Attenuated Total Reflectance (ATR)

Definition

Attenuated Total Reflectance (ATR) is a sampling technique used in infrared (IR) spectroscopy that allows for the analysis of solid, liquid, and viscous samples without the need for complex sample preparation. It works by directing the IR beam into a crystal with a high refractive index, causing the beam to reflect internally and interact with the sample at the surface of the crystal.

5 Must Know Facts For Your Next Test

1. ATR allows for the analysis of a wide range of sample types, including liquids, solids, and viscous materials, without the need for complex sample preparation.
2. The IR beam is directed into a high refractive index crystal, such as diamond or germanium, causing the beam to reflect internally and interact with the sample at the surface of the crystal.
3. The evanescent wave that extends beyond the surface of the ATR crystal interacts with the sample, allowing for the detection of surface and near-surface information.
4. The penetration depth of the evanescent wave is dependent on the wavelength of the IR radiation and the refractive indices of the crystal and sample, typically ranging from 0.5 to 5 micrometers.
5. ATR is particularly useful for the analysis of samples that are difficult to analyze using traditional transmission IR spectroscopy, such as highly absorbing or opaque materials.

Review Questions

• Explain how the internal reflection of the IR beam in an ATR setup allows for the analysis of solid, liquid, and viscous samples.
  • In an ATR setup, the IR beam is directed into a high refractive index crystal, such as diamond or germanium. The beam reflects internally within the crystal, creating an evanescent wave that extends beyond the surface of the crystal and interacts with the sample. This allows for the analysis of a wide range of sample types, including solids, liquids, and viscous materials, without the need for complex sample preparation. The evanescent wave can penetrate the sample to a depth of 0.5 to 5 micrometers, depending on the wavelength of the IR radiation and the refractive indices of the crystal and sample, enabling the detection of surface and near-surface information.
• Describe the role of the evanescent wave in the ATR technique and how it allows for the analysis of sample information.
  • The evanescent wave is a key component of the ATR technique. When the IR beam is reflected internally within the ATR crystal, an evanescent wave is generated that extends beyond the surface of the crystal and interacts with the sample. This evanescent wave allows for the detection of surface and near-surface information about the sample, as it can penetrate the sample to a depth of 0.5 to 5 micrometers. The depth of penetration is dependent on the wavelength of the IR radiation and the refractive indices of the crystal and sample. By utilizing the evanescent wave, ATR enables the analysis of a wide range of sample types, including solids, liquids, and viscous materials, without the need for complex sample preparation.
• Analyze how the choice of ATR crystal material and the refractive indices of the crystal and sample can impact the performance and applications of the ATR technique.
  • The choice of ATR crystal material and the refractive indices of the crystal and sample are critical factors that can impact the performance and applications of the ATR technique. The crystal material must have a high refractive index to ensure internal reflection of the IR beam and the generation of an evanescent wave that can interact with the sample. Common ATR crystal materials include diamond, germanium, and zinc selenide, each with their own unique properties and refractive indices. The refractive index of the sample relative to the crystal also plays a role, as a higher refractive index difference between the crystal and sample will result in a shallower penetration depth of the evanescent wave. This can be advantageous for the analysis of surface and near-surface information, but may limit the ability to analyze deeper into the sample. By carefully selecting the ATR crystal material and considering the refractive indices of the crystal and sample, researchers can optimize the ATR technique for their specific analytical needs and sample types.

Related terms

Internal Reflection Spectroscopy (IRS): A technique in which the sample is placed in contact with the surface of a transparent crystal, and the infrared beam is reflected multiple times within the crystal, interacting with the sample at each reflection.

Evanescent Wave: The electromagnetic field that extends beyond the surface of the ATR crystal and interacts with the sample, allowing for the detection of surface and near-surface information.

Penetration Depth: The depth to which the evanescent wave extends into the sample, which is dependent on the wavelength of the IR radiation and the refractive indices of the crystal and sample.