18-07-2012, 12:37 PM
X-Ray Fluorescence Spectroscopy
X-Ray Fluorescence Spectroscopy.ppt (Size: 240.5 KB / Downloads: 45)
INTRODUCTION
It is otherwise called as x-ray emission method.
It is a non-destructive technique used for quantitative and qualitative elemental analysis.
In many cases, application of x-ray analysis yields information which is unattainable from other instrumental procedures.
It frequently requires very little sample preparation before the analysis can be carried out.
PRINCIPLE
When an element is placed in a beam of x-rays, the x-rays are absorbed. The absorbing atoms become excited and emit x-rays of characteristic wavelength. This process is called x-ray fluorescence.
Since the wavelength of the fluorescence is characteristic of the element being excited, measurement of this wavelength enables us to identify the fluorescing element.
The intensity of the fluorescence depends on the concentration of the element in the x-ray beam.
X-Ray Source
Three types of sources are used in x-ray instruments
- x-ray tube/Coolidge tube
- radioisotopes
- secondary fluorescence sources
X-ray tube :
It is the most common source of x-rays for analytical work. It is a highly evacuated tube which consists of a cathode and anode.
Choice of a target material
Depends on the sample to be examined.
Target material whose atomic number greater than that of the elements being examined should be selected.
Energy of x–rays emitted by the target material should be greater than that required to excite the elements being irradiated.
Monochromator
The monochromator used is an analyzing crystal. This crystal separates x-rays of different wavelengths by diffracting them at different angles.
For best sensitivity, curved crystal is used. This refocuses x-ray fluorescence after diffraction at the crystal.
Curved crystals have been made from salts such as