What FTIR is, how the interferometer actually works, what lives inside the instrument, and how to read a spectrum — the fundamentals in one referenced, citation-friendly page. Written for students, educators and working analysts; free to use in coursework and training.
Fourier-transform infrared (FTIR) spectroscopy is a technique for obtaining the infrared absorption or emission spectrum of a solid, liquid or gas. Chemical bonds vibrate — they stretch, bend, rock and twist — at frequencies that fall in the infrared region, and each bond type absorbs infrared light at characteristic frequencies. Measure which frequencies a sample absorbs, and you obtain a molecular fingerprint: evidence of which functional groups are present, and often the identity of the compound itself.
The "Fourier transform" in the name refers to the mathematics, not the chemistry. An FTIR spectrometer does not scan through wavelengths one at a time; it measures all of them simultaneously using an interferometer, then a computer applies a Fourier transform to convert the raw signal (the interferogram) into the familiar spectrum. The first commercial FTIR spectrometer, Digilab's FTS-14, appeared in 1969 — made practical only once minicomputers could perform the transform.
Infrared work is quoted in wavenumbers (cm⁻¹) — the number of wavelengths per centimetre. Higher wavenumber means higher frequency and higher photon energy. The infrared region divides into three working ranges:
| Region | Range | What it probes | Typical use |
|---|---|---|---|
| Near-IR (NIR) | ~12,800–4,000 cm⁻¹ | Overtones and combination bands | Process control, moisture, chemical imaging |
| Mid-IR (MIR) | ~4,000–400 cm⁻¹ | Fundamental molecular vibrations | The classic FTIR range — identification and QC |
| Far-IR | ~400–10 cm⁻¹ | Lattice modes, heavy-atom and metal–ligand vibrations | Inorganics, crystallinity, THz research |
Routine laboratory FTIR lives in the mid-IR, which is why standard optics and windows are specified for 4,000–400 cm⁻¹ — and why window material choice matters so much (see the window materials table in our Sample Preparation Hub).
At the heart of every FTIR instrument sits a Michelson interferometer: a beamsplitter, a fixed mirror and a moving mirror.
Two clever details make this precise. A helium–neon laser (633 nm) travels through the same interferometer; its sinusoidal fringes trigger the sampling of the infrared signal at exactly equal path intervals, and simultaneously calibrate the wavenumber axis. And because a real measurement first records a background spectrum (no sample), the final result is a ratio — the instrument's own profile, atmospheric CO₂ and water vapour largely cancel out.
Older dispersive instruments used a monochromator to scan one narrow wavelength band at a time through slits. Fourier-transform instruments displaced them for three textbook reasons:
The practical consequence: a bench FTIR records a good-quality mid-IR spectrum in seconds, and co-adding a handful of scans polishes it further (signal-to-noise grows with the square root of the number of scans).
| Component | Common choice (mid-IR) | Notes |
|---|---|---|
| Source | Globar — silicon carbide element at ~1,200 K | Near-blackbody emitter. NIR uses tungsten–halogen lamps; far-IR uses mercury discharge lamps. |
| Beamsplitter | KBr substrate with germanium coating | Covers ~4,000–400 cm⁻¹ but is hygroscopic — one reason instruments are purged or desiccated. ZnSe resists moisture (to ~500 cm⁻¹); CaF₂ serves the near-IR; CsI or polymer films extend to the far-IR. |
| Detector (routine) | DTGS (deuterated triglycine sulfate) pyroelectric | Room temperature, robust, adequate for most work. |
| Detector (high performance) | MCT (mercury cadmium telluride), liquid-N₂ cooled | Much faster and more sensitive — an interferogram in as little as ~10 ms; the choice for microscopy, kinetics and weak signals. |
| Reference laser | Helium–neon, 633 nm | Triggers data sampling and fixes the wavenumber scale (the Connes advantage). |
Everything after the interferometer — the sample interface — is the accessory world: ATR units, transmission cells, gas cells, diffuse-reflectance optics. That is the layer Specac has engineered since 1971, and several instrument manufacturers integrate Specac-designed sampling modules into their own spectrometers as OEM components.
Resolution is set by how far the mirror travels: spectral resolution (cm⁻¹) equals the reciprocal of the maximum optical path difference (cm). A 0.25 cm OPD gives 4 cm⁻¹ — the routine setting for solids and liquids, whose bands are naturally broad. Gas-phase work uses 2 cm⁻¹ or finer to resolve rotational structure; research instruments reach 0.001 cm⁻¹.
