Fourier Transform Infrared spectrometry (FTIR)

Ana López Pirez, summer semester 2024/25

FTIR stands for Fourier Transform InfraRed, the preferred infrared spectroscopy method in Non-Destructive Testing methods. Infrared light is electromagnetic radiation (EMR) with wavelengths longer than visible light, ranging from 780 nanometers to 1 millimeter ^[1].

In this technique, some infrared radiation that passes through a sample is absorbed, and some is transmitted. The resulting spectrum represents the molecular absorption and transmission, creating a molecular fingerprint of the sample. And just like a fingerprint, no two unique molecular structures produce the same infrared spectrum. It can identify unknown materials, determine the quality or consistency of a sample, and determine the amount of components in a mixture ^[2].

In summary, FTIR is a powerful and widely used NDT technique, particularly valuable for the chemical analysis and characterization of various materials, especially organic and polymeric ones.

• 1 Physical principles
  • 1.1 IR radiation
  • 1.2 Fourier Transform Infrared Spectrometry
• 2 FTIR device
• 3 Fields of application
  • 3.1 Failure analysis
  • 3.2 Polymers
  • 3.3 Coating
  • 3.4 Composites
  • 3.5 Other fields of application
• 4 Literature

Physical principles

IR radiation

Absorption of IR radiation is typical of molecular species with a slight energy difference between the rotational and vibrational states. A criterion for IR absorption is a net change in dipole moment in a molecule as it vibrates or rotates.

The dipole moment is determined by the magnitude of the charge difference and the distance between the two charge centers. As the molecule vibrates, there is a fluctuation in its dipole moment; this causes a field that interacts with the electric field associated with radiation. Suppose there is a match in frequency of the radiation and the natural vibration of the molecule; absorption occurs, which alters the molecular vibration's amplitude. This also happens when the rotation of asymmetric molecules around their centers results in a dipole moment change, which permits interaction with the radiation field ^[2].

Infrared light can be divided into three categories: Near infrared (NIR), mid infrared (MIR), and far infrared (FIR). NIR has the shortest wavelength with higher wavenumbers, while FIR has the longest wavelength with lower wavenumbers. Typically, when discussing IR spectroscopy, MIR is the type of IR light used. The IR light in this range is helpful because it coincides with an essential property of chemical compounds: their vibrations ^[3].

The bond of a molecule experiences various types of vibrations and rotations. This causes the atom not to be stationary and to fluctuate continuously. Vibrational motions are defined by stretching and bending modes. These movements are easily defined for diatomic or triatomic molecules. This is not the case with large molecules.

• Stretching vibration: continuous change in the interatomic distance along the axis of the bond between two atoms. Two stretching vibrations exist: symmetric and antisymmetric
• Bending vibration: change in the angle occurring between two bonds. Four bending vibrations exist: wagging, twisting, rocking, and scissoring

Molecular vibrational frequencies lie in the IR region of the electromagnetic spectrum, and they can be measured using the IR technique. In IR, polychromatic light (light having different frequencies) is passed through a sample, and the intensity of the transmitted light is measured at each frequency. When molecules absorb IR radiation, transitions occur from a ground vibrational state to an excited vibrational state.

For a molecule to be IR active, there must be a change in dipole moment because of the vibration that occurs when IR radiation is absorbed. Dipole moment is a vector quantity and depends on the orientation of the molecule and the photon's electric vector. The dipole moment changes as the bond expands and contracts. When all molecules are aligned, as in a crystal, the photon vector points along a molecular axis, such as z.

IR deals with the interaction between a molecule and radiation from the electromagnetic spectrum. The wavenumber  is defined as 1/wavelength, and its scale is used for these cases and can range from 4000- 40 cm^-1. A linear wavenumber is often used due to its direct relationship with frequency and energy. The frequency of the absorbed radiation causes the molecular vibrational frequency for the absorption process. The relationship is given below:

$//<![CDATA[ \begin{array}{l}σ \left ( cm^{-1} \right )=\frac{1}{λ(cm)}×10^4(\frac{μm}{cm})=\frac{v(Hz)}{c(cm/s)}\end{array} //]]>$

IR spectroscopy is an excellent method for identifying compounds, especially for identifying functional groups. Therefore, we can use group frequencies for structural analysis. Group frequencies are vibrations that are associated with specific functional groups. It is possible to identify a functional group of a molecule by comparing its vibrational frequency on an IR spectrum to an IR stored data bank ^[2].

