INTRODUCTION
TO ENERGY-DISPERSIVE
X-RAY FLUORESCENCE (XRF)
WHAT IS XRF?
• X-ray Fluorescence Spectrometry
• An elemental analysis technique
• An advanced, highly automated, portable analytical
tool that can be used by scientists, lab staff, field
investigators, and even non-experts to support their
job functions
TYPICAL APPLICATIONS OF XRF
XRF is currently used in many different disciplines:
Geology
• Major, precious, trace element analysis
• Characterization of rocks, ores, and soils
Environmental Remediation
• Pb in paint
• Heavy metals in soil
Recycling
• Alloy identification
• Waste processing
Miscellaneous
• Art and archeology
• Industry
• Forensics
THE ELECTROMAGNETIC SPECTRUM
How does light affect molecules and atoms?
X-RAY INTERACTIONS WITH MATTER
When X-rays encounter matter, they can be:
• Absorbed or transmitted through the sample
(Industrial X-Rays – used to see inside materials)
• Diffracted or scattered from an ordered crystal
(X-Ray Diffraction – used to study residual stress)
• Cause the generation of X-rays of different “colors”
(X-Ray Fluorescence – used to determine elemental composition)
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
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)
K, L, M Spectral Lines
Ø
K - alpha lines: L shell e-
transition to fill a vacancy
in K shell. Most frequent
transition, hence most
intense peak.
Ø
K - beta lines: M shell e-
transitions to fill a
vacancy in K shell.
Ø
L - alpha lines: M shell e-
transition to fill a vacancy
in L shell.
Ø
L - beta lines: N shell e-
transition to fill a vacancy
BOX DIAGRAM OF XRF INSTRUMENT
X-ray Digital Pulse XRF Results
Source Detector Spectrum (elements
Processor software
(cps vs keV) and conc’s)
Sample
• X-ray tube source
High energy electrons fired at anode (usually made from Ag or Rh)
Can vary excitation energy from 15-50 kV and current from 10-200 µA
Can use filters to tailor source profile for lower detection limits
• Silicon Drift Detector (SDD) and digital pulse processor
Energy-dispersive multi-channel analyzer – no monochromator needed, Peltier-
cooled solid state detector monitors both the energy and number of photons over
a preset measurement time
The energy of photon in keV is related to the type of element
The emission rate (cps) is related to the concentration of that element
• Analyzer software converts spectral data to direct readout of results
Concentration of an element determined from factory calibration data, sample
thickness as estimated from source backscatter, and other parameters
DIFFERENT TYPES OF XRF INSTRUMENTS
Handheld/ Portable/ Benchtop/Lab model/
• EASY TO USE (“point and shoot”) • COMPLEX SOFTWARE
• Used for SCREENING • Used in LAB ANALYSIS
• Can give ACCURATE RESULTS when used • Designed to give
by a knowledgeable operator ACCURATE RESULTS
(autosampler, optimized
excitation, report generation)
XRF SPECTRA
Consecutive elements in periodic table
15
Zn
Ga
Ge
As
Se
10
Intensity (cps)
0
5 6 7 8 9 10 11 12 13 14 15
Energy (keV)
• Plotting only a portion of the XRF spectra of several different elements
• Note periodicity - energy is proportional to Z2 (Moseley’s law)
XRF ENERGIES FOR VARIOUS ELEMENTS
Generalizations based on use of field portable analyzers
• ORGANIC ELEMENTS (i.e., H, C, N, O) DO NOT GIVE XRF PEAKS
Fluorescence photons from these elements are too low in energy to be
transmitted through air and are not efficiently detected using conventional Si-
based detectors
• LOW Z ELEMENTS (i.e., Cl, Ar, K, Ca) GIVE ONLY K PEAKS
L peaks from these elements are too low in energy (these photons are not
transmitted through air and not detected with conventional Si-based detectors)
• HIGH Z ELEMENTS (i.e., Ba, Hg, Pb, U) GIVE ONLY L LINES
K peaks from these elements are too high in energy (these electrons have high
binding energies and cannot be removed with the limited voltage available in
