Principle of XRF Analysis

Descriptions
Here we introduce the principle and application examples of X -ray
fluorescence.

1. Principle
X-rays are a type of electromagnetic wave comparable to visible light
rays but with an extremely short wavelength that measures from 100A
to 0.1A. Compared to norm al electromagnetic waves, X -rays easily pass
through substances and become stronger as the atomic number of a
substance through which it passes decreases. X -ray fluorescence
analysis is a method that uses characteristic X -rays (fluorescent X -rays)
generated when X-rays irradiate a substance. Fluorescent X -rays are
electromagnetic waves that are created when irradiated X -rays force
inner-shell electrons of the constituent atoms to an outer shell and outer
shell electrons promptly move to inner shells to fill the vacancies.
Figure 1 shows how fluorescent X -rays are produced. These fluorescent
X-rays possess energies that are characteristic to each type of element
enabling qualitative analysis by using Moseley's law and quantitative
analysis by using the intens ity (number of photons) of each X -ray
energy.
X-ray fluorescence analysis can be thought of as spectrochemical
analysis within an X -ray region. It has the same characteristics as
atomic absorption spectrometry and optical emission spectrometry
except that the sample does not need to be dissolved in a solution to be
analyzed. Flameless atomic absorption spectrometry (FLAAS) atomizes
the elements in a sample in a 2000 to 3000C flame. ICP optical
emission spectrometry (ICP -OES), excites a sample in a 6000 to 9 000C
plasma flame. X-ray fluorescence likewise excites the sample to obtain
information from X -rays.
 Figure 1 X-ray generation
2. Instrument Configuration
X-ray fluorescence analysis instruments can be largely categorized into
wavelength-dispersive X-ray spectroscopy (WDX) and energy -dispersive
X-ray spectroscopy (EDX). (See Figure 2.) WDX disperses the
fluorescent X-rays generated in a sample using an analyzing crystal and
a goniometer, resulting in the instrument being large in size. On the
other hand, the detector in EDX has superior energy resolution and
requires no dispersion system, which allows the instrument to be
smaller in size.

Figure 2 WDX
and EDX types of instruments

2-1 X-Ray Generation

X-rays are generated when the X -ray tube (Figure 3) accelerates
electrons at a high voltage and bombards them against a metal anode
(anti-cathode). There are two types of X-ray tubes, the side window type
and the end window type. Both are designed to irradiate intense X -rays
on the sample surface as evenly as possible.
A beryllium foil is commonly used as a window for retrieving incident X -
rays. Tungsten, rhodi um, molybdenum and chromium are examples of
anti-cathodes. Anti-cathode is also referred to as the gtargeth. The anti -
cathode is chosen based on what type of samples will be analyzed. X -
ray tubes with anti-cathodes the same as elements being analyzed
should not be used.

Figure 3. X-ray tube

2-2 Detector
Figure 4 shows the basic structure of a Si (Li) detector. The Si (Li)
detector features a p -i-n-type diode. The diode can only pass an electric
current in one direction (rectification mechanism). When a voltage is
applied against the current (reverse bias) and light is allowed to enter,
the electrons in the forbidden band are excited into a conductive band
and only the current for the excited electrons will travel. X -ray detection
is performed by measuring, one by one, each current pulse that
corresponds to an incident X-ray photon. The instantaneous current
value of a single pulse is proportional to the incident X -ray energy, and
thus the X-ray energy can be found by measuring the pulse height of
the current pulse.
The Si (Li) semiconductor detector is a di ode with Li drifted over a high -
purity single Si crystal, at a diameter of 3 to 6mm and thickness of 3 to
5mm, cooled by liquid nitrogen, and maintained in a vacuum. The field -
effect transistor is also cooled by liquid nitrogen. Reports about the first
developed semiconductor detector indicated that damage was caused
by applying a high voltage resulting from a shortage of liquid nitrogen
and consequent rise in temperature. The surface temperature of todayfs
detectors is monitored so that when it rises above a certain
temperature, a protection circuit shuts off the high voltage to the
detector, eliminating damage to the detector from an accidental high
voltage. At low frequency of use, the detector can be used 30 minutes
after supplying it with liquid nitroge n.
 Figure 4. Si (Li) detector
diode

