Infrared (IR) Spectroscopy- Definition, Principle, Parts, Uses

Infrared (IR) spectroscopy is a powerful and versatile analytical technique that can be used to study the structure, composition and interactions of molecules. IR spectroscopy is based on the principle that molecules absorb or emit infrared radiation at specific wavelengths that correspond to their vibrational modes. By measuring the intensity and frequency of the infrared radiation that passes through or reflects from a sample, one can obtain information about the types and amounts of chemical bonds, functional groups and molecular environments present in the sample.

IR spectroscopy can be applied to various types of samples, such as solids, liquids or gases, and can be performed in different modes, such as transmission, reflection, emission or absorption. IR spectroscopy can also be combined with other techniques, such as mass spectrometry, chromatography or microscopy, to enhance the sensitivity and specificity of the analysis.

IR spectroscopy has many advantages over other spectroscopic methods, such as:

• It is non-destructive and requires minimal sample preparation
• It can provide qualitative and quantitative information in a single measurement
• It can identify unknown compounds and confirm the identity of known compounds
• It can detect functional groups and molecular structures that are not easily accessible by other methods
• It can monitor chemical reactions and physical changes in real time
• It can cover a wide range of frequencies and wavelengths, from near-infrared to far-infrared

IR spectroscopy has a wide range of applications in various fields of science and industry, such as:

• Organic chemistry: IR spectroscopy is used to determine the structure and purity of organic compounds, to identify functional groups and to characterize reaction mechanisms and intermediates
• Inorganic chemistry: IR spectroscopy is used to study the coordination and bonding of metal complexes, to identify minerals and crystals and to investigate the structure and properties of solid-state materials
• Biochemistry: IR spectroscopy is used to analyze the structure and function of biomolecules, such as proteins, lipids, carbohydrates and nucleic acids, to monitor enzyme activity and substrate binding and to detect biomarkers of diseases
• Pharmaceutical: IR spectroscopy is used to verify the identity and quality of drugs, to monitor their stability and degradation and to study their interactions with biological systems
• Forensic: IR spectroscopy is used to identify substances involved in crimes, such as drugs, explosives, poisons and fibers, to compare samples from different sources and to provide evidence in court cases
• Environmental: IR spectroscopy is used to measure the concentration and composition of pollutants in air, water and soil, to detect greenhouse gases and ozone depletion agents and to monitor environmental changes
• Food: IR spectroscopy is used to determine the quality and safety of food products, to measure their nutritional value and composition and to detect adulteration and contamination

In this article, we will discuss the principle, instrumentation and applications of IR spectroscopy in more detail. We will also provide some examples of how IR spectroscopy can be used to solve various analytical problems.

Infrared (IR) spectroscopy is based on the principle that molecules absorb specific frequencies of infrared light that are characteristic of their structure. These frequencies correspond to the vibrational modes of the molecular bonds, such as stretching, bending, twisting, and rocking. When an infrared photon with the same energy as a vibrational mode hits a molecule, it can be absorbed and cause a transition from a lower to a higher vibrational state. This results in a decrease in the intensity of the transmitted infrared light at that frequency, which can be measured by a detector.

The frequency of a vibrational mode depends on several factors, such as the mass of the atoms involved, the strength of the bond, and the geometry of the molecule. Different functional groups, such as alcohols, carboxylic acids, ketones, and alkenes, have distinct infrared absorption patterns that can be used to identify them. For example, the C=O bond in ketones has a strong absorption around 1700 cm-1, while the O-H bond in alcohols has a broad absorption around 3300 cm-1. By analyzing the infrared spectrum of a compound, one can determine its molecular structure and functional groups.

Infrared spectroscopy can also be used to quantify the amount of a substance in a sample by measuring the intensity of its infrared absorption. The intensity of an absorption peak is proportional to the concentration of the absorbing molecules, according to the Beer-Lambert law. Therefore, by comparing the intensity of a known peak with that of a standard solution, one can calculate the concentration of the substance in the sample.

Infrared spectroscopy can be performed in different modes, depending on how the sample is prepared and how the infrared light is delivered. The most common modes are:

• Transmission mode: The sample is placed in a transparent cell and the infrared light passes through it. This mode is suitable for liquid and gas samples.
• Reflection mode: The sample is placed on a reflective surface and the infrared light is reflected from it. This mode is suitable for solid samples.
• Attenuated total reflectance (ATR) mode: The sample is pressed against a crystal with a high refractive index and the infrared light undergoes multiple internal reflections within the crystal. A small fraction of the light penetrates into the sample and is absorbed by it. This mode is suitable for solid and liquid samples that are difficult to prepare for transmission or reflection modes.

