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

SECTION V
SAMPLE PREPARATION
XRF analysis is a physical method which directly analyses almost all chemical elements
of the periodic system in solids, powders or liquids. These materials may be solids such as
lass, ceramics, metal, rocks, coal, plastic or liquids, like petrol, oils, paints, solutions or blood.
With XRF spectrometer both very small concentrations of very few ppm and very high
concentrations of up to 100 % can directly be analyzed without any dilution process.
Therefore XRF analysis is a very universal analysis method, which, based on simple and fast
sample preparation, has been widely accepted and has found a large number of users in the
field of research and above all in industry.
The quality of sample preparation in X-ray fluorescence analysis is at least as important
as the quality of measurements.
An ideal sample would be prepared so that it is:
• Representative of the material;
• Homogeneous;
• Thick enough to meet the requirements of an infinitely thick sample;
• Without surface irregularities;
• Composed of small enough particles for the wavelengths to be measured.
The care taken to determine the best method of sample preparation for a given material,
and the careful adherence to that method, will often determine the quality of results obtained.
It is safe to say that sample preparation is likely the single most important step in an analysis.
A wide variety of sample types may be analyzed by X-ray spectrometer; hence, a wide variety
of sample preparation techniques is required (Figure V.1).
Samples are often classified into two types based upon their thickness as measured by
the attenuation of X-rays. Infinitely thick samples are those which completely attenuate Xrays
emitted from the back side of the sample before they emerge from the sample. Further
increase in the thickness yields no increase in observed X-ray intensity. Clearly, the critical
value for infinite thickness will depend upon the energy of the emitted X-radiation and the
mass absorption coefficient of the sample matrix for those X-rays.
On the other hand, a thin sample has been defined as one in which m⋅µm ≤ 0.1, where m
is the mass per unit area (g/cm 2 ) and µm is the sum of the mass absorption coefficient for the
incident and emitted X-radiation. Although there are many advantages to thin samples, it is
rarely feasible to prepare them for routine samples. Many samples fall between these two
cases and require extreme care in preparation.