field portable analyzers)
• MIDDLE Z ELEMENTS (i.e., Rh through I) MAY GIVE BOTH K AND L LINES
L LINE SERIES
~10% Pb in imported Mexican tableware
700
Pb Lα line Pb Lβ line
10.55 keV 12.61 keV
600
500
Intensity (cps)
400
300
200
100 Pb Lγ line
0
0 5 10 15 20 25 30 35 40
Energy (keV)
• K lines not observed (75.0 and 94.9 keV - too high in energy to be excited)
• Lα and Lβ peak energies are often further apart (2.1 keV apart for Pb)
• L lines observed for high Z elements (i.e., Hg, Pb, Th)
• Lα and Lβ peaks have typical ratio of ~ 1 to 1
ARTIFACT PEAKS
Arising from X-ray tube source
• Electrons with high kinetic energy (typically 10-50 kV) strike
atoms in the X-ray tube source target (typically Rh or Ag) and
transfer energy
• The interaction of X-ray source photons with the sample
generates several characteristic features in an XRF spectrum
which may include the following:
Bremsstrahlung
Rayleigh peaks
Compton peaks
BREMSSTRAHLUNG
Continuum/backscatter
100
40 kV
E0 > 90
20 kV
10 kV
80
70 Bremsstrahlung
60
Intensity (cps)
50
40
30
20
10
0
Adapted from Thermo Scientific Quant’X EDXRF training manual 0 5 10 15 20 25 30 35 40
E0 = initial energy of electron in X-ray tube source Energy (keV)
E1 , E2 = energy of X-ray
• Very broad peak due to backscattering of X-rays from sample to
detector that may appear in all XRF spectra
• Maximum energy of this peak limited by kV applied to X-Ray tube,
maximum intensity of this peak is ~ 2/3 of the applied keV
• More prominent in XRF spectra of less dense samples which scatter
more of X-ray source photons back to the detector
RAYLEIGH PEAKS
Elastic scattering from metal alloy sample
50
metal sample
45 Cr, Fe, Ni
peaks from
40
metal sample
35
30
Rayleigh Peaks
Intensity (cps)
25
(Rh Lα and Lβ lines)
20
15
10
5
E0 = initial energy of X-ray from target
element in x-ray tube source 0
0 5 10 15 20 25 30 35 40
E1 = energy of X-ray elastically scattered Energy (keV)
from (typically dense) sample
• Peaks arising from target anode in X-ray tube source (Rh in this case) that
may appear in all XRF spectra acquired on that instrument
• No energy is lost in this process so peaks show up at characteristic X-ray
energies (Rh Lα and Lβ at 20.22 and 22.72 keV in this case)
• Typically observed in spectra of dense samples as weak peaks (due to
increased absorption of X-ray source photons by sample)
COMPTON PEAKS
Inelastic scattering
100
cellulose sample
90
Compton Peaks
(E’s < Rh Lα and Lβ lines )
80
70
PHOTO
ELECTRON 60
Intensity (cps)
50
Rayleigh Peaks
40 (Rh Lα and Lβ lines)
30
20
E0 = initial energy of X-ray from target 10
element in x-ray tube source 0
0 5 10 15 20 25 30 35 40
E1 = energy of X-ray inelastically scattered
Energy (keV)
from (typically non-dense) sample
• Peaks arising from target element in X ray tube (again, Rh in this case)
that may appear in all XRF spectra acquired on that instrument
• Some energy is lost in this process so peaks show up at energies slightly
less than characteristic X-ray tube target energies
• Typically observed in spectra of low density samples as fairly intense
peaks (note these peaks are wider than Rayleigh peaks)
Recommended Practice-API RP-578-Material Verification Program-MVP/PMI
New Construction
Existing Piping Systems
Existing Piping Systems
X-RAY FLUORESCENCE
Specifiche tecniche:
-generatore di HT da 50 kV e corrente sino a 200 mA, tubo raggi-X con anodo in Rh
(Rodio)
-rivelatore S.D.D. ( Silicon Drift Detector ) avente risoluzione in energia migliore di 175
eV per la Mn Ka, velocità di acquisizione sino a 250.000 cps.
- Videocamera Integrata
- Docking Station
-GPS integrato
- Applicazioni e calibrazioni per leghe
metalliche, matrici non metalliche (terreni,
rifiuti, vetri, plastiche, liquidi), leghe di
metalli preziosi, saldature.
Energy Dispersive System (EDS)
PRINCIPLE
• EDS is an analytical technique used for the elemental analysis of a
sample.
