12.1
Ch.12 Atomic X-ray Spectrometry
• X-ray measurement on absorption, fluorescence
for elements larger than Na
12A. X-ray
(10-5~100
o
A)
o
X-ray spectrometry: 0.1~25 A
based on radiation by electron deceleration
or by electron transition from inner orbital.
by Prof. Myeong Hee Moon
12.2
12A-1. Emission of X-rays
• X-ray sources in analytical purposes
1) Bombardment of a metal target with high E. electron beam
2) Exposure to primary X-ray beam to get
secondary beam of X-ray fluorescence
3) Radioactive source – X-ray emission
• types of X-ray
1) Continuum (white radiation)
2) Line spectra
by Prof. Myeong Hee Moon
12.3
12A-1. Emission of X-rays
☞ Continuum spectra from electron beam
e- 100kV
metal anode
heated cathode
X-rays
continuum
W target
Line spectrum
Mo target
by Prof. Myeong Hee Moon
12.4
12A-1. Emission of X-rays
• Short wavelength limit (l0)
: depends on voltage applied
but not on target materials
l0(Mo) 35kV = l0(W) 35kV
• Photon energy of X-ray
hc
 Ve
0
Duane-Hunt law
Ve: (voltage x e- charge) – kinetic energy
h: planck constant
0 
by Prof. Myeong Hee Moon
12,398
V
(in angstroms)
12.5
12A-1. Emission of X-rays
☞ Line spectra from electron beam sources
• Features
1) Most elements give two typical series (K, L)
o
Mo --- 0.63, 0.67
A
K
L series
(exception: elements <23 show only K series
since no filled d orbital)
Why?
L series originate from transition of higher level
M, or N shell (3rd or 4th periods) to L shell
by Prof. Myeong Hee Moon
12.6
12A-1. Emission of X-rays
by Prof. Myeong Hee Moon
12.7
12A-1. Emission of X-rays
2) Min. Acceleration Voltage increases with atomic number
o
i.e.) W at 50kV: no lines in 0.1~1.0 A
70kV:
0.18, 0.21
i.e.) W at 50kV: no lines in 0.1~1.0
70kV:
0.18, 0.21
X-ray spectra originates from
inner most atomic orbital
thus, nothing to do with
sample states
(pure metal, oxide, sulfide)
by Prof. Myeong Hee Moon
12.8
12A-1. Emission of X-rays
4d
3d
2S
by Prof. Myeong Hee Moon
12.9
12A-1. Emission of X-rays
• Line spectra from fluorescence source
get less energy  (fluorescence) by radiating X-ray
later section
• From radioactive sources
X-radiation: product of radioactive decay (from intranuclear rxn)
similar to X-rays
emission
excite nucleus
nucleus releases one or more quanta of -rays
as it returns to ground states
by Prof. Myeong Hee Moon
12.10
12A-1. Emission of X-rays
electron capture or K-capture
nucleus capture K-electrons – produces X-radiation
then formation of the next lower element
If isotope formed has too few neutrons, can get
positron decay or K– electron capture.
11
6C 
11
B+
5
0
e
1
or
11
C+
6
11
0
e  B
5
–1
Results: e- transition to empty K shell
X-ray spectrum from newly formed element
by Prof. Myeong Hee Moon
12.11
12A-1. Emission of X-rays
Results: e- transition to empty K shell
X-ray spectrum from newly formed element
55Fe
54Mn + hn
(half life 2.6yrs)
o
Ka = 2.1 A: radioisotopic source
by Prof. Myeong Hee Moon
12.12
12A-2 Absorption spectra
Absorption edge: sharp discontinuous
Absorption Edge of Pb, Ag
• absorption process
: absorption causes
ejection of one of the
innermost e- from an atom,
– produces excited ion
Energy distributed
: kinetic energy of e: potential energy of excited ion
by Prof. Myeong Hee Moon
12.13
12A-2 Absorption spectra
• Mass absorption coefficient
Beer’s law
P0: power of incident beam
x: sample thickness
: linear absorption coeff.
