FTIR, or Fourier Transform Infrared Spectroscopy, is a technique that identifies what a material is made of by measuring how it absorbs infrared light. Every type of chemical bond absorbs infrared light at specific frequencies, almost like a molecular fingerprint. FTIR reads that fingerprint and tells you which chemical groups are present in a sample, making it one of the most widely used tools for identifying unknown substances in chemistry, manufacturing, and forensics.
How Infrared Light Reveals Molecular Identity
Molecules are not static. Their atoms are constantly vibrating, stretching, and bending at specific frequencies determined by the types of bonds holding them together, the mass of each atom, and the overall shape of the molecule. When infrared light hits a sample, any frequency that matches the natural vibration of a bond gets absorbed. The rest passes through.
By measuring which frequencies were absorbed and how strongly, you get a spectrum: a pattern of peaks and valleys unique to that substance. A peak at one frequency might indicate an oxygen-hydrogen bond (the kind found in alcohols), while a peak at another might point to a carbon-oxygen double bond (found in acids, esters, and other carbonyl compounds). FTIR is particularly good at identifying the “chemical family” a substance belongs to, even in mixtures.
Inside the Instrument
Older infrared spectrometers worked by scanning one wavelength of light at a time, which was slow and produced weak signals. FTIR solved both problems by using a device called a Michelson interferometer, which sends all wavelengths of infrared light through the sample simultaneously.
The interferometer has three key parts: a beam splitter, a fixed mirror, and a moving mirror. Infrared light enters and hits the beam splitter, which sends half the light toward the fixed mirror and half toward the moving mirror. Both beams bounce back and recombine at the beam splitter before passing through the sample and reaching the detector. Because the moving mirror is constantly shifting position, the two beams travel slightly different distances. That difference causes some wavelengths to reinforce each other and others to cancel out, creating a complex signal called an interferogram.
The interferogram doesn’t look like a useful spectrum yet. It’s a raw signal that encodes intensity information across all wavelengths at once. A computer then applies a mathematical operation, the Fourier transform, to convert this raw signal into the familiar infrared spectrum with peaks at specific frequencies. This conversion takes the data from a time-like domain into a frequency domain, separating out the contribution of each wavelength so you can see exactly where absorption occurred.
Why FTIR Replaced Older Methods
FTIR has two major advantages over the older style of infrared spectrometer. The first is speed. Because the interferometer collects data from all wavelengths at once rather than scanning them one by one, a complete spectrum can be recorded in seconds. This is known as the multiplex advantage. It also means you can run dozens of scans and average them together, which dramatically improves the signal quality.
The second advantage is sensitivity. FTIR instruments don’t need narrow slits to isolate wavelengths and use fewer mirrors overall, so much more light actually reaches the sample and detector. This higher throughput means the instrument can detect weaker signals and work with smaller samples. Together, these two advantages made FTIR faster, more sensitive, and more reliable than anything that came before it.
Sample Preparation and ATR
Traditionally, running an FTIR measurement required pressing a sample into a pellet with potassium bromide (KBr), a salt that’s transparent to infrared light. This worked, but it was tedious. You had to grind your sample into a fine powder, mix it thoroughly, press it under high pressure, and keep everything dry, since moisture in the air could interfere with results.
Most modern FTIR instruments use a technique called Attenuated Total Reflectance, or ATR. Instead of passing light through a prepared pellet, ATR bounces the infrared beam through a crystal. At the surface where the crystal contacts your sample, a tiny amount of light penetrates into the material and interacts with it before bouncing back to the detector. You simply press your sample (solid, liquid, paste, or powder) against the crystal surface and run the scan. No grinding, no pellets, no drying. ATR has become the standard approach because it’s faster, requires almost no preparation, and works with a much wider variety of sample types.
Reading an FTIR Spectrum
An FTIR spectrum is displayed as a graph with wavenumber (a unit of frequency, measured in inverse centimeters) on the horizontal axis and either absorbance or transmittance on the vertical axis. Different regions of the spectrum correspond to different types of molecular vibrations, and experienced users can identify functional groups by where peaks appear.
Some of the most recognizable features include:
- Broad peak around 3200 to 3550 cm⁻¹: Typically an oxygen-hydrogen stretch, indicating alcohols or phenols
- Peaks near 3300 to 3500 cm⁻¹: Nitrogen-hydrogen stretches, pointing to amines or amides
- Strong, sharp peaks between 1630 and 1780 cm⁻¹: Carbon-oxygen double bonds, one of the most distinctive features in IR spectra, found in acids, esters, ketones, and aldehydes
- The fingerprint region (600 to 1500 cm⁻¹): A complex zone of overlapping peaks unique to each molecule, useful for confirming exact identity rather than just functional group type
The carbonyl (C=O) absorption is one of the strongest and most useful peaks in infrared spectroscopy. Its exact position within the 1630 to 1780 range narrows down what type of compound you’re looking at. An ester, for example, absorbs in a slightly different spot than a carboxylic acid.
Quantitative Analysis
FTIR isn’t limited to identifying what’s in a sample. It can also tell you how much. The principle is straightforward: the stronger the absorption at a given frequency, the more of that substance is present. This relationship follows the Beer-Lambert law, which states that absorbance is proportional to concentration multiplied by the path length the light travels through the sample.
In practice, you measure the height or area of a characteristic peak and compare it to a calibration curve built from samples with known concentrations. For complex mixtures where peaks overlap, statistical methods like partial least squares regression can tease apart contributions from multiple components and achieve prediction accuracy within fractions of a percent by weight.
Common Applications
FTIR’s combination of speed, minimal sample prep, and molecular specificity makes it useful across a wide range of fields. In pharmaceutical manufacturing, it’s routinely used to verify that incoming raw materials are what the supplier claims. Every batch of a chemical ingredient can be scanned in seconds and compared against a reference library. The same approach helps detect counterfeit drugs, since fakes often contain different fillers or active ingredients that show up immediately in the spectrum.
Forensic scientists rely on FTIR to identify trace evidence. Paint chips, textile fibers, and unknown powders can all be characterized without destroying the sample. Recent work has used FTIR to distinguish between different types of paint-derived microplastics found in marine environments, identifying materials like acrylate, epoxy, urethane, and alkyd coatings based on their fingerprint region absorptions.
Environmental monitoring is another major use. FTIR can identify microplastics in water, measure pollutant concentrations in air samples, and characterize soil contaminants. Food safety labs use it to detect adulteration, checking whether olive oil has been diluted with cheaper oils or whether honey contains added sugars. The technique is fast enough for production-line quality control, where hundreds of samples may need testing in a single shift.
Portable and Handheld FTIR
One of the biggest shifts in recent years has been the miniaturization of FTIR instruments. Portable and handheld devices now allow real-time analysis outside the laboratory. First responders use them to identify unknown chemicals at spill sites. Customs agents scan suspicious powders at borders. Conservators analyze pigments on paintings without removing any material.
These portable units are becoming more affordable and easier to operate, with some models incorporating cloud-based spectral libraries and AI-assisted identification that can match an unknown sample against thousands of reference spectra in seconds. The market for portable FTIR spectrometers is growing at roughly 12% annually, driven by demand in pharmaceuticals, environmental monitoring, and food safety.