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Advanced instrumental techniques
for micro, surface and trace analysis
P. Van Espen
Dept. of ChemistryUniversity of Antwerp, [email protected]
X-ray fluorescence analysis:A general purpose trace analysis method
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Course outline
1. Introduction
2. X-ray physics
3. Instrumentation
4. XRF Configurations
5. Quantitative analysis
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X-ray Fluorescence (XRF)
Atoms in the sample are excited and emit characteristic x-rays
x-ray source
x-ray detector
Energy of characteristic x-rays
How does it work?
1. Introduction
concentration(quantitative analysis)
Number of x-rays for each element
type of elements present(qualitative analysis)
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Fluorescence linesScatter Excitation(Ag K-lines)
Continuum
Fe 3.38%Cu 2950 ppm
Zn 6952 ppmPb 5532 ppm
0 5 10 15 20 25 30 35 40
600
1200
1800
2400
3000
3600
4200
4800
5400
keV
Counts
Pb
SrRb Zr
Fe
Cu
Zn
Mn
Typical XRF spectrum
NIST SRM 2710 Montana SoilCd-109 source, Si(Li) detector
How do the x-ray spectra look?
1. Introduction
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Analytical characteristics of XRF
instrumental analytical technique - multi-element capability elemental analysis of solids and liquids minimal sample treatment concentration range: ppm to % (trace, minor and major elem.) element range: from boron to uranium (in theory)
Why and when to use XRF?
Industrial process control (metallurgy, cement, glass, industry) mining and exploration geology and geochemistry materials research environmental analysis archaeology
14 000 XRF instruments in operation world wide
1. Introduction
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Types of XRF instruments
How can we classify XRF methods?
Based on the excitation
Tube excited XRF Radio-isotope excited XRF Secondary target, Synchrotron,
Total reflection...
Based on the detection
Wavelength dispersive (WD-XRF) Energy dispersive (ED-XRF) Filter instruments (proportional counter)
1. Introduction
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Historical background
1895: Wilhelm Conrad Roentgen
(University of Wrzburg)discovery of X-rays while experimenting withdischarge tubesX = unknown X-radiation or X-rays
1913: CoolidgeDemonstration of the possibility of XRF using a X-ray tube1913: Moseley law
1948: Friedmann and BirksFirst prototype wavelength-dispersive XRF spectrometer
1965: Lawrence Berkeley Laboratories, USADevelopment of Si(Li) detectors, first energy-dispersive systems
1. Introduction
How did we got there we are now?
K Z( )2
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1. Introduction
Summary
What to remember from this?
1. in XRF x-rays are used to excite the sample2. characteristic x-rays are measured in the form of a spectrum
3. the spectrum tells which elements are present and how much
4. element range: B U; concentration range ppm - %
5. used in various fields
6. there are wavelength and energy-dispersive instruments
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X-ray are part of the electromagnetic spectrum betweenultraviolet radiation and gamma-rays.
The Nature of X-rays
When dealing with diffraction we best consider X-rays aselectromagnetic waves with wavelength
When discussing absorption and scattering of X-raysthey are best considered asphotons with a certain energyE.
h Planck's constant (6.6254x10-34 J s)c the velocity light the wave (3.00x108 m/s)
wavelength in metersE Energy in joules.
Relation between energy and wavelength:hc=E
2. X-rays
What are x-rays?
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Wavelength in ngstrom units (1 = 0.1 nm = 10-10 m)
Energy in kilo-electronvolt (keV) 1 J = 6.24x1015 keV
][
4.12[keV]
=E
Be K E= 0.11 keV = 113
Fe K E= 6.40 keV = 1.94
U K E= 98.4 keV = 0.126
X-rays in XRF2. X-rays
What is the range of x-rays used in XRF?
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Bohr approximation of the atom
The electrons in an atom occupy discrete energy levels.
electrons are grouped in shells designated K, L, M, N, O, P(principal quantum numbern = 1, 2, 3, 4, 5, and 6).
A shell can have at maximum 2n2 electrons.
K-shell electrons are more tightly bound than the L-shell electrons
The K-shell has 1 energy level, the L-shell has 3 (L1, L2, L3)and the M-shell has 5
2. X-rays
What is the relation between x-rays and matter?
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The binding energy of inner electrons in the atom is of the same order ofmagnitude as the energy of the X-ray photons.
X-rays can interact with the inner shell electrons.
Sub-shell K L1 L2 L3
Binding energy (keV) 8.981 1.102 0.953 0.933
Energy levels in copper
Compare this with:IR spectrometry: vibration rotation levels in moleculesUV-VIS spectrometry: binding energy levels
2. X-rays
Why x-rays interact with atoms?
