IR Spectroscopy- Comprehensive Guide

This article describes about IR Spectroscopy, It’s principle, Components, Different Types, applications and limitations.

What is mean by IR Spectroscopy and It’s Principle ?

It is a analytical technique that uses interaction of infrared radiation with matter to study the vibrational and rotational motions of molecules. This technique provides information about  functional groups, chemical bonds, and molecular structure of a wide range of compounds in the pharmaceutical industry.

Principle of IR Spectroscopy : The Technique is based on the principle that molecules absorb specific frequencies of IR radiation and produce characteristic absorption patterns that can be used to identify and quantify the chemical composition of the sample.

How Does IR Spectroscopy works ?

It works on the principle that when a molecule is exposed to IR radiation, the energy of the radiation is absorbed by the molecule, causing the molecular bonds to vibrate. Different bonds in the molecule will vibrate at different frequencies, depending on their strength and the masses of the atoms involved and provides information about  functional groups, chemical bonds, and molecular structure.

What are the Components of an IR Spectroscopy :

It consists of below Major components as below :

  • A source of IR radiation: This can be a light bulb, a laser, or an IR emitter.
  • A sample holder: This is the device that holds the sample in place and exposes it to the IR radiation.
  • A detector: This detects the amount of radiation that is absorbed by the sample.

What are the Types of IR Spectroscopy 

There are several types of this spectroscopy which are using in the pharmaceutical industry as below :

  1. Fourier transform infrared (FTIR) Spectroscopy :

    This is the most common and advanced type of IR spectroscopy, which uses a Fourier transform algorithm to convert the raw data obtained from the detector into an IR spectrum.

  2. Transmission IR spectroscopy:

    This is the one type of IR spectroscopy, in which a sample is placed between two IR transparent plates and exposed to IR radiation. The amount of radiation that is transmitted through the sample is measured, and the spectrum is obtained from the difference in the transmitted and incident radiation.

  3. Attenuated total reflection (ATR) IR spectroscopy:

    This technique is used for the analysis of solid and liquid samples. A sample is placed on a crystal that is designed to absorb IR radiation, and the radiation is reflected back and forth between the crystal and the sample, creating an evanescent wave. The wave interacts with the sample, and the absorbed energy is measured.

  4. Reflection IR spectroscopy:

    In this technique, IR radiation is directed at a sample, and the reflected radiation is analyzed.

  5. Dispersive IR spectroscopy:

        This type of spectroscopy uses a prism or grating to separate the IR radiation into its different wavelengths, which are then detected by a detector array.

What is the scanning Range of IR spectroscopy ?

Scanning Range of IR is 4000 Centimeter Inverse to 400 Centimeter Inverse

Testing Sample Preparation for IR Spectroscopy

Before analyzing the sample, it must be prepared appropriately.

  1. Solid Samples are typically ground into fine powder, mixed with non-absorbing material such as potassium bromide (Kbr).
  2. Liquid Samples are placed in a small container called a cuvette, and
  3. Gases Samples are typically analyzed using specialized gas cells.

Applications of IR Spectroscopy in Pharmaceutical Industry

It is widely used in the pharmaceutical industry for the analysis of drugs and drug formulations. It can be used to identify and quantify active ingredients i.e. (Identification test of Functional group), as well as to detect impurities and contaminants.

How many regions available in IR spectroscopy graphs?

There are two regions available i.e. first one is Finger Print region (having scanning range of 400 cm inverse to 1400 cm inverse) and Functional group region (having scanning range 1400 cm inverse to 4000 cm inverse).

For IR spectroscopy calibration what we are using and Why ?

Polystyrene film with the thickness of 0.38mm is using for calibration. It is stable in high temperature and Highly Durable.

What are the limitations of this spectroscopy ?

  1. Limited sensitivity and Inability to identify functional groups in certain compounds : It may not be able to detect trace amounts or certain functional groups of some compounds.
  2. Overlapping spectra for complex mixtures : In some cases, the spectra of different compounds may overlap, making it difficult to distinguish between them. This can be especially problematic when analyzing complex mixtures.
  3. Sample preparation: It requires that the sample be in a specific physical state, such as a thin film or a solution. Preparing the sample in the correct state can be time-consuming and may require specialized equipment.
  4. Water Interference: Water is a strong absorber of IR radiation, which can interfere with the analysis of some samples.

What is the Difference between IR Spectroscopy and FTIR Spectroscopy ?

It is a general term for any technique that uses infrared radiation to analyze chemical compounds. While FTIR spectroscopy is a specific type of IR spectroscopy that uses Fourier transform technology to provide more accurate and precise spectra. It is often used in research, industrial and quality control applications where high sensitivity, resolution, and accuracy are required.

How Many types of Vibrations will happen in IR spectroscopy ?

Two Types of Vibrations will happen in this spectroscopy i.e.

1. Stretching Vibrations : 

Variation in bond distance due to vibration.

Again it has been classified as

  • Symmetric Vibrations
  • Asymmetric Vibrations

2. Bending Vibrations :

Variation in bond angle due to vibrations.

Again it has been classified as

  • Scissoring Vibration
  • Rocking Vibration
  • Twisting Vibration
  • Wagging Vibration

Why Potassium Bromide (KBr) is using in sample preparations.

Reasons are as 1) IR Inactive – No peak will show in IR Region. 2) Dipole Movement is Zero 3) Stable and Temperature Resistant.

What is ratio of Sample Vs Potassium bromide (KBr) while sample preparations

It is 1-2 mg of Sample Vs 300 to 400 mg of KBr

Is IR Rays are Visible in IR Spectroscopy?

No these rays are invisible. It can not seen by Necked eyes.

What is measuring unit of IR Spectroscopy ?

Centimeter Inverse or per centimeter  is the measuring unit for this spectroscopy.

Abbreviations :

IR  : Infrared

KBr : Potassium Bromide

FTIR : Fourier Transform Spectroscopy

Note :

Spectroscopy/Spectrophotometer/Spectrometer are like synonyms

IR spectroscopy can be termed as IR Visible spectrophotometer or IR Visible spectrometer.


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