X-Ray Fluorescence (XRF)

K V GOPINATH M Pharm PhD,CPhTTirumala Tirupati Devasthanams

TIRUPATIe-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.

TheoryWhen 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 XRF3) Analyzing crystal rotates to

accurately reflect eachwavelength and satisfyBragg’s Law n ƛ=2dsin θ

4) Detector measures position and intensity of XRF peaks

5) XRF is diffracted by a crystal at different θ toseparate X-ray l and toidentify elements

By Laue Method - To Determine the Orientation of Single Crystals

Back-reflection Laue Transmission Laue

Pattern

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