Topic 6 X-Ray Spectrometry

7/29/2019 Topic 6 X-Ray Spectrometry

1/45

SKA6014

ADVANCED ANALYTICAL CHEMISTRY

TOPIC 6X-ray Spectrometry

Azlan Kamari, PhD

Department of ChemistryFaculty of Science and Mathematics

Universiti Pendidikan Sultan Idris


2/45

Outline X-ray absorption/fluorescence processes

Auger electron emission

Photoelectron emission

Excitation of X-rays

X-ray fluorescence, X-ray emission

X-ray Detection and Spectrometer Design Energy-dispersive (ED) spectrometers

Wavelength-dispersive (WD) spectrometers

Methods and Applications

Topics mentioned here but discussed in detail during the SurfaceAnalysis and Microscopy Lecture:

Scanning electron microscopy an X-ray emission microprobe

Auger electron spectrometry (electron energy)

X-ray photoelectron spectrometry (again, electron energy)


3/45

The Electromagnetic Spectrum

X-rays

(Also gamma rays)


4/45

X-rays

What are X-rays? High energy photons.

Note: gamma rays are just high-energy X-rays

Advantages of X-ray spectrometric methods:

The X-ray spectrum is notvery sensitive to molecular effects or

chemical state, or excitation conditions

This is because core electrons are usually involved in X-ray

transitions physical and chemical state have only minuteeffects (I.e. gas vs solid, oxide vs. element).

Atomization is not necessary for elemental analysis

Precision and accuracy are good, spectra are simple

Surface-sensitive (penetration of 100 um at most) Disadvantages of X-ray methods:

Surface-sensitive, if you want bulk analysis (often not a problem)

Modest limits of detection, compared to other elemental methods

(e.g. AA, ICP-OES, ICP-MS)


5/45

X-ray Production X-ray are commonly

produced by bombarding a

target with electrons

The target emits a

spectrum with two

components:

Characteristic radiation

Continuous radiation(also called white

radiation,

Bremsstrahlung

(braking radiation)

The Duane-Hunt limitexplains the cutoff of the

continuous radiation:

max

min

0

c

hh

eV (where V0 is the electron accelerating voltage)


6/45

X-ray Generation: Characteristic Radiation

The characteristics lines in X-ray spectra

result from electronic transitions between

inner atomic orbitals.

The X-ray spectra for most heavy

elements are much simpler than the

UV/Vis spectra observed in ICP-OES, for

example. (Only a few lines!!!)

Big difference between X-ray and UV-Vis:

The radiation is ionizing, and doesnt just

excite electrons to higher levels.

Moseleys law: Predicts the basic

relationship of atom number and thefrequency of the characteristic lines.

ZKwhere Z is the atomic number, and K and are

constants that vary with the spectral series.


7/45

X-ray Processes: when an X-ray strikes an atom


8/45


X-ray transitions:

(Here denoted using

the Siegbahnnotation)

Remember the

quantum numbers:

n principal quantum

number

l angular

momentum quantum

number

s spin quantum

number (1 and 2have s = -1/2 and s =

+1/2)

jinner quantum

number, from

coupling ofland s


9/45


X-ray transitions,for gold (Z=79),

using both optical

and X-ray

(Siegbahn)

notation.


10/45

X-ray Generation: Nomenclature

Example notations for Copper (K series) in different notations

Transition Siegbahn IUPAC

2p3/2 1s K1 KL3

2p1/2 1s K2 KL2

3p3/2 1s K1 KM3

3p1/2 1s K3 KM2


11/45



12/45

X-ray Generation: X-ray Tubes X-ray tubes: fire electrons at targets that are selected for their x-

ray emission properties as well as their robustness, heat

conductivity, etc

(Note modern tubes are more efficient, no water cooling needed)


13/45

X-ray Generation: The Future

Goals

Short pulsed sources (femtoseconds)

Brilliant sources

Coherent

Small beam sizes

One way of getting there capillary optics (polycapillary

lenses)

