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

The powder can also be packed in cells or spread out

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of the spectrometer. The powder can also be packed in cells or spread out on a thin film ma- terial (so-called “slurry” technique). The slurry technique works for water insoluble materials. A water slurry is prepared out of a few milligrams of powder and a few milliliters of water. A turbid suspension is made and then filtered through, e.g., a Nuclepore filter. The objective is to produce a uni- formly thick, homogenous specimen. Processing large bulk samples poses the biggest problem. The first major challenge is to remove a portion of the bulk which is representative of the bulk material. This is not an easy task and requires great care when there is a large bulk of material to work with. The next challenge is to pulverize the sample so as to produce fine particles of uniform size and without contaminating the sample or segregating it. The sample must also have a homogenous distribution of all elements throughout the entire sample. Grinding A major objective during the preparation of a sample from a bulk is the reduction of the powder to a uniform particle size. For a routine trace XRF analysis, a particle size of less than 60 µ m is commonly accepted. Reduction of particle size is usually accomplished by grinding. Various methods for grinding samples are available. For routine work, manual grinding, using a mortar and pestle (agate, corundum, mullite), is traditionally done, while for finer (1–10 µ m) and controlled particle sizes, a variety of mills are commercially available. SPEX Industries (see e.g. ) produces a grinder, called a mixer mill, which does an excellent job of producing particles of a uniform size. It is necessary to note that this grinder can only be used when there are several grams of material to work with. When dealing with extremely small amounts of sample, hand grinding is the best option although
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6 Specimen Preparation 423 it is tedious and time-consuming, and requires care to produce particles with a uniform size distribution. Materials that are difficult to treat, like rocks and ores, are ground better after the introduction of an additive such as sodium stearate. It is found that manual grinding of, e.g., inorganic aluminosilicates by using a mortar and pestle reduces the particle size to about 40 µ m which is acceptable even for X-ray diffraction methods (XRD). However, as mentioned above, the particle size distribution must be controlled or else preferential absorption of secondary radiation can occur in the particles of different size. In the case of coal, samples are prepared as a powder (5 g sample ground in a tungsten carbide rotary swing mill together with 1 g of a boric acid binder and 100 mg of sodium stearate). A grinding time of about 6 min will reduce the particle size to 50 µ m. In some analyses, it is important to use a uniform grinding time for all samples to assure reproducible results. For materials that may be damaged or undergo alteration during size reduction, grinding is usually carried out in ethyl or isopropyl alcohol using a pestle and mortar.
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