Scans: co-adding N scans improves signal-to-noise by √N. Sixteen to thirty-two scans is a sensible routine default; quadrupling scan count doubles quality.
Apodization multiplies the interferogram by a taper function before transforming, suppressing the sidelobe artefacts around sharp bands at the cost of slightly broadened peaks — the reason instrument software offers Happ–Genzel, Blackman–Harris and similar options.
The background scan is not optional housekeeping: it captures the source profile, beamsplitter efficiency, detector response and the atmosphere inside the bench. Ratioing sample against background removes them. When you see CO₂ at 2,349 cm⁻¹ or water-vapour combs in a result, the atmosphere changed between background and sample — re-run the background, or purge. Our hub's housekeeping section covers this for Indian humidity specifically.
The spectrometer is only half the measurement; the sampling accessory determines whether the light and the sample meet on useful terms.
| Technique | How it works | Best for |
|---|---|---|
| Transmission | Beam passes through the sample: KBr pellets, mulls, cast films, fixed-path liquid cells, gas cells | Quantitative work, gases, classic library-matched spectra |
| ATR (attenuated total reflectance) | Beam totally internally reflects inside a high-index crystal; an evanescent wave probes ~0.5–2 µm of whatever touches the surface | The modern default: solids, pastes, liquids, polymers with near-zero preparation |
| Diffuse reflectance (DRIFTS) | Collects light scattered from powders — the Praying Mantis™ geometry is the recognised standard for heterogeneous catalysis studies | Powders, rough surfaces, in-situ reaction chambers |
| Specular / grazing angle | Mirror-like reflection off flat or coated surfaces, at grazing incidence for ultra-thin layers | Coatings, films and monolayers on metal |
| Gas cells | Fixed 10 cm paths for concentrated gases; multipass 2.5–10 m cells for trace analysis | Emissions, purity, trace contaminants |
| Microsampling | Diamond compression cells and beam condensers shrink the beam onto microgram specks | Forensics, particles, inclusions |
Choosing between them is a two-question decision — sample form, then goal. Our Accessory Selector automates it, the Sample Preparation Hub FTIR Encyclopaedia gives the full SOPs, and the ATR vs transmission guide settles the most common either/or.
An IR spectrum plots transmittance (or absorbance) against wavenumber, conventionally 4,000 → 400 cm⁻¹ left to right. Two mental zones: above ~1,500 cm⁻¹, bands map cleanly to functional groups; below it lies the fingerprint region — a dense, molecule-specific pattern ideal for library matching even when individual bands defy assignment.
| Band position (cm⁻¹) | Assignment | Character |
|---|---|---|
| 3,200–3,550 | O–H stretch (alcohols, water) | Broad |
| 2,500–3,300 | O–H stretch (carboxylic acids) | Very broad |
| 3,300–3,500 | N–H stretch (amines, amides) | Medium; one or two bands |
| 3,000–3,100 | C–H stretch, aromatic/alkene | Just above 3,000 |
| 2,850–2,960 | C–H stretch, aliphatic | Strong; just below 3,000 |
| 2,210–2,260 | C≡N nitrile stretch | Medium, sharp |
| 2,100–2,260 | C≡C alkyne stretch | Weak–medium |
| 1,650–1,750 | C=O carbonyl stretch | Strong — the most diagnostic band in IR |
| 1,620–1,680 | C=C alkene stretch | Variable |
| 1,000–1,300 | C–O stretch (esters, ethers, alcohols) | Strong |
| < 1,500 | Fingerprint region | Molecule-specific pattern |
Worth remembering: a vibration only absorbs infrared light if it changes the molecule's dipole moment. That is why symmetric homonuclear diatomics — N₂, O₂, H₂ — are invisible to FTIR, and why the technique pairs so well with Raman spectroscopy, whose selection rules are complementary.
FTIR quantifies as well as identifies. Absorbance obeys the Beer–Lambert law, A = ε·l·c — absorbance equals molar absorptivity × path length × concentration. Fix the path length (a sealed liquid cell with a known spacer, or a gas cell of defined length), calibrate against standards, and band intensity reads out concentration. It is how oil-condition labs track additive depletion, how geologists quantify dissolved H₂O and CO₂ in volcanic glasses, and how gas analysers report ppm levels — the long-path multipass cells in our gas cell range exist precisely to stretch l when c is tiny.
The practical caveats: work in the linear range (very strong bands saturate), keep pathlength genuinely constant, and for aqueous systems keep paths short — water absorbs mid-IR ferociously (≤ 0.025 mm, as detailed in the hub).
