Understanding the Basics: What is IR and FTIR and How Do They Differ?

Among the various forms of absorption spectroscopy is infrared spectroscopy. IR spectroscopy is an analytical technique used to determine the qualitative and quantitative analysis of drug substances by using the infrared region of the electromagnetic spectrum. It is used to identify a compound by determining its structure and functional groups. Functional groups have separate and unique fingerprint structures. The infrared region in the electromagnetic spectrum is divided into three regions.

Region	Near-Region	Mid-Region	Far-Region
Λ (µm)	0.7 to 2.5 µm	2.5 to 25 µm	25 to 1000 µm
Λ (cm⁻¹) Wavenumber	14000 to 4000 cm⁻¹	4000 to 400 cm⁻¹	400 to 10 cm⁻¹
Energy required	High energy	Mostly used in IR spectra	low

Mid-range IR is used to study fundamental vibrations and associated rotational vibrations. Those compounds selected for IR spectroscopy should have matching frequencies and dipole moments. From 400 cm⁻¹ to 1600 cm⁻¹ is the fingerprint region, and from 1600 cm⁻¹ to 4000 cm⁻¹ is the functional group region.

Principle of IR spectroscopy

When IR radiation passes through an IR-active compound, the compound absorbs the radiation when the frequency of the IR matches the vibrational frequency of the compound (the compound's band) and becomes excited; this corresponds to the fundamental vibration.

Fundamental Vibration

They are obtained when the compound absorbs IR radiation of matched frequency and the bond of the compound shows these vibrations.

There are two types: (I) Stretching and (II) Bending.

(I) Stretching

In this, the distance between the two atoms increases or decreases, but on the same axis. They are further divided into two types.

a. Symmetric: The movement of atoms is in the same direction.

b. Asymmetric: The movement of atoms is in the opposite direction.

(II) Bending

In this, the position of atoms changes concerning the original bond axis. The stretching absorption of a bond appears at higher frequencies than the bending absorption. They are further divided into four types:

a. Scissoring: A scissoring motion occurs when two atoms get close to one another.

b. Rocking: Atoms are moving in the same direction. 

c. Wagging: In this process, two atoms oscillate concerning the central atom up and down the plane. 

d. Twisting: In this case, one atom moves toward the center of the plane while the other moves in the opposite direction. 

Sample Handling Techniques:

There are techniques by which we can place the sample in a sample holder for IR spectroscopy.

i. Solid Sampling Technique

ii. Liquid Sampling Technique

iii. Gas Sampling Technique

iv. Solution Sampling Technique

I. Solid Sampling Technique:

In this technique, we prepare solid samples.

a. Direct sampling: In this, the solid sample is directly placed in a sample holder.

b. Pelletization technique: In this, the solid sample is mixed with KBr and then passed under very high pressure in a press to form small 1-2 mm thick pellets (1 cm diameter). These pellets are transparent to IR radiation. For IR spectroscopy, use only KBr pellets, not KBr+ drug pellets.

c. Mulling techniques: In this method, the solid sample is mixed with heavy mineral oil (Nujol) to form a paste, which is placed between two salt plates. Nujol is transparent in most of the IR region. Other mulling agents are CFC, hexachlorodiene, etc. 

d. As a solid film: In this technique, the sample solution is placed on a KBr or NaCl surface, and the solvent is evaporated. On the surface of the cell, the solid sample consequently leaves behind a thin layer.

II. Liquid Sampling Technique:

In this, a liquid sample is squeezed or sandwiched between two sodium chloride plates and forms a thin film (0.10-0.3 mm). Sodium chloride is transparent to IR. Calcium fluoride plates are used for water-containing samples.

III. Gas Sampling Technique:

In this, a gaseous sample is placed into a gas cell (10 cm) having sodium chloride walls. NaCl allows IR.

IV. Solution Sampling Technique:

In this, the sample is dissolved in various solvents and then analyzed as a solution. E.g., chloroform, carbon tetrachloride, and carbon disulfide. 

Factor Affecting Vibrations

There are some factors that can affect the vibration.

I. Coupled Interaction

II. Hydrogen Bonding

III. Resonance Effect

IV. Electronic Effect

I. Coupled Effect: 

Normally, there is one vibrational frequency, i.e., C-H and C=O bonds. But in the case of -CH₂ and CO₂... It has two fundamental stretching vibrations, one of which is symmetrical (lower wave no.) and the other is asymmetrical (higher wave no.).

ii. Hydrogen Bonding: 

The attraction between molecules is a unique kind of dipole-dipole attraction. 

Intermolecular: Bond between different molecules. It gives broad bonds that depend upon concentration.

Intramolecular: Bond between the same molecules. Sharp and well-defined bonds.

H-bonding decreases frequency. e.g., amines show an N-H→3500 cm⁻¹. After H-bonding, N-H→3300 cm⁻¹

iii. Resonance Effect: 

It will increase the bond length, which decreases the frequency.

iv. Electronic Effect: 

The frequency will change by altering the substituents on neighboring groups. Also known as inductive effects (I effect). +I effect decreases the frequency. - The I effect increases the frequency.

