LECTURE 3.pptx ATR lecture for enhancing on FTIR usage in quantification
- 1. 09/22/2024 08:26 AM LECTURE 4 Attenuated total reflectance (ATR)
- 2. 09/22/2024 08:26 AM Attenuated total reflectance (ATR) • Attenuated total reflectance or ATR is one of the most common sampling techniques in Fourier transform infrared (FTIR) spectroscopy. • ATR is based on total internal reflection where infrared (IR) light and sample interact only at the point where IR light is reflected. • Transmission, on the other hand, is based on IR light passing through the sample. • ATR FTIR spectroscopy allows solid or liquid samples to be measured with only minimal sample preparation.
- 3. 09/22/2024 08:26 AM Principles of attenuated total reflectance • In ATR FTIR spectroscopy the sample is in contact with the ATR crystal. • The IR radiation travels through the crystal and interacts with the sample on the surface in contact with the ATR crystal. • Because of the differences in refractive indices of both materials, total internal reflection occurs. • This reflection forms the so-called “evanescent wave” which extends into the sample. • Based on the sample’s composition, a small part of the infrared light is absorbed when the evanescent wave interacts with the sample, resulting in a slightly attenuated total reflection.
- 4. 09/22/2024 08:26 AM Total internal reflection • When a propagating wave (infrared light beam) hits the surface between two optical media which are characterized by two different refractive indices at a certain angle of incidence, the light is totally internally reflected. • This angle is called the critical angle and can be calculated using Snell’s law. • Snell’s law states that the ratio of two refractive indices is equal to the inverse ratio of the angle of incidence and the angle of refraction. • For the special case of no refraction, the angle of incidence becomes the so-called critical angle.
- 5. 09/22/2024 08:26 AM If the α > α critical then incident light rays are totally reflected If the α<α critical, then the incident light is partly refracted • Many organic samples have mid-IR refractive indices of around 1.5 • Therefore, the critical angle for ATR crystal materials can be calculated for a refractive index of 1.5.
- 6. 09/22/2024 08:26 AM Common materials for ATR crystals Cell material Refractive index Critical angle (degrees) Silver chloride AgCl 1.90 49° KRS-5 2.37 40° Zinc selenide (ZnSe) 2.40 40° Zinc sulfide (ZnS) 2.2 43° Germanium (Ge) 4.00 22° Silicon (Si) 3.41 26° Diamond 2.41 40° • To obtain total internal reflection, ATR crystal materials must have a higher refractive index than the tested sample material. • Because of their high refractive indices, ATR crystals are said to be optically dense whereas typical samples have low refractive indices and therefore are optically rare. • The angle of incidence of the IR beam has to match the calculated critical angle for each cell material to create total internal reflection.
- 7. 09/22/2024 08:26 AM Evanescent wave • Total internal reflection does not explain the interaction of the IR beam and sample because the IR beam never leaves the ATR crystal. • Instead, the interaction of the IR beam and sample occurs through an evanescent field, often called “evanescent wave”. • Upon total reflection of the incident light at the interface where the crystal touches the sample a small fraction of the light extends into the sample as an evanescent wave. • When a wave cannot propagate regularly into the sample, it is concentrated in proximity to the point of total reflection and starts decaying exponentially.
- 8. 09/22/2024 08:26 AM • The evanescent wave sticks out into the sample. • In areas where the sample is in contact with the evanescent wave specific parts of the IR beam are absorbed based on the sample’s composition. • The totally reflected IR light lacks the absorbed parts and thus is attenuated, hence the name “attenuated total reflectance” (ATR).
- 9. 09/22/2024 08:26 AM Penetration depth • The penetration depth is a measure of how far the evanescent wave extends into the sample. • Its exact value is determined by the wavelength of light, the angle of incidence on the interface between crystal and sample, and the indices of refraction for the ATR crystal and the sample medium. • The depth of penetration is defined as the distance to the point at which the evanescent wave’s amplitude has decreased to 1/e (i.e. about 37 %) of its maximum value and can be calculated with the following equation: • λ … wavelength of incident light in vacuum n1 … refractive index of ATR crystal (dense medium) n2 … refractive index of sample (rare medium) … angle of incidence ϴ
- 10. 09/22/2024 08:26 AM • According to the above equation the penetration depth for an ATR cell made of zinc selenide, a sample with a refractive index of 1.5 and a wavelength of 10 µm is approximately 2.0 µm. • Most organic samples have a refractive index of 1.5; therefore this value is widely used for calculation. • The penetration depth is subjected to changes depending on varying incident wavelengths. • Since the IR beam used in FTIR spectroscopy contains a broad range of wavelengths, the penetration depth is different for each wavelength.
- 11. 09/22/2024 08:26 AM • In regular transmission cells the IR beam passes through the sample; the space where the IR beam and sample interact depends only on the length of this transmission cell. • The path length is the same for every wavelength. Therefore, absorbance spectra measured with transmission cells are different from spectra measured with ATR cells. • In a first approximation this can be accounted for by a multiplication of the ATR spectra with wavenumbers. This is often called a simple ATR correction.