Fourier Transform Infrared Spectrometry

Fourier Transform Infrared (FT-IR) spectrometry was developed to overcome the limitations (slow scanning process) encountered with dispersive instruments. A method for measuring all the infrared frequencies simultaneously, rather than individually, was needed. A solution that employed a simple optical device called an interferometer was developed. The interferometer produces a unique signal type with all the infrared frequencies “encoded” into it. This signal can be measured quickly, usually in one second or so. Thus, the time element per sample is reduced to a few seconds rather than several minutes. The resulting signal is called an interferogram, with the unique property that every data point (a function of the moving mirror position) makes up the signal and has information about every infrared frequency that comes from the source. This means that all frequencies are measured simultaneously as the interferogram is measured. Thus, the use of the interferometer results in high-speed measurements. Because the analyst requires a frequency spectrum (a plot of the intensity at each frequency) to identify the sample, the measured interferogram signal cannot be interpreted directly. A means of “decoding” the individual frequencies is required. This can be accomplished via a well-known mathematical technique called the Fourier transform. A computer performs this transformation, presenting the user with the desired spectral information for analysis ^[4].

Invented more than one hundred years ago, the two-beam Michelson interferometer is still the heart of most modern Fourier transform infrared (FTIR) spectrometers. It consists of a fixed mirror, a moving mirror, and a beamsplitter, as illustrated in Figure 3. The beamsplitter is a laminate material that reflects and transmits light equally. The collimated infrared (IR) beam from the source (S) is partially transmitted to the moving mirror (M2) and partially reflected to the fixed mirror (M1) by the beamsplitter (G). The two IR beams are then reflected to the beamsplitter by the mirrors. The detector (D) then simultaneously sees the transmitted beam from the fixed mirror and the reflected beam from the moving mirror.

The two combined beams interfere constructively or destructively depending on the wavelength of the light (or frequency in wavenumbers) and the optical path difference introduced by the moving mirror. The latter is referred to as retardation, $//<![CDATA[ \begin{array}{l}δ\end{array} //]]>$ (cm). To obtain an interferogram $//<![CDATA[ \begin{array}{l}I(δ)\end{array} //]]>$ the detector signal is digitized and recorded as a function of retardation. In other words, the interferogram represents the IR light intensity against the optical path difference between the two light beams. The interferogram intensity of a polychromatic source is mathematically described as:

$//<![CDATA[ \begin{array}{l}I(δ)=\int^+^∞_-_∞ B(σ) cos(2πσδ)dσ\end{array} //]]>$

where $//<![CDATA[ \begin{array}{l}B(σ)\end{array} //]]>$  is the spectral intensity at a wavenumber $//<![CDATA[ \begin{array}{l}σ\end{array} //]]>$(cm^-1). The interferogram $//<![CDATA[ \begin{array}{l}I(δ)\end{array} //]]>$ is a simple sinusoidal wave when a monochromatic source is used. For a continuum (or polychromatic) source, $//<![CDATA[ \begin{array}{l}I(δ)\end{array} //]]>$ is a superposition of sinusoidal waves for IR light at all wavenumbers $//<![CDATA[ \begin{array}{l}σ\end{array} //]]>$. At zero path difference (ZPD) or zero optical retardation, all the sinusoidal waves are constructive, producing a centerburst on the interferogram. Fourier transformation (FT) of $//<![CDATA[ \begin{array}{l}I(δ)\end{array} //]]>$ gives the single-beam IR spectrum expressed as below:

