Introduction to X-ray fluorescence (XRF) analysis

Introduction to
X-ray fluorescence (XRF)
analysis
Outline
•
X-rays, what are they? What is XRF?
•
History of discoveries in the field of X-rays
•
Basics of XRF theory
•
Evaluation of XRF spectra
•
XRF instrumentation
•
Analysis software
Discovery of X-rays
•
November 11, 1895,
Wilhelm Conrad
Roentgen
First Nobel prize in
physics
in 1901.
December 22, 1895
photograph of Anna Röntgen’s hand
Electromagnetic spectrum
X-rays, what are they?
Bremsstrahlung
Characteristic radiation
Spectrum of X–ray tube
•
Bremsstrahlung
Short wavelength limit
depends on tube voltage
•
Characteristic radiation
Depends of anode material
What is XRF?
•
The XRF is the emission of characteristic (or fluorescent)
X-rays from a material that has been excited with X-rays.
History of discoveries in the field of X-rays
•
Max Theodor Felix von Laue
Nobel Prize in Physics in 1914
for his discovery of the diffraction
of X-rays by crystals.
History of discoveries in the field of X-rays
•
Sir William Henry
Bragg
shared a Nobel
Prize in Physics in
1915 with his son
William Lawrence
Bragg
History of discoveries in the field of X-rays
•
Sir William Lawrence Bragg
shared a Nobel Prize in
Physics in 1915 with his father
Sir William Henry Bragg
The youngest Nobel Laureate
having received the award at the
age of 25.
History of discoveries in the field of X-rays
•
Charles Glover Barkla
Nobel Prize in Physics in
1917
For his discovery of the
characteristic X-rays of elements.
History of discoveries in the field of X-rays
•
Karl Manne Georg Siegbahn
Nobel Prize in Physics in 1924
For his discoveries and research
in the field of X-ray spectroscopy
History of discoveries in the field of X-rays
•
Arthur Holly Compton
Nobel Prize in Physics in 1927
For his discovery of the
Compton effect
History of discoveries in the field of X-rays
•
Henry Gwyn Jeffreys Moseley
Moseley law in X-ray spectra
(1924)
Moseley was shot and killed during
the Battle of Gallipoli , Turkey, on 10
August 1915, at the age of 27. Some
prominent authors have speculated that
Moseley could have been awarded
the Nobel Prize in Physics in 1916, had
he not died in the service of the British
Army.
Characteristic (fluorescent) X-rays
A source X-ray strikes an
inner shell electron and
removes it from the atom.
Higher energy electrons cascade
to fill vacancy, giving off
characteristic X-rays.
ATOMIC STRUCTURE
•
An atom consists of a nucleus (protons and neutrons) and electrons
•
Electrons spin in shells at specific distances from the nucleus
•
Electrons take on discrete (quantized) energy
•
Inner shell electrons are bound more tightly and are harder
to remove from the atom
Adapted from Thermo Scientific Quant’X EDXRF training manual
ELECTRON SHELLS
Shells have specific names (i.e., K, L, M) and
only hold a certain number of electrons
n
principal quantum number
2n2
number of electrons
2n-1 number of sublevels
K shell, n=1, 2 electrons, 1 level
L shell , n=2, 8 electrons, 3 sublevels
M shell, n=3 , 18 electrons, 5 sublevels
N shell , n=4, 32 electrons, 7 sublevels
X-rays typically affect only inner shell (K, L) electrons
Adapted from Thermo Scientific Quant’X EDXRF training manual
MOVING ELECTRONS TO/FROM SHELLS
Binding Energy versus Potential Energy
•
•
The K shell has the highest binding energy and hence it takes more energy to remove an electron from a K shell (i.e., high energy X-ray) compared to an L shell (i.e., lower energy X-ray) The N shell has the highest potential energy and hence an electron falling from the N shell to the K shell would release more energy (i.e., higher energy X-ray) compared to an L shell (i.e., lower energy X-ray)
Adapted from Thermo Scientific Quant’X EDXRF training manual
K, L, M Spectral Lines
Ø
Ø
Ø
Ø
K - alpha lines: L shell etransition to fill a vacancy
in K shell. Most frequent
transition, hence most
intense peak.
