100–400 MPa (1–4 tons per cm 2 ) can be employed. High-pressure pelletizing in a die, or directly in a sample cup, often results in a fracture of the pellet following the removal of pressure from the die. The release of pressure should be slow and always at the same rate in order to produce reproducible data.
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424 J. Injuk et al. It is sometimes necessary to add a binder before pelletizing to form stable pellets, and prevent caking of the sample on the surface of the die. Binders are normally composed of low Z-elements or organic materials. As mentioned above, these types of materials can produce scattered X-rays and thereby increase the background counts. This effect can become important in the de- termination of trace elements in the low-Z region. Therefore, the choice of the binding agent should be made with special care. It must be free of contami- nation and should have a low absorption coefficient for the analyte lines. The most useful binders are starch, cellulose, lucite, polyvinyl, urea, boric acid, graphite, etc. Liquid binders like ethyl alcohol or diethyl ether can be mixed with the sample manually, whereas powdered binders are recommended to be mixed mechanically to form a homogeneous mixture. A recommended pro- cedure is the addition of 2–10% by weight of binder to the sample. Further
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