$//<![CDATA[ \begin{array}{l}B(σ) =\int^+^∞_-_∞ I(δ) cos(2πσδ)dδ\end{array} //]]>$

In practice, a discrete Fourier transform (DFT) is used as a digital approximation of the continuous Fourier series. Thus, the equation reduces to the following: the constant variables are the wavenumber $//<![CDATA[ \begin{array}{l}σ\end{array} //]]>$, and optical retardation $//<![CDATA[ \begin{array}{l}δ\end{array} //]]>$, have been replaced by discrete values $//<![CDATA[ \begin{array}{l}k\end{array} //]]>$ and $//<![CDATA[ \begin{array}{l}n\end{array} //]]>$, respectively. The summation is then over the total number of discrete data points $//<![CDATA[ \begin{array}{l}N\end{array} //]]>$^[6].

$//<![CDATA[ \begin{array}{l}B(k\cdot \Delta \sigma )=\sum_{n=0}^{N-1}I\left ( n\cdot \Delta \delta \right ) cos \left ( \frac{2\pi k n}{N} \right )\end{array} //]]>$

An infrared spectrum represents a fingerprint of a sample with absorption peaks corresponding to the frequencies of vibrations between the bonds of the atoms making up the material. Because each different material is a unique combination of atoms, no two compounds produce the same infrared spectrum. High‑resolution FTIR imaging maps molecular distributions, phase orientations, and microstructural features, which are critical to understanding anisotropy at small scales ^[7].

Therefore, infrared spectroscopy can result in the identification (qualitative analysis) of different kinds of material. In addition, the size of the peaks in the spectrum is a direct indication of the amount of material present (quantitative analysis). With modern software algorithms, infrared is an excellent tool for quantitative analysis ^[6].

FTIR device

The normal instrumental process is as follows:

• The Source: Infrared energy is emitted from a glowing black-body source. This beam passes through an aperture which controls the amount of energy presented to the sample (and, ultimately, to the detector).
• The Interferometer: The beam enters the interferometer, where the “spectral encoding” occurs. The resulting interferogram signal then exits the interferometer.
• The Sample: The beam enters the sample compartment where it is transmitted through or reflected off the sample's surface, depending on the analysis being accomplished. This is where specific frequencies of energy, which are uniquely characteristic of the sample, are absorbed.
• The Detector: The beam finally passes to the detector for final measurement. The detectors used are specially designed to measure the special interferogram signal.
• The Computer: The measured signal is digitized and sent to the computer, where the Fourier transformation occurs. The final infrared spectrum is then presented to the user for interpretation and further manipulation.

A background spectrum must also be measured because there needs to be a relative scale for the absorption intensity. This is usually a measurement with no sample in the beam. This can be compared to the measurement with the sample in the beam to determine the “percent transmittance.” This technique results in a spectrum that has all the instrumental characteristics removed. Thus, all present spectral features are strictly due to the sample. A single background measurement can be used for many sample measurements because this spectrum is characteristic of the instrument ^[4].

Fourier Transform Infrared (FTIR) spectroscopy can be used through different sampling techniques, the most common being:

• Transmission: this is the most traditional method to analyze samples. Incident infrared light passes through the sample, and the amount transmitted through is measured, producing an FTIR spectrum in %Transmission. Transmission FTIR can be used for liquids, solids, and gases. Whilst still a commonly used technique, and essential for gas measurements, the limitations of transmission FTIR have resulted in it no longer being the most favorable sampling technique for solids and liquids. ATR-FTIR has replaced transmission FTIR as the standard sampling technique.
• Attenuated Total Reflectance (ATR): this is the dominant FTIR method for analysis of solids and liquids as it requires little to no sample preparation and is non-destructive. With ATR-FTIR, an Internal Reflective Element (IRE) is used as the ATR crystal into which the beam of radiation is directed. This must have a high refractive index, higher than the sample's, to prevent the beam from going through the sample. When the light is shone into the IRE and is reflected from its internal surface, an evanescent wave is created that projects orthogonally into the sample. The sample absorbs a certain amount of energy, and the evanescent wave becomes attenuated. The attenuated beam is reflected out of the IRE and to the detector.
• Diffuse Reflectance (DRIFTS Spectroscopy): this is a second external reflectance technique used for collecting IR spectra from strongly absorbing samples with rough surfaces, such as powders. The IR incident beam penetrates the sample and subsequently scatters in all directions. The main application area of DRIFTS is qualitative analysis of powders in pharmaceutical and forensic laboratories ^[8].

