Introduction to XRF
Introduction to
X-Ray
Fluorescence
Analysis
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Introduction to XRF
When an element is placed in a beam of
x-rays, the x-rays are absorbed.
The
absorbing atoms become ionized (e.g. due
to the x-ray beam ejects the electron in the
inner shell).
An electron from higher energy shell
(e.g., the L shell) then fall into the position
vacated by dislodged inner electron and
emit x-rays or characteristic wavelength.
This process is called x-ray fluorescence.
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Introduction to XRF
The wavelength of fluorescence is
characteristic of the element being excited,
measurement of this wavelength enable
us to identify the fluorescing element.
The intensity of the fluorescence depends
on how much of that element is in x-ray
beam.
Hence measurement of the fluorescence
intensity makes possible the quantitative
determination of an element.
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Introduction to XRF
The process of detecting and analyzing
the emitted x-rays is called “X-ray
Fluorescence Analysis.”
In most cases the innermost K and L
shells are involved in XRF detection.
A typical x-ray spectrum from an irradiated
sample will display multiple peaks of
different intensities.
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Introduction to XRF
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Introduction to XRF
The characteristic x-rays are labeled as K, L, M or N to
denote the shells they originated from.
Another designation alpha (α), beta (β) or gamma (γ) is
made to mark the x-rays that originated from the
transitions of electrons from higher shells.
Hence, a Kα x-ray is produced from a transition of an
electron from the L to the K shell, and a Kβ x-ray is
produced from a transition of an electron from the M to a
K shell, etc.
Since within the shells there are multiple orbits of higher
and lower binding energy electrons, a further designation
is made as α1, α2 or β1, β2, etc. to denote transitions of
electrons from these orbits into the same lower shell.
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Introduction to XRF
The X-Ray Fluorescence Process
Example: Titanium Atom (Ti = 22)
1) An electron in the K shell is
ejected from the atom by an
external primary excitation
x-ray, creating a vacancy.
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Introduction to XRF
2) An electron from the L
or M shell “jumps in” to
fill the vacancy. In the
process, it emits a
characteristic
x-ray
unique to this element
and in turn, produces a
vacancy in the L or M
shell.
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Introduction to XRF
3) When a vacancy is created
in the L shell by either the
primary excitation x-ray or by
the previous event, an
electron from the M or N shell
“jumps in” to occupy the
vacancy. In this process, it
emits a characteristic x-ray
unique to this element and in
turn, produces a vacancy in
the M or N shell.
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Introduction to XRF
“Auger” Electron
The excitation energy from
the inner atom
is transferred to one of the
outer electrons
causing it to be ejected
from the atom.
This process is a
competing process to the
XRF.
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Introduction to XRF
• X-ray
fluorescence's
spectroscopy
provides a means of identification of an
element,
by
measurement
of
its
characteristic X-remission length or energy
• The method allows the quantification of a
given element by first measuring the
emitted characteristic line intensity and
then relating this intensity to elemental
concentration
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Introduction to XRF
• The energy of the peaks leads to the
identification of the elements present in
the sample (qualitative analysis),
• while the peak intensity provides the
relevant or absolute elemental
concentration (semi-quantitative or
quantitative analysis).
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Introduction to XRF
Advantages of X-ray Fluorescent Analysis
•
•
•
•
Rapid analysis
Nondestructive analysis
No spectrum is affected by chemical bonding
Easily analysis of the element among the same
family elements
• High accurate analysis (5B to 92U can be
analysis)
• Easy qualitative analysis
• Easy sample preparation
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Introduction to XRF
• Elemental oxygen can be analyzed but
oxides content is estimated by assuming
the sample contains certain oxides.
• Consequently oxides content is estimated
result because XRF can only determine
elements.
• Elemental carbon and sulfur can also be
analyzed but not CO3=, SO4=, SO3= .
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Introduction to XRF
• Schematic figure of an x-ray fluorescence
spectrophotometer
BASIC PRINCIPLE:
n  2d . sin 
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Introduction to XRF
X-ray generator part
X-ray
generator
Sample
chamber
collimator
Spectrometer Part
Analyzing
crystal
collimator
To
spectrometer
part
To
counting
and
recording
part
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Introduction to XRF
X-RAY GENERATOR
• X-ray tube for XRF spectrometer is a
diode (vacuum tube) consist of the
filament generating thermo- electron and
the anode (target) generating x-rays.
• Near the target, there is a window to pass
x-rays through to the outside tube. The
window material, Beryllium, is employed
because of its nature for having the
excellent transmission (penetration) of xrays.
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Introduction to XRF
• There two types of x-ray tubes:
- Side Window Type X-ray Tube
- End Window Type X-ray Tube
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Introduction to XRF
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Introduction to XRF
Comparison on the grounding method and
window thickness of x-ray tubes
Type
Target Ground Window
Material -ing
Thicknes
s
Max
Load
Cooling
Water
Side
window
W, Cr,
Anode
Mo, Au,
etc
1000μm,
300μm
(Cr)
3 kW
City
water
End
window
Rh, etc
127μm
3kW
Deionize
d water*
*
Cathod
e
Less than 2 μS/cm (higher than 50x104Ωcm
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Introduction to XRF
• Rh target end-window type x-ray tube has the
features that since it is effectively sensitive to the
element less than the atomic number 16 (S) and
it can also obtain relatively the good sensitivity to
the heavy elements. It can make the
measurements from heavy elements to light
elements without exchanging X-tube.
