Attenuated total reflectance (atr)
- 1. Submitted By : Arisha Sharma Dept. Of Analysis
- 2. INTRODUCTION Attenuated Total Reflectance (ATR) accessories remove the need for transmission cells and KBr pellets when performing measurements on liquid, semi-solid and solid materials. The ATR spectroscopy works especially well for opaque samples or samples that are too thick for transmission measurements. The technique gives information about the surface properties and condition of materials and can be useful for both qualitative and quantitative measurements.
- 3. BASIC PRINCIPLES ATR spectroscopy uses the phenomenon of total internal reflection. A beam of radiation passes through a crystal (there are a variety of crystals used for ATR measurements) and undergoes total internal reflection through the crystal. An evanescent wave is created which protrudes only a few micros beyond the crystal surface. The distance that the wave extends from the crystal surface depends upon the material being used. The evanescent wave which is formed decays rapidly from the crystal surface, consequently, a good contact between the crystal and the sample must be maintained when performing ATR measurements.
- 4. Hence, many ATR accessories employ a pressure arm to do this. The radiation that penetrates a fraction of a wavelength beyond the surface of the crystal enters the sample that is placed on its surface. Part of the radiation is absorbed by the sample which will then produce an absorption spectrum. The depth of penetration for ATR is a function of the wavelength, refractive index of the crystal being used and the angle of incident of the beam.
- 7. TYPE OF MATERIALS USED FOR ATR CRYSTAL The crystal used in an ATR cell needs to have a high refractive index. In general materials used for a cell need to have a higher refractive index than the sample being measured. The table below shows typical physical properties of the materials that are commonly used in ATR cells.
- 9. DIAMOND: Diamond has excellent chemical and physical properties when used as ATR material. Its hard scratch resistant properties make it suitable for a wide range of applications. It can withstand highly acidic and basic samples and does not react with strongly oxidizing or complex agents. Diamond ATRs can be used to analyze hard powder solid samples. The only disadvantage with diamond is it’s relatively high absorbance in the region of 2,500cm-1 to 1,650cm-1. GERMANIUM: Due to it high refractive index, germanium is excellent for highly absorbing samples e.g. rubber materials containing carbon black. Due to it low depth of penetration germanium ATR is useful for analysis of a single layer in laminate samples.
- 10. KRS-5: Thallium-bromoiodide has the widest spectral range of commonly used ATR crystals, however, it is soft and can be easily damaged. KRS-5 is insoluble in water but will form a surface bloom which makes it unsuitable for use with aqueous samples. Care must be taken when using KRS-5 due to its high toxicity. ZINC SELENIDE: This material has the advantage of being resistant to many commercially available which makes it easy to clean and ideal for wet or aqueous samples. Zinc selenide has a similar refractive index to that of diamond, which make it a cost effective alternative for acids and alkalis and is water resistant.
- 11. SILICON: Silicon has a high refractive index which makes it applicable for high absorbing samples. Silicon is also scratch and water resistance. With its low depth of penetration, silicon is ideal for the analyses of thin film samples. However, the material is affected by strong acids and alkali and also has a limited spectral range. AMTIR: This crystal is manufactured from selenium, arsenic and germanium and is insoluble in water. It has a similar refractive index to that of zinc selenide and is mainly used in measurement with strong acids.
- 12. TYPE OF ATR • Horizontal ATR (H-ATR) • Universal ATR (UATR) • Diffuse Reflectance Infrared Transform Spectrocopy (DRIFTS)
- 13. ADVANTAGES • Faster sampling with no preparation. • Excellent sample-to-sample reproducibility. • Minimal operator-induced variations. DISADVANTAGES • The ATR crystals absorb energy at low energy level. • If the sample does not have good contact with the crystals, the data will not be accurate. • Most ATR crystals have pH limitations.
- 14. • Identification of drug substances and functional group. • Used in identification of paints and varnishes. • Useful for substances that are difficult to handle by conventional sampling technique. • Used in forensic and biomedical field.