Xrf

XRF

1. INTRODUCTION TO X-RAY FLUORESCENCE TECHNIQUE BY TEJASVI

2. Contents Fluorescence Introduction to Method Basic Principle Instrumentation Forensic Applications Advantages Disadvantages References

3. Fluorescence Fluorescence is a an optical phenomena in which the molecular absorption of a photons triggers the emission of another photon with a longer wavelength.

4. X ray Fluorescence is an atomic spectrometric method based on detection of emitted X ray radiation from excited atoms. This technique is a two step process: 1. Removal of inner shell electron of atom. 2. Transition of electron from outer shell to inner shell.

5. Basic Principle Primary X rays, emitted by an X ray tube, are directed at the sample to be analyzed. The chemical composition present in the sample are excited such as to emit their own characteristics X rays. These are analyzed in spectrometer.

6. Instrumentation Energy dispersive spectrometry

7. Instrumentation Two different type of measurement system are used in XRF. 1. Wavelength Dispersive X Ray Fluorescence 2. Energy Dispersive X Ray Fluorescence

8. WDXRF In WDXRF, the emitted X rays are dispersed based on their wavelength. Gas Ionization and Scintillation detectors are found in this system. High resolution is produced.

9. Wavelength Dispersive XRF

10. EDXRF The energy dispersive system collects all X ray photons simultaneously on to the detector. Each photon generates an electrical pulse with an amplitude proportional to energy of X ray photon. Solid State Semiconductors detectors are found in this system.

11. Source The most common source of X rays are X ray tube. It is highly evacuated tube. Cathode – Tungsten filament Anode – Chromium, Silver, Iron etc. Filament is heated and electrons are accelerate to target. Production of new X rays occurs.

12. Samples State – Solid or Liquid Amount – Microgram to Gram Preparation – 1. For simple analysis, no preparation is necessary. 2. For minimum pretreatment, grind, homogenize and press into the pellets. 3. For maximum pretreatment, dry, fuse and cast into glass disks.

13. Detectors Three basic detectors are used in XRF instrumentation : 1. Gas Ionization 2. Scintillation 3. Solid State Semiconductors

14. Gas Ionization Gas Ionization detector consist of two electrodes, a wire anode in the center of metal cylinder cathode and an argon and methane mixture. In this type of detector the X ray photon enters through in window in the cylinder and ionize the gas.

15. The resulting ions and electrons are collected and the current is proportional to intensity of the X ray photon.

16. Scintillation It consist of Thallium – doped Sodium Iodide crystal on the front of a photo- multiplier tube. The X ray from the sample strike Sodium Iodide crystal and generate photons. These photon generate photo- electrons that are amplified and detected. The number of photons produced is proportional to energy of X rays.

17. Solid State Semiconductors It is a Lithium – drifted Silicon wafer. The X ray strikes the detector and generates a series of pulses that corresponds to X ray energy. The pulse height is proportional to X ray energy. The concentration of element is determined by counting the pulses.

18. Signal Processors & Readout A Microprocessor or Minicomputer can be used for steering of the device. It can calculate element concentration.

19. Forensic Applications To determine the distance of shots fired at close range. In the examination of glass fragments. To determine the alterations in the gold. In the analysis of paintings, coins, archeological specimens and other valuable objects.

20. In the examination of hair, fiber, soil and building materials. In the examination of fingerprint. In the examination of documents.

21. Advantages Non destructive method Simple spectra Speed Accuracy Minimal sample preparation Solids samples can be directly analyzed.

22. Disadvantages High cost Matrix interference can prevent detection of some elements. Standards for quantitative analysis do not always match unknown matrix. Elemental range is limited to Boron and up.

23. References Advances In Forensic Science, Instrumental Analysis by Lee & Gaensselen The Use Of X-Ray Technique In Forensic Investigation by Grahm D. Handbook Of Instrumental Technique For Analytical Chemistry by Settle, F.A. Fundamentals Of Analytical Chemistry by Skoog & Holler