Instrumental Chemistry
Chapter 12
Atomic X-Ray Spectrometry
Brief Summary
X-ray spectroscopy is a form of
optical spectroscopy that utilizes
emission, absorption, scattering,
fluorescence, and diffraction of Xray radiation
About X-Rays
• X-rays are short-wavelength (hence, high frequency, and
hence, relatively high energy) electromagnetic radiation.
Two ways to produce X-rays:
1) Deceleration of high-energy electrons
2) Electronic transitions involving inner-orbital (e.g. d or f) electrons
• The wavelength range of X-Rays is from about 10-5 Å to
100Å
• Conventional X-Ray spectroscopy is largely confined to
the region of about 0.1 Å to 25 Å
For Analytical Purposes, X-rays
are Generated in Three Ways:
1) Bombardment of metal target with
high-energy electron beam
2) Exposure of target material to primary
X-ray beam to create a secondary beam of
X-ray fluorescence
3) Use of radioactive materials whose
decay patterns include X-ray
emission
Schematic of an X-ray tube
Energy-level iagram showing common
transitions producing X-rays
Energy-Level Diagram Showing
Common Transitions Producing X-rays
Common X-Ray Transitions
• Partial energy level
diagram showing
common transitions
leading to X–
radiation.
The most intense lines
are indicated by the
widest arrows
Wavelengths/Å for Intense X–ray
Emission Lines
Note that all possible electronic transitions
are not of equal probability, i.e., the nature of
a spectrum depends on specific selection
rules, so that the complexity of a spectrum is
not as great as might be expected from first
consideration of an energy level diagram.
Wavelengths/Å for Intense X–ray
• The fact that the wavelength of a line of
given type decreases as the atomic number
of the element increases is rather important
in that it means that an X-ray from a given
element must be able to cause inner shell
ionization and, hence, emission of radiation
of lower energy from any lighter element.
Characteristics and Identification
Of Wavelengths
• Identification and measurement of concentration
of elements based on the fact that primaryemission x-rays emitted by an element excited by
an electron beam have a wavelength characteristic
of that element and an intensity related to its
concentration. It may be performed by an electron
probe microanalyzer, an electron microscope
microanalyzer, or by an electron microscope, or
scanning electron microscope, fitted with an x-ray
spectrometer.
Electron Beam Sources
In electron beam sources, X-rays are
produced by heating a cathode to produce
high-energy electrons; these electrons are
energetic enough to ionize off the cathode
and race towards a metal anode (the target)
where, upon collision, X-rays are given off
from the target material in response to the
colliding electrons.
The Duane-Hunt law
The maximum photon energy corresponds
to total stopping of the electron and is given
by:
hvo = (hc)/o = Ve
vo is the maximum frequency
V = accelerating voltage
e = electron charge
Continuum Spectra from Electron
Beam Sources
• In an X-ray tube, electrons produced at a heated cathode
are accelerated toward a metal anode by a potential as
great as 100kV; upon collision, part of the energy of the
electron beam is converted into X-Rays. Under some
conditions only a continuum spectrum is results. The
continuum X-Ray spectrum is characterized by a welldefined, short wavelength limit, which is dependent upon
the accelerating voltage but independent of the target
material. The continuum radiation from an electron beam
source results from collisions between the electrons of the
beam and the atoms of the target material.
Line Spectra from Electron Beam
Sources
• Bombardment of a molybdenum target produces intense emission
lines. The emission behavior of molybdenum is typical of all elements
having atomic numbers greater than 23, that is, the X-Ray line spectra
are similar when compared with ultraviolet emission and consist of
two series of lines.
• Line spectra are composed of distinct lines of color, or in the case of
our graphs, sharp peaks of large intensity at a particular wavelength.
Line spectra are characteristic of elements and compounds when
excited (energized) under certain conditions. These spectra helped
develop the current atomic theories. Line spectra thus provide a
“fingerprint” unique to each element, and as with continuous spectra,
the combination of the prominent lines in the spectrum produce the
observe light color.
Line Spectra from Electron Beam
Sources
X-ray Fluorescence
Since X-rays are rather energetic,
excitation of sample electrons will
give rise to fluorescence as the
sample electrons are excited and
return to their ground states in a
series of electronic transitions.
Bragg Equation
sin  = (n)/2d
= angle of incidence
 = wavelength
d = interplane distance of crystal
Diffraction of X-rays by a crystal
X-ray Monochromator and Detector
References
http://www.anachem.umu.se/jumpstation.ht
m
http://userwww.service.emory.edu/~kmurra
y/mslist.html
http://www.chemcenter/org
http://www.sciencemag.org