X RAY SATURATION DEPTH or PENETRATION DEPTH
The term ceramics was originally used for pottery and earthenware collectively. Today, it
comprises a much wider range of materials, including metallic oxides, nitrides and carbides. These
materials are used in application areas ranging from household items (porcelain, sanitaryware,
artware) to high-performance tools for industrial use (ball bearings, cutting tools, isolators,
catalysts). In addition to their great hardness, ceramics are also resistant to thermal and chemical
influences, making them highly suitable for applications where the product is subjected to high
mechanical or thermal stress.
Another important factor is the purity of the material, as even slight impurities can lead to rejects
during the manufacturing process. Such impurities may not only influence the physical and
chemical properties of the product but can also prove to be harmful to the health of the user.
Therefore, quality control with regard to the composition of the ceramic material is a challenge for
each manufacturer. It is necessary to prepare ceramic materials quickly and reproducibly in order to
carry out representative and reliable quality checks.
Analysis Requirements
The successful use of spectrometers in the analytical laboratory requires an understanding of the
method used and a great deal of practical experience, and routine tasks often do not leave users with
enough time to develop and optimize the methods involved. Some analysis technologies require
extensive external training for the laboratory analysts, which delays the implementation and
acceptance of the method by the laboratory staff.
X-ray fluorescent analysis (XRF) is an exception because the sample is analyzed in solid form and
the measurements are easy to carry out. XRF is therefore well established in areas where quick
results are essential, such as for quality checks during production. Since XRF measurements are so
simple to carry out, the importance of reliable sample preparation is often neglected. This can lead
to poor reproducibility and even incorrect analysis results.
For XRF analysis, the laboratory sample of a few grams often has to represent a total amount that
could be several tons. In addition to the quality of the spectro-meter, the quality of the sample
preparation has a decisive influence on the precision and reproducibility of the analysis results. It is
important to consider which mill and size reduction principle is suitable for a particular material.
Figure 1. Saturation depth. Only a part of the fluorescent light leaves the sample.
28000 μm = 0,028 m = 28 mm (1 μm = 10-6 m)
3 mm = 3000 μm
Saturation Depth
The deeper the X-ray enters the sample, the more it interacts with the sample’s atoms. An
increasing portion of the X-ray is absorbed by the sample until a specific depth is reached beyond
which the X-ray light can no longer penetrate. This also applies to the fluorescent light that must
exit the sample in order to be detected.
The lowest detectable sample layer is called the saturation depth (see Figure 1). The saturation
depth depends on the intensity of the X-rays, the wavelength (i.e., the type of detected atom) and
the density of the sample’s surroundings (the matrix). If different elements are analyzed in the same
surroundings, the saturation depth increases according to the atomic number of the element in
question. Table 1 illustrates this correlation for porcelain.
Table 1. X-ray saturation depth of different elements in a porcelain sample.
Saturation depth generally decreases with the atomic number, which means that the element
becomes more difficult to detect and explains why elements such as carbon and boron emit very
weak fluorescent signals. Changing the matrix, by analyzing iron in zirconium oxide or tungsten
carbide instead of porcelain, for example, has a great influence on the saturation depth. Heavy
elements in the matrix decrease the saturation depth considerably, making a correct analysis much
more difficult.
Figure 2. Vibratory disc mill. Inside the grinding jar, the grinding tools (usually a puck and a ring)
are moved in such a way that the sample is crushed by impact and friction effects.
Sample Preparation
When preparing samples for XRF analysis, care should be taken to ensure that the size of the
particles to be examined lies within the saturation depth of the X-rays in order to obtain a
representative analysis result. For a porcelain sample, for example, a fineness of 80 microns is only
necessary if elements lighter than potassium have to be analyzed. Otherwise, a grind size of 100
microns, which can be quickly and easily obtained with any suitable laboratory mill, is sufficient.
Sample materials frequently come in large amounts with large feed sizes, making preliminary size
reduction necessary. Jaw crushers are very suitable as a preliminary grinder for ceramic materials;
they crush the material through pressure and friction between two breaking jaws, one moving and
one stationary.
After preliminary size reduction, a part of the sample is subjected to fine grinding. This part of the
sample must have the same properties as the original bulk material in order to obtain reliable
information about the composition of the total sample. The selection of the sample division method
and instrument depends on the material and the amount. Dry, pourable bulk samples can be fed to
rotating dividers via vibratory feeders, whereas sample splitters are suitable for sticky and nonflowing materials.
The part sample is then subjected to pulverization. It is important to use a suitable mill and grinding
tools that will not alter the material properties to be determined in any way during the sample
preparation process. A thorough knowledge of the instruments is required, as is some experience in
the preparation of different materials. Finally, care should be taken to ensure that possible abrasion
from the grinding tools does not interfere with the analysis results.
The most frequently used mill for the size reduction of hard and brittle sample materials for
subsequent XRF analysis is the vibratory disc mill (see Figure 2). Inside the grinding jar, the
grinding tools (usually a puck and a ring) are moved in such a way that the sample is crushed by
impact and friction effects. The required reproducible analytical fineness can be achieved after very
short grinding times. The quick turnaround provides a decisive advantage when analysis results are
needed quickly, such as for a product approval.
Grinding aids can be used during this stage, particularly if the material has a tendency to cake
during grinding. One grinding aid that seems to have received wide acceptance is Vertrel©XF, a
DuPont product that helps prevent caking and conveniently evaporates after grinding.