K - beta lines: M shell etransitions to fill a
vacancy in K shell.
L - alpha lines: M shell etransition to fill a vacancy
in L shell.
L - beta lines: N shell etransition to fill a vacancy
in L shell.
Moseley law in X-ray spectra
Evaluating Spectra
In addition to elemental peaks,
other peaks appear in the spectra:
•
K & L Spectral Peaks
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Rayleigh Scatter Peaks
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Compton Scatter Peaks
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Escape Peaks
•
Sum Peaks
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Bremsstrahlung
K & L Spectral Peaks
K-Lines
Ka 20.214 keV
Kb 22.721 keV
L-lines
2.694 keV
2.834 keV
Rh X-ray Tube
Rayleigh Scatter
•
•
•
•
X-rays from the X-ray tube or
target strike atom without
promoting fluorescence.
Energy is not lost in collision. (EI =
EO)
They appear as a source peak in
spectra.
AKA - “Elastic” Scatter
EO
EI
Rh X-ray Tube
Compton Scatter
•
•
•
•
Rh X-ray Tube
X-rays from the X-ray tube or
target strike atom without
promoting fluorescence.
Energy is lost in collision. (EI >
EO)
Compton scatter appears as a
source peak in spectra, slightly
less in energy than Rayleigh
Scatter.
AKA - “Inelastic” Scatter
EO
EI
Ka E0=20.214 keV E’=19.445 keV shift= 0.769 keV
Kb E0=22.721 keV E’=21.754 keV shift= Compton effect
SUM PEAKS
Example from analysis of Fe sample
Detector
Fe Kα peak
6.40 keV
Fe
Kα πηοτον
6 .4 0 κες
Fe
Kα πηοτον
6 .4 0 κες
Sum peak
12.80 keV
Sum Peak = Fe + Fe
12.80 = 6.40 + 6.40
Adapted from Thermo Scientific Quant’X EDXRF training manual
•
•
•
Fe sum peak
12.80 keV
Artifact peak due to the arrival of 2 photons at the detector at exactly the same time (i.e., Kα + Kα, Kα + Kβ )
More prominent in XRF spectra that have high concentrations of an element
Can be reduced by keeping count rates low
ESCAPE PEAKS
Example from analysis of Pb sample
700
Detector
600
Si Kα photon
1.74 keV
500
Pb Lα photon
10.55 keV
Escape Peak = Pb – Si
8.81
= 10.55 – 1.74
Adapted from Thermo Scientific Quant’X EDXRF training manual
•
•
Pb escape peak
(from Lβ)
400
Intensity (cps)
Escape peak
8.81 keV
•
Pb Lα line Pb Lβ line 10.55 keV 12.61 keV
300
Pb escape peak
(from Lα)
200
100
0
0
5
10
15
20
25
30
35
40
Energy (keV)
Artifact peak due to the absorption of some of the energy of a photon by Si atoms in the detector (Eobserved = Eincident – ESi where ESi = 1.74 keV)
More prominent in XRF spectra that have high concentrations of an element and for lower Z elements
X-ray absorption
Io
Ix
Properties of absorption coefficient
XRF Instrumentation
•
•
The basic concept for all XRF spectrometers is a
source, a sample, and a detection system.
The source irradiates the sample, and a detector
measures the fluorescence radiation emitted from
the sample.
