Fourier Transform Infrared Spectrometer Components (FTIR)

A typical FTIR spectrometer includes a source, an interferometer, a sample chamber, a detector, an amplifier, an A / D converter, and a computer. Creates a radiation source that passes the sample through the interferometer and reaches the detector.

 The signal is then converted to digital signal by the amplifier and the analog-to-digital converter, respectively. Finally, the signal is transmitted to a computer where the Fourier transform is performed. The following is a block diagram of an FTIR spectrometer. The main difference between the FTIR spectrometer and the IR spectroscopy is the Michelson interferometer scatter.

The infrared part of the electromagnetic spectrum is usually divided into three regions. Near, middle, and far infrared, named because of their association with the visible spectrum. Higher energies close to IR, approximately 14000-4000 cm  ^-1  (wavelength 0.8-2.5 μm) can trigger color or harmonic vibrations.

 The average infrared, approximately 4000-400 cm-  ¹  (2.5-25 μm) may be used to study the underlying vibrations and the associated rotational vibration structure. The far infrared, approximately 10–400 cm-  ¹  (25–1000 μm), adjacent to the microwave area, has low energy and may be used for rotational spectroscopy.

 The names and classifications of these subregions are conventional and are based solely on relative molecular or electromagnetic properties.

Infrared spectroscopy theory

Infrared spectroscopy uses the fact that molecules absorb certain frequencies that characterize their structure. These absorptions are resonant frequencies, meaning that the frequency of the absorbed radiation corresponds to the frequency of the bond or vibrational group. Energies are determined by the shape of the molecular potential energy levels, the mass of the atoms, and the associated vibrational bonds.

In particular, in the created Oppenheimer and harmonic approximations, that is, when the molecular Hamiltonian corresponding to the electronic ground state can be approximated by a harmonic oscillator in the vicinity of the molecular geometry, the resonance frequencies with the natural surface-related states Molecular electronics potential energy levels.

However, resonant frequencies can be the first approach to the bond strength and mass of atoms at both ends. Therefore, the frequency of vibrations can be related to a particular type of bond.

For a vibrational state to be “active” in an IR molecule, it must be accompanied by permanent bipolar changes.

A molecule can vibrate in different ways, and each path is called a vibrational state. Linear molecules have a 3N – 5 degree vibrational state while nonlinear molecules have a 3N – 6 degree vibrational state (also called the degree of freedom vibration). For example H  ₂  6 = 3 degrees of vibrational freedom, or the state – O, a nonlinear molecule, 3 × 3.

Simple two-atom molecules have only one bond and only one vibrating band. If the molecule is symmetric, for example N  ₂  , the group is not observed in the infrared spectrum, but only in the Raman spectrum. Asymmetric diatomic molecules, such as CO, are absorbed in the IR spectrum. Complex molecules have many bonds and their vibrational spectra are more complex, meaning that large molecules have many peaks in their IR spectrum.

Atoms in a CH  ₂ group, usually present in organic compounds, can vibrate in six different ways: symmetrical and antisymmetric stretching, shearing, warping, shaking and twisting.

How does an infrared spectrometer work?

Step 1  : The  first step is sample preparation. The standard method for preparing a solid sample for an FTIR spectrometer is to use KBr. About 2 mg of the sample and 200 mg of KBr are dried and ground. The particle size should be the same and less than two micrometers.

 The mixture is then compressed to form clear pellets that can be measured directly. For liquids with a high boiling point or viscous solution, it can be added between two NaCl pellets. The sample is then fixed and tilted in the cell. The volatile liquid, for example, dissolves in CS  ₂  or CCl  _4.

To form a solution 10 then the solution is injected into the liquid cell for measurement. The gas sample should be measured in a gas cell with two KBr windows on each side. The gas cell must first be vacuumed. The sample can then be introduced to the gas cell for measurement.

Step 2:  The second step is to obtain the background spectrum by collecting the interference and then converting it to frequency data by inverse Fourier transform. 

We obtain the background spectrum because the solvent in which we place our sample contains traces of soluble gases as well as solvent molecules that share information that is not our sample.

 The background spectrum contains information about species of gases and solvent molecules, which may then be removed from our sample spectrum to obtain sample information. The following figure  shows an example of an FTIR background spectrum.

The background spectrum also takes into account several other factors related to the performance of the device, including source information, interferometer, detector, and ambient water share (note the two groups of irregular lines, about 3600 cm ^-1  and about 1600 cm  ⁾ . Cm  ^-1 in the figure below) and carbon dioxide (double in 2360 cm  ^-1  and sharp spike in 667 cm  ^-1  in the figure.  6

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 ) are available on the light bench.

Step 3: In the  next step , we will collect a single beam spectrum of the sample that will contain the absorbing bands of the sample as well as the background (gaseous or solvent).

Step 4: The ratio between the single-beam sample spectrum and the single-beam background spectrum gives the sample spectrum (Figure below)

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Source: https://www.ru.nl/systemschemistry/equipment/optical-spectroscopy/infrared/

https://www.hamamatsu.com/eu/en/applications/infrared-spectroscopy.html