Fourier change infrared spectroscopy (FTIR) identifies chemical bonds and functional groups in materials by measuring how they absorb infrared light. It reveals molecular structure through unique vibrational fingerprints, enabling precise material analysis across industries.
FTIR works by passing infrared radiation through a sample. Molecules absorb specific wavelengths based on their atomic bonds-bonds between carbon-hydrogen, oxygen-hydrogen, or nitrogen-carbon vibrate at characteristic frequencies. The instrument records the transmitted or reflected light, then uses a mathematical transformation called the Fourier change to convert raw data into a readable spectrum. This process improves signal-to-noise ratio and speeds up measurement compared to older dispersive spectrometers.
The core component is the interferometer, which splits the infrared beam into two paths. One reflects off a moving mirror, the other off a fixed mirror. When the beams recombine, they create interference patterns. These patterns encode all frequency information. A computer processes the resulting interferogram using the Fourier change algorithm to produce a spectrum showing absorption intensity versus wavenumber (cm⁻¹).
FTIR is used in pharmaceutical development to verify drug purity and detect counterfeit medications. For example, the U.S. Pharmacopeia (USP) requires FTIR for identity testing of active pharmaceutical ingredients (APIs). A single scan can confirm whether a tablet contains the correct compound by matching its spectral pattern against a reference library.
In environmental science, FTIR analyzes pollutants in air and water. Researchers at the National Institute of Standards and Technology (NIST) use FTIR to quantify volatile organic compounds (VOCs) in emissions. One study detected benzene at concentrations below 1 ppm using attenuated total reflectance (ATR)-FTIR, a variant that requires minimal sample preparation.
Manufacturers rely on FTIR for quality control. In plastics production, it verifies resin composition and detects contaminants. A batch of polyethylene pellets was flagged during routine screening because its spectrum showed unexpected carbonyl absorption-indicating oxidation from improper storage. Correcting the handling protocol prevented defective product shipment.
FTIR systems differ by sampling method. Transmission mode requires thin samples, such as KBr pellets or films. Attenuated total reflectance (ATR) uses a diamond or zinc selenide crystal to probe surface layers. ATR is ideal for solids, liquids, and irregular surfaces-no grinding or pressing needed.
| Mode | Best For | Sample Prep Required | Example Use Case |
|---|---|---|---|
| Transmission | Thin films, powders | High (grinding, pelletizing) | Verifying API identity in tablets |
| ATR | Solids, liquids, coatings | Low (clean surface only) | Detecting oil contamination on metal parts |
| Reflection | Metals, rough surfaces | None | Measuring paint layer thickness on automotive panels |
Diffuse reflectance (DRIFT) captures scattered light from powdered samples. It's used in soil analysis to identify organic matter content. The technique measures how deeply infrared light penetrates the sample and scatters back-ideal for heterogeneous materials.
Many assume FTIR identifies elements. It does not. It detects molecular vibrations, not elemental composition. To identify elements, use techniques like X-ray fluorescence (XRF) or inductively coupled plasma mass spectrometry (ICP-MS).
Another misconception is that FTIR requires large sample sizes. Modern instruments with ATR accessories analyze spots as small as 1 mm². A forensic lab in Chicago used ATR-FTIR to compare trace fibers from a suspect's clothing to those found at a crime scene-both matched within 0.5 cm⁻¹ of peak position.
Some confuse FTIR with Raman spectroscopy. Both detect molecular vibrations, but Raman relies on inelastic scattering of laser light, while FTIR depends on absorption. Raman excels with aqueous samples and crystalline materials; FTIR performs better with polar bonds and gases. For example, a research team at MIT used both methods to characterize graphene oxide-Raman confirmed layer count, FTIR verified oxygen-containing functional groups.
In a 2021 incident, a food processing plant reported a strange odor in packaged soup. Samples were sent to a lab for analysis. Using ATR-FTIR, analysts obtained a spectrum with strong peaks at 1740 cm⁻¹ (ester C=O stretch), 1240 cm⁻¹ (C-O-C bend), and 1100 cm⁻¹ (C-O stretch). These features matched ethyl acetate-a solvent used in flavor extraction.
Further investigation revealed a leak in a nearby mixing tank. The FTIR result confirmed contamination without needing complex separation steps. The batch was recalled, and the system was repaired. The entire analysis took under 15 minutes, including data processing.
I've used FTIR in a university lab to validate synthesized polymers. After synthesizing a copolymer of styrene and maleic anhydride, I ran an ATR-FTIR scan. The spectrum showed a sharp peak at 1780 cm⁻¹-the signature of anhydride C=O stretching. Without this, we couldn't confirm successful reaction completion. The test saved weeks of time compared to NMR.
When choosing analytical tools, consider that FTIR is not a standalone solution. Combine it with mass spectrometry or nuclear magnetic resonance (NMR) for definitive structural confirmation. But for rapid, non-destructive screening, few techniques match FTIR's speed and accessibility.
For more on alternative spectroscopic methods, a short comparison between FTIR and Raman spectroscopy shows how each complements the other in material characterization.