FTIR Sample Techniques: Attenuated Total Reflection (ATR)

How does it work?

An ATR accessory operates by measuring the changes that occur in an internally reflected IR beam when the beam comes into contact with a sample. An IR beam is directed onto an optically dense crystal with a high refractive index at a certain angle. This internal reflectance creates an evanescent wave that extends beyond the surface of the crystal into the sample held in contact with the crystal.

In regions of the IR spectrum where the sample absorbs energy, the evanescent wave will be attenuated. The attenuated beam returns to the crystal, then exits the opposite end of the crystal and is directed to the detector in the IR spectrometer. The detector records the attenuated IR beam as an interferogram signal, which can then be used to generate an IR spectrum.

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What types of samples can you analyze with ATR?

ATR is ideal for strongly absorbing or thick samples which often produce intense peaks when measured by transmission. ATR works well for these samples because the intensity of the evanescent waves decays exponentially with distance from the surface of the ATR crystal, making the technique generally insensitive to sample thickness.

Other solids that are a good fit for ATR include homogeneous solid samples, the surface layer of a multi-layered solid or the coating on a solid. Even irregular-shaped, hard solids can be analyzed using a hard ATR crystal material such as diamond. Ideal solids include:

Laminates
Paints
Plastics
Rubbers

Coatings
Natural powders
Solids that can be ground into powder

In addition, ATR is often the preferred method for liquid analysis because it simply requires a drop of liquid to be placed on the crystal. ATR can be used to analyze:

Free-flowing aqueous solutions
Viscous liquids

Coatings
Biological materials

What are the advantages of ATR?

Minimal sample preparation—place the sample on the crystal and collect data
Fast and easy cleanup—simply remove the sample and clean the surface of the crystal
Analysis of samples in their natural states—no need to heat, press into pellets, or grind in order to collect spectra
Excellent for thick or strongly absorbing samples—ideal for difficult samples like black rubber

ATR on-demand webinars

Spectroscopy Simplified: ATR Sampling in FTIR

Attenuated Total Reflection (ATR) sampling revolutionized FTIR spectroscopy, with its simplicity of sample handling and near-universal applicability. This webinar will introduce the theory and basic use, ideas for ATR, and will show many examples. Register for this on-demand webinar and learn more.

ATR: Watch and learn with this video series

Basics

Sample preparation

Data collection

Introducing Mike

Dr. Michael Bradley received his BS degree in chemistry from the University of South Carolina and his PhD in physical chemistry from the University of Illinois, and also completed his MBA in management. He taught graduate and undergraduate chemistry for 15 years, prior to becoming a field applications scientist with Thermo Nicolet, subsequently Thermo Fisher Scientific, in 2002.

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