: density of sample
M: mass absorption coeff.
independent of phy.chem.state
P
ln 0  x
P
 Mx
Br = HBr, NaBr
by Prof. Myeong Hee Moon
12.14
12A-3. X-ray fluorescence
Excited ion (vaccant K shell)
h
fluorescence
absorption
atom
fl > abs
Cutoff 0 < absorption edge
To get K lines for Ag
V
by Prof. Myeong Hee Moon
12,398
 25,560 V
0.485
12.15
12A-4. Diffraction of X-rays
X-ray diffraction
from X-ray scattering through
a crystal surface
• Bragg’s law
diffraction
AP  PC  n
AP  PC  d sin
n  2d sin
n
sin 
2d
by Prof. Myeong Hee Moon
12.16
12B. Instrument components
12B-1. Sources
X-ray tubes, radioisotopes,
secondary fluorescent sources
• X-ray tubes (Coolidge tube)
Cathode – tungsten filament
Anode – copper block plated
with metals
W, Cr, Cu, Mo, Rh,
Sc, Ag, Fe, Co
by Prof. Myeong Hee Moon
12.17
12B. Instrument components
• radioisotope
by Prof. Myeong Hee Moon
12.18
12B-2. Filters
• Zr filter (0.01 cm thick)
-- pure K line
• filter combinations
by Prof. Myeong Hee Moon
12.19
12B-3. X-ray Monochromators
• a pair of beam collimators
(similar to slits)
• dispersing element
(single crystal <known d>
on a goniometer)
Rotation of single crystal at 
Rotation of detector at 2
Closely spaced metal plates
by Prof. Myeong Hee Moon
12.20
12B-3. X-ray Monochromators
• Crystals
Dispersion:
by Prof. Myeong Hee Moon
d
d
: change of angle w.r.t.
change of wavelength
12.21
12B-4. X-ray transducers & signal processors
classic X-ray eq. ---- photographic emulsions for
convenient, speedy, accurate
Modern transducers – photon counting
gas filled transducers
scintillation counters
semiconductor transducers
• photon counting
Production of
Pulsed charge
(# of counts/time)
from radiation
transducers (fast response)
Digital recording : for weak source of radiation
more accurate than measuring av. currents
by Prof. Myeong Hee Moon
12.22
12B-4. X-ray transducers & signal processors
• gas filled transducers
X-ray
inert gas
Ar, Xe, Kr
positive gas ions &
photoelectrons
Mica
Be, Al, Mylar
by Prof. Myeong Hee Moon
12.23
12B-4. X-ray transducers & signal processors
Ionization chambers
Proportional counters
Geiger tubes
• Scintillation counters
Scintillator : chemicals to
transfer radiation E
to light energy
NaI (with 0.2% TlI3)
Scintillation counter
: scintillator + photomultiplier
by Prof. Myeong Hee Moon
12.24
12C. X-ray Fluorescence (XRF) methods
Rather than putting sample into target of X-ray tube
Use fluorescence --- widely used
excitation by X-ray absorption
– emit characteristic lines
: for semi-quantitative or quantitative elemental analysis
: non destructive
by Prof. Myeong Hee Moon
12.25
12C-1. Instruments
• wavelength dispersive instruments
tubes for compensating large loss due to collimator
single channel, sequential – Fig 12-9
multichannel, simultaneous detection of ~ 24 elements
• energy dispersive instruments
Ad: simple, no moving parts
no collimators, crystal diffraction
- 100 fold increase in E.
- good for weak radioactive source or
lower power X-ray tubes
Disadv: lower resolution (>1A)
by Prof. Myeong Hee Moon
12.26
12C-1. Instruments
Energy dispersive X-ray Spectrometer
a) Excitation from
X-ray tube
by Prof. Myeong Hee Moon
12.27
12C-2. Qualitative & Semi-quantitative Analysis
by Prof. Myeong Hee Moon
12.28
12C-3. Quantitative Analysis
• internal standards
• applications of XRF :
liquid sample: Pb, Br -- in aviation gasoline
Ca, Ba, Zn -- lubricating oils, hydrocarbon
pigments in paints
air sample: (collecting with filters)
elements (> Na) in rocks & soil
: Mars pathfinder mission (Sojourner)
by Prof. Myeong Hee Moon
12.29
Advantages
Disadvantages
•Simple spectra
•Nondestructive
•Good for barely visible speck
of sample
•Speedy, convenience
•Superior accuracy, precision
•Low sensitivity than optical tech.
(< few parts per million)
0.01~100%
•Inconvenient for lighter elements
•High cost of instruments
by Prof. Myeong Hee Moon
12.30
12D. XRA (X-ray absorption)
X-ray absorption method: relatively free of matrix
but time consuming
good for only low matrix effect sample
12D-1. XRD (X-ray diffraction)
Structural elucidation
XRD: atomic space & arrangement in crystal
-- physical properties of metal, polymer, solids
steroids, vitamin, etc.
X-ray powder diffraction :
for compounds present in solid sample
by Prof. Myeong Hee Moon
12.31
12D-2. Identification of Crystalline compounds
• sample preparation
Crystal – ground to fine powder, molded with binder
or filled in thin walled glass or capillary tube
• photographic recording
From  to d (lattice distance)
by Prof. Myeong Hee Moon
12.32
by Prof. Myeong Hee Moon