X-Ray Fluorescence

Question 1

What is the process of X-ray fluorescence?

Answer 1

1. Sample is bombared with x-rays
2. The absorption of x-rays produces electronically excited ions that return to ground state via transitions of electrons from higher energy levels giving characteristic x-rays identical to those produced by electron bombardment.
3. The primary source must be energetic enough to remove core electrons.

Question 2

What is characteristic of Wavelength Dispersive XRF?

Answer 2

• Can be a manual or automatic process.
• Multichannel instruments are large/expensive but provide simultaneous detection of up to 24 elements in seconds
• Individual channels of crystal and detector are arranged radially around an x-ray source and sample holder.
• Useful for determining several components in industrial materials such as steel, cement, petroleum etc.
• Samples can be metals, powders, films, liquids
• Can be held in mylar or cellophane windows

Question 3

What is characteristic of Energy Dispersive XRF?

Answer 3

• EDXRF makes use of the fact that pulse height of the detector signal is proportional to x-ray photon energy, which correlates with wavelength.
• Thus optical path is simpler as no crystals or goniometers are needed and fluorescence photons from the sample hit the detector directly.
• Elements and their concentration are identified by counting pulses at different energy levels. Usually multichannel analyser is used. Each channel counts number of photons with a certain energy level simultaneously

Question 4

What are the advantages/disadvantages of EDXRF?

Answer 4

• Advantages:
  
  • simple with no moving parts
  • no collimators/crystal diffractors = more energy reaching the detector - can use weaker source such as radioactive materials.
  • Cheaper and less radiation damage to the sample.
• Disadvantages:
  
  • Low resolution (50x less than WDXRF)

Question 5

What are the applications of EDXRF?

Answer 5

• Very powerful tool for all but lightest of elements.
• Quick and accurate e.g. corrections to alloys during manufacture.
• Russians sent EDXRF instrument to Venus.

Question 6

What is characteristic of Nondispersive XRF?

Answer 6

• Relatively cheap but crude.
• E.g. routine analysis of sulfurs in petrol
• Samples are irradiated with 55Fe radioactive source. Fluorescent sulfur line at 5.4 angstroms
• Radiation passes through a pair of filters, one adsorption edge just below and the other just above 5.4 angstroms