| 1. Principle
X-ray is a type of electromagnetic waves such
as visible light ray, but the key difference is
its extremely short wavelength, measuring from
100A to 0.1A. And compared to normal electromagnetic
waves, X-ray easily passes through material and
it becomes stronger as the material's atomic number
decreases. X-ray fluorescence analysis is a method
that uses the characteristic X-ray (fluorescent
X-ray) that is generated when X-ray is irradiated
on a substance. The fluorescent X-ray is the excess
energy irradiated as electromagnetic field, which
is generated when the irradiated X-ray forces the
constituent atom's inner-shell electrons to the
outer shell and the vacant space (acceptor) falls
on the outer-shell electrons. The generation of
fluorescent X-ray is shown in Figure 1. These rays
possess energy characteristic to each element,
and qualitative analysis using Mosley's Equation
and quantitative analysis using the energy's X-ray
intensity (number of photons) are possible.
X-ray fluorescence analysis can be considered as spectrochemical analysis
of an X-ray region. It has the same characteristics as atomic absorption
spectrometry and optical emission spectrometry which conduct measurement
by putting the sample into solution. For example, in flameless atomic
absorption spectrometry (FLAAS), elements in the sample are atomized
in 2000 to 3000C flame and in ICP atomic emission spectrometry (ICP-AES),
sample is excited in 6000 to 9000C plasma flame. X-ray fluorescence likewise
excites the sample using X-ray to obtain information.
(Figure 1 X-ray generation)
2. Device Structure.
X-ray fluorescence analysis devices can be largely
categorized into wavelength-dispersive X-ray spectroscopy
(WDX) and energy-dispersive X-ray spectroscopy
(EDX). (Shown in Figure 2.) WDX disperses the fluorescent
X-ray generated in the sample using dispersion
crystal and measures it using a goniometer, resulting
in a large size. On the other hand, the detector
in EDX has a superior energy resolution and requires
no dispersion system, which enables downsizing
of the device.
(Figure 2 WDX and EDX types)
2-1 X-Ray Generation
X-ray is generated when the X-ray tube (Figure
3) accelerates the electrons at high voltage and
bombards them against the metal anode (anti-cathode).
There are two types of X-ray tubes, side window
type and end window type, and both are designed
to irradiate intense X-ray on the sample surface
as evenly as possible.
Beryllium foil is commonly used for the window
for retrieving the incident X-ray. For the anti-cathode,
(sometimes referred as gtargeth) tungsten, rhodium,
molybdenum and chrome are used. These anti-cathode
are chosen based on the analysis sample. X-ray
tubes with anti-cathode similar to analysis element
are essentially not used.
. 
(Figure 3. X-ray tube bulb)
2-2 Detector
Figure 4 shows the basic structure of a Si (Li)
device. Si (Li) device features a p-i-n+structure
diode. Diode can only pass electric current in
one direction (rectification mechanism). When voltage
is applied against the current (reverse bias) and
light enters in this state, the electrons in the
forbidden band are excited into conductive band
and only the current for the excited electron travel.
For X-ray detection, each current pulse corresponding
to an incident X-ray photon is measured one by
one. The instantaneous current value of a single
pulse is relative to the incident X-ray energy,
so X-ray energy can be found by measuring the wave
height of the current pulse.
Si (Li) semiconductor detector is a diode with
Li spread over high-purity single Si crystal, diameter
3 to 6mm and thickness 3 to 5mm, cooled with field-effect
transistor and liquid nitrogen and maintained in
vacuum. When semiconductor detector was first developed,
damage caused by application of high voltage that
resulted from shortage of liquid nitrogen and consequent
temperature rise, was reported. With current devices,
the surface temperature of the detector is monitored
and when it rises above a certain temperature,
protection circuit shuts off the high voltage to
the detector, eliminating damage to the detector
from accidentally applying high voltage. At low
frequency of use, it can be used about 30 minutes
after supplying it with liquid nitrogen.
(Figure 4. Si (Li) device structure)
2-3 Sample Chamber & Measurement Atmospher
There are two types of sample chambers, top-surface
irradiation type that irradiate X-rays from above,
and bottom-surface irradiation type that irradiate
from below. There are not many differences between
the two types in detection concentration, but for
sample observation and measurements conducted while
moving the stage, the top-surface irradiation is
better.
In most devices, atmosphere in sample chambers
can be decompressed. This is because X-rays are
absorbed and lose intensity in atmosphere. For
light element measurements, setting the measurement
atmosphere is vital.
2-3. Qualitative Analysis
In defining X-ray fluorescence analysis, the wavelength
of the characteristic X-ray or the regularity of
the energy and atomic number are used. Most devices
are equipped with the automatic identification
(definition) feature but it is important to note
various interfering spectrums.
Depending on the element types contained in the
sample, energy position of characteristic X-rays
may be close to each other or spectrums may overlap.
Figure 5 shows an example with As and Pb spectrums.
(Figure 5 As and Pb Spectrums)
As shown above, if Pb is contained
in the sample, the energy position of As's K line
overlap with Pb's L line, and would lead to identifying
As by mistake. There are multiple characteristic
X-rays of an element, such as K line, K line,
L line, L line etc.
In cases such as this, confirmation with a KLM marker shown in Figure
5 is necessary. A KLM marker compares the intensity and theoretical energy
positions of multiple characteristic X-rays. Figure 6 shows an example
with the KLM marker of Pb displayed on the spectrum.
(Figure 6. Pb KLM marker)
The X-ray intensity of Pb is shown
and if the sample contains Pb, a peak would be
present at each energy position at about the same
interval as the KLM marker. If peaks are not present
at Pb energy positions other than Pbfs L line,
it can be judged that Pb does not exist in the
sample. Likewise, if peaks do not exist on the
Asfs K line as well as the K line, the sample
does not contain As. As above, by displaying the
KLM marker and observing the intensity comparison
of multiple characteristic X-rays, qualitative
analysis can be performed accurately.
2-4. Quantitative Analysis
The following is an overview of conducting quantitative
analysis using fluorescent X-ray.
When a sample that contains element A is irradiated
primary X-ray, fluorescent X-ray of element A is
generated, but the intensity of this fluorescent
X-ray is dependent on the amount of element A in
the sample. The more element A contained in the
sample, the higher the intensity of the fluorescent
X-ray that is generated. Taking this into account,
if the fluorescent X-ray intensity and concentration
of an element contained in a sample is known, then
we can go in reverse and find how much element
A contained in another sample by its fluorescent
X-ray intensity.
When conducting quantitative analysis with fluorescent
X-ray, there are two basic methods. The first is
to create a standard curve. This method involves
measuring several samples with a known element
concentration, and finding the relationship between
the intensity of the measured element's fluorescent
X-ray and the concentration. By referring this
relationship, element concentration of unknown
sample is obtained only with information on its
fluorescent X-ray intensity.
The other method is known as the fundamental parameter
method of theoretical calculation, or the FP method.
With this method, if the type and properties of
all elements that compose a sample are known, then
the intensity of each fluorescent X-ray can be
derived theoretically. By utilizing this method,
the composition of unknown sample can be extrapolated
by its fluorescent X-ray intensity of each element.
3. Conclusion
Since X-ray fluorescence analysis can analyze a sample
non-destructively and quickly, it can be applied
to a wide range of use such as manufacturing and
quality control. Recently, as techniques for high-sensitivity,
technologies such as filtering and lamination have
been applied to eliminate the interference of background,
which made measurement of trace amounts possible.
This analysis method will become more widespread
particularly in measurements of hazardous metals
in materials and soil. |
