Basics of FTIR spectroscopy

Infrared spectroscopy is the study of the interaction of infrared light with matter. It is based on the ability of almost all molecules to absorb infrared radiation at specific wavelengths. Each molecule has characteristic absorption properties, which are often linked to its functional groups. These absorption features can be used like a "fingerprint" to identify substances. An infrared spectrometer measures these absorptions and produces an infrared spectrum, which can be used for identifying and determining the structure of chemical compounds.^[6]

In routine work, the most important region is the mid-infrared, typically from 4,000 to 400 cm⁻¹. 

For a detailed discussion on this subject, see "What is light – a particle or a wave?"
Electromagnetic radiation propagates through space carrying a certain amount of electromagnetic energy. This energy is proportional to its frequency ν of oscillation, which is connected to the wavelength λ by the speed of light c (Equation 1):
 

Therefore, a light wave (or photon) carries more energy E the larger the frequency is or, alternatively, the smaller the wavelength is (Equation 2).

For historical reasons, spectroscopists also like to use the wavenumber ṽ, which is defined as the reciprocal of the wavelength. The wavenumber is directly proportional to the energy of the photon (Equation 3) and usually expressed in units of reciprocal centimeters (cm^–1) to give easy-to-read numbers.

The above description is valid for a single light wave or photon. However, a light beam consists of many light waves with different frequencies propagating in the same direction. Each frequency contributes to the beam with intensity I (i.e., a certain number of photons per time interval). The intensity of a light beam is the quantity that is ultimately measured with the detector of a spectrometer.

The intensity distribution of all frequencies is the spectrum of the light beam. Only a small part of the light frequencies can be seen by the human eye (“visible light”). Other spectral regions include microwave, infrared, ultraviolet (UV), or Röntgen (X-ray) radiation. For IR spectroscopy, infrared (IR) light is used for the excitation.^[8],[9],[10]
 

An infrared spectrum is a plot of measured intensity versus wavenumber. When the spectrum is shown in absorbance, peaks point upward and indicate where the sample absorbed infrared light. When the same data are shown in transmittance, the peaks point downward. For most analytical work, absorbance is preferred because it is the correct basis for quantitative analysis, spectral subtraction, and library searching. 

Each peak corresponds to a molecular vibration, belonging to a functional group of the molecule. The overall pattern indicates the chemical structure of a sample. This is why FTIR is often described as a fingerprint method. At the same time, peak height is related to concentration through Lambert–Beer’s law:

 A = absorbance 
ε = constant (absorptivity) 
l = pathlength 
c = concentration 

This combination makes FTIR useful not only for identifying materials, but also for measuring how much of a component is present.
 

An FTIR spectrometer consists of the following components:

• Light source
• Interferometer with beam splitter
• Sample cell
• Detector
• Computer unit for Fast Fourier Transform

At the heart of every FTIR spectrometer is an interferometer. Instead of separating light wavelength by wavelength with slits and gratings, the beam splitter of the interferometer splits the beam into two paths, changes the optical path difference between them with a moving mirror, and recombines them. Before reaching the detector, the IR light passes through the sample in the sample chamber, where specific molecular vibrations absorb characteristic wavelengths of the radiation. As a result, the transmitted light carries the absorption information of the sample. The detector records the changing signal as an interferogram. A mathematical operation, the Fourier transform, converts that interferogram into the familiar spectrum.

In routine use, the instrument first measures a background spectrum and then a sample spectrum. By ratioing these, most contributions from the instrument and environment are removed, leaving the spectrum of the sample. A built-in laser acts as an internal wavenumber reference, which is one reason FTIR peak positions are highly reproducible.

Such a design ensures that more light reaches the detector, all wavelengths are measured simultaneously, and wavenumber measurements remain precise thanks to the laser reference.

These are the main reasons instruments used for FTIR spectroscopy can outperform older dispersive IR systems in everyday laboratory work.

Why molecules absorb infrared radiation?