The main parts of the IR spectrometer are as follows:

• Radiation source: This is the device that emits IR radiation, which must be steady, intense enough for detection, and extend over the desired wavelength range. Various sources of IR radiation are used, such as Nernst glower, incandescent lamp, mercury arc, tungsten lamp, glober source, and nichrome wire.
• Sample cells and sampling of substances: This is where the sample to be analyzed is placed. IR spectroscopy can be used for the characterization of solid, liquid, or gas samples. Different techniques are used for preparing and holding the samples, such as pressed pellet technique, solid run in solution, solid films, mull technique, liquid sample cell, and gas sample cell. The sample cells are usually made of alkali halides, such as potassium bromide or sodium chloride, which are transparent to IR radiation. Aqueous solvents cannot be used as they will dissolve alkali halides. Only organic solvents like chloroform can be used.
• Monochromators: This is the device that separates the IR radiation into its component wavelengths or frequencies. Various types of monochromators are used, such as prism, gratings, and filters. Prisms and gratings are made of alkali halides, which have high refractive indices and disperse IR radiation well. Filters are made of lithium fluoride, which has a low refractive index and absorbs most of the IR radiation except for a narrow band.
• Detectors: This is the device that measures the intensity of unabsorbed IR radiation after it passes through the sample. Detectors must be sensitive to IR radiation and respond quickly to changes in intensity. Various types of detectors are used, such as thermocouples, bolometers, thermistors, Golay cell, and pyro-electric detectors. These detectors convert the IR radiation into an electrical signal that can be amplified and recorded.
• Recorders: This is the device that records the IR spectrum, which is a plot of measured IR intensity versus wavelength or frequency of light. Recorders can be either analog or digital. Analog recorders use a pen or a needle to trace the spectrum on a paper chart. Digital recorders use a computer to store and display the spectrum on a screen.

IR instruments require a source of radiant energy that emits IR radiation that must be steady, intense enough for detection, and extend over the desired wavelength range. Various sources of IR radiation are as follows:

• Nernst glower: A ceramic rod that is electrically heated to produce IR radiation. It has a broad emission spectrum that covers the entire IR region.
• Incandescent lamp: A tungsten filament that is heated by an electric current to emit IR radiation. It has a continuous spectrum that is more intense in the near-IR region.
• Mercury arc: A high-pressure mercury lamp that produces IR radiation by an electric discharge through mercury vapor. It has a line spectrum that consists of discrete wavelengths in the mid- and far-IR regions.
• Globar source: A silicon carbide rod that is electrically heated to about 1500°C to emit IR radiation. It has a smooth and continuous spectrum that covers the mid- and near-IR regions.
• Nichrome wire: A coiled wire of nickel-chromium alloy that is heated by an electric current to produce IR radiation. It has a similar spectrum to the globar source but less intense.

IR spectroscopy can be used to analyze samples in different physical states, such as solid, liquid, or gas. However, different techniques are required to prepare the samples and hold them in the path of the IR beam. The sample cells are usually made of alkali halide materials, such as sodium chloride or potassium bromide, which are transparent to IR radiation. However, these materials are soluble in water and some organic solvents, so they cannot be used with aqueous or polar samples.

Some of the common techniques for sampling substances in IR spectroscopy are:

• Pressed pellet technique: This technique is used for solid samples that are finely ground and mixed with a small amount of alkali halide powder. The mixture is then pressed into a thin disc or pellet, which is placed in a metal holder and inserted into the IR spectrometer. This technique allows for a uniform distribution of the sample and a good transmission of IR radiation.

• Solid run in solution: This technique is used for solid samples that are soluble in organic solvents, such as chloroform or carbon tetrachloride. The sample is dissolved in a small volume of solvent and placed in a liquid sample cell with thin alkali halide windows. The solvent must be transparent to IR radiation and not interfere with the absorption bands of the sample.

• Solid films: This technique is used for solid samples that can be melted or evaporated to form thin films on alkali halide plates. The sample is heated in a crucible or a boat and deposited on a clean plate by sublimation or condensation. The plate is then mounted on a metal holder and inserted into the IR spectrometer. This technique allows for a high sensitivity and resolution of the spectrum.

