X-Ray Fluorescence Analytical Techniques - CNSTN : Centre ...

eground and pressed or cast as a disk. For precise quantitative determinations, loose powders
are rarely acceptable, especially when low energy X-rays are detected. Pressed briquets are
much more reliable.
Briquets or pressed powders yield better precision than powder samples and are
relatively simple and economical to prepare. In many cases all that is needed is a hydraulic
press and a suitable die. In the simplest case, the die diameter should be the same as the
sample holder so that the pressed briquets will fit directly into the holder. The amount of
pressure required to press a briquet which yields maximum intensity depends upon the sample
matrix, the energy of the X-ray to be used, and the initial particle size of the sample.
Therefore, prior grinding of the sample to a small particle size (< 100 micrometers) is
advisable. In cases of materials which will not cohere to form stable briquets, a binding agent
may be required. A wide, variety of binding agents have been used such as: powdered
cellulose, detergent powders, starch, stearic acid, boric acid, lithium carbonate, polyvinyl
alcohol and commercial binders.
III. Fused Materials
Fusion of materials with a flux may be done for several reasons. Some refractory
materials cannot be dissolved, ground into fine powders, or otherwise put in a suitable,
homogeneous form for X-ray spectrometric analysis. Other samples may have compositions
which lead to severe interelement effects, and dilution in the flux will reduce these. The fused
product cast into a glass button, provides a stable, homogeneous sample well suited for X-ray
measurements. The most severe disadvantages to fusion techniques are the time and material
costs involved, and the dilution of the elements which can result in an order of magnitude
reduction in X-ray intensity. However, when other methods of sample preparation fail, fusion
will often provide the required results.
More common are the glass-forming fusions with lithium borate, lithium tetraborate or
sodium tetraborate. Flux to sample ratios range from 1:1 to 10:1. The lithium fluxes have
lower mass absorption coefficients and, therefore, less effect on the intensity of the low
energy X-rays. Lithium carbonate may be added to render acidic samples more soluble in the
flux, whereas lithium fluoride has the same effect on basic sample. Lithium carbonate can
also reduce the fusion temperature. Oxidants such as sodium nitrate, potassium chlorate or
others may be added to sulfides and other mixtures to prevent loss of these elements.
IV. Filters and Ions-Exchange Resins
Various filters, ion-exchange resin beads, and ion-exchange resin impregnated filter
papers have become important sampling substrates for samples for X-ray spectrometric
analysis. Filter materials may be composed of filter paper, membrane filters (i.e., Nuclepore,
Millipore), glass fiber filters, and others. Filters are used in a variety of applications.
One of the most widely used applications of filters is in the collection of aerosol
samples from the atmosphere. Loadings of several milligrams of sample on the filter may
correspond to sampling several hundred cubic meters of atmosphere. Many elements may be
determined directly o these filters by X-ray spectrometric analysis.