In IR spectroscopy, molecular vibrations are excited by the absorption of infrared radiation. This leads to changes in bond lengths and bond angles within a molecule. Two main categories of vibrations are distinguished (see Figure 4)^[2]:

1. Valence vibrations (stretching vibrations): These involve changes in the bond lengths between atoms

   • Symmetric stretching: All involved atoms move toward or away from each other at the same time
   • Asymmetric stretching: One atom moves toward another, while the other moves away

    

2. Deformation (bending) vibrations: These involve changes in the bond angles.
   • Bending vibration: Two atoms move toward or away from each other without changing the bond lengths
   • Wagging vibration: The atoms swing back and forth in the same plane
   • Rocking vibration: The motion occurs out of the molecular plane
   • Twisting vibration: A rotating motion around a bond axis

Number of degrees of freedom

The simplest example of a molecule is a diatomic system such as molecular oxygen (O₂), nitrogen (N₂), or carbon monoxide (CO). These molecules have only one vibrational degree of freedom along the bond axis, as the two atoms can only move relative to each other along their mutual distance. This motion is called a stretching or valence vibration.^[3] In multi-atomic molecules, additional independent vibrational modes are possible due to the presence of more atoms. These are referred to as normal modes. The number of these normal modes depends on the ways in which each atom can move. Each atom has three degrees of freedom, corresponding to movement in the three spatial directions. A molecule with N atoms therefore has a total of 3N degrees of freedom.^[3],[4] Since the atoms in a molecule are connected by chemical bonds, they cannot move completely independently. However, the molecule is not rigid, so relative movements between atoms are possible. As a result, the total degrees of freedom of a molecule can be divided as follows^[4]:

• Three degrees of freedom correspond to translation (i.e., movement of the entire molecule in the x-, y-, and z-directions)
• Two degrees of freedom (for linear molecules) or three (for non-linear molecules) are used for rotation around spatial axes
• The remaining degrees of freedom are associated with vibrational motion of the atoms within the molecule. This results in linear molecules that have 3N−5 vibrational modes, while bent (non-linear) molecules have 3N−6 vibrational modes.5

An example is the linear carbon dioxide molecule (CO₂), which consists of three atoms. It therefore has 3 × 3 − 5 = 4 vibrational modes, including the symmetric and asymmetric stretching vibrations, as well as two bending vibrations, one in the plane and one out of the plane of the diagram. In contrast, the bent water molecule (H₂O) also consists of three atoms but has 3 × 3 − 6 = 3 vibrational modes.

Types of vibrations and their effects on the absorption region

The carbonyl group is a common functional group in chemistry that, due to its permanent dipole moment, gives rise to very distinctive bands caused by the stretching vibration of the C=O bond. The wavenumber of this band (or bands) is strongly influenced by the chemical environment of the carbonyl group. Delocalization effects can alter the bond strength of the C=O bond, resulting in a shift of the IR band. Typical wavenumber ranges for various functional groups can be seen in Table 2 and Figure 6.^[9]

Table 2: Overview of important vibrations.[9]

A major advantage of FTIR spectroscopy is that little sample preparation is needed for routine sampling methods. 
The three main sampling methods are:

• Transmission
• Attenuated total reflectance (ATR)
• Specular reflectance and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS)

Transmission

Transmission methods are widely trusted because they can produce excellent spectra with flat baselines and clear bands. They are also broadly applicable. However, their main limitation is the opacity problem: The sample must be thin enough, or diluted enough, so that it absorbs neither too little nor too much light. 

Solids are often pressed into KBr pellets while liquids are filled into transmission cells with pathlengths ranging from 25 µm to 200 µm. Gases require dedicated gas cells with much longer pathlengths ranging from 5 cm to 20 m.^[1]

ATR

Attenuated total reflectance (ATR) has become the most popular general-purpose FTIR sampling method because it allows measurements of solids, powders, and liquids, and does not require sample preparation. 

In ATR, the infrared beam undergoes internal reflection inside a crystal, creating an evanescent wave that penetrates only a few micrometers into the sample. 

Diamond ATR is highly versatile because diamond is chemically resistant, mechanically robust, and difficult to scratch. For routine work, measurement with a diamond ATR is often the first technique to try.^[6]
 

DRIFTS and specular reflectance

For powders and rough solids, diffuse reflectance (DRIFTS) can be useful. For smooth metallic surfaces or coatings on shiny metals, specular reflectance and reflectance-absorbance methods are valuable. These techniques can reduce preparation time, but they are generally more surface-sensitive and can produce noisier spectra than good transmission measurements.

Overall, the most practical rule is simple: For solids, powders, polymers, liquids, and semi-solids, ATR is usually the preferred starting point; for gases, transmission gas cells remain the standard approach.^[1]