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A Fourier transform infrared (FTIR) spectrophotometer is a laboratory instrument that identifies materials by how they absorb infrared light. This page covers what is inside one, how the detector choice changes what it can do, and how to read the specification lines on a datasheet. It is aimed at lab staff, students and anyone writing an instrument specification.
What an FTIR spectrophotometer is
An FTIR instrument uses interferometry to record information about a material placed in the infrared beam, then applies a Fourier transform to turn the detector output into a spectrum a person can interpret (Thermo Fisher Scientific, FTIR basics). Each molecule gives a distinct spectral pattern, which is what allows a sample to be identified.
Who uses it: analytical and quality-control laboratories that need to identify or compare materials from their infrared spectra. The instrument is laboratory equipment, not a clinical device.
How it is built
Three ideas describe the signal path. The interferometer modulates the infrared beam, the sample sits in that beam, and a detector records the result as an interferogram. Software then applies the Fourier transform to convert the interferogram into the spectrum (Thermo Fisher Scientific, FTIR basics).
- Interferogram: the raw detector signal, before the transform. Saving it lets you reprocess the data later with different settings.
- Apodization: a processing step applied to the interferogram. At a resolution of 4 cm-1 or coarser, heavy apodization does not badly distort spectra, but for narrow gas-phase lines its effect can be significant (Thermo Fisher Scientific).
- Optics and windows: inorganic peaks below 400 cm-1 need far-infrared optics. For protein work the useful region is roughly 1400 to 1750 cm-1, where BaF2 windows transmit (Thermo Fisher Scientific).
Detector types
The detector is the specification choice that most changes how an instrument behaves. Thermo Fisher describes two common types:
| Detector | Cooling | What the source says |
|---|---|---|
| MCT-A (mercury cadmium telluride) | Liquid nitrogen | High sensitivity |
| DLaTGS | Room temperature | Wider spectral range, lower sensitivity |
Source: Thermo Fisher Scientific, FTIR basics, read 2026-10-29.
Reading the specifications on a datasheet
As a worked example, here are the lines published for one manufacturer’s instrument, the Shimadzu IRXross. They are shown to illustrate what a datasheet lists, not as a recommendation.
| Specification | Published value (Shimadzu IRXross) | What to look for |
|---|---|---|
| Wavenumber range | 7,800 to 350 cm-1 | The span of the spectrum the instrument records |
| Resolution settings | 0.25, 0.5, 1, 2, 4, 8, 16 cm-1 | The selectable steps; the right setting depends on the sample, since gas-phase lines are narrow |
| Beamsplitter | Germanium-coated KBr | The material in the interferometer |
| Standard detector | DLATGS with temperature control | Matches the room-temperature type in the table above |
| Optional detector | MCT (Hg-Cd-Te), liquid nitrogen cooled | Matches the high-sensitivity type above |
Source: Shimadzu IRXross product page, read 2026-10-29. Values are the manufacturer’s own.
Shimadzu also states that a KRS-5 window option is compliant with pharmacopoeia wavenumber range requirements (350 to 7,800 cm-1) and that its software enables easy compliance with USP, EP, JP and ChP regulations. These are the manufacturer’s statements; if your lab works to a pharmacopoeia, check the requirement in the pharmacopoeia itself.
Sampling options
Thermo Fisher notes that attenuated total reflectance (ATR) needs minimal sample preparation, while transmission sampling is economical. The choice between them is covered in detail in ATR-FTIR vs transmission FTIR sampling.
How to choose
Only criteria stated in the sources above can be offered:
- Match the detector to the job: MCT-A for sensitivity, DLaTGS where a wider range and no liquid nitrogen matter more (Thermo Fisher).
- Check that the wavenumber range covers the region you need. Far-infrared optics are required below 400 cm-1 (Thermo Fisher).
- Check the window material against your region of interest, for example BaF2 for protein work around 1400 to 1750 cm-1 (Thermo Fisher).
- Check the resolution settings suit your sample type (see apodization above).
Common mistakes
- Discarding the interferogram, which removes the option to reprocess the data with different settings.
- Applying heavy apodization to narrow gas-phase lines without checking its effect.
- Using a window or optics that do not transmit in the region of interest.
Where to go next
Once you have a spectrum, the IR functional-group frequency table shows how to read it. For how infrared absorption compares with another vibrational technique, see Raman vs FTIR spectroscopy. Broader context: what is analytical chemistry.
Sources
- Thermo Fisher Scientific, FTIR basics (spectroscopy learning center): thermofisher.com, read 2026-10-29.
- Shimadzu, IRXross FTIR product page: ssi.shimadzu.com, read 2026-10-29.