Achieve a higher spectral efficiency and small spot size for

a given X-ray beam Best as of 2004 19 keV focussed onto a 20-30 um spot


14/45

Design of X-ray Instrumentation

Two major types:

Wavelength dispersive spectrometers

Analogous to dispersive spectrometers encountered in

IR and UV-Vis spectroscopy

Radiation

SourceSample

Wavelength

SelectorDetector

Energy dispersive spectrometers

No real analogy in dispersive spectrometry

Detects portions of a spectrum directly through its energy

Radiation

SourceSample Detector


15/45

Design of X-ray Instrumentation

Most substances have refractive indices of unity (1) at X-

ray frequencies.

The reason X-radiation is so high-frequency that there is

no time for the electronic polarization needed to cause a

refractive index.

Therefore, mirrors and lenses for X-rays cannot be made(in general), and other ways to control X-rays must be

found

X-rays can be diffracted by crystals.

Compare this to the rulings and gratings used in optical

spectroscopy the wavelength of X-rays is so short, that

only molecular diffraction gratings (crystals) can be used.


16/45

Energy-Dispersive Analyzers

Energy-dispersive (ED) analyzers are heavily used in:

X-ray fluorescence (XRF), especially portable or small-footprint

Electron microprobe (SEM)

The spectrometer is just a Si(Li) detector.

Si(Li) detectors are made of silicon doped with Li, usually cooledusing LN2 or a refrigeration system

Usually called lithium-drifted silicon, also drifted germanium.

The detector is polarized with a high voltage

When x-ray photons hit the detector, electron-hole pairsare created that drift through the potential, creating a

pulse thats magnitude is directly proportional to the x-

ray energy


17/45

Energy-Dispersive Analyzers

The Si(Li) detector:


18/45

Energy-Dispersive Analyzers: Typical Spectra

An ED X-ray spectrum from a Si(Li) detector, for

qualitative analysis:


19/45

Wavelength-Dispersive Analyzers

General layout of a WD X-ray monochromator and

detector:

Sample

(source of X-rays)

Wavelength-dispersing

crystal

Detector

(pulse height

detector)

Total = 2

sin2dn

d

n

2sin

Reflection occurs when:


20/45

Wavelength-Dispersive Analyzers

The Rowland design:


21/45

Wavelength-Dispersive Analyzers: Typical Spectra

WD offers much higher energy

resolution than ED, better sensitivity,

and better reproducibility (precision) forquantitative analyses

Figures from McSwiggen and Associates, www.mcswiggen.com


22/45

Comparison of WD and ED X-ray Detectors

Most important advantages of WD: Higher resolution, sensitivity

Most important advantages of ED: Cheaper, faster (except for

multichannel WD) Other differences (more detailed comparison):

The future CdTe and CdZnTe materials as ED detectors

Energy-Dispersive Wavelength-Dispersive

Fast qualitative analysis Slow qualitative analysis

Non-focusing spectrometer Focusing spectrometerAnalyzes all elements at once Analyzes one/few element(s) at a time

Low count rates (~2000 counts/sec) High count rates (~50000 counts/sec)

Poor resolution (140-150 eV/channel) Good resolution (5 eV/channel)

Limited detection limits (1% w/w) Good detection limits (0.01% w/w)

Adequate quantitative analysis Excellent quantitative analysis (0.03%)Poor light element detection (typically down

to boron with windowless designs)

Excellent light element detection, including

quantitative analysis down to beryllium

Higher background (lowers S/N) Lower background (increases S/N)

Less expensive (simpler) More expensive (complex)


23/45

X-ray Fluorescence (XRF) Spectrometry

Review of the principles:

if an X-ray photon (the primary X-ray) is absorbed by

an atom, and it has enough energy, it can eject an

electron, leaving a vacancy

A higher energy electron will drop down to replace it,

emitting a secondary X-ray

The energy of the secondary X-ray (if it can be

detected) is the difference of the binding energy of

the two shells!!!