Interaction of X-rays with matter
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X-rays interact with atomic electrons in two fundamentally different ways
absorption of the photon
photoelectric absorption is the dominant interaction, causes the
generation of the characteristic X-rays in the sample.
scattering of the photon
responsible for most of the continuum observed in XRF spectra
(part of the exciting radiation is scattered by the sample andenters the detector system)
Types of interaction2. X-rays
How do x-rays interact with matter?
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Photoelectric absorption
a photon is completely absorbed bythe atom, an (inner shell) electron is
ejected.Part of the photon energy is used toovercome the binding energy of theelectron, the rest is transferred to theelectron in the form of kinetic energy
2. X-rays
What happens during photoelectric absorption?
After the interaction, the atom(actually ion) is in a highly excitedstate. A vacancy has been createdin one of the inner shells.
The atom will almost immediatelyreturn to a more stable electronconfiguration emission of an Augerelectron or a characteristic X-rayphoton.
Photoelectric absorption can only occur if EPhoton > Eab !!!
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At high energy the probability of ejecting a electron is rather low
Photoelectric absorption cross section
Cu
2. X-rays
How big is the interaction probability?
At 8.98 keV there is an abrupt decrease in the cross section
X-rays with lower energy can only interact with the L- and M- electrons.
At E slightly greater than 8.98 keV the cross section is higher !!!
absorptionedges
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Elastic and Inelastic Scattering
Scattering causes the photon to change direction.
Elastic or Rayleigh scattering The energy of the photon is the same before and after scattering Forms the basis of X-ray diffraction.
Inelastic or Compton scattering
The photon loses some of its energy A photon having an initial energy E, afterundergoing inelastic scatter will deflectover an angle with an energy E'given
by the Compton equation:
e-K L M
> Compton Scatter
Rayleigh Scatter
'
)cos1(511
1 -E+
E=E
E= 20 keV, = 90 E'= 19.25
2. X-rays
What happens during scattering?
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X-ray attenuation
X-rays pass through matter photons will be lost by photoelectric absorption
by scattering
)exp(0 dII =
is the density and is called the mass attenuation coefficient.The total mass attenuation coefficient is the contribution from photo-electric absorption, coherent and incoherent scattering
IncCohPhoto ++=
Lambert-Beer law
I I0d
2. X-rays
What is the consequence of all this?
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After photoelectric absorption the atom is in a highly excited state.
The vacancy will be filled by an electron from a higher shell.
The energy difference between those two states, (vacancy in the K-shelland vacancy in the L3-shell) can be emitted as an X-ray photon.
These X-rays are called "characteristic" because their energy is differentfor each element, as every element has its own energy levels.
The emission is governed by quantum mechanical selection rules.
n > 0, l= 1, and j= 0 or 1.
Characteristic X-rays emission2. X-rays
How do we get characteristic x-rays?
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2. X-rays
Can an atom emit different x-ray lines?
Characteristic X-rays lines
K - alpha lines: L shell e-transition to fill vacancy in K
shell. Most frequent
transition, hence most intense
peak.
K - beta lines: M shell e-transitions to fill vacancy in K
shell.
L Shell
K Shell L - alpha lines: M shell e-transition to fill vacancy in L
shell.
L - beta lines: N shell e-transition to fill vacancy in L
shell.
K alpha
K beta
M Shell
L alpha
N Shell
L beta
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2. X-rays
What did we learned from this?
Summary
1. x-rays are electromagnetic radiation with an energy in the samerange as the binding energy or inner (K, L-shell) electrons
2. the main interaction is the photo-electric effect
3. the decay of the vacancy created the PE-effect causes the emission
of characteristic x-rays4. there are K, L and M x-ray lines
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XRF instruments
sample holder
x-ray source (excitation of the sample)
spectrometer(measures energy/wavelength and counts x-rays) wavelength-dispersive spectrometers WD-XRF energy-dispersive spectrometers ED-XRF
XRF Instrumentation3. Instrumentation
What are the essential parts of an XRF?
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X-ray tubes
The tungsten filament (the cathode) and anode are in an vacuum glass tube. The anode: very pure metal (Cr, Mo, Rh, Ag, W) The filament is heated by the current from a low voltage power supply
causing emission of electrons from the W-wire. The negative high-voltage (e.g., -30 kV) applied to the filament,
accelerated the electrons to the anode at ground potential The generated X-rays escape from the tube via a beryllium window.
3. Instrumentation
How does an x-ray tube works?
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X-ray production
Due to the interaction of the electrons with the atoms of the anode
1) characteristic X-rays: emission of characteristic X-rays from the anode2) continuous X-rays: deceleration of the electrons in collisions with the target atom"Bremsstrahlung" (German, break-radiation)
spectrum from a Rh-anode X-ray tube operated at 45 kV
The shape depends on: applied voltage anode material Be window thickness
Short wavelength limitaccelerating voltage V=45 kV Emax = 45 keV
ormin = 12.4/V = 0.28
X-ray tube spectra3. Instrumentation
How is the x-ray spectrum of an x-ray tube looking?
