Attenuated total reflectance (ATR) is a sampling technique used in infrared (IR) spectroscopy, particularly with Fourier Transform Infrared (FTIR) spectroscopy. It allows for the direct analysis of solid or liquid samples without extensive preparation. ATR utilizes the principle of total internal reflection, where light is passed through a crystal with a high refractive index and interacts with the sample at the crystal’s surface. This interaction creates an evanescent wave that penetrates the sample, allowing for the measurement of its infrared spectrum.
Principle
ATR relies on the phenomenon of total internal reflection (TIR), which occurs when light travels from a medium with a high refractive index to one with a lower refractive index at an angle greater than the critical angle. In ATR, a beam of infrared light is directed through a crystal (often diamond, zinc selenide, or germanium) with a high refractive index, and then reflected at the interface between the crystal and the sample.
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Evanescent Wave
During total internal reflection (TIR), a portion of the infrared light penetrates the sample at the crystal-sample interface, forming an “evanescent wave” that extends a short distance (typically a few micrometers) into the sample. This wave carries information about the sample’s absorbance at specific infrared frequencies.
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Sampling
ATR is known for its ease of sampling. Liquids can be placed directly on the crystal surface, while solids can be pressed against it using a pressure arm. No extensive sample preparation, like making KBr pellets in traditional transmission methods, is usually needed.
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Applications
ATR-FTIR is widely used in various fields, including materials science, chemistry, and biology, for analyzing the composition and structure of a wide range of samples. It is particularly useful for analyzing samples that are thick, strongly absorbing, or difficult to prepare for traditional transmission measurements.
Key advantages of ATR
Minimal sample preparation:
ATR simplifies the process of sample handling and reduces the time required for analysis.
Direct measurement:
ATR allows for the direct measurement of solid and liquid samples without the need for dissolving or diluting them.
Surface sensitivity:
ATR provides information about the surface properties of materials due to the short penetration depth of the evanescent wave.
Versatility:
ATR can be used to analyze a wide variety of materials, including polymers, coatings, and biological samples.
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