• The frequency of using this Rh target X-ray tube
is high
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Introduction to XRF
• In case of side window type x-ray tube, the
W and Mo target x-ray tubes will be
applied for the analysis of heavy elements
and the Cr target x-ray tube will be applied
for the analysis of light elements.
• Especially , the Mo target x-ray tube will
be frequently applied for the analysis of
environmental pollution such as Hg, Pb,
As and so on.
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Introduction to XRF
Analyzing crystal
• The diffraction phenomenon of x-ray
through the single crystal is utilized for the
dispersion of x-rays. This crystal is called
the analyzing crystal. In accordance with
the wavelength region of the fluorescence
x-rays, the optimum analyzing crystal is
employed respectively.
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Introduction to XRF
• Analyzing crystals such as TAP, RAP, ADP,
EDDT, PET, NaCl etc. are sensitive to humidity.
If they are left in the air, their surface will
deliquesce, leading to lower x-ray reflection
intensities and deteriorated resolution. It is
therefore necessary to keep the interior of the
spectroscopic chamber in a vacuum even during
the time that the x-ray spectrometer is not used.
• Samples which are highly acid or alkaline, or
which will sublime at low temperatures will
deteriorate analyzing crystal leading to lower
reflection intensities.
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Introduction to XRF
• The fluorescence x-ray excited from the
specimen are diffracted through the analyzing
crystal by scanning goniometer.
• The diffracted x-rays are detected by the counter
and through the electronic circuit, the intensities
are recorded automatically on the charge
recorder.
• The characteristic wavelength (λ) emitted by
each element in the sample is analyzed by
applying a diffracting (an analyzing) crystal
which has a certain d value.
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Introduction to XRF
• Diffracting angles (θ) are measured and λ
of each element is determined using
Bragg’s law.
• By determining the elemental spectra
recorded on a chart, we can learn the
name of elements containing in the
specimen.
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Introduction to XRF
Example of a qualitative measurement
result.
Fluorescent spectrum recording of a stainless steel
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Introduction to XRF
• The primary radiation was supplied by tungstentarget tube operated at 50 kV, and the sample
was stainless steel containing 18%Cr and 8%
Ni.
• The K lines of all the major constituents (Fe, Cr
and Ni) and of some of the minor constituents
(Mn and Co) are apparent.
• In addition tungsten L lines can be seen; these
always be present when a tungsten tube is used.
The copper K lines are due to copper existing as
an impurity in the tungsten target
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Introduction to XRF
Consider the quantitative analysis result of a dry
limestone sample which is obtained from XRF analysis ;
ELEMENT SPECTRUM
Mg
Ca
Fe
Si
•
•
Mg Kα
Ca Kα
Fe Kα
Si Kα
WT%
1.2
90.0
2.3
6.5
Mg and Ca are found as carbonates, while Fe and Si as
oxides. Convert the data to mol % of the actual
substances (CaCO3, MgCO3, Fe2O3 and SiO2) present in
the limestone. Mol weight : Ca = 40.1; Mg = 24.3; C =
12.0; O = 16.0 and Fe = 55.8, Si=28.1.
Spectra of oxygen and carbon are not considered.
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Introduction to XRF
Concentration
Quantitative Analysis
•XRF is a reference method,
standards are required for
quantitative results.
•Standards
are
analysed,
intensities obtained, and a
calibration plot is generated
(intensities vs. concentration).
• XRF instruments compare the
spectral intensities of unknown
samples to those of known
standards.
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Introduction to XRF
Sample Preparation
Powders:
•Grinding (<400 mesh if possible) can
minimise scatter affects due to particle size.
Additionally, grinding insures that the
measurement is more representative of the
entire sample, vs. the surface of the sample.
•Pressing
(hydraulically
or
manually)
compacts more of the sample into the analysis
area, and ensures uniform density and better
reproducibility..
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Introduction to XRF
• Solids:
– Orient surface patterns in same manner so
as minimise scatter affects.
– Polishing surfaces will also minimise
scatter affects.
– Flat samples are optimal for quantitative
results.
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Introduction to XRF
• Liquids:
– Samples should be fresh when analysed
and analysed with short analysis time - if
sample is evaporative.
– Sample should not stratify during analysis.
– Sample should not contain
precipitants/solids, analysis could show
settling trends with time.
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Introduction to XRF
XRF Application
• During the last two decades, the development in
X-ray detectors has established the XRF method
as a powerful technique in a number application
fields, including:
• Ecology and environmental management:
measurement of heavy metals in soils,
sediments, water and aerosols
• Geology and mineralogy: qualitative and
quantitative analysis of soils, minerals, rocks etc.
• Metallurgy and chemical industry: quality control
of raw materials, production processes and final
products
• Paint industry: analysis of lead-based paints
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Introduction to XRF
XRF Application
• Jewelry: measurement of precious metals
concentrations
• Fuel industry: monitoring the amount of
contaminants in fuels
• Food chemistry: determination of toxic metals in
foodstuffs
• Agriculture: trace metals analysis in soils and
agricultural products
• Archaeology and archaeometry
• Art Sciences: study of paintings, sculptures etc.
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