FTIR Spectroscopy Explained: Principles, How It Works & Applications
What Is FTIR Spectroscopy?
Fourier Transform Infrared (FTIR) Spectroscopy is one of the most powerful and widely used analytical techniques for identifying organic and inorganic materials, characterising chemical functional groups, and detecting molecular structural changes in solids, liquids, and gases. It operates by measuring the absorption of infrared radiation at characteristic wavelengths corresponding to the vibrational frequencies of molecular bonds — producing a “fingerprint” spectrum unique to each chemical compound.
FTIR is routinely used for polymer identification, contamination characterisation, failure analysis, coating analysis, and chemical reaction monitoring across virtually every materials-intensive industry.
How FTIR Works
The Interferometer Principle
Unlike dispersive spectrometers that scan one wavelength at a time, FTIR instruments use a Michelson interferometer to collect all wavelengths simultaneously. The interferometer contains a beamsplitter that divides the infrared source beam into two paths — one reflected onto a fixed mirror, the other transmitted to a moving mirror. When the beams recombine after traveling different path lengths, they interfere constructively or destructively depending on the wavelength and the optical path difference (retardation).
As the moving mirror travels, the detector records the combined beam intensity as a function of the mirror’s position —an interferogram. A mathematical Fourier transform converts the interferogram to the conventional absorption spectrum (absorbance or transmittance vs. wavenumber in cm⁻¹).
Infrared Absorption and Molecular Vibrations
Infrared radiation (wavenumber range 400–4000 cm⁻¹ for mid-IR) causes molecules to vibrate at their natural bond frequencies. Stretching vibrations (C-H at 2850–3000 cm⁻¹, C=O at 1700–1750 cm⁻¹, O-H at 3200–3500 cm⁻¹) and bending vibrations (C-H bending at 1350–1470 cm⁻¹) produce characteristic absorption bands. The fingerprint region (400–1500 cm⁻¹) contains complex, overlapping bands unique to each compound’s overall molecular structure.
FTIR Sampling Techniques
Transmission FTIR
The traditional method — IR beam passes through a thin specimen (KBr pellet, pressed film, solution cell). Suitable for powders, liquids, and thin films. Requires sample preparation.
ATR-FTIR (Attenuated Total Reflection)
The most common modern technique. An evanescent wave from total internal reflection in a crystal (diamond, germanium, zinc selenide) penetrates 1–5 µm into the sample pressed against the crystal — no sample preparation required. Ideal for polymer surfaces, coatings, adhesives, residues, and bulk materials. Direct contact analysis of solids, liquids, and gels.
DRIFTS (Diffuse Reflectance Infrared Fourier Transform Spectroscopy)
For powder samples and rough surfaces, the diffusely reflected IR beam is collected and analysed. Useful for catalyst characterisation, pharmaceutical powders, and soil analysis.
Microscopy (FTIR Microscopy / Microspectroscopy)
An infrared microscope enables spatially resolved FTIR spectra from areas as small as 10 µm × 10 µm — used for contamination particle identification, defect characterisation, and laminate layer analysis in failure investigation.
Industrial Applications of FTIR
Polymer Identification and Quality Control
ATR-FTIR identifies polymer types (PE, PP, PVC, PET, ABS, nylon, silicone, etc.) from their characteristic spectrum patterns matched against NIST or commercial spectral library databases — the fastest and most definitive polymer identification method for incoming material inspection and failure analysis.
Contamination Analysis
FTIR microscopy identifies the chemical nature of small contamination particles (>10 µm) found in products, on surfaces, or in process fluids — distinguishing silicone (Si-O bands at 1000–1100 cm⁻¹), lubricant residues (C-H bands), tape adhesive, biological matter, and inorganic deposits from their spectra.
Coating Analysis and Degradation Assessment
ATR-FTIR monitors oxidative degradation of polymer coatings by tracking carbonyl index (C=O band at ~1700 cm⁻¹) increase after UV weathering or thermal ageing — as used in ASTM D6387 geomembrane OIT assessment.
Adhesive and Sealant Characterisation
FTIR verifies curing degree of thermosetting adhesives (epoxy, polyurethane, acrylate) by tracking reactive group consumption — epoxide ring opening (C-O-C at 910 cm⁻¹ decrease), isocyanate consumption (N=C=O at 2270 cm⁻¹ decrease).
Conclusion
FTIR spectroscopy is a powerful, rapid, and versatile analytical technique that provides detailed molecular-level insight into material composition and structure. By identifying functional groups and generating unique spectral fingerprints, it enables accurate material identification, contamination analysis, and degradation assessment across a wide range of industries. Its minimal sample preparation, fast analysis time, and broad applicability make FTIR an essential tool for quality control, failure analysis, and research — delivering reliable chemical characterisation for both routine and advanced applications.
Why Choose Infinita Lab for FTIR Analysis?
Infinita Lab provides ATR-FTIR, transmission FTIR, FTIR microscopy, and DRIFTS analysis through our nationwide accredited analytical chemistry and materials characterisation laboratory network. Our spectroscopists provide expert spectrum interpretation and library matching for definitive compound identification.
Looking for a trusted partner to achieve your research goals? Schedule a meeting with us, send us a request, or call us at (888) 878-3090 to learn more about our services and how we can support you.
Frequently Asked Questions (FAQs)
Can FTIR identify unknown polymers in a blended or compounded material? FTIR can identify the major polymer component(s) and many additives in compounded materials. Simple blends where one polymer dominates produce recognisable spectra. Complex multi-component blends may require spectral subtraction, principal component analysis, or complementary characterisation (DSC, py-GC-MS) to resolve overlapping spectra from multiple components.
What is the detection limit of FTIR for contaminants in a polymer matrix? For contaminants with strong, distinct absorption bands in an otherwise clean spectral region, FTIR can detect concentrations of 0.1–1% by mass in a polymer matrix using ATR-FTIR. Weaker absorbers or contaminants whose bands overlap the matrix bands require concentrations of 1–5% for reliable detection. FTIR microscopy enables identification of individual contamination particles >10 µm diameter.
How does FTIR confirm the cure state of an epoxy adhesive? Uncured epoxy resin shows a characteristic absorption band at approximately 910 cm⁻¹ from the epoxide ring C-O-C stretch. As the epoxy cures (epoxide rings open and react with the hardener), this band decreases in intensity. The ratio of the 910 cm⁻¹ band to an internal reference band (typically aromatic C-C at 1510 cm⁻¹) provides the degree of cure — approaching zero as cure reaches completion.
What is the difference between mid-IR and near-IR FTIR, and when is each used? Mid-IR (400–4000 cm⁻¹) probes fundamental molecular vibrations — providing the primary fingerprint spectral information for compound identification and functional group analysis. Near-IR (4000–14000 cm⁻¹) probes overtone and combination bands — weaker absorbers that permit deeper penetration into samples and non-invasive analysis of intact products. NIR is widely used for rapid at-line quality control of incoming raw materials and products through transparent packaging.
Can FTIR distinguish between different grades of nylon (PA6 vs PA66 vs PA12)? Yes. Different nylon grades have slightly different FTIR spectra in the amide I and amide II band regions (1600–1700 cm⁻¹ and 1500–1560 cm⁻¹) and in the C-H stretching and bending regions — reflecting their different repeat unit lengths and hydrogen bonding densities. Spectral library matching can reliably distinguish PA6, PA66, PA12, PA11, and PA46 from their characteristic spectral differences.
