ATR-FTIR
Also known as: ATR-IR, attenuated total reflectance, FTIR spectroscopy
Attenuated Total Reflectance (ATR) Fourier Transform Infrared (FTIR) spectroscopy is a variant of conventional FTIR that measures infrared absorption through evanescent-wave interrogation of samples in direct contact with a high-refractive-index crystal. Developed by Harrick and Fahrenfort in the 1960s, ATR-FTIR is now the dominant form of FTIR spectroscopy, enabling rapid, non-destructive characterization of organic compounds, polymers, coatings, and biological materials without extensive sample preparation.
Key highlights
- Minimal sample preparation: solids, powders, liquids, and pastes analyzed directly
- Fast analysis: spectra acquired in seconds
- Surface-sensitive: probes only the top 1-10 micrometers, useful for coating and surface analysis
- Non-destructive: samples recovered unmodified after analysis
Intuition
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How it works
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When to use it
Apply ATR-FTIR to characterize solid, liquid, and paste samples rapidly: polymers, coatings, minerals, pharmaceutical tablets, food components, and biological tissues. ATR is ideal when samples are opaque or too thick for transmission FTIR, or when minimal sample preparation is desired. Assumes good contact between sample and crystal and that the sample refractive index is lower than the crystal.
Strengths & limitations
- Minimal sample preparation: solids, powders, liquids, and pastes analyzed directly
- Fast analysis: spectra acquired in seconds
- Surface-sensitive: probes only the top 1-10 micrometers, useful for coating and surface analysis
- Non-destructive: samples recovered unmodified after analysis
- Quantitative interpretation difficult: peak intensity depends on sample contact, refractive index, and geometry
- Refractive index effects: spectra differ from conventional transmission FTIR; library matching may fail
- Crystal surface degradation: hard samples scratch diamond or sapphire crystals; replacement is expensive
- Spectral distortion from high absorbing samples: baseline artifacts and band distortion occur for strong absorbers
Common pitfalls
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Applications
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Frequently asked
What materials are used for ATR crystals?
Common materials include diamond (hardest, most durable, most expensive), sapphire, germanium, and zinc selenide. Diamond is preferred for hard samples; germanium for aqueous solutions (higher refractive index, shorter penetration); zinc selenide for biological and organic samples. Choice depends on sample type and cost considerations.
Why do ATR spectra sometimes look different from transmission FTIR spectra of the same material?
ATR spectra are distorted by the optical properties of the sample (refractive index, absorption) and the ATR geometry. High-absorbing bands appear distorted; peak ratios change compared to transmission. This is why ATR spectra cannot always be directly matched to transmission FTIR libraries without correction factors.
How deep does ATR probe into the sample?
The penetration depth (distance where intensity falls to 1/e) typically ranges from 1 to 10 micrometers and depends on wavenumber, incident angle, and sample refractive index. Higher wavenumbers and steeper angles give shallower penetration. This makes ATR surface-sensitive, ideal for coatings and thin films.
Sources
- 1.Harrick, N. J. (1960). Study of physics of internal reflection from metals. Journal of Physics and Chemistry of Solids, 13(2), 143-155.
- 2.Fahrenfort, J. (1961). Attenuated total reflection: a new principle for the production of useful infra-red reflection spectra of organic compounds. Spectrochimica Acta, 17(6), 698-709.
- 3.Humecki, H. J. (Ed.). (1995). Practical Guide to Infrared Microspectroscopy. CRC Press.
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ScholarGate. (2026, June 3). ATR-FTIR. ScholarGate. https://scholargate.app/spectroscopy/atr-ftir