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Radioactive sources
Radio-nuclides emitting X-rays or low energy gamma rays
transitions in the nucleus gamma rays:electron capture emission of X-rays
Electron capture decay55Fe (26 p+ and 29 n) captures an (K) orbital electron
p+
+ e-
nresulting in 55Mn (25 p+ and 30 n ) and a vacancy in the K-shell emission of Mn K-L3,2 or Mn K-M3,2 X-ray.
Activity
expressed in becquerels
(1 Bq = 1 disintegration per second = 2.7x10-11
Ci).typically 100 to 300 MBq (~3 to 10 mCi).Half-life, t
After a time equal to t, the intensity of the source is reduced to 50% ofits initial value
Usageportable ED-XRF systems that can be operated in the field.
radio-isotope sources3. Instrumentation
Can we use radio-nuclides as x-ray sources?
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Some sources commonly used in radioisotope excited XRF
13.61,17.22
U-L x-rays60238Pu
13.95,17.7460
Np-L X-raysg-ray
433241Am
22.10,24.9988
Ag-K X-rays-ray
1.27109Cd
5.89, 6.49Mn-K X-rays2.755Fe
Energy,keV
RadiationHalf-life,T (y)
Radioisotope
radio-isotope sources3. Instrumentation
Which sources can we use?
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Wavelength-dispersive spectrometers
Crystal spectrometer based on the principle of Bragg diffraction.
)21(sin2 K,,=nn=d
the x-rays reflected from the second plane (b) travel
a distance xyz = 2dsin further
x-ray waves will out off phase after reflection (intensity 0)
except if the difference = an integer number of wavelengths
(constructive interferences) condition for Bragg diffraction:
3. Instrumentation
Who are x-rays reflected by a crystal?
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Layout of an WD-spectrometer
Crystal and
plane
2d(nm) wave-length
range ()
Typical element
range
LiF (200)
Lithium
fluoride
0.402 3.88 - 0.52 K - Cd (K-lines)
Sn - U (L-lines)
PET (002)Pentaerythritol
0.874 8.44 - 1.14 Al - Cl
3. Instrumentation
How does a WD spectrometer works?
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x-ray detectors for WD spectrometers
Flow proportional counter
Scintillation counter
3. Instrumentation
How are the x-rays counted?
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WD spectrum of a geological material between 0.69 and 0.98
3. Instrumentation
How do WD spectra look?
Measurement of WD spectra
Resolution very good little peak overlapMeasurement sequential takes much time
measure only at peaks and some background points
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3. Instrumentation
What is de principle behind ED spectrometers?
Energy-dispersive spectrometers
The heart of an ED-spectrometer is a semi-conductor crystal (Si, Ge)a high voltage is applied over the crystal (bias -600V)and the crystal is cooled (e.g. at liquid nitrogen temperature)
When x-rays enter the crystal electron-hole pairs are formedthe number is proportional to the energy of the x-ray
because of the bias the electrons are swept out of the crystal
For each photon an electric
pulse is produced with an amplitudeproportional to the energy
Measuring the amplitude and
counting produces the ED-spectrum
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3. Instrumentation
How does a the detector looks like?
Si(Li) crystal
-
+ ++
++
+++
+-- -
---
-
Be-window7.5 m
X-ray photonPreamp
-500 V
-196 C
Liquid Nitrogen cooled
Area: 30 - 100 mm2 Thickness 3 - 5 mmResolution ~150 eV @ Mn Ka
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3. Instrumentation
How is the detector cooled?
Detector and cryostat
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2 4 6 8 10 12 14 16 18
1
10
100
1k
10kMo, scatter
SrRbTi,Ba
ZnAl
SiCa
KFe
Energy (keV)
3. Instrumentation
Energy-dispersive spectrum
ED-spectrum of geological standard JG1
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Si Pin-diode detectorsArea 5 - 10 mm2, Thickness ~0.3 mm
Resolution: ~250 eV @ Mn Ka
3. Instrumentation
Are there other types of semi-conductor detectors?
Si PIN diode detector
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3. Instrumentation
What is best?
ED versus WD spectrometers
Wavelength dispersive spectrometers very good resolution (10-20 eV) -> no peak overlap sequential complex mechanical design
simple electronics
Energy-dispersive spectrometers lower resolution (150 200 eV) -> much peak overlap simultaneous
simple mechanically electronically complex
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3. Instrumentation
What did we learned from this?
Summary
1. An XRF needs a x-ray source, a sample holder and a detector2. Based on the detector we distinguish between wavelength and
energy dispersive spectrometers (WD-XRF and ED-XRF)
3. As x-ray sources we can x-ray tubes or radio-isotopes
4. WD spectrometers rely on Bragg reflection to disperse the x-raysthey have good resolution but are mechanically complex
5. ED spectrometers use a semi-conductor crystal to measure the
energy and count the x-rays. Their resolution is not so good butthey are small and simpler in use.