Instrumentation:

It is used to measure the sample's intensity in IR spectroscopy. The main components of an IR spectrometer are 

i. Source Radiation

ii. Monochromators (wavelength selector)

iii. Sample cells

iv. Detectors

v. Recording system

i. Source of radiation:

The IR spectrometer requires a source of radiant energy for narrow frequency bands.

To produce infrared radiation, an incandescent solid is used.

The following sources can be used:

a. Nernst glower: It consists of a rod or hollow tube (2 cm long and 1 mm in diameter) made by sintering a mixture of cerium, zirconium, thorium, etc. It is heated between 1000 and 1800°C. It provides maximum radiation at 7100 cm⁻¹.

b. Globar: It is a silicon carbide rod 5 cm long and 0.5 cm in diameter, which is also electrically heated between 1300 and 1700°C.

c. Nichrome wire: A coil of this wire is heated by a passing current.

d. Rhodium wire: The wire is sealed in a cylinder.

e. Tungsten filament lamp: It is used for near-infrared regions.

ii. Monochromators:

Also known as a wavelength selector. The source of radiation emitted radiation of various frequencies. For IR, we required certain frequencies, so the monochromator (wavelength selector) passed the desired frequencies only, and the other frequencies should be rejected. 

a. Prism Monochromator: It is used as a dispersive element. The elements should be made up of materials that transmit in IR regions, e.g., various metal halide salts like sodium chloride (4-15 cm).

b. Grafting Monochromator: It causes linear dispersion, and higher dispersion is achieved. Grafting is a series of parallel straight lines cut into a plane surface.

iii. Sample cells and sample handling

In this, we have to put the sample in an IR spectrometer.

iv. Detectors:

These are those devices that are basically used to generate the signals that we can read in the recording system. Thermal detectors are the best choices for IR. The names of some detectors that are used

a. Golay cells

b. Bolometer

c. Thermocouple

d. Thermistor

e. Pyroelectric detectors

a. Golay cells: It has a small cylinder with one end of the blackened metal plate and the other with a flexible metalized diaphragm mirror. It is filled with xenon gas and sealed. Now, IR radiation falls on the metal plate, and then the gas heats up and expands. This increases the pressure and deforms the metalized diaphragm mirror. Now, light from a lamp is allowed to fall on the diaphragm, which reflects the light onto a photocell. Now the diaphragm motion changes on the output of the cell, which depends on the intensity of the sources of IR.

b. Bolometer: It reflects the principle that the electrical resistance of a metal increases by 0.4% for every degree Celsius increase in temperature. A bolometer consists of a thin metal conductor whose temperature changes when IR radiation falls on it, which further changes the resistance.

c. Thermocouple: It works on the principle that two dissimilar metal wires are connected together at both ends. So a temperature differential exists, and an electric current flows between the two ends. IR radiation is exposed through non-junctions. The other end, the cold junction, which is thermally insulated, is carefully screened from stray light. The electric current is directly proportional to the energy difference between the two connections.

d. Thermistor: It is made up of fused mixtures of metal oxides. It is almost the same bolometer, but in this case, the electrical resistance of the mixture decreases with an increase in temperature. Temperature is inversely proportional to resistance.

e. Pyroelectric: It contains a pyroelectric material that produces electricity when exposed to thermal energy. In this dielectric material is placed between the electrodes. Now, when IR radiation is exposed to the black coating, it generates thermal energy and temperature changes. Due to this, a charge developed in the dielectric, which formed a pyroelectric material, and this led to the following conductivity. So the effects of this detector depend upon the rate of temperature changes. It is used in the FTIR spectrometer. \

v. Recording system: 

In this, we get the result in the form of a graph. 

Application of IR spectroscopy

It is widely used in industries and in research work.

a. Qualitative analysis:

i. Identification of organic compounds: By using fingerprint regions.

ii. Structure determination: By identifying all the functional groups.

iii. Detection of impurities: By comparing with a pure sample.

iv. Distinction between two types of hydrogen bonds.

b. Quantitative analysis: 

It helps in evaluating an organic mixture quantitatively.

i. Study of chemical reaction: The process of chemical reaction can be studied by examining the IR spectra of a small amount of the mixture. For example, a ketone forms a strong band at 1710 cm⁻¹, and upon reduction, it forms butane-2-ol.

c. Study of Keto-Enol Tautomerism

These compounds contain bonds C=O, OH, and C=C. So they show IR spectra.

d. Geometrical Isomerism: Based on the change in their dipole moments.

e.g., cis-isomers have some dipole moments, but trans-isomers have zero.

Fourier Transform IR Spectroscopy (FTIR):

When compared to dispersive infrared spectroscopy, the Fourier Transform is a mathematical conversion that has an advantage. The frequency domain is converted from the time domain in mathematics. Path differences between the two optical pathways produce interference, which can be either constructive or destructive, and this is the fundamental idea underlying FTIR. The path difference is directly correlated with the radiation intensity entering the sample, which is subsequently translated into wave number.

Zero Path Difference: Constructive Interference (Intensity High)

When moving and stationary mirrors are at the same distance, the path difference is zero. When IR radiation passed, it reached the sample with high intensity.

