What Is FTIR Spectroscopy?
FTIR stands for Fourier Transform Infrared spectroscopy. It is a technique that measures how a sample absorbs infrared light across a wide range of wavelengths simultaneously, using an interferometer and a mathematical process called the Fourier transform.
When infrared light passes through or reflects off a sample, certain frequencies are absorbed by the chemical bonds present. The pattern of absorbed frequencies creates a unique molecular fingerprint that identifies the substance and reveals its chemical structure.
FTIR spectroscopy (also written as FT-IR spectroscopy) became the dominant method for mid-infrared analysis during the 1980s. Today, virtually every laboratory mid-infrared spectrometer sold is an FTIR instrument. The key distinction from older dispersive IR spectrometers is that FTIR measures all wavelengths simultaneously (multiplexing), which gives faster acquisition and better signal-to-noise; an internal laser reference adds superior wavenumber precision.
A typical FTIR spectrum covers the mid-infrared range from 4000 to 400 cm⁻¹ (wavenumbers), though near-infrared (NIR, 12000–4000 cm⁻¹) and far-infrared (below 400 cm⁻¹) measurements are also possible with appropriate sources and detectors. The mid-infrared range is where most organic functional groups produce their strongest, most characteristic absorptions.
How FTIR Spectroscopy Works
At the heart of every FTIR spectrometer is a Michelson-type interferometer — an optical device containing a beamsplitter, a fixed mirror, and a moving mirror. The beamsplitter divides the infrared beam into two paths. When the reflected beams recombine, the varying path difference creates an interference pattern called an interferogram.
The interferogram encodes intensity information about every infrared wavelength in a single signal. A mathematical operation — the Fourier transform — then decomposes this signal into a conventional spectrum of intensity versus wavenumber, revealing which frequencies the sample absorbs.
A single scan at 4 cm⁻¹ resolution takes about a second, and multiple scans can be averaged to improve the signal-to-noise ratio. The result is a highly reproducible infrared spectrum suitable for both qualitative identification and quantitative analysis.
Why FTIR Replaced Dispersive IR
Three fundamental advantages made FTIR instruments superior to their dispersive predecessors:
Fellgett's Advantage (Multiplex)
All wavelengths are measured simultaneously. When detector noise dominates (as with mid-IR thermal detectors), this improves signal-to-noise by up to the square root of the number of resolution elements compared with scanning one wavelength at a time, or gives the same signal-to-noise in far less time. A complete spectrum can be collected in about one second.
Jacquinot's Advantage (Throughput)
FTIR uses a circular aperture instead of narrow slits, allowing much more infrared energy to reach the detector. This higher throughput translates to significantly better signal-to-noise ratio.
Connes' Advantage (Precision)
An internal helium-neon (HeNe) laser precisely tracks the moving mirror position, providing wavenumber precision of about 0.01 cm⁻¹. This ensures excellent reproducibility between scans and instruments.
Principles of FTIR Spectroscopy
FTIR spectroscopy rests on two physical principles: the interaction of infrared radiation with molecular vibrations, and the use of interferometry to encode spectral information.
Molecular Vibrations and IR Absorption
Chemical bonds vibrate at specific frequencies determined by the bond strength and the masses of the connected atoms. When infrared light matches a bond's natural vibration frequency and the vibration causes a change in dipole moment, the molecule absorbs that light. This selective absorption is what produces the peaks in an FTIR spectrum.
Interferometry and the Fourier Transform
Rather than measuring one wavelength at a time (as dispersive spectrometers do), FTIR instruments use a Michelson interferometer to create an interference pattern that encodes all wavelengths simultaneously. The Fourier transform — a mathematical operation — then decomposes this pattern into individual frequency components, producing the final spectrum. This multiplexing approach is the source of FTIR's signal-to-noise and speed advantage (Fellgett's advantage). See detailed interferometer diagrams →
Sample Interaction Modes
FTIR spectroscopy can measure samples in several ways: transmission (IR beam passes through the sample), ATR (attenuated total reflectance, where an evanescent wave probes the sample surface), DRIFTS (diffuse reflectance for powders), and specular reflectance (for coatings on reflective substrates). Each technique suits different sample types. Compare sampling techniques →
A Brief History of FTIR
Albert Michelson builds the interferometer that, in modified forms, is still at the heart of nearly every FTIR instrument. (In 1887 he used it with Edward Morley in the Michelson–Morley experiment.)
Peter Fellgett computes the first Fourier-transform spectrum and identifies the multiplex advantage; Pierre Jacquinot describes the throughput advantage.
The first commercial Fourier-transform spectrometers appear, mostly for the far-infrared, where dispersive instruments performed poorly. The Cooley–Tukey FFT algorithm (1965) makes the computation fast.
Digilab's FTS-14, with HeNe laser referencing and a dedicated minicomputer, becomes the first commercial mid-infrared FTIR.
