FTIR spectroscopy

Fourier transform infrared (FTIR) spectroscopy is an advanced and more user-friendly version of infrared spectroscopy. In this article, we have tried to comprehensively summarize the fundamental working principle, the usage, and applications of FTIR spectroscopy. Additionally, we have also discussed how is FTIR spectroscopy different from conventional dispersive infrared spectroscopy. So, let’s start reading to know everything you need to know about FTIR spectroscopy.

What is FTIR spectroscopy

As its name implies, the Fourier transform infrared spectroscopy is based on a mathematical Fourier transformation operation that breaks down a waveform into its constituent frequencies or wavelengths.

All the characteristic wavelengths of IR radiation (2.5 to 16 μm) are passed through the sample in one go with the help of a Michelson interferometer. Sample molecules absorb a specific wavelength from the entire wavelength range and undergo vibrational energy changes. All the remaining wavelengths are transmitted and recorded as an interferogram. The interferogram is then converted into the conventional IR spectrum via Fourier transformation.

What is the working principle of FTIR spectroscopy

Below are mentioned all the major components of an FTIR spectrophotometer and how these components work in sync to perform FTIR spectroscopy.

1. Radiation source

• A blackbody or heated silicon carbide (SiC) is used as the IR radiation source.
• It emits IR radiations in the range of 2.5 to 16  μm (or 4000-625 cm^-1).
• These are comparatively low energy (10¹⁰ to 10¹⁴ Hz) radiations as opposed to UV or visible radiations.

2. Michelson interferometer

• The interferometer consists of a beam splitter (a semi-transparent mirror) along with two reflectors. One reflector is held fixed while the other is movable.
•  As a collimated IR beam falls on the beam splitter, it is split into two beams of equal intensity.
• One beam is transmitted to the fixed mirror while the second is reflected off of the beam splitter to reach the movable mirror.

• Eventually, both the beams are simultaneously reflected by the fixed and movable reflectors such that both coincide at the semi-transparent mirror and constructive interference occurs.
• Different interference patterns are consequently obtained, each denoting a specific wavenumber which is calculated by a highly specialized mathematical software.
• The data obtained is recorded as a detector signal against mirror tilt and it is known as an interferogram.
• A raw signal is first obtained without inserting the sample. Fourier transformation is then applied to convert this interferogram into the reference IR spectrum.
• This process is then repeated with the sample to obtain the required sample spectrum.

3. Sample port

• Latest FTIR spectrophotometers are often fascinated with an attenuated total reflectance ATR port.
• The ATR port makes FTIR spectroscopy more user-friendly. It eliminates the need for complex sample preparation and handling unlike that required for dispersive IR spectroscopy.
• The ATR port is a delicate port made up of a crystal such as diamond, germanium, or zinc selenide (ZnS). The ATR port however must have a higher refractive index than the samples under investigation.
• Samples without any pre-treatment are directly introduced onto the crystal surface.
• IR radiation passes through the crystal and undergoes total internal reflection. This creates an evanescent field in which at a time, only a small portion of the IR radiation penetrates into the sample.
• The intensity of the evanescent field decreases exponentially with time which allows a quick interaction between sample molecules and the IR radiations.
• A specific wavelength is absorbed while the rest are transmitted/ reflected back from the sample to reach the detector.

• The detector uses the transmitted radiation to gain information about IR absorption by the sample. This information is recorded as an electrical signal.
•  An interferogram is obtained which is subsequently transformed into an IR spectrum.

Sampling techniques such as specular and diffuse reflectance can also be used in FTIR spectroscopy, other than ATR. Each sampling technique has its own characteristic strengths and weaknesses.

4. Infrared spectrum

• The IR spectrum is a plot of absorbance against wavenumber.
• Neatly distinguishable sharp peaks obtained in the fundamental group region (4000-1600 cm^-1) of the IR spectrum help find the principal functional groups present in the targeted sample.
• Different functional groups absorb characteristic wavelengths of IR radiations. For e.g., a broad peak at 3650 cm^-1 on an FTIR spectrum often represents the hydroxyl (OH) functional group. Similarly, the sharp sword-like peaks around 1600-1700 cm^-1 are characteristic peaks of carbonyl (CO) functional groups present in aldehydes and ketones.  These peaks appear due to asymmetric stretching vibrations of bonded atoms in the target molecules.  

The tongue and sword trick proves quite useful in interpreting an IR spectrum.

• Conversely, the small, heavily entwined peaks in the fingerprint region (1600-625) cm^-1 appear due to bending vibrations of targeted molecules by IR absorption.
• The data provided by both fundamental as well as the fingerprint region is finally combined to reveal the identity of the unknown chemical compound.

You can use this standard IR frequency table to identify the different functional groups present in a molecule.

Advantages of FTIR spectroscopy over conventional IR spectroscopy

FTIR spectroscopy	Dispersive IR spectroscopy
The use of an interferometer in FTIR spectroscopy increases the speed of the process multifold	A simple monochromator is used in dispersive IR spectroscopy. The slow scanning process offers a limited speed. Only a single wavelength can pass through the sample at one time and then the process is repeated to cover all the wavelengths in the IR region
High signal-to-noise ratio for a specific scanning time. The higher the ratio better the signal quality thus more reliable results obtained	A comparatively lower signal-to-noise ratio thus poorer signal quality. Less precise results
Higher wavelength accuracy	Lower wavelength accuracy
Great adaptability. The presence of an ATR port facilitates experimental setup change instantly, as per requirement	Lower adaptability. A complex instrument setup that can only be operated by a skilled technician is required
Any cumbersome sample preparation is usually not required. A diverse range of samples can be analyzed within seconds in their original form	Extensive sample preparation is often required in conventional IR spectroscopy. Solid samples need to be pressed into KBr pellets. Liquid sample solutions should be prepared in an IR transparent solvent such as CCl4
Trace amount of impurities can be instantly detected using FTIR spectroscopy	The samples must be extremely pure otherwise small impurities can result in a misleading structural identification

What information FTIR spectroscopy gives

• FTIR spectroscopy is primarily used for functional group identification and structural elucidation of a variety of chemical compounds.
• FTIR spectroscopy is an important characterization tool in research and development and also in the industrial sector.
• It is used to study newly synthesized polymers, foodstuff, pharmaceutical drugs, petroleum fractions, nanomaterials, biomolecules (polysaccharides, amino acids, proteins ), etc., and for detecting any impurities present in them.
• FTIR spectroscopy can also be performed for studying inorganic chemical substances like minerals, glass, and metal oxides.

Thus, FTIR spectroscopy is a fast, highly efficient, non-destructive, and cost-effective spectroscopic technique that is there to stay for long in the field of chemical structural studies.

Here is a video tutorial for you on FTIR spectroscopy.

If you are interested to know more about spectroscopy, you can also check out our articles:

• Introduction to spectroscopy- Everything you need to know about spectroscopy
• NMR spectroscopy

References

1.Berthomieu, C. and R. Hienerwadel (2009). “Fourier transform infrared (FTIR) spectroscopy.” Photosynth Res 101(2-3): 157-170.

2. Peak, D. (2013). Fourier Transform Infrared Spectroscopic Methods of Soil Analysis. Reference Module in Earth Systems and Environmental Sciences, Elsevier.