X ray fluorescence (X R F)

X-Ray Fluorescence (XRF)
K V GOPINATH M Pharm PhD,CPhT
Tirumala Tirupati Devasthanams
TIRUPATI
e-mail:gopinath.karnam@gmail.com

Introduction
 X-ray fluorescence (XRF) spectrometry is an elemental analysis
technique with broad application in science and industry.
 XRF is routinely used for the simultaneous determination of
elemental composition and film thickness.
 Modern XRF instruments are capable of analyzing solid, liquid, and
thin-film samples for both major and trace (ppm-level) components.
 The analysis is rapid and usually sample preparation is minimal or
not required at all.

Principle
 XRF is based on the principle that individual atoms, when excited by
an external energy source, emit X-ray photons of a characteristic
energy or wavelength. By counting the number of photons of each
energy emitted from a sample, the elements present may be identified
and quantitated.
Theory
When an electron beam of high energy strikes a material, one
of the results of the interaction is the emission of photons which have
a broad continuum of energies. This radiation, called “braking
radiation”, is the result of the deceleration of the electrons inside the
material. The bremsstrahlung continuum is illustrated as a function of
electron acceleration voltages for a molybdenum target .

Theory of XRF
 Another result of the interaction between the electron beam and the
material is the ejection of photoelectrons from the inner shells of the
atoms making up the material. These photoelectrons leave with a
kinetic energy (E-φ) which is the difference in energy between that
of the incident particle (E) and the binding energy (φ) of the atomic
electron. This ejected electron leaves a “hole” in the electronic
structure of the atom, and after a brief period, the atomic electrons
rearrange, with an electron from a higher energy shell filling the
vacancy. By way of this relaxation the atom undergoes fluorescence,
or the emission of an X-ray photon whose energy is equal to the
difference in energies of the initial and final states. Detecting this
photon and measuring its energy allows us to determine the element
and specific electronic transition from which it originated

Instrumentation
 Most of the XRF instruments in use today fall into two categories:
energy-dispersive (ED) and wavelength-dispersive (WD)
spectrometers.
 Within these two categories is a tremendous variety of differing
configurations, X-ray sources and optics, and detector technologies.
 WD Spectrometers :The instrument operates based on the principle
of Bragg diffraction of a collimated X-ray beam, in this case the
beam emanating from the sample. A detector is angularly scanned
relative to the analyzing crystal, registering the spectrum.
 Energy Dispersive (ED): The entire polychromatic spectrum from
the sample is incident upon a detector that is capable of registering
the energy of each photon that strikes it. The detector electronics and
data system then build the X-ray spectrum as a histogram, with
number of counts versus energy.

Instrumentation
1) X-ray irradiates specimen
2) Specimen emits characteristic
X-rays or XRF
3) Analyzing crystal rotates to
accurately reflect each
wavelength and satisfy
Bragg’s Law n =2dsinƛ θ
4) Detector measures position and
intensity of XRF peaks
5) XRF is diffracted by a
crystal at different θ to
separate X-ray l and to
identify elements

By Laue Method - To Determine the
Orientation of Single Crystals
Back-reflection Laue
Transmission Laue

Advantages of XRF
 XRF is a versatile, rapid technique .
 It is non destructive method of chemical analysis. Important as in
case of samples in limited amounts, or valuable or irreplaceable.
 It is precise and with skilled operations it is accurate.
 Applicable to a wide variety of samples from powders to liquids.
 It is convenient and economical to use.
 With the major input cost being the hardware itself, which averages
around $75,000 for a modern industrial-use spectrometer or
$125,000 for a research-quality instrument.
 The instruments have few moving parts, tend to be low-maintenance,
and on a regular basis consume only liquid nitrogen and electricity.

Disadvantages
 Disadvantages include fairly high limits of detection (LODs) when
compared to other methods.
 Possibility of matrix effects, although these can usually be accounted
for using software-based correction procedures. LODs for graphite
furnace atomic absorption spectroscopy (GFAAS) beat XRF by
several orders of magnitude, but analyses can exhibit substantial
matrix effects. GFAAS is also relatively slow, with one element
determined at a time, and is destructive
 PXRF instruments are capable of producing results comparable in
many ways to the lab-based XRF at a fraction of the cost. PXRF
instruments can be purchased for about $30,000 to $50,000 complete
with vacuum systems, sample changer, and accompanying
computer.

Applications of XRF
 It is a method of elemental (metal and Non metal ) analysis with
atomic number greater than 12.
 Quantitative analysis can be carried out by measuring the intensity of
fluorescence at the wavelength characteristics of the element being
determined, especially applicable to most of the element in the
periodic table.
 In medicine
– Direct determination of sulfur in protein. The sulfur content of each of the many
different forms in which protein exists in human blood varies considerably.
– XRF indicates protein distribution and provides a diagnostic link for the medical
practitioner.
– Determination of chloride in blood serum
– Determination of strontium in blood serum and bone tissue
– Elemental analysis of tissues, bones and body fluids.
– It is used for determination of trace elements in plants and foods.
– It is used for detection of pesticides on fruits and herbal drugs

X ray fluorescence (X R F)

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