Infrared Spectroscopy: Principle and Types | Soil Mineralogy

In this article we will discuss about:- 1. Principle of Infrared Spectroscopy 2. Equipment of Infrared Spectroscopy 3. Types 4. Sample Preparation 5. Uses and Applications.

Principle of Infrared Spectroscopy:

IR spectroscopy works on the principle that molecules absorb specific frequencies that are characteristic of their structure. At temperatures above absolute zero, all the atoms in molecules are in continuous vibration with respect to each other. The IR spectrum of a sample is recorded by passing a beam of IR radiation through the sample.

When the frequency of a specific vibration is equal to the frequency of the IR radiation directed on the molecule, the mol­ecule absorbs the radiation. The examination of the transmitted light reveals how much energy was absorbed at each frequency (or wavelength). Using various sampling accessories, IR spectrometers can accept a wide range of sample types such as gases, liquids, and solids.

Equipment of Infrared Spectroscopy:

An IR spectrometer consists of three basic components – radiation source, monochromator, and detector. The common radiation source for the IR spectrometer is an inert solid, such as rare-earth oxides, silicon carbide or nichrome coil, heated electrically to 1000°C-1800°C.

The monochromator is a device used to disperse a broad spectrum of radiation and provide a continuous cali­brated series of electromagnetic energy bands of determinable wavelength or frequency range. Prisms or gratings are the dispersive components used in conjunction with variable-slit mechanisms, mirrors, and filters. For example, a grating rotates to focus a narrow band of frequencies on a mechanical slit. Narrower slits enable better resolution, while wider slits provide better system sensitivity.

Detectors measure the heating effect produced by infrared radiation and include thermocouples, thermistors, and Golay detectors. A variety of physical property changes are quantitatively determined-expansion of a non- absorbing gas (Golay detector), electrical resistance (thermistor), and voltage at junction of dissimilar metals (ther­mocouple).

Types of IR Spectrometers:

There are basically two types of spectrometers used in IR spectroscopy – Dispersive IR (DIR) spectrometers and Fourier transform IR (FTIR) spectrometers.

In a typical dispersive IR spectrometer, radiation from a broadband source passes through the sample and is dispersed by a monochromator into component frequencies. Then, the beams fall on the detector that generates an electrical signal and results in a recorder response. Most dispersive spectrometers have a double-beam design. Two equivalent beams from the same source pass through the sample and reference chambers, respectively. Using an optical chopper (such as a sector mirror), the reference and sample beams are alternately focused on the detector.

Commonly, the change of IR radiation intensity due to absorption by the sample is detected as an off-null signal that is translated into the recorder response through the actions of synchronous motors.

Fourier transform infrared (FTIR) spectrometers have recently replaced dispersive instruments for most appli­cations due to their superior speed and sensitivity. Instead of viewing each component frequency sequentially, as in a dispersive IR spectrometer, all frequencies are examined simultaneously in FTIR spectroscopy.

There are three basic spectrometer components in an FT system – radiation source, interferometer, and detector. The same types of radiation sources are used for both dispersive and Fourier transform spectrometers. However, the source is more often water-cooled in FTIR instruments to provide better power and stability. The interferom­eter produces interference signals that contain infrared spectral information generated after passing through a sample.

The interferometer consists of three active components- a moving mirror, a fixed mirror, and a beam splitter. When the mirror is moved at a constant velocity, the intensity of radiation reaching the detector varies in a sinusoi­dal manner to produce the interferogram output. A mathematical operation known as Fourier transform converts the interferogram to the final IR spectrum.

FTIR instruments have the following distinct advantages over dispersive spectrometers:

1. Better Speed and Sensitivity:

A complete spectrum can be obtained during a single scan of the moving mirror, while the detector observes all frequencies simultaneously. Because multiple spectra can be readily collected in 1 min or less, sensitivity can be greatly improved.

2. Increased Optical Throughput:

The beam area of an FT instrument is usually 75 – 100 times larger than the slit width of a dispersive spectrometer. Thus, more radiation energy is made available.

3. Internal Laser Reference:

The use of a helium neon laser as the internal reference in many FTIR systems pro­vides an automatic calibration in an accuracy of better than 0.01 cm–1 cm. This eliminates the need for external calibrations.

4. Simpler Mechanical Design:

There is only one moving part, the moving mirror, resulting in less wear and better reliability.

5. Powerful Data Station:

Modern FTIR spectrometers are usually equipped with a powerful, computerized data system. It can perform a wide variety of data processing tasks such as Fourier transformation, interactive spec­tral subtraction, baseline correction, smoothing, integration, and library searching.

FTIR spectrometers are the preferred choice for samples that are energy-limited or when increased sensitivity is desired.

Sample Preparation for IR Spectroscopy:

The soil sample can be solid, liquid, or gaseous in form. Minimum of 50-200 mg is desirable for solid sample. The specimen is prepared by grinding the solid sample with KBr or dissolve sample in a suitable solvent such as CCl₄. Water should be removed from sample if possible. Most samples can be prepared for infrared (IR) analysis in approximately 1-5 min.

Uses and Applications of Infrared Spectroscopy:

IR spectroscopy is a simple and reliable technique that is widely used in both organic and inorganic chemistry in research and industry. IR region is the most useful for the analysis of organic compounds having a wavelength range from 2500 nm to 16000 nm. IR spectroscopy is an important and popular tool for structural elucidation and compound identification. Estimated time to obtain spectrum from a routine sample varies from 1 to 10 min depend­ing on the type of instrument and the resolution required.

A store of thousands of reference spectra in some instruments will also automatically identify the substance being measured. The instruments are now small and can be transported, even for use in field trials.