Fields of application

Failure analysis

Infrared spectroscopy is highly useful in failure analysis and can be a great asset in cases where it is critical to understand why and how a material failure occurred. FTIR is also useful for identifying contaminants and chemical agents in contact with failed parts, chemical bonds, and sequences resulting from degradation, such as heat, water, oxygen, oxidizers, or UV light. Applications of FTIR in failure analysis investigations include:

• Material identification
• Qualitative assessment of polymers, copolymers, and blends
• Identification and verification of resin formulation ingredients
• Identification of contaminants in a material
• Determination and quantification of molecular degradation
• Identification of chemical agents in contact with failed parts ^[9]

One practical example is in the determination of fire damage in concrete structures. There is a study in which the equivalent maximal temperature that concrete has sustained during a fire is determined. This is useful in forensic investigations, particularly for fires in buildings. This is done using a handheld FTIR instrument to establish the thermal history of concrete structures. The results in this study have demonstrated the development of a model using multivariate data analysis for the non-destructive evaluation of thermally damaged and degraded specimens. Three successful multivariate methods were developed for three concrete formulations typically deployed for specific civil engineering applications. These models show good potential, which could be further extended to other types of concrete ^[10].

Polymers

Release agents are applied as a liquid to form a thin film coating, aiding the removal of carbon-fiber-reinforced polymer (CFRP) parts from a mold or peel ply. This process can lead to inevitable contact transfer, causing the release agent to remain on the part. Retained release agent can lead to a reduced bond strength of joined parts or poor topcoat paint adhesion. FTIR can be used to quantify the release agent on a carbon-fiber reinforced polymer ^[11]. FTIR is also useful in polymers for the following tasks:

• Identify, verify, and authenticate polymer components
• Measure the degree of cross-linking and cure
• Determine the composition of copolymers
• Analyze rubber and other elastomers – even those containing carbon particles
• Quantify phthalate plasticizer in polymeric materials used in consumer products
• Verify composition and authenticity of seals, gaskets, and O-rings
• Establish the identity and composition of carbon-filled polymers recycled from electronics.

Coating

Another practical example of FTIR is monitoring, evaluating, and quantifying various factors throughout a coating’s life cycle. Modern coatings are often multilayer systems with each layer having a specific role. These layers are often two-part formulations, especially for higher performing grades containing a resin, binder, or film former (Part A) and the curative or hardener (Part B). Before these two parts are mixed and applied, the surface condition of the material must be devoid of contaminants and in a suitable condition to be coated. Handheld FTIR instruments are used to monitor, evaluate, and quantify various factors throughout a coating’s life cycle from the unused product to its end of useful life ^[12]. Other operations that can be done on coatings are:

• Track the cleaning of contaminants from inorganic and organic surfaces
• Ensure the correct coating has been applied to the finished product
• Test whether primers and coatings are properly cured
• Measure thickness and uniformity on metal surfaces
• Evaluate monolayer coatings for coverage and uniformity
• Monitor paint aging and weathering
• Identify lacquers, paints, and pigments used in art conservation and restoration
• Determine the presence of residual solvent following coating cure ^[13]

Composites

FTIR is used in composite materials, for example, to characterize the materials used to produce FRP (Fiber‑Reinforced Polymer) and to evaluate the potential degradation of the polymeric matrix during manufacturing. Especially in natural fiber composites, moisture or humidity can negatively influence the mechanical properties of the composite, as it can lead to interface degradation. The natural fibers have hydrophilic properties and absorb more water than the resin commonly used in natural FRP. This condition is reflected in swelling of fibers, micro-cracking in the composite, and loss of interfacial adhesion due to the stress induced by water absorption ^[14].