2-3 Sample Chamber & Measurement Atmospher

There are two types of sample chambers: the top -surface type that
irradiates X-rays from above and bottom -surface type that irradiates X -
rays from below. There are not many differences between these two
types in terms of detection, but sample observation a nd measurements
conducted with stage travel are easier with top -surface irradiation.
In most X-ray fluorescence analysis instruments, the atmosphere in
sample chambers can be reduced to vacuum conditions. The reason for
this is because X-rays are absorbed and lose intensity under normal
atmospheric conditions and measurement of lighter elements requires
vacuum conditions.

3. Qualitative Analysis
Characteristic X-ray peak energies increase as the atomic number of
the substance increases The Qualitative Analysis of fluorescent X -ray
analysis uses this regularity. Computer software qualitatively
determines the composition of the sample. Most X -ray fluorescence
analysis instruments are equipped with an automatic identification
(definition) feature but it is important to be aware of various interfering
spectrums.
Energy positions of characteristic X -rays may be close to each other or
their peaks may ove rlap depending on which elements are contained in
the sample. Figure 5 shows an example of As and Pb spectrums.

Figure 5 As and Pb
Spectrums
The figure above shows a sample that contains Pb but not arsenic. The
energy position of the Kα line of arsenic (As) overlaps with the Lα line
of Pb, resulting in the possibility of arsenic being identified by mistake.
An element will have multiple charact eristic X-rays such as Kα line, Kβ
line, Lα line, Lβ line. Confirmation with the KLM marker shown in Figure
6 is necessary in such cases. The KLM marker compares the intensity
and theoretical energy positions of multiple characteristic X -rays. Figure
6 shows an example of the KLM marker of Pb displayed on the
spectrum.

Figure 6. Pb KLM marker

The X-ray intensities of Pb are shown here. If the sample contains Pb, a
peak should be present at each energy position and at the same
intervals as indicated by the KLM markers. If a peak is present only at
the Lα line of Pb but not at the other Pb energy positions, then we can
conclude that the sample does not actually contain Pb. Likewise, if
there are no peaks at the Kα line and Kβ line of arsenic, then the
sample does not contain aresenic. Qualitative analysis, thus, can be
accurately performed by displaying the KLM marker and observing the
intensity comparison of multiple characteristic X -rays.

4. Quantitative Analysis
This section gives an overview of how quantitative analysis is performed
using fluorescent X-rays.
Fluorescent X-rays of element A, for example, i s generated when a
sample that contains element A is irradiated by primary X -rays. The
intensity of the fluorescent X -rays depends on how much of element A
the sample contains. The intensity of the fluorescent X -rays of element
A will be higher as more of element A is contained in the sample. This
process can be done in reverse if the fluorescent X -ray intensity and
concentration of an element contained in a sample are known, allowing
us to find how much of element A is contained in another sample by its
fluorescent X-ray intensity.
There are two basic methods of quantitative analysis by with fluorescent
X-rays. The first method is to create a calibration curve. This method
involves measuring several samples of known element concentration
and finding the relationship between the intensity of the measured
element's fluorescent X -rays and the concentration. This relationship
allows you to obtain the element concentration of an unknown sample
from its fluorescent X -ray intensity.
The other method is known as the fundamental parameter method of
theoretical calculation or the FP method. This method allows you to
theoretically derive the intensity of the fluorescent X -rays if the type
and properties of all elements that compose a sample are known. The
composition of the unknown sample can be extrapolated by the
fluorescent X-ray intensities of each element.

5. Conclusion
Since X-ray fluorescence analysis can analyze a sample non -
destructively and quickly, it can be applied to a wide range of uses such
as manufacturing and quality control. Recently, the quantitative analysis
of trace elements became possible because of high -sensitivity
technologies such as filtering, which eliminates background
interference, and thin film methods. In the future, X -ray fluorescence
analysis will become more widespread particularly in measuring
hazardous metals in materials and soils.