X-ray Spectrometry docx

Lecture Date: February 4 th , 2008X-ray Spectrometry Notes applications of X-ray spectrometry  X-ray diffraction is only briefly discussed here - it is covered in its own lecture along

Lecture Date: February 4 th , 2008

X-ray Spectrometry

Notes

applications of X-ray spectrometry

 X-ray diffraction is only briefly discussed here - it is

covered in its own lecture along with its applications to

crystallography and solid-state structural analysis

in advance of its own lecture

– Auger electron emission

– Photoelectron emission

 Excitation of X-rays

– X-ray fluorescence, X-ray emission

– Energy-dispersive (ED) spectrometers

– Wavelength-dispersive (WD) spectrometers

Surface Analysis and Microscopy Lecture:

– Scanning electron microscopy – an X-ray emission “microprobe”

– Auger electron spectrometry (electron energy)

– X-ray photoelectron spectrometry (again, electron energy)

The Electromagnetic Spectrum

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

– The X-ray spectrum is not very 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 minute effects

(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)

– 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)

X-ray Production

 X-ray are commonly

produced by bombarding

a target with electrons

 The target emits a

spectrum with two

 The Duane-Hunt limit

explains the “cutoff” of

the continuous radiation:

max min

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

doesn’t just excite electrons to higher

levels.

 Moseley’s law: Predicts the basic

relationship of atom number and the

frequency of the characteristic lines

  K Z where Z is the atomic number, and K and  are constants that vary with the spectral series.

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

X-ray Generation: Characteristic Radiation

X-ray Generation: Nomenclature

 Example notations for Copper (K series) in different notations

R Jenkins, et al., Pure Appl Chem., 63, 736-746 (1991).

X-ray Generation: Characteristic Radiation

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)

X-ray Generation: The Future

– 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

I Szaloki, J Osan, and R E Van Grieken, “X-ray Spectrometry”, Anal Chem., 76, 3445-3470 (2004).

Design of X-ray Instrumentation

– Wavelength dispersive spectrometers

– Energy dispersive spectrometers

 Detects portions of a spectrum directly through its energy

Radiation

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….

(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.

Energy-Dispersive Analyzers

– 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 cooled

using LN2or 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 pairs

are created that drift through the potential, creating a

“pulse” that’s magnitude is directly proportional to the

x-ray energy

Energy-Dispersive Analyzers

 The Si(Li) detector:

Energy-Dispersive Analyzers: Typical Spectra

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

analysis:

J I Goldstein, D E Newbury, P Echlin, D C Joy, A.D Romig, Jr., C E Lyman, C Fiori, and E Lifshin , Scanning

Electron Microscopy and X-Ray Microanalysis,” 2nd Edition, Plenum Press, 1992.

Detector (pulse height detector)

Wavelength-Dispersive Analyzers

Diagram from Strobel and Heineman, Chemical Instrumentation, A

Systematic Approach, Wiley, 1989.

Wavelength-Dispersive Analyzers: Typical Spectra

WD offers much higher energy

resolution than ED, better sensitivity,

and better reproducibility (precision) for

quantitative analyses

Figures from McSwiggen and Associates, www.mcswiggen.com

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 spectrometer

Analyzes 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)

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

K of number

produced photons

K of number



K



K Auger 

  1 

XRF: Typical Spectra

 An ED XRF spectrum of a calibration standard:

Advantages and Disadvantages of XRF

– Can be applied in-situ and

nondestructively to analytes with

little or no sample preparation

– Speed – very fast

– Good accuracy and precision

– 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

Applications of XRF to Qualitative and

Quantitative Analysis

 Matrix Effects

– Fluorescent X-rays can be produced by both the analyte and the

matrix

(elemental impurities) in silicon

 Machinery – analysis of metal composition, effects of

machining, defects and abnormalities

 Petrochemicals – analysis of liquids, catalysts, etc…

carbonate antacid tablets

– Entire tablets can be analyzed in situ

 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

Applications of Hand-Held XRF Technology

B Dubey, H M Solo-Gabriele, and T G Townsend, “Quantities of Arsenic-Treated Wood in Demolition Debris Generated by Hurricane Katrina”,

Environ Sci Technol 41(5) 1533–1536 (2007).

arsenate (CCA, now banned),

which was used to pressure-treat

– They penetrate about 1 um

– They knock loose K and L

shell electrons

energy electrons drop down

to fill the “hole”

Electron-Induced X-ray Emission

X-ray Emission in Electron Microscopy

 X-ray Emission is just one of a

multitude of processes that can

occur when electrons hit a

target

following are possible:

– X-ray emission spectrometry with

mapping

– Formation of images from

backscattered electrons

– Diffractometric analysis

 Will be discussed in the

“Surface Analysis” Lecture

X-ray Emission: PIXE

 PIXE: particle (proton) induced

x-ray emission

at Harvard: 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-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 - )

X-ray Emission: APXS

spectrometry

 Alpha particles better for

exciting light elements:

spectrometer for light-medium

elements with a radioactive

curium-244 source

Images from www.nasa.gov (2006)

X-ray Emission: APXS

Images from www.nasa.gov (2006)

X-ray Absorption

for totally different

applications that X-ray

fluorescence and

emission

x P

where  is the linear absorption coefficient

(depends on the element and #of atoms):

where Mis the mass absorption coefficient, which is

independent of the element’s 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:

X-ray 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.

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 atom’s x-ray

absorption probability due to the chemical and physical

state of the atom

 XAFS spectra are sensitive to the oxidation state,

coordination chemistry, and the distances, coordination

number and species of the atoms immediately

surrounding the atom of interest

(a synchrotron)

X-ray Absorption Fine Structure (XAFS)

Two regions of the

Slide from M Newville, “Fundamentals of XAFS”, University of Chicago, 2003.

EXAFS

J J., Ankudinov, A L., Progress in the theory and

interpretation of XANES Coordination Chemistry Reviews,

Jan 2005

Diagram from M Newville, “Fundamentals of XAFS”, University of Chicago, 2003.

X-ray Photoelectron Spectroscopy and Related

Techniques

UPS, ECSA, and

more…

 All are surface analysis

methods and will be

discussed during the

Homework Problems

From Chapter 12 of Skoog et al.:

12-212-9

Further Reading

I Szaloki, et al., “X-ray Spectrometry”, Anal Chem., 2002,

74, 2895-2918.