XRF is a similar process to the photoelectric effect where an x-ray is absorbed and transfers all of its

energy to an electron

Both ED and WD spectrometers are widely available


24/45

X-ray Fluorescence


25/45

X-ray Fluorescence (XRF)

The XRF yield is

actually influencedby the degree of

Auger electron

formation

Auger electrons

predominate atlower Z

XRF can be produced by:

X-rays Alpha particles (APXS)

Protons (PIXE)

Electron beams (SEM electron

microprobe)

createdvacanciesshellKofnumber

producedphotonsKofnumberK

KAuger 1


26/45

XRF: Typical Spectra

An ED XRF spectrum of a calibration standard:


27/45

Advantages and Disadvantages of XRF

Advantages:

Can be applied in-situ and

nondestructively to analytes withlittle or no sample preparation

Speed very fast

Good accuracy and precision

Disadvantages:

Not as sensitive as UV/Vis

methods for elemental analysis

(only gets down to ppm level in

some cases)

Auger process reduces sensitivity

for lighter elements (Z < 23)

Windows and other spectrometer

components can limit elements to

those with atomic numbers

greater than 5-6 (i.e. carbon)

Philips PW2400 WDS


28/45

Applications of XRF to Qualitative and

Quantitative Analysis Matrix Effects

Fluorescent X-rays can be produced by both the analyteand the matrix

Electronic materials measurement of defects (elemental

impurities) in silicon

Machinery analysis of metal composition, effects ofmachining, defects and abnormalities

Ceramics elemental composition and impurities

Biological specimens and foods

Petrochemicalsanalysis of liquids, catalysts, etc Example: Calcium quantitative analysis in calcium carbonate

antacid tablets

Entire tablets can be analyzed in situ


29/45

Hand-Held XRF Technology

Miniaturized XRF technology

applications are growing:

Mining

Geology

Environmental analysis

Alloy analysis

Utilize lightweight x-ray

tubes and Si PiN diode

detector No radioactive isotopes

http://www.spectroscopymag.com/spectroscopy/article/articleDetail.jsp?id=406625

The Innov-X Systems Alpha Series, see http://www.innov-xsys.com
http://www.innov-xsys.com/http://www.innov-xsys.com/http://www.innov-xsys.com/http://www.innov-xsys.com/


30/45

Applications of Hand-Held XRF Technology

Rapid, non-

invasive XRF

analysis of woodwaste found in

Hurricane Katrina

debris for arsenic

Wood contains chromated copper

arsenate (CCA, now banned),

which was used to pressure-treat

lumber Detection limit for As in low-density

samples is 10-100 ppm

Using K and K lines at 10.54 and

11.73 keV

S i El t Mi d X


31/45

Scanning Electron Microscopy and X-ray

Microanalysis

A scanning electron

microscope is a popular

excitation source for X-ray

emission

Electrons (5 keV 30 keV) hit

a sample. They penetrate about 1 um

They knock loose K and L shell

electrons

X-rays are emitted as higherenergy electrons drop down

to fill the hole


32/45

Electron-Induced X-ray Emission

X E i i i El t Mi


33/45

X-ray Emission in Electron Microscopy

X-ray Emission is just one of a

multitude of processes that can

occur when electrons hit atarget

In an SEM/TEM/STEM, the

following are possible:

X-ray emission spectrometry

with mapping

Formation of images from

backscattered electrons Diffractometric analysis

X E i i PIXE


34/45

X-ray Emission: PIXE PIXE: particle (proton) induced x-

ray emission

Diagram is from the PIXE system atHarvard: requires a particle

accelerator (5-10 meters long)

PIXE is heavily used in art

conservation and archaeology

Diagram of PIXE Instrument from www.mrsec.harvard.edu (2006)

X E i i PIXE
http://www.mrsec.harvard.edu/http://www.mrsec.harvard.edu/


35/45

X-ray Emission: PIXE

PIXE: Just like electron-

induced x-ray emission, only

more efficient Less damaging to the sample but

more sensitive

Less charging than electrons

Less lateral deflection (protons

are not multiply scattered like e-)