• Mull technique: This technique is used for solid samples that are insoluble in organic solvents and cannot be pressed into pellets or formed into films. The sample is finely ground and mixed with a liquid paraffin or mineral oil to form a paste or mull. The mull is then smeared on an alkali halide plate and placed in a metal holder. This technique allows for a good contact between the sample and the IR beam, but may introduce some absorption bands from the oil.

• Liquid samples: Liquid samples can be held using a liquid sample cell made of alkali halide materials with thin windows. The cell has a fixed or variable path length, depending on the concentration and absorption strength of the sample. The cell is filled with the liquid sample and sealed with rubber or Teflon caps. The cell is then placed in a metal holder and inserted into the IR spectrometer. Aqueous solvents cannot be used as they will dissolve alkali halides. Only organic solvents like chloroform can be used.

• Gas samples: Gas samples can be analyzed using a gas sample cell made of alkali halide materials with thin windows. The cell has a long path length, typically 10 cm or more, to increase the interaction between the gas molecules and the IR beam. The cell is connected to a gas supply system that allows for the introduction and removal of the gas sample under controlled pressure and temperature conditions. The cell is then placed in a metal holder and inserted into the IR spectrometer.

Monochromators are devices that separate the IR radiation into its component wavelengths and select a narrow range of wavelengths to pass through the sample. Monochromators are essential for obtaining high-resolution IR spectra that can reveal the fine details of the molecular vibrations.

There are three main types of monochromators used in IR spectroscopy: prisms, gratings and filters.

Prisms

Prisms are transparent solids that refract the IR radiation at different angles depending on the wavelength. Prisms can be made of alkali halides, such as potassium bromide (KBr), sodium chloride (NaCl) or cesium iodide (CsI), which are transparent in the IR region. Prisms can cover a wide range of wavelengths, but they have some disadvantages, such as:

• They are hygroscopic, meaning they absorb water from the air and become cloudy.
• They have a low dispersion, meaning they do not separate the wavelengths very well.
• They have a high refractive index, meaning they reflect a lot of radiation and reduce the intensity.

Gratings

Gratings are optical elements that diffract the IR radiation into different directions depending on the wavelength. Gratings can be made of metal or alkali halides, and they consist of a large number of parallel grooves or slits that act as a series of prisms. Gratings have some advantages over prisms, such as:

• They have a high dispersion, meaning they separate the wavelengths very well and produce sharp peaks in the spectrum.
• They have a low refractive index, meaning they transmit most of the radiation and maintain the intensity.
• They can be rotated to select different wavelengths without changing the optical alignment.

Filters

Filters are materials that selectively transmit or absorb certain wavelengths of IR radiation. Filters can be made of various substances, such as lithium fluoride (LiF), polyethylene (PE) or quartz (SiO2), which have characteristic absorption bands in the IR region. Filters have some advantages and disadvantages, such as:

• They are simple and inexpensive to use.
• They are not hygroscopic and do not degrade over time.
• They can cover a narrow range of wavelengths with high resolution.
• They cannot cover a wide range of wavelengths with one filter and require multiple filters to scan the whole spectrum.
• They have a low transmission and reduce the intensity.

7. Detectors

Detectors are devices that measure the intensity of the infrared radiation that passes through the sample cell. They convert the infrared energy into an electrical signal that can be amplified and recorded. Different types of detectors have different characteristics, such as sensitivity, response time, wavelength range, and noise level. Some of the common detectors used in IR spectroscopy are:

• Thermocouples: These are pairs of wires made of different metals that generate a voltage when heated by infrared radiation. They are cheap, fast, and sensitive, but they have a low signal-to-noise ratio and require frequent calibration.
• Bolometers: These are thin metal films that change their resistance when heated by infrared radiation. They are more sensitive and stable than thermocouples, but they have a slower response time and a limited wavelength range.
• Thermistors: These are semiconductors that change their resistance when heated by infrared radiation. They are similar to bolometers, but they have a higher sensitivity and a wider wavelength range. However, they also have a slower response time and a higher noise level.
• Golay cell: This is a pneumatic device that consists of a sealed chamber filled with gas and a flexible membrane. When infrared radiation enters the chamber, it heats up the gas and causes the membrane to bulge. The displacement of the membrane is measured by a light beam and converted into an electrical signal. The Golay cell is very sensitive and has a wide wavelength range, but it has a slow response time and requires cooling.
• Pyroelectric detectors: These are crystals that generate an electric charge when heated or cooled by infrared radiation. They are fast, sensitive, and have a wide wavelength range, but they have a low signal-to-noise ratio and require chopping of the radiation source.