• The secondary x‐rays (X‐ray florescence) are directed to a detector. A
detector is used to convert X‐ray energy into voltage signals; this
information is sent to a pulse processor, which measures the energy of
the signals and passes them onto an analyzer. The analyzer converts
the analog into a digital signal which is proportional to the energy of
the incoming pulse. Received pulses are actually amplified, converted
into digital signals and then sorted by energy with help of multi‐
channel analyzer (energy is characteristic for each element) and
frequency of appearance (characteristic for concentration) and sent to
data display and analysis. The most common detector now is Si(Li)
detector cooled to cryogenic temperatures with liquid nitrogen;
however newer systems are often equipped with silicon drift detectors
(SDD) with Peltier cooling systems.
EDS spectrometer
ED‐XRF
SEM‐EDX
Si(Li) detector with LN cooling
Si(Li) detector with Peltier cooling
Detection limit for EDXRF
• Analysis of elements from Sodium (Na) to
Uranium (U).
• Concentration range from 100 % down to the
ppm‐level (not valid for light elements). Limits
of detection depend upon the specific element
and the sample matrix, but as a general rule,
heavier elements will have better detection
limits.
SEM-EDS vs ED-XRF
SEM-EDS ED-XRF
Corpuscular (electron) induced X-rays Wave (X-rays or γ-rays) induced X-rays
NECESSARY for nonconductive samples surface No coating necessary
coating with electro conductive thin film (carbon,
gold…)
Spot, line or surface analysis (scanning system) Mainly surface analysis (spot in case of μ-XRF)
Can work only with vacuum Vacuum not necessary but it can work with vacuum
Small excitation energy 0,06 W !! (up to 2 μA beam High excitation energies up to 4 kW (up to 120 mA
current, up to 30 kV excitation potential) beam current, up to 80 kV excitation potential)
Fair energy resolution Fair energy resolution
Analysis of surface layers (shallow penetration up to Analysis of dipper layers – real bulk analysis
2 μm) (penetration up to 3mm)
Short time for quali- and quanti-tative analysis Short time for quali- and quanti-tative analysis
Good detection limit. Less optimal for light elements. Good detection limit in vacuum. Less optimal for
light elements.
Analysis of elements from Sodium (Na) to Uranium Analysis of elements from Sodium (Na) to Uranium
(U) (U)
Concentration range from 100 % down to the ppm- Concentration range from 100 % down to the ppm-
level (not valid for light elements). level (not valid for light elements).
Concentration range from 100 % down to the 10 or
100 ppm-level with no vacuum.
Analysis of solid samples Analysis of solids and liquids
Manual to semiautomatic procedures Possible full automatic procedures
HANDHELD instruments available
Definizione delle tensioni residue
Si definiscono “tensioni residue” o “tensioni interne” quelle tensioni
che esistono in un corpo o in parte di esso in assenza di carichi
esterni e che sono autobilanciate.
Le tensioni residue, a seconda della porzione di materiale che
interessano, si distinguono in:
Tipo I macrostress, sono costanti in grandezza e direzione su
un’ampia area, dell’ordine di parecchi grani
Tipo II microstress, sono costanti su un’area dell’ordine di un grano
Tipo III microstress, esistono all’interno di un grano e non sono costanti
in esso; sono essenzialmente dovuti alla presenza di
dislocazioni o altri difetti cristallini
Genesi delle tensioni residue
Genesi delle tensioni residue
In generale, le tensioni residue sono di origine:
meccanica:
fresatura, tornitura, pallinatura, rettifica, lavorazioni plastiche a
freddo (estrusione, forgiatura)
termica:
sono dovute a processi di riscaldamento e raffreddamento non
uniformi (tempra, saldatura…)
chimica:
sono generate da variazioni volumetriche associate a reazioni
chimiche, precipitazione o trasformazione di fase; deposizione di
coatings
Tensioni residue da saldatura
Tensioni residue da saldatura
Evoluzione delle tensioni residue in funzione della variazione di
temperatura ∆T all’avanzare del processo.
• Elevate tensioni di trazione
nella Zona Affetta
Termicamente (HAZ)
possono provocare la
rottura prematura per
frattura fragile, stress
corrosion, fatica.
• Le distorsioni e le tensioni
di compressione possono
ridurre la resistenza a
buckling.
Tensioni residue da saldatura
Distorsioni da saldatura:
1. Ritiro trasversale perpendicolare
al cordone (a),
2. Distorsione angolare: rotazione
attorno alla linea di saldatura
dovuta alla distribuzione non
uniforme della temperatura nello
spessore (b,c).