PIXE images from www.ipp.phys.ethz.ch and www.tiara.taka.jaeri.go.jp (2006)

X E i i APXS
http://www.ipp.phys.ethz.ch/http://www.tiara.taka.jaeri.go.jp/http://www.tiara.taka.jaeri.go.jp/http://www.ipp.phys.ethz.ch/


36/45

X-ray Emission: APXS

APXS: alpha particle x-ray

spectrometry

Alpha particles better for excitinglight elements:

Na, Mg, Al, Si

X-rays better in exciting heavier

elements Fe, Co, Ni

Relative effectiveness crosses

over at chromium

APXS a compact EDspectrometer for light-medium

elements with a radioactive

curium-244 source

Images from www.nasa.gov (2006)
http://www.nasa.gov/http://www.nasa.gov/


37/45

X-ray Emission: APXS

APXS spectra from Mars: easy detection from sodium to iron

Images from www.nasa.gov (2006)
http://www.nasa.gov/http://www.nasa.gov/


38/45

X-ray Absorption

X-ray absorption is used for

totally different applications

that X-ray fluorescence andemission.

Beer-Lambert law:

xP

P

0

ln

x

P0 Px

P

PM

0ln

where is the linear absorption coefficient

(depends on the element and no of atoms):

where M is the mass absorption coefficient, which is

independent of the elements state and is the density

3

4

AE

Z

(E is the energy of the x-rays, A is the atomic mass

and Z is the atomic number). Also:


39/45

X-ray Absorption

Why do X-ray and atomic/molecular UV-Vis absorption

spectra look so different, with all that the two techniques

have in common?

Atomic absorption/UV-Vis spectra have peaks

X-ray absorption spectra have edges

Answer: the ionization!

Optical AA has a peak with a narrow bandwidth because an outer

shell electron is excited to a higher energy level a discrete

quantum process

X-ray absorption is caused by photoelectron ionization not as

discrete of a process since energy in excess of that required for

ionization appears as kinetic energy of the photoelectron.


40/45

X-ray Absorption Fine Structure (XAFS)

X-ray absorption fine structure (XAFS) refers to the details of how x-

rays are absorbed by an atom at energies near and above the core-level binding energies of that atom.

Specifically, XAFS is the modulation of an atoms x-ray absorption

probability due to the chemical and physical state of the atom.

XAFS spectra are sensitive to the oxidation state, coordinationchemistry, and the distances, coordination number and species of the

atoms immediately surrounding the atom of interest.

XAFS needs an intense, energy-tunable source of X-rays (a

synchrotron).


41/45

X-ray Absorption Fine Structure (XAFS)

Two regions of the XAFS

spectrum:

EXAFS (extended x-ray

absorption fine

structure): Sensitive to

distances, coordination

number, and identity of

surrounding atoms

XANES (X-ray

absorption near edge

spectroscopy):

Sensitive to oxidationstate and coordination

(e.g. tetrahedral vs.

octahedral coordination

of an atom).

EXAFS


42/45

EXAFS

EXAFS


43/45

EXAFS

XANES


44/45

XANES

XANES often empirically interpreted

X Ph t l t S t d R l t d


45/45

X-ray Photoelectron Spectroscopy and Related

Techniques

Scanning Auger, XPS,UPS, ECSA, and

more

All are surface analysismethods and will be

discussed during the

Microscopy and

Surface Analysislecture.

Diagram from Charles Evans and Associates website (http://www.cea.com)

http://www.cea.com/cai/augtheo/caiatheo.htm
http://www.cea.com/http://www.cea.com/cai/augtheo/caiatheo.htmhttp://www.cea.com/cai/augtheo/caiatheo.htmhttp://www.cea.com/

Date post:	14-Apr-2018
Category:	Documents
Upload:	nurshuhada-nordin
View:	222 times
Download:	0 times

Topic 6 X-Ray Spectrometry

Documents