Introduction to X-ray fluorescence (XRF) analysis

Outline •

X-rays, what are they? What is XRF?

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History of discoveries in the field of X-rays

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Basics of XRF theory

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Evaluation of XRF spectra

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XRF instrumentation

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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

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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

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An atom consists of a nucleus (protons and neutrons) and electrons

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Electrons spin in shells at specific distances from the nucleus

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Electrons take on discrete (quantized) energy

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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

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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

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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 •

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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 •

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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

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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

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Pb escape peak (from Lβ)

400

Intensity (cps)

Escape peak 8.81 keV

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Pb Lα  line Pb Lβ  line  10.55 keV 12.61 keV

300

Pb escape peak (from Lα)

200 100 0 0

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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 •

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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

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Very small magnitude of signals

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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 •

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).

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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.

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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

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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)

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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.