3. Ritiro longitudinale parallelo al
cordone (d),
1. Il piatto base si espande durante la saldatura; quando il cordone solidifica il
metallo base deve ritirarsi e questo ritiro avviene principalmente a causa della
contrazione trasversale al cordone nel piatto base, mentre il ritiro del cordone è solo il
10%.
2. La contrazione longitudinale in giunti di testa è circa 1/1000 della lunghezza del
cordone e molto minore di quella trasve8rsale.
Tensioni residue da pallinatura
L'introduzione delle tensioni residue
nel pezzo avviene secondo un
meccanismo che è l'unione di due
fenomeni:
1. allungamento degli strati
superficiali causati dall' impronta
dei pallini;
2. plasticizzazione degli strati
sub-superficiali per effetto della
pressione hertziana.
La proporzione volutamente
marcata dell'uno o dell'altro
fenomeno dà origine ad una
distribuzione di tensioni residue
di compressione
Metodi di misura delle tensioni residue
I metodi di analisi sperimentale si differenziano in base al grado di
danneggiamento che l’applicazione del metodo provoca nei componenti.
Metodi diffrattometrici a raggi X (XRD)
Condizioni di diffrazione:
1. la diffrazione si ottiene quando un reticolo cristallino viene investito da
raggi incidenti con un certo angolo θ
2. raggio incidente, raggio diffratto e normale devono giacere sullo stesso piano
e θi = θd
3. devono risultare in fase anche tutte le onde diffuse dai successivi piani
reticolari
Metodi diffrattometrici a raggi X (XRD)
Se si verificano le condizioni di diffrazione, i raggi diffratti che investono
due piani cristallini posti a una distanza d rimangono in fase se la differenza
di cammino ottico è un multiplo intero della lunghezza d’onda λ:
A’YB’-AXB=GYH = GY+YH = 2dsenθ
nλ=2dsen θ legge di Bragg
Metodi diffrattometrici a raggi X (XRD)
Un corpo metallico contiene diversi grani con piani cristallini
diversamente orientati.
Si può definire per il generico grano un sistema di riferimento locale
concorde all’orientazione del piano cristallino.
Consideriamo un grano generico il cui piano cristallino è inclinato di φ
rispetto alla direzione di σ1.
Metodi diffrattometrici a raggi X (XRD)
Metodi diffrattometrici a raggi X (XRD)
Metodi diffrattometrici a raggi X (XRD)
Metodi diffrattometrici a raggi X (XRD)
Metodi diffrattometrici a raggi X (XRD)
Metodi diffrattometrici a raggi X (XRD)
Metodi diffrattometrici a raggi X (XRD)
Metodi diffrattometrici a raggi X (XRD)
L’analisi diffrattometrica a vari angoli ψ di misura, cioè l’angolo tra la normale
al provino e la direzione di misura, produce uno spettro del segnale di uscita
con una serie di picchi.
Le grandezze fondamentali che caratterizzano la figura di diffrazione sono:
a) intensità dei picchi principali;
b) posizione del picco;
c) larghezza a metà altezza del picco (FWHM).
Riguardo al primo parametro, non è l'intensità massima assoluta la
grandezza importante in tali misure, ma il rapporto picco-fondo, cioè delle
altezze rispetto allo zero, che è funzione del materiale e deve essere pari
ad almeno 3 o 4 per avere una buona misura.
Metodi diffrattometrici a raggi X (XRD)
Esempio di spettro di misura di diffrazione Raggi x (43°<2θ<46°), ad angolo ψ fissato.
Metodi diffrattometrici a raggi X (XRD)
La posizione del picco prodotto dal
provino contenente le TR è shiftata rispetto al
materiale scarico e viene determinata in
maniera molto precisa (nell’ordine del
centesimo di grado) elaborando i punti della
figura di diffrazione. Lo Shift è il parametro
fondamentale nella misura delle TR.
L’FWHM, parametro indice della definizione del
picco, deve essere basso, infatti il picco
risulterebbe molto stretto e risulta più
semplice individuare precisamente la
posizione del massimo. Anche l’FWHM è
influenzato dalle TR presenti nel pezzo;
tuttavia mentre la posizione del picco è
legata ai macrostress (TR), la larghezza
dipende dalle distorsioni del reticolo,
ovvero dai microstress.
