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The intensisty of fluorscent x rays from a sample is influenced by particle size and homegeneity of the sample. The intensity typically decreases with increased particle size. Also, the lower the atomic number, the more pronounced the effect will be. A general rule considers this effect to be negligable if the particle size is less than 60 microns.
Loose Powders
For powders the required error of analysis will define the way of sample preparation. The required error of analysis may be the main deciding factor as to whether or not analysis can be made on loose powders. The powder is treated in exactly the same way as liquids and simply poured into a sample cup fitted with mylar or polypropylene fim.
Pressed pellets
Consideration for the use of pressed pellets will depend on a number of factors: the required precision; the homogeneity of the material and the atomic number of the elements to be measured.For the measurement of heavy elements in a rock sample, a fine (<60 microns) loose powder analysis will usually be adequate. For measurement of light or trace elements, a pressed pellet will always be needed.
To produce stable pellets the powder is homogenized with a binder such as Hoescht wax. For the analysis of main components from Na - Fe the powder can be pressed directly into an aluminium cup’ the advantage beingto avoid the step of adding a binder and the possible errors associated with weighing and mixing. For heavier elements, the aluminium can cause undesirable amounts of backscatter.
Fusion
There are limits to the use of pellets for XRF, often due to a varation in the matrix of the sample, effecting the results. The solution to this problem is the use of fusions. Fusions however cause a significant dilution of the sample and are not the method of choice for measurement of trace elements.
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