Optical Path Difference: Destructive Interference

When moving the mirror, it increases the path differences.

Component of FTIR:

i. Sources

ii. Collimator

iii. Interferometer

iv. Sample Cell

v. Detector

Same as IR, FTIR also has a source that produces IR radiation. This IR radiation is passed through one of the devices that is called a collimator. This collimator makes the radiation parallel to the sample cell, and once this radiation passes through this collimator, it can go parallelly into the undercompressed interferometer. We can observe that a monochromator is not present in this FTIR; instead, an interferometer is going to be placed between the source and cell. After passing through the interferometer, the transmitted radiation is perpendicular to the interferometer, and then it is transmitted into the sample; finally, it reaches the detector.

i. Source: 

Generally, we are going to use the mid-IR region, so for the mid-IR region, we can use the gglobalsource as well as the Nernst Glower. For the near-IR region, we can use a tungsten-halogen lamp. For the far-IR region, a high-pressure mercury vapor lamp is used. 

ii. Michelson Interferometer: 

It just acts like a monochromator, but it works by a different mechanism. It has mainly had three important components. They are: 

a. Beam Splitter

b. He-Ne Laser Beam (made up of germanium coated on KBr)

c. Two mirrors (fixed mirror and movable mirror)

A beam splitter splits the incident radiation into two equivalent radiations. It has a helium-neon laser beam, which is responsible for the accuracy of the FTIR. Finally, we have two mirrors; one is a fixed mirror, and the other is a movable mirror. Among the two equivalent components from the beam splitter, one of the components passes towards the fixed mirror, and another component travels towards the movable mirror. Once this radiation strikes the mirror, it is going to be reflected from the fixed mirror as well as the movable mirror. What is the path difference? Suppose both the fixed mirror and the movable mirror are at equal distances from the beam splitter; then the path difference is zero. Now the two reflected beams are combined, and they are going to be transmitted perpendicular to the interferometer. Now this radiation is passed into the sample, which finally reaches the detector. 

But after time t, this movable mirror is going to move; then what happens? Now the movable mirror is nearer to the beam splitter, so the reflected beams will have different path lengths. Because of the difference in their path length, there is a path difference between the two reflected beams coming from the fixed mirror as well as the movable mirror. This path difference produces interference within the spectra, which may be constructive or destructive. Here, the position of the movable mirror is accurately measured by a helium-neon laser beam. This laser beam is going to be passed toward the movable mirror, which can reflect it so that it can detect the accurate position of the movable mirror. Optical path difference is the difference between the path lengths of two optical waves, which may produce interference in the spectrum, and this interference may be constructive or destructive. 

Constructive Interference: They have similar crests and troughs. When they are going to interfere in a similar way, they produce constructive interference. Due to which the amplitude is increased. 

Destructive Interference: Destructive Interference: Destructive Interference: Destructive Interference: The first wave has a trough, gh, gh, and gh, and the second wave has a crest. So when they are going to interact, they produce a distraction. Here, both crests and troughs are going to cancel their amplitudes; we cannot serve any significant amplitude in the wave. Due to which the amplitude is reduced. 

iv. Detector: 

Here, fast-acting detectors are used. Pyroelectric detector, which is made up of deuterium triglycine sulfate (DTGS), and photon detector, which is filled with mercury cadmium telluride (MCT).

Advantages of FTIR:

Capable of identifying organic functional groups and often specific organic compounds. 

Extensive spectral libraries for compound identification. 

Ambient condition (not vacuum, good for volatile compounds)

Typically non-destructive.

Improved frequency resolution.

Higher energy throughput.

Faster operation 

IR spectroscopy interview questions with answers

👉️ SOP for the operation of the IRe IR instrument

Difference between Dispersive IR and FTIR:

Properties	Dispersive IR	FTIR
Components	Many moving parts result in mechanical slippage...	Only the moving mirror during the experiment
Calibration	Calibration against reference spectra is required to measure the frequency.	The use of laser provides high-frequency precision...
Stray light	Cause spurious reading in the instrument	Does not affect the detector, since all signals are modulated
Resolution	To improve resolution, a small amount of IR beam is passed through the slit...	Much larger beam aperture used:
No. of frequency	Only narrow-frequency radiation falls on the detector at any one time	All frequencies of radiation fall on the detector simultaneously...
Scan Speed	Slow scan speed	Rapid scan speed
Source effects	Due to the length of the scan time, the sample gets thermal effects from sources...	Short scan time; the sample doesn’t get a thermal effect.
IR from the sample itself	Any emission of IR radiation by the sample will fall on the detector due to the conventional positioning of the sample before the monochromator.	Any emission of IR radiation by the sample will not be detected.
Advantages of Beam Optics	Double-beam optics permits continuous real-time background subtraction.	Single-beam optic; background spectrum collected separately in time from the sample spectrum.

Reference:

Allen DW, Early EA, Hanssen LM, Kaplan SG, Nadal E. Comparison of Near-Infrared Transmittance and Reflectance Measurements Using Dispersive and Fourier Transform Spectrophotometers. Metrologia. 2002;39:157–164. �

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