X-ray Spectrometry docx

1
Lecture Date: February 4
th
, 2008
X-ray Spectrometry
Notes
 See Chapters 12 and 21 (mostly Chapter 12) of Skoog
 This lecture covers both atomic and molecular
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
 Surface analysis and microscopy is also briefly discussed
in advance of its own lecture
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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
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

 X-rays
 (Also gamma rays)
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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 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)
 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)
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 limit
explains the “cutoff” of
the continuous radiation:
max
min
0
c


h
h
eV 
(where V
0
is the electron accelerating voltage)
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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



 ZK
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…
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X-ray Generation: Characteristic Radiation
 X-ray transitions:
(Here denoted using
the Siegbahn
notation)
 Remember the
quantum numbers:
 n – principal quantum
number
 l – angular momentum
quantum number
 s – spin quantum number
(
1
and 
2
have s = -1/2
and s = +1/2)
 j – “inner” quantum

number, from coupling of
l and s
X-ray Generation: Characteristic Radiation
 X-ray transitions,
for gold (Z=79),
using both optical
and X-ray
(Siegbahn)
notation.
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X-ray Generation: Nomenclature
 Example notations for Copper (K series) in different notations
Transition Siegbahn IUPAC
2p
3/2
 1s K
1
KL3
2p
1/2
 1s K
2
KL2
3p
3/2
 1s K
1
KM3
3p
1/2

 1s K
3
KM2
R. Jenkins, et al., Pure Appl. Chem., 63, 736-746 (1991).
X-ray Generation: Characteristic Radiation
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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
 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
I. Szaloki, J. Osan, and R. E. Van Grieken, “X-ray Spectrometry”, Anal. Chem., 76, 3445-3470 (2004).
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Design of X-ray Instrumentation
 Two major types:
– Wavelength dispersive spectrometers
 Analogous to dispersive spectrometers encountered in IR and UV-
Vis spectroscopy
Radiation

Source
Sample
Wavelength
Selector
Detector
– Energy dispersive spectrometers
 No real analogy in dispersive spectrometry
 Detects portions of a spectrum directly through its energy
Radiation
Source
Sample Detector
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.
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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 cooled
using LN
2
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 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:
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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.
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
2
sin



Reflection occurs when:
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Wavelength-Dispersive Analyzers
 The Rowland design:
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
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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
 Both ED and WD spectrometers are widely available
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X-ray Fluorescence

X-ray Fluorescence (XRF)
 The XRF yield is
actually influenced
by the degree of
Auger electron
formation
– Auger electrons
predominate at
lower Z
 XRF can be produced by:
– X-rays
– Alpha particles (APXS)
– Protons (PIXE)
– Electron beams (SEM
electron microprobe)
created

vacancies
shell
K

of
number
produced photonsK ofnumber

K

KAuger

 1

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XRF: Typical Spectra
 An ED XRF spectrum of a calibration standard:
Advantages and Disadvantages of XRF
 Advantages:
– Can be applied in-situ and
nondestructively to analytes with
little 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
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Applications of XRF to Qualitative and
Quantitative Analysis
 Matrix Effects
– Fluorescent X-rays can be produced by both the analyte and the
matrix
 Electronic materials – measurement of defects
(elemental impurities) in silicon

 Machinery – analysis of metal composition, effects of
machining, defects and abnormalities
 Ceramics – elemental composition and impurities
 Biological specimens and foods
 Petrochemicals – analysis of liquids, catalysts, etc…
 Example: Calcium quantitative analysis in calcium
carbonate antacid tablets
– Entire tablets can be analyzed in situ
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
/>The Innov-X Systems “Alpha Series”, see
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Applications of Hand-Held XRF Technology
 Rapid, non-invasive
XRF analysis of
wood waste found
in Hurricane
Katrina debris for
arsenic
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).
 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
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 higher
energy electrons drop down
to fill the “hole”
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.
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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

 In an SEM/TEM/STEM, the
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
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X-ray Emission: PIXE
 PIXE: particle (proton) induced
x-ray emission
 Diagram is from the PIXE system
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
-
)

PIXE images from www.ipp.phys.ethz.ch and www.tiara.taka.jaeri.go.jp (2006)
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X-ray Emission: APXS
 APXS: alpha particle x-ray
spectrometry
 Alpha particles better for
exciting light elements:
– Na, Mg, Al, Si
 X-rays better in exciting
heavier elements
– Fe, Co, Ni
 Relative effectiveness crosses
over at chromium
 APXS – a compact ED
spectrometer for light-medium
elements with a radioactive
curium-244 source
Images from www.nasa.gov (2006)
X-ray Emission: APXS
 APXS spectra from Mars: easy detection from sodium to iron
Images from www.nasa.gov (2006)
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X-ray Absorption
 X-ray absorption is used
for totally different
applications that X-ray
fluorescence and
emission.
 Beer-Lambert law:
x

P
P


0
ln
x
P
0
P
x
P
P
M


0
ln
where  is the linear absorption coefficient
(depends on the element and #of atoms):
where 
M
is 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
 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.
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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.
 XAFS needs an intense, energy-tunable source of X-rays
(a synchrotron).

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 oxidation
state and coordination
(e.g. tetrahedral vs.
octahedral coordination
of an atom).
Diagram from M. Newville, “Fundamentals of XAFS”, University of Chicago, 2003.
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EXAFS
Slide from M. Newville, “Fundamentals of XAFS”, University of Chicago, 2003.
EXAFS
Slide from M. Newville, “Fundamentals of XAFS”, University of Chicago, 2003.
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XANES
 XANES – often empirically interpreted – see new ref Rehr,
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
 Scanning Auger, XPS,
UPS, ECSA, and
more…
 All are surface analysis
methods and will be
discussed during the
“Microscopy and
Surface Analysis”
lecture.
Diagram from Charles Evans and Associates website ()
/>24
Homework Problems
From Chapter 12 of Skoog et al.:
12-2
12-9
Further Reading
I. Szaloki, et al., “X-ray Spectrometry”, Anal. Chem., 2002,
74, 2895-2918.