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Short answer. ATR-FTIR spectroscopy is Fourier-transform infrared spectroscopy performed with an attenuated total reflectance (ATR) accessory. The sample is pressed against a high-refractive-index crystal, and infrared light probes only the thin layer touching the crystal, so most samples need little or no preparation.
What ATR-FTIR stands for and what it covers
ATR stands for attenuated total reflectance; FTIR stands for Fourier-transform infrared. The combined term, also written ATR FTIR, FTIR-ATR, FTIR ATR or ATR spectroscopy, names one technique: an FTIR spectrometer fitted with an ATR sampling accessory. Infrared light identifies the molecular composition of a sample held in contact with the crystal (Specac, read 2026-11-08). For the spectrometer itself, see the Fourier-transform infrared spectrophotometer guide.
How does ATR-FTIR spectroscopy work?
The infrared beam travels through the crystal and reflects off its underside, where the sample sits. The angle of incidence has to exceed the critical angle for total internal reflection. At each reflection an evanescent wave extends a short distance into the sample, and the sample absorbs at its characteristic frequencies, which attenuates the reflected beam (Specac, 2026-11-08). The attenuated light goes on to the detector, and the instrument converts the signal into a spectrum.
Sampling depth is shallow. Specac describes penetration “typically on the order of a few micrometres”, and a 2020 review of biopharmaceutical applications gives 0.5-5 µm for the interaction with the sample surface (Tiernan, Byrne and Kazarian, Spectrochimica Acta Part A, 2020, PMC7308041, read 2026-11-08).
What is the difference between ATR-FTIR and FTIR?
It is not an either/or choice. FTIR describes how the spectrum is acquired; ATR describes how the sample is presented to the beam. A conventional transmission FTIR measurement passes the beam through the sample, which for solids may mean slicing, grinding or dissolving it. ATR-FTIR is the same spectroscopy with a different sampling mode, and it is usually easier because it removes most of that preparation, provided the sample makes good contact with the crystal (Specac, 2026-11-08). For protein solutions, the 2020 review calls ATR-FTIR the most commonly used technique, and notes that transmission mode for solutions can also be laborious.
| Point | Transmission FTIR | ATR-FTIR |
|---|---|---|
| How the sample meets the beam | Beam passes through it | Sample sits on a crystal; an evanescent wave samples the surface layer |
| Preparation | May need slicing, grinding or dissolving | Little or no preparation |
| Depth sampled | Path through the sample | A few micrometres (0.5-5 µm cited) |
| Relative band intensities | Reference behaviour | Wavelength-dependent, so they differ from transmission spectra |
Sources: Specac and Tiernan et al., as cited above. For the depth, crystal and correction details behind choosing between the two, see ATR-FTIR versus transmission FTIR sampling selection.
Which crystals are used?
Specac names diamond (robust, broad range), germanium (good mid-infrared transmission, more delicate), zinc selenide (relatively soft) and silicon. The 2020 review likewise lists diamond, Ge, Si and ZnSe as the usual internal reflection elements.
Where ATR-FTIR is used in laboratories
Specac lists pharmaceuticals, chemicals, forensics, environmental monitoring, adhesive cure monitoring and biological samples. The method is non-destructive and, per the 2020 review, label-free: in one cited example, 0.5 µl of dried IgG on a diamond element was used to characterise glycosylation without labelling. Because it reports on a surface layer, it suits thin films, coatings and surfaces. To assign the bands you record, use the IR functional-group frequency table. For how it compares with a complementary vibrational technique, see Raman versus FTIR spectroscopy.
Common misunderstandings
- “ATR is a different spectroscopy from FTIR.” It is a sampling mode of FTIR.
- “It measures the whole sample.” It reports on the layer next to the crystal, a few micrometres deep.
- “The spectrum will match a transmission spectrum exactly.” The strength of the ATR effect depends on wavelength, so relative intensities differ from transmission spectra.
- “Distribution in the sample does not matter.” In the 2020 review, IgG aggregates distributed unevenly near a ZnSe element surface, so spectral processing had to account for it.
- “Contact is a detail.” Good crystal contact is the main practical requirement.
FAQ
What is FTIR spectroscopy?
FTIR spectroscopy measures how a sample absorbs infrared light across a range of frequencies and uses a Fourier transform to produce the spectrum, which identifies molecular composition through characteristic absorptions. See what analytical chemistry is for the wider field.
What does ATR stand for in spectroscopy?
Attenuated total reflectance.
What is ATR in FTIR spectroscopy?
A sampling accessory in which the beam reflects inside a high-refractive-index crystal and an evanescent wave probes the sample in contact with it.
What is the price of an ATR-FTIR instrument?
The sources used here do not state a price, and none is given on this page. Cost depends on the spectrometer and accessory configuration, so request a quotation from the supplier.
Sources
- Specac, “Everything You Need to Know About ATR-FTIR Spectroscopy”, specac.com/en/learn/everything-you-need-to-know-about-atr-ftir-spectroscopy, read 2026-11-08.
- Tiernan H, Byrne B, Kazarian SG. “ATR-FTIR spectroscopy and spectroscopic imaging for the analysis of biopharmaceuticals.” Spectrochimica Acta Part A, 2020. PMC7308041, read 2026-11-08.
