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