Chapter 12 Atomic X-Ray Spectroscopy. X-ray spectroscopy, like optical spectroscopy, is based on...
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Transcript of Chapter 12 Atomic X-Ray Spectroscopy. X-ray spectroscopy, like optical spectroscopy, is based on...
Chapter 12
Atomic X-Ray Spectroscopy
X-ray spectroscopy, like optical spectroscopy,
is based on measurement of emission,
absorption, scattering, fluorescence,
and diffraction of electromagnetic radiation. X-ray
fluorescence and X-ray absorption methods are
widely used for the qualitative and quantitative
determination of all elements in the periodic table
having atomic numbers greater than that of sodium.
With special equipment, elements with atomic numbers in the range of 5 to 10 can also be determined.
12A FUNDAMENTAL PRINCIPLES
X-Rays are short wavelength electromagnetic radiation produced by the deceleration of high-energy electrons or by electronic transitions of electrons in the inner orbitals of atoms.
12A-1 Emission of X-Rays
For analytical purposes, X-rays are obtained in four ways:
1. by bombardment of a metal with a beam of high-energy electrons,
2. by exposure of a substance to a primary beam of X-rays in order to generate a secondary beam of X-ray fluorescence,
3. by use of a radioactive source whose decay process results in X-ray emission,
4. from a synchrotron radiation source.
Continuum Spectra from Electron Beam Sources
0
l0 = 12,398/V
Duane-Hunt Law
•Independent of material
•Related to acceleration voltage E
Ee = E’e + hAt o, E’e = 0
h0 = hc/o = Ve
V: accelerating voltagee: charge on electron
FIGURE 12-1 Distribution of continuum radiation from anX-ray tube with a tungsten target. The numbers above thecurves indicate the accelerating voltages.
FIGURE 12-2 Line spectrum for an X-ray with a molybdenum target.
Atomic number >23
2 line series K and L
E K> EL
Atomic number < 23
K only
A minimum acceleration voltage is required for
From electron transitions involving inner shells
A minimum acceleration voltage required for each element increases with atomic number
0
Line Spectra from Electron Beam Sources
FIGURE 12-4
Partial energy level diagram showing common transitions producing X-rays. Themost intense lines are indicated by the wider arrows.
Line Spectra from Fluorescent Sources
Spectra from Radioactive Sources
X-Radiation is often a product of radioactive decay processes.
TABLE 12-2 Common Radio isotopic Sources for X-ray Spectroscopy
12A-2 Absorption Spectra
When a beam of X-rays is passed through a thin layer of matter, its intensity or power is generally diminished as a consequence of absorption and scattering.
The absorption spectrum of an element, like its emission spectrum, is simple and consists of a few well-defined absorption peaks.
The Absorption Process
The Mass Absorption Coefficient
Ln P0/P = μX
μ is the linear absorption coefficientis characteristic of the Element and # of atoms in the path of the beam.X is sample thickness
Ln P0/P = μMηX η is density of the sample
μM is mass absorption coefficient
FIGURE 12-5 X-ray absorption spectra for lead and silver.
12A-3
X-ray Fluorescence
12A-4
Diffraction of X-rays
d
d
dPCAP
PCAP
2
nsin
sin2n
sin
n
Bragg's Law
12B INSTRUMENT COMPONENTS
12B-1 Sources
The X-ray Tube
Radioisotopes
Secondary Fluorescent Sources
100KV!
Controlling the intensity of X-Ray
Determining the energy of the X-Ray
12B-2
Filters for X-rays
12B-3
X-ray Monochromators
d2
nsin
FIGURE 12-9 An X-ray monochromator and detector.Note that the angle of the detector with respect to thebeam (2ө) is twice that of the crystal face. For absorptionanalysis, the source is an X-ray tube and thesample is located in the beam as shown. For emissionmeasurements, the sample becomes a source of X-rayfluorescence as shown in the insert.
12B-4 X-ray Transducers and Signal Processors
Photon CountingPhoton Counting
Gas-Filled Transducers Gas-Filled Transducers
The Geiger TubeThe Geiger Tube
Proportional CountersProportional Counters
Ionization ChambersIonization Chambers
Scintillation CountersScintillation Counters
Semiconductor Transducers
FIGURE 12-12 Vertical cross section of alithium-drifted silicon detector for X-raysand radiation from radioactive sources.
Distribution of Pulse-HeightsDistribution of Pulse-Heights
from X-ray Transducersfrom X-ray Transducers
12B-5
Signal Processors
Pulse-Height SelectorsPulse-Height Selectors
Pulse-Height AnalyzersPulse-Height Analyzers
Scalers and CountersScalers and Counters
12C X-RAY FLUORESCENCE
METHODS
12C-1 Instruments
Wavelength-Dispersive Wavelength-Dispersive InstrumentsInstruments
Energy-Dispersive InstrumentsEnergy-Dispersive Instruments
FIGURE 12-14 Energy-dispersive X-ray fluorescence spectrometer. Excitation by X-rays from(a) an X-ray tube and (b) a radioactive substance
FIGURE 12-15
12C-2 Qualitative and
Semi quantitative Analysis
FIGURE 12-16 X-Ray fluorescence spectrum for a genuine bank note recorded with a wave- length dispersive spectrometer.
FIGURE 12-17 Spectrum of an iron sample obtained with an energy-dispersive instrumentwith a Rh anode X-ray tube source. The numbers above the peaks are energies in keV.(Reprinted with permission from J. A. Cooper, Amer. Lab., 1976,8 (11),44. Copyright 1976by International Scientific Communications, Inc.)
12C-3 Quantitative Analysis
Matrix EffectsMatrix Effects
Calibration Against StandardsCalibration Against Standards
Use of Internal StandardsUse of Internal Standards
Dilution of Samples and StandardsDilution of Samples and Standards
Some Quantitative ApplicationsSome Quantitative Applications
of X-ray Fluorescenceof X-ray Fluorescence
Advantages and DisadvantagesAdvantages and Disadvantages
of X-ray Fluorescence Methodsof X-ray Fluorescence Methods
12D X-RAY ABSORPTION METHODS
12E X-RAY DIFFRACTION METHODS
12E-1 Identification of Crystalline Compounds
Automatic DiffractometersAutomatic Diffractometers
Photographic RecordingPhotographic Recording
FIGURE 12-18 Schematic of (a) a Debye-Scherrer powder camera; (b) the film stripafter development. D2, D1 and T indicatepositions of the film in the camera.
12E-2 Interpretation of Diffraction Patterns
12F THE ELECTRON MICROPROBE