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sophisticated mechanical design
rotation of the goniometer
selection crystals, detectors, collimators and filter
Sequential (or single channel) WD-XRF instruments
one goniometer.
goniometer moved to the correct 2 angle for each element the intensity is measured for 1 to 100 s entire measurement of a sample ~30 minutes
simultaneous (or multi-channel) instruments
a number of crystal-detector combinations at fixed 2up to 30 channels.
multi-element analysis of a fixed set of elements in a fewseconds
ideally for process control, e.g., in the steel industry
WD-XRF Configurations
4. XRF configurations
What do we need to make a WD-XRF?
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Combined sequential and fixed WD-XRF spectrometer
WD-XRF spectrometer
4. XRF configurations
How are the parts put together?
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WD-XRF instruments
4. XRF configurations
How do they look in reality?
Philips instrument with automatic sample changer
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ED-XRF configurations
4. XRF configurations
How can we make configurations with the ED detector?
x-ray
tube
collimatorfilter
sample
detector
Direct tube excitation
Sample
Detector
X-ray tube
Secondarytarget
Collimator& filter
Secondary targetexcitation
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ED-XRF configurations
4. XRF configurations
Are there other configurations possible?
Radio-isotopeexcitation
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4. XRF configurations
Can it be even more crazy?
Micro-XRF
synchrotronradiation
x-ray capillaries
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Philips MiniPal
Rh x-ray tubeSi PIN Diode detector
Examples of ED-XRF instruments
4. XRF configurations
How do they look like?
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RadioisotopesEnergy-dispersive detectors Si(Li) Detector Pin diode
Portable ED-XRF instruments
4. XRF configurations
Are there instruments we can use in the field?
4 XRF fi i
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What did we learned from this?
Summary
4. XRF configurations
1. WD instruments are complex, they operate many sequential
2. There are many ED-XRF configurations including portable andlaboratory made
5 XRF l i
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i
iw
I
M
XRF analysis
Intensity in cps of the Fe K-L3,2 linefor an iron concentration of 1% in various matrices
2079221081200Int. Fe K-L3,2PbNiCrAlCMatrix
5. XRF analysis
How do we obtain analytical information?
Wavelength (2 angle) / Energy (keV)=> which element present
(qualitative analysis)
Intensity (number of x-ray per unit of time)=> how much of each element
(quantitative analysis)
Relation depends on the sample composition (matrix)
Ii,EFe K
Io,Eo
12
xx + dx
d
X-ray sourceDetector
Sample: 95 % Al,5% Fe
2
1
5 XRF l i
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Analysis of carbon in steel
2373.46
2623.80
2042.87
2743.89
2433.45
2092.93
1582.32
C K X-rayintensity incounts/s
Cconcentrationin wt%
straight line calibration:
Int C K (cps) = (-0.05 10) + (69.9 3.1) x Conc
150
200
250
300
2.00 2.50 3.00 3.50 4.00
Concentration C, Wt%
IntensityCK
a
XRF calibration
5. XRF analysis
Are there simple cases?
5 XRF l i
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XRF fundamental parameter quantization
5. XRF analysis
Is there a general method?
Fundamental relation between x-ray intensity and concentration
relies on many physical constants and parameters
need sophisticated computer programs to solve (iteratively)
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Example fundamental parameter quantization
5. XRF analysis
Can we obtain good results with this method?
Standardless quantitative analysis of NIST 1200 Cr-Co-Ni alloyusing Rh tube-excitation (Spectrace 5000 XRF) WinFund
Concentrations in Wt%
Elem-Line Calculated Certified Diff
Ti-Ka 0.11 0.03 -0.08
Cr-Ka 20.87 19.9 -0.97
Mn-Ka 1.67 1.34 -0.33
Fe-Ka 3.18 3.19 0.01
Co-Ka 42.12 42 -0.12
Ni-Ka 19.91 20 0.09
Nb-Ka 3.34 3.18 -0.16
Mo-Ka 4.55 4 -0.55
Ta-La 1.06 1.08 0.02
W -La 3.17 3.86 0.69
5 XRF anal sis
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Spectrum evaluation in ED-XRF
5. XRF analysis
Are there other problems?
We need net x-ray without background and interference
Computer programs to evaluate the ED-XRF spectra
5 XRF analysis
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What did we learned from this?
Summary
5. XRF analysis
1. There exists matrix effects so that the relation betweenconcentration and intensity is not so simple
2. For a constant matrix we can use simple calibration
3. We can also use the fundamental parameter relation4. We need to analyze the ED spectra because of the peak overlap