Cheaper minicomputers and microcomputers bring FTIR from research labs into routine use. By the end of the 1980s FTIR has largely replaced dispersive mid-IR instruments.
Modern desktop FTIR instruments become smaller, more affordable, and more capable. Handheld FTIR devices enter the market for field analysis.
Explore the FTIR Spectroscopy Guide
How FTIR Works
The Michelson interferometer, from interferogram to spectrum
Spectrum Interpretation
Systematic guide to reading and interpreting FTIR spectra
Peak Identifier
Enter wavenumbers, get ranked functional group assignments
Sampling Techniques
Transmission, ATR, DRIFTS, and specular reflectance
ATR-FTIR
Attenuated total reflectance: crystals, penetration depth, and when to use it
FTIR vs Raman
Selection rules, strengths of each technique, and when to choose which
Instruments
Spectrometer types, detectors, specifications, and approximate costs
Applications
FTIR uses in pharma, polymers, food, environment, and forensics
Troubleshooting
Fix water vapor, CO₂ interference, noisy spectra, and more
Frequently Asked Questions
What does FTIR stand for?
FTIR stands for Fourier Transform Infrared. It refers to a method of infrared spectroscopy where an interferometer collects all wavelengths of infrared light simultaneously, and a mathematical technique called the Fourier Transform converts the raw data (an interferogram) into a conventional infrared spectrum.
How does FTIR spectroscopy work?
An FTIR spectrometer uses a Michelson-type interferometer with a fixed mirror and a moving mirror. The moving mirror creates a varying optical path difference between the two beams, producing constructive and destructive interference. The detector records the resulting interferogram — a plot of intensity vs. optical path difference. A Fourier Transform then converts this interferogram into a spectrum of intensity vs. wavenumber (cm⁻¹), revealing which infrared frequencies the sample absorbs.
What is the difference between FTIR and IR spectroscopy?
IR spectroscopy is the general field of using infrared light to study materials. FTIR is a specific method within IR spectroscopy that uses an interferometer and Fourier Transform mathematics. Older 'dispersive' IR instruments used a monochromator to scan wavelengths one at a time. FTIR is faster, more sensitive, and more precise — it has almost entirely replaced dispersive instruments for mid-infrared work in modern laboratories.
What is FTIR spectroscopy used for?
FTIR spectroscopy is used for identifying unknown chemicals, verifying material identity, and quantifying concentrations. Major applications include pharmaceutical quality control, polymer identification for recycling, food adulteration detection, environmental pollutant monitoring, forensic trace evidence analysis, and academic research. It works on solids, liquids, gases, and thin films.
What is the principle of FTIR spectroscopy?
FTIR spectroscopy is based on two principles: (1) molecules absorb infrared light at frequencies matching their bond vibrations, and (2) a Michelson interferometer encodes all wavelengths into a single signal (the interferogram, recorded against optical path difference), which the Fourier transform then decodes into a spectrum. Multiplexing gives FTIR Fellgett's advantage (better signal-to-noise in a given time); the circular aperture gives Jacquinot's throughput advantage; and the HeNe reference laser gives Connes' wavenumber-accuracy advantage.
What wavelength range does FTIR cover?
Standard FTIR spectroscopy covers the mid-infrared range from 4000 to 400 cm⁻¹ (2.5 to 25 micrometers). With appropriate optics, FTIR instruments can also measure the near-infrared (12,000–4,000 cm⁻¹) and far-infrared (below 400 cm⁻¹) regions. The mid-IR range is where most organic functional groups produce their strongest absorptions.
How much does an FTIR spectrometer cost?
Approximate 2026 prices: $15,000 to $25,000 for a basic benchtop instrument with a DTGS detector, $30,000 to $60,000 for a higher-end benchtop model with an MCT detector, $80,000 to $200,000+ for research/high-resolution systems, and $100,000 to $250,000+ for FTIR microscopes. Portable handheld FTIR units typically cost $30,000 to $50,000. Costs vary by manufacturer, detector type, accessories, and software packages.
What is the difference between FTIR and Raman spectroscopy?
FTIR measures infrared light absorbed by a sample (requiring a change in dipole moment), while Raman spectroscopy measures light scattered by a sample (requiring a change in polarizability). They are complementary: some vibrations are IR-active but Raman-inactive, and vice versa. FTIR is generally better for identifying functional groups like O-H, N-H, and C=O, while Raman excels at measuring symmetric vibrations, carbon frameworks, and samples in aqueous solution.
References
- Griffiths, P. R.; de Haseth, J. A. Fourier Transform Infrared Spectrometry, 2nd ed. Wiley, 2007.
- Smith, B. C. Fundamentals of Fourier Transform Infrared Spectroscopy, 2nd ed. CRC Press, 2011.
- Skoog, D. A.; Holler, F. J.; Crouch, S. R. Principles of Instrumental Analysis, 7th ed. Cengage, 2017.