Small sample volumes can also be processed in a mixer mill. Here, the grinding jars perform radial
oscillations in a horizontal direction. The inertia of the grinding balls causes them to impact with
high energy on the sample material at the rounded ends of the grinding jars.
Pellet Production
For most XRF applications, pellets with a plane surface are used. In contrast to loose powder, a
pellet is advantageous in that the element concentration detected by the X-ray is higher because the
material is more compact. In addition, a smooth surface is preferable to a rough one from an optical
point of view.
Pellets are usually produced either through fusion of the sample with salt or by pressing the sample
into a pellet. Fusion of the sample with lithium tetra borate is a very effective method of producing
a bead. The sample is weighed together with the flux in a platinum crucible, and then the crucible is
heated in a fusion machine to more than 1000°C. This process, which destroys the original matrix
and creates a homogeneous borate glass, yields highly reproducible results regardless of the original
material.
Fusion has a few disadvantages, however. Volatile elements like thallium or cadmium tend to
escape during the fusion process and cannot be detected. Moreover, the sample is heavily diluted
with lithium salt (factor 10-50), which impairs the detection limit when compared to pellets. Certain
elements (e.g., boron, iron, carbides) could even damage the very expensive platinum crucible.
Finally, it takes much more time to produce a bead than a pellet (15 minutes compared to
approximately two minutes).
Pellet pressing is the most common procedure for many applications, even though calibration of the
spectrometer can be more involved due to the sample matrix. A pressed pellet should be
homogeneous; absolutely solid, since loose particles pollute the X-ray tube; stable; and storable.
The pressing of a sample can be carried out with or without additives. Pressing without additives
(free pressing) is not very common because the pellets are usually not sufficiently stable. The most
frequently used materials are cellulose- or paraffin-based. Cellulose has the advantage of also acting
as a grinding aid, which helps avoid the caking of the sample inside the grinding jar. Cellulose can
be used in vibratory disc mills as well as mixer mills.
Wax is added after the sample has been ground, either manually or by mixing it with the help of
polyamide balls in a plastic jar in the mixer mill. The addition of wax makes the pellet’s surface
indelible. Moreover, wax is more inexpensive than cellulose and is not hygroscopic, which is
important if the pellets are to be stored. Either steel rings or aluminum cups are used to stabilize the
pellets. The cups can be labeled on the reverse side and are useful for storing the pellets.
Successful Analysis
The size reduction techniques described here are an essential precondition to producing
representative samples for XRF analysis. Close attention should be paid to the particle size of the
ground sample, since particles that are too coarse tend to impair the reproducibility of the analysis.
However, it is not recommended that the material be ground to sizes finer than necessary, as this
practice results in needless time and effort for sample preparation.
Jaw crushers are widely used for the preliminary size reduction of ceramic samples. A
representative part sample is obtained from this first step with the help of a sample divider and is
then subjected to fine grinding in a vibratory disc mill or mixer mill. The finely ground sample can
then be pressed to pellet form and analyzed by XRF.
Penetration Depths
It is known that X-Rays will penetrate some way into a material. For XRF analysis, the important
question is from what depth within the sample does the spectrum arise. Unfortunately this is not a
simple question, as there are many factors involved.
The two main points to consider are (a) the depth of penetration of the primary X-Ray beam into the
sample, and (b) the escape depth from which fluorescent X-Rays can be detected. Both of these are
directly linked to the energy of the X-Rays - the higher the X-Ray energy the deeper the X-Ray
penetrates. In general, it is fair to assume that X-Rays will penetrate a few micrometers down to
several millimeters, depending on the sample matrix. At best fluorescent X-Rays will be detectable
from a few millimeters within the sample, but in many situations this could be reduced to a few
micrometers or less.
Penetration of the primary X-Rays
The primary X-Rays should be considered in two parts, both of which are effected by the voltage
setting of the X-Ray generator.
First of all, the characteristic X-Rays from the anode target material are at a fixed energy. If the
generator voltage is sufficient to excite multiple lines (eg, K and L) then both high energy (K) and
low energy (L) X-Rays will be incident on the sample. Usually the K lines will be more intense, and
so these will dominate in considerations about penetration. If however, the voltage is reduced to
such an extent that the higher energy X-Rays are no longer excited, then the characteristic X-Rays
will be low energy L lines only - as a result the expected penetration will be greatly reduced.
Secondly, the bremsstrahlung (or continuum) X-Rays must be considered. As their name suggests,
these X-Rays have a continuous energy range (up to a maximum equal to the accelerating voltage of
the generator. The continuum spectrum is most intense towards the higher energy cut off - by
reducing the voltage it is possible to reduce this "average energy" of the continuum, and thus reduce
penetration.
Escape of fluorescent X-Rays
The ability of fluorescent X-Rays to penetrate through and escape from the sample itself depends
again on their energy, which directly relates to which elements are being detected. The lighter
elements (eg, Na, Mg, Al, Si) have very low energy X-Rays, and thus will be difficult to detect
even at relatively small depths within the sample. Heavier elements (eg, Cu, Ag, Au) have much
more energetic X-Rays which will be able to pass through large distances within the sample.
Clearly the sample composition itself is also an important factor. The higher the concentration of
heavier elements which absorb strongly, the more reduced the chance of X-Rays escaping from
deep within the sample.
To summarize, heavy elements (ie, energetic fluorescent X-Rays) will be detectable relatively
deep within a sample matrix primarily comprised of light elements (ie, low absorption). Light
elements (ie, low energy fluorescent X-Rays) will be detectable only at the surface of a sample
matrix comprised of heavy elements (ie, strong absorption).