Two types of instruments:
Wavelength Dispersive XRF spectrometers
Energy Dispersive XRF spectrometers
Wavelength Dispersive XRF
spectrometer
Energy Dispersive XRF spectrometer
Detector for EDXRF
•
Great efforts in the design of detector and signal processing system
•
Very small magnitude of signals
•
Example: Ca (Z=20) Ka =3.691 keV
Energy required to produce 1 electron-hole pair =3.86eV
Number of electron-hole pairs produced=3691/3.86=956
Electron charge= 1.6∙10-19 C
Charge from Ca Ka interaction Q= 956 ∙ 1.6∙10-19 =1.53 ∙ 10-16 C
With capacitance 0.1pF the output voltage Q/0.1pF=1.53mV
Example: Na (Z=11) Ka =1040eV, Mg (Z=12) Ka =1253 eV E=213eV
Energy resolution of 200 eV requires 80 V voltage resolution
Silicon drift detector(SDD)
200µ
Ultralow capacitance detector: C=0.01- 0.1pF / cm2
The concept of the SDD was introduced in 1984 :
E. Gatti, P. Rehak, “Semiconductor drift chamber – an application of a novel
charge transport scheme,” Nucl. Instrum. Meth. 225, pp 608-614 (1984).
Energy Dispersive Electronics
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Fluorescence generates a current in the detector.
In a detector intended for EDXRF, the height of the pulse
produced
•
is proportional to the energy of the respective incoming X-ray.
Multi-Channel Analyser
•
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Detector current pulses are translated into counts
(counts per second, “CPS”).
Pulses are segregated into channels according to energy
via the MCA (Multi-Channel Analyser).
DIFFERENT TYPES OF XRF INSTRUMENTS
Bruker Tracer V
http://www.brukeraxs.com/
Benchtop/Lab model/
Portable/
Handheld/
Innov-X X-50
Thermo/ARL Quant’X
http://www.innovx.com/
http://www.thermo.com/
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EASY TO USE (“point and shoot”)
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COMPLEX SOFTWARE
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Used for SCREENING
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Used in LAB ANALYSIS
•
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Can give ACCURATE RESULTS when used by a knowledgeable operator
Primary focus of these materials
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Designed to give ACCURATE RESULTS (autosampler, optimized excitation, report generation)
ARL QUANT’X Energy Dispersive XRF spectrometer
Rh anode X-ray tube
• Voltage 50kV
Power 50W
• Peltier cooled Si (Li)detector
(6 stages)
Temperature : -90o C, Energy resolution 150ev
• Vacuum pump 10-3 bar • Warm-up time 2 hours
• Sample size:30x40x5 cm max
Element range and chamber atmosphere
• Organic elements (i. e. , H, C, N, O)
do not give XRF peaks
• Air absorbs low energy X-rays from light
elements particularly below Ca, (Z=20).
•
•
For detection of light elements 2 kinds
of chamber atmosphere can be used:
Vacuum - For solids or pressed pellets.
• Helium - For liquids or powdered samples
Qualitative Analysis
Quantitative Analysis
Concentrati
on
•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).
Intensity
• XRF instruments compare the spectral intensities of unknown Analysis software
ARL QUANT’X has two types of software for
quantitative analysis
•
WinTrace
Calibration standards are required.
Composition of standards must be similar to
that of the sample under study.
•
UniQuant - Standardless analysis software
Pre-calibrated at the factory – no standards required.
Uniquant analysis software
•
Automatically collects and processes spectra
with 8 excitation conditions for each sample
•
Each excitation condition (tube voltage and filter)
is optimized for a range of elements of different
atomic numbers Z (low Z, mid Z, high Z)
•
Measurement time: 15 min per sample
Analytes and conditions
Sample specific information
is provided by the user
Example of analysis report
Results of XRF analysis for 14 ceramics
samples from Byurakan
Typical
error
XRF analysis result: composition diagram
(average for 14 samples)
Summary
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•
•
•
XRF is based on detection of X-rays that are
characteristic of the elements in the sample.
These characteristic X-rays (fluorescence) are excited
in the sample by an external radiation source
(X-ray tube or isotope source)
Energy Dispersive XRF systems detect elements
between Sodium (Na, Z=11) and Uranium (U, Z=92).
XRF is a fast, non-destructive, and usually requires
only minimal sample preparation.