Some more examples of the use of FTIR in composite materials are:

• Detecting damage caused by excess exposure to heat
• Mapping thermal damage on surfaces
• Guiding sanding, scarfing, and patching repairs
• Measuring oxidative damage from UV light and other environmental factors
• Confirming the effectiveness of plasma treatment in preparing composite surfaces for bonding
• Detecting hydrocarbon and silicone oil contamination
• Assessing moisture levels
• Determining the extent of pre-preg curing
• Identifying and verifying composition ^[13]

The reader can find even more uses of FTIR for an array of composite materials in the literature ^[15].

Other fields of application

• Food: measure soil composition and chemistry, analyze fruit and vegetable ripeness
• Art and historical object conservation: confirm authenticity, analyze paints, pigments, fillers, and lacquers, determine paper and textile composition
• Energy and chemicals: identify engineering components (such as gaskets, seals, and O-rings) by composition, monitor UV degradation of polymers used in solar panel arrays, identify and qualify films and coatings for critical applications in oil and gas exploration and handling, measure thermal damage and coating wear on composite wind turbine blades
• Mining and Geology: measure soil composition, analyze rocks, minerals, and ores ^[13]

Literature

1. Bundesamt für Strahlenschutz: Optical radiation, (n.d.). https://www.bfs.de/EN/topics/opt/opt_node.html
2. Liu, X.: 2: Infrared (IR) Spectroscopy Theory. Chemistry LibreTexts, (n.d.). https://chem.libretexts.org/@go/page/1847?pdf
3. Bruker Corporation: Guide to Infrared Spectroscopy, (n.d.). https://www.bruker.com/en/products-and-solutions/infrared-and-raman/ft-ir-routine-spectrometer/what-is-ft-ir-spectroscopy.html
4. Thermo Nicolet Corporation: Introduction to Fourier Transform Infrared Spectrometry, (2001).
5. Nilesh, S.: Schematic of Michelson Interferometer. Wikimedia Commons, (2022). https://commons.wikimedia.org/wiki/File%3ASchematic_of_Michelson_Interferometer.svg
6. Nicolet CZ: Advanced FT‑IR Spectroscopy: Principles, Experiments and Applications. Nicolet CZ, (2018).
7. Ludwig, M.: Elasticity tensor in anisotropic materials. (2020). https://collab.dvb.bayern/x/UpgtB
8. Edinburgh Instruments: Common Sampling Techniques of FTIR Spectroscopy. (2023). https://www.edinst.com/resource/common-sampling-techniques-of-ftir-spectroscopy/
9. S. Held: Fourier‑Transform Infrared (FTIR) Technology & Analysis. J.S. Held, May (2022).
10. Tang, P. L., Alqassim, M., Nic Daéid, N., Berlouis, L., & Seelenbinder, J.: Nondestructive handheld Fourier transform infrared (FT‑IR) analysis of spectroscopic changes and multivariate modelling of thermally degraded plain Portland cement concrete and its slag and fly ash‑based analogs. Applied Spectroscopy, (2016).
11. Agilent Technologies: Applications of FTIR. Agilent, (2019).
12. Agilent Technologies: Handheld FTIR Spectroscopic Applications of Modern Coatings. Agilent, (2018).
13. Agilent Technologies: At-Site. Immediate Results. True Nondestructive Analysis. Agilent, (2025).
14. Verma, D., & Senal, I.: Natural fiber-reinforced polymer composites: feasibility study for sustainable automotive industries. Biomass, biopolymer-based materials, and bioenergy: construction, biomedical, and other industrial applications, UK: Woodhead Publishing, (2019), p. 103–122.
15. Murakoshi Takematsu, M.:Analytical approaches to fiber-reinforced polymer composites: a short review. Departamento de Química, Instituto Tecnológico da Aeronáutica – ITA, São José dos Campos, SP, Brasil, (2023).