What is ATR or Attenuated Total Reflectance?

Principle of ATR:

FTIR Sample Techniques- Attenuated Total Reflection (ATR)

FTIR : Fourier-transform infrared’ spectroscopy

  • ATR is the full form of “Attenuated Total Reflection” and ATR is one of the most standard technique used in sampling technologies for FTIR Spectroscopy method.
  • The infrared light passes through a crystal of a certain material (Like, Diamond, ZnSe (zinc selenide) or Germanium) and interacts with the sample, which is pressed onto the crystal.
Attenuated Total Reflectance (ATR)
  • ATR uses a nature of total internal reflection resulting in an evanescent wave
  • An Infra-Red beam is focused onto an optically dense crystal with a high refractive index at a some defined angle.
  • This internal reflectance generates an evanescent wave that outspreads beyond the surface of the crystal into the sample held in contact with the crystal.
  • The evanescent (Means: having been reduced in force) wave will be attenuated in area of the IR spectrum where the sample absorbs energy.
  • The attenuated beam back to the crystal then exits the opposite end of the ATR-crystal and is directed to the ATR-Detector in the IR spectrometer.
  • The detector generates an infrared spectrum by recording the attenuated IR beam as an interferogram signal.

Why Diamond is the best crystal used in ATR (Attenuated Total Reflectance)?

  1. Its robustness and durability in use.
  2. Instrument’s lifetime replacement costs is minimal in diamond.
  3. Effect of cleaning agent minimum than other. Easy to clean and can avoid contamination.

What are the benefit of Germanium (Ge) in Attenuated Total Reflectance as crystal?

  1. Germanium (Ge) is a much better working pH range.
  2. It can be used to analyse weak acids and alkalis.
  3. Germanium has the highest refractive index of all the ATR materials available. That leads to the effective depth of penetration is approximately 1 µ (micron)

Minimum Requirement of ATR:

  1. Because the evanescent wave or bubble only extends 0.5 – 5 mm beyond the ATR crystal, the sample must be in direct touch with it.
  2. Internal reflectance does not occur until the refractive index of the crystal is significantly greater than that of the substance or material; otherwise, light will be transmitted rather than internally reflected in the crystal. At 2000 cm-1, ATR crystals typically have refractive index ranges of 2.38 to 4.01. The majority of solids and liquids, it is safe to infer, have substantially lower refractive indices.

What types of samples can you analyze with Attenuated Total Reflectance?

  • Crystal as diamond.

Ideal solids include:

  • Laminates 
  • Paints 
  • Plastics
  • Rubbers
  • Coatings
  • Natural powders
  • Solids that can be ground into powder

For liquid analysis

  • Viscous liquids
  • Coatings
  • Biological materials

What are the advantages of Attenuated Total Reflectance?

  • Nominal sample preparation
  • Fast and easy clean-up, just remove and clean with suggested cleaning agents.
  • Analysis of samples in their ideal states (That is solid or Liquid) no need to heat, press into pellets, or grind in order to collect spectra
  • Exceptional for thick or strongly absorbing samples—perfect for difficult samples like black rubber

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