01_Intro_XRF

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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


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


How are the parts put together?

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WD-XRF instruments


How do they look in reality?

Philips instrument with automatic sample changer

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ED-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


Are there other configurations possible?

Radio-isotopeexcitation

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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


How do they look like?

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RadioisotopesEnergy-dispersive detectors Si(Li) Detector Pin diode

Portable ED-XRF instruments


Are there instruments we can use in the field?

4 XRF fi i

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Summary


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)

00000

max

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),()(1

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rEWEGN

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KiiiFi

K=

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5 XRF l i

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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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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

01_Intro_XRF

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