The choice of detector depends on the requirements of the analysis, such as the speed, accuracy, resolution, and cost. The most commonly used detectors in modern IR spectrometers are pyroelectric detectors, which offer a good balance of performance and convenience.

8. Detectors

Detectors are devices that measure the intensity of the infrared radiation that passes through the sample cell. They convert the infrared energy into an electrical signal that can be amplified and recorded. Different types of detectors have different characteristics, such as sensitivity, response time, wavelength range, and noise level. Some of the common detectors used in infrared spectroscopy are:

• Thermocouples: These are pairs of wires made of different metals that generate a voltage when heated by infrared radiation. They are cheap, fast, and sensitive, but they have a low signal-to-noise ratio and require frequent calibration.
• Bolometers: These are thin metal films that change their resistance when heated by infrared radiation. They are more sensitive and stable than thermocouples, but they have a slower response time and a limited wavelength range.
• Thermistors: These are semiconductors that change their resistance when heated by infrared radiation. They are similar to bolometers, but they have a higher sensitivity and a wider wavelength range. However, they also have a slower response time and a higher noise level.
• Golay cell: This is a sealed chamber filled with gas that expands when heated by infrared radiation. The expansion pushes a flexible membrane that moves a mirror, which reflects a beam of visible light onto a photocell. The photocell generates an electrical signal proportional to the intensity of the infrared radiation. The Golay cell is very sensitive and has a wide wavelength range, but it has a slow response time and a high cost.
• Pyroelectric detectors: These are crystals that generate an electric charge when heated or cooled by infrared radiation. They are fast, sensitive, and have a wide wavelength range, but they have a low signal-to-noise ratio and require a chopper to modulate the infrared beam.

The choice of detector depends on the requirements of the specific application, such as the resolution, speed, accuracy, and cost of the measurement.

Infrared (IR) spectroscopy is a versatile and powerful analytical technique that can be used for a variety of purposes. Some of the applications of IR spectroscopy are:

• Protein characterization: IR spectroscopy can provide information about the secondary structure, conformation, and interactions of proteins. For example, IR spectroscopy can be used to monitor the folding and unfolding of proteins, detect protein aggregation and amyloid formation, and study protein-ligand binding and enzyme catalysis.
• Nanoscale semiconductor analysis: IR spectroscopy can be used to characterize the optical and electronic properties of nanomaterials such as quantum dots, nanowires, and graphene. For example, IR spectroscopy can be used to measure the band gap, carrier concentration, mobility, and doping level of nanoscale semiconductors.
• Space exploration: IR spectroscopy can be used to study the composition and origin of extraterrestrial objects such as planets, moons, asteroids, comets, and interstellar dust. For example, IR spectroscopy can be used to identify organic molecules and water on Mars, detect minerals and ices on asteroids and comets, and probe the molecular clouds and star-forming regions in the galaxy.
• Analysis of gaseous, liquid or solid samples: IR spectroscopy can be used to identify and quantify various compounds in different phases of matter. For example, IR spectroscopy can be used to measure the concentration of pollutants and greenhouse gases in the atmosphere, determine the purity and composition of pharmaceuticals and food products, and characterize the structure and properties of polymers and ceramics.
• Identification of compounds: IR spectroscopy can be used to determine the functional groups and molecular structure of unknown compounds. For example, IR spectroscopy can be used to identify functional groups such as alcohols, ketones, carboxylic acids, amines, etc., by their characteristic absorption bands in the IR spectrum. IR spectroscopy can also be used to deduce the molecular skeleton of a compound by analyzing the fingerprint region of the spectrum.
• Quantitative analysis: IR spectroscopy can be used to measure the amount of a substance in a sample by comparing its absorbance with a standard curve or a calibration factor. For example, IR spectroscopy can be used to determine the concentration of a solute in a solution, the percentage of a component in a mixture, or the degree of polymerization or crystallinity in a material.

These are some of the applications of IR spectroscopy that demonstrate its usefulness and importance in various fields of science and technology. IR spectroscopy is a simple, fast, non-destructive, and sensitive technique that can provide valuable information about the molecular structure and interactions of matter.

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