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.
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:
- 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
- Biological materials
- 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
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.
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. He is the Global Training Manager for FTIR products.
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