Strengths: fast (seconds per spectrum), non-destructive, minimal or zero sample preparation with ATR, applicable to solids, liquids and gases, quantitative when calibrated, and inexpensive to run.
Limitations: homonuclear diatomics and noble gases give no spectrum; water dominates mid-IR and constrains aqueous work; complex mixtures overlap and may need chromatographic separation or chemometrics; detection limits trail mass spectrometry for trace organics; and glass/quartz containers are opaque below ~2,000 cm⁻¹, which is why dedicated IR windows and cells exist at all.
Fourier-transform infrared spectroscopy. An interferometer measures all infrared wavelengths simultaneously, and a Fourier transform — a mathematical operation — converts that raw interferogram into the spectrum.
How strongly a sample absorbs infrared light at each frequency. Absorptions correspond to molecular bond vibrations, so the spectrum is a fingerprint of the functional groups present — and, matched against libraries, of the compound itself.
A broadband IR beam enters a Michelson interferometer, where a moving mirror modulates every wavelength at a distinct rate. The beam passes through the sample to a detector, producing an interferogram; a fast Fourier transform converts it into intensity versus wavenumber. Ratioing against a background scan removes the instrument and atmosphere, leaving the sample's spectrum.
Three compounding advantages: multiplex (all wavelengths measured at once — Fellgett), throughput (no slits — Jacquinot) and wavenumber accuracy (laser-referenced axis — Connes). Together they deliver better spectra in a fraction of the time.
Vibrations that don't change the dipole moment: N₂, O₂, H₂ and noble gases are invisible. Highly dilute analytes in strongly absorbing matrices (especially water) are also difficult without pathlength or accessory strategies.
ATR for speed and convenience on solids, pastes, films and neat liquids — press and measure, ~1–2 µm sampled. Transmission for gases, quantitative liquid work in fixed-path cells, and legacy library matching. Our full comparison covers the edge cases.
Potassium bromide transmits across the whole mid-IR (to ~400 cm⁻¹) and sinters into a clear disc under pressure. Its weakness is hygroscopicity — it dissolves in water and fogs in humid air, so pellets are pressed from dried powder and stored desiccated. The full KBr SOP is in the hub.
4 cm⁻¹ and 16–32 scans is the sensible routine default for solids and liquids. Use 2 cm⁻¹ or finer for gases with rotational fine structure. Signal-to-noise improves with the square root of scan count.
It records everything that isn't the sample — source profile, beamsplitter, detector response, atmospheric CO₂ and water vapour — so it can be ratioed out. Bands at 2,349 cm⁻¹ (CO₂) or water combs in your result mean the atmosphere shifted since the background: re-run it or purge.
Essentially non-destructive, particularly by ATR, where the sample lifts off intact. Quantities are small: micrograms to a few milligrams for solids, microlitres for liquids, and diamond compression cells push this to single particles.
Yes — via the Beer–Lambert law (A = ε·l·c) with a fixed pathlength and calibration standards. Sealed liquid cells and defined-path gas cells exist precisely to hold l constant.
Almost certainly. Standard slide-mount accessories fit most instruments from Thermo Fisher, PerkinElmer, Bruker, Shimadzu, Jasco and Agilent, among others. Tell us your model when requesting a quote and we confirm the exact baseplate.
| Wavenumber | Waves per centimetre (cm⁻¹); the standard IR frequency axis. Higher = more energetic. |
| Interferogram | The raw detector signal versus mirror position, before Fourier transformation. |
| OPD | Optical path difference between the interferometer's two arms; its maximum sets resolution. |
| Background | A no-sample reference spectrum ratioed out of every measurement. |
| ATR | Attenuated total reflectance — surface sampling via an evanescent wave at a high-index crystal. |
| Evanescent wave | The shallow (~0.5–2 µm) field that leaks from an ATR crystal into the sample. |
| DRIFTS | Diffuse reflectance infrared Fourier-transform spectroscopy — the powder and catalysis technique. |
| Apodization | Tapering the interferogram to suppress sidelobe artefacts, trading a little resolution. |
| DTGS / MCT | The workhorse room-temperature detector / the cooled high-sensitivity detector. |
| Fingerprint region | Below ~1,500 cm⁻¹, where complex band patterns uniquely identify molecules. |
| Beer–Lambert law | A = ε·l·c — the basis of quantitative IR analysis. |
| Fellgett / Jacquinot / Connes | The multiplex, throughput and wavenumber-accuracy advantages of FT instruments. |
The encyclopaedia explains the instrument; the rest of this site equips it. Step-by-step preparation SOPs, an accessory selector, and the authorised Specac range — with applications advice from Mumbai.
