What is Infrared (IR) Spectroscopy?
Infrared (IR) microscopy, also known as vibrational spectroscopy is a method of analysis that takes advantage of the vibrational transitions of a molecule. It is among the most widely employed spectroscopic techniques utilized by organic and inorganic chemical scientists due to its effectiveness in determining the structure of molecules and in identifying them. The technique or method of using infrared spectroscopy involves an instrument known as an infrared-spectrometer (or a spectrophotometer) to create the infrared spectrum.
Principle of Infrared (IR) Spectroscopy
Infrared Spectroscopy is the analysis of infrared light that interacts with molecules. The infrared spectrum that is most suitable to study organic compounds has an infrared wavelength of between 2,500 and 16,000 nm and a frequency range of 1.9*1013 to 1.2*1014 the frequency of which is Hz. The photon energies that are that are associated with this portion of infrared (from 1 to 15 kcal/mole) aren’t enough to trigger electrons, however, they could trigger vibrational excitation of the covalently bonded atoms and groups.
It is well-known that, in addition to the easy rotation of groups around one bond, molecules undergo an array of vibrational motions that are characteristic of their atoms. In turn, all organic compounds absorb infrared radiation , which corresponds in energy to the vibrations. Infrared spectrometers are similar in concept to other spectrometers allow chemists to get absorption spectra for compounds which reflect an exclusive representation of the molecular structures.
The most fundamental measurement made by Infrared spectroscopy is the infrared spectrum. This is a graph of the measured infrared intensities versus the frequency (or frequency) of light. IR Spectroscopy examines the vibratory patterns of atoms and on this , it’s possible to identify which functional groups. The more strong bonds and lighter atoms oscillate at a higher stretch frequency (wavenumber).
Instrumentation of Infrared (IR) Spectroscopy

The principal components of IR the spectrometer are:
- Radiation source
- Sample cells and sampling of substances
- Monochromators
- Detectors
- Recorder
1. IR radiation sources
IR instruments require the use of a source of energy that emits IR radiation that must be constant and intense enough to be detected and extend to what is needed in the wavelength.
The various types of IR radiations are listed below.
- Nernst glower
- Incandescent lamp
- Mercury arc
- Tungsten lamp
- Glober source
- Nichrome wire
2. Sample cells and sampling of substances
IR spectrum has been employed to characterize gas, liquid or solid samples.
- Solid – A variety of methods are employed for the preparation of solid samples, including the pressed pellet method solid run-in-solution mull technique, solid films and so on.
- Liquid – Liquid samples can be stored in the liquid sample cell that is composed of alkali halides. Solvents that are aqueous cannot be used since they dissolve alkali halids. Only organic solvents, such as chloroform can be utilized.
- Gas– It is similar to the sampling of liquids.
3. Monochromators
There are many kinds of monochromators: prisms, gratings, and filters. Prisms are constructed of Potassium bromide Sodium chloride, or Caesium Iodide. Filters are comprised of Lithium Fluoride, and Diffraction gratings comprise alkali Halides.
4. Detectors
Detectors can be used to measure the strength of the infrared radiation. They include thermocouples, Bolometers, thermisters Golay cells, as well as pyroelectric detectors are employed.
In an infrared spectrometer, the detector is a device that measures the intensity of the infrared light that is transmitted or absorbed by the sample.
There are several different types of detectors that are used in IR spectroscopy, including:
- Thermal detectors: Thermal detectors, also known as bolometers, are devices that measure the change in temperature of a material as a result of absorbed radiation. They are typically used to measure the absorption of infrared light by a sample, and they are sensitive to a wide range of wavelengths.
- Photovoltaic detectors: Photovoltaic detectors, also known as photodiodes, are devices that convert absorbed radiation into an electrical current. They are typically used to measure the absorption of infrared light by a sample, and they are sensitive to a wide range of wavelengths.
- Photoconductive detectors: Photoconductive detectors are devices that measure the change in electrical conductivity of a material as a result of absorbed radiation. They are typically used to measure the absorption of infrared light by a sample, and they are sensitive to a wide range of wavelengths.
- Photoresistive detectors: Photoresistive detectors are devices that measure the change in resistance of a material as a result of absorbed radiation. They are typically used to measure the absorption of infrared light by a sample, and they are sensitive to a wide range of wavelengths.
The specific type of detector used in IR spectroscopy depends on the specific requirements of the measurement, including the sensitivity, wavelength range, and dynamic range of the detector. In general, detectors that are sensitive to a wide range of wavelengths and have a high dynamic range are preferred for IR spectroscopy.
5. Recorders
Recorders are devices used to capture all aspects of the IR spectrum.
In an infrared spectrometer, the detector measures the intensity of the infrared light that is transmitted or absorbed by the sample, and the recorder is a device that records the measurement data.
There are several different types of recorders that are used in IR spectroscopy, including:
- Strip chart recorders: Strip chart recorders are devices that record data on a continuously moving strip of paper. They are often used to record the absorption spectrum of a sample in real-time, and they are relatively simple and inexpensive.
- Digital oscilloscopes: Digital oscilloscopes are devices that record and display data in the form of a graph on a computer screen. They are often used to record and analyze the absorption spectrum of a sample in real-time, and they offer a higher resolution and a wider dynamic range than strip chart recorders.
- Data acquisition systems: Data acquisition systems are devices that collect and store data from multiple sensors or detectors. They are often used to record and analyze the absorption spectrum of a sample in real-time, and they offer a high resolution and a wide dynamic range.
The specific type of recorder used in IR spectroscopy depends on the specific requirements of the measurement, including the resolution, dynamic range, and data storage needs of the recorder. In general, recorders that offer a high resolution and a wide dynamic range are preferred for IR spectroscopy.
Graph of the IR spectrum
Below is a small example of the typical Infrared Absorption Frequency.

So, IR spectroscopy involves the gathering of absorption data and the analysis of that information as an IR spectrum.
Types of Infrared Spectrophotometry
There are several different types of infrared (IR) spectrophotometry that are used to measure the absorption of IR light by molecules. Some common types of IR spectrophotometry include:
1. Fourier transform infrared (FTIR) spectrophotometry
- This is a type of IR spectrophotometry that is based on the Fourier transform, a mathematical technique that allows the spectrum to be analyzed in the frequency domain rather than the time domain.
- FTIR spectrophotometry is widely used due to its high sensitivity and wide dynamic range, and is often used in conjunction with other analytical techniques, such as Raman spectroscopy, to provide a more complete understanding of the sample.
2. Attenuated total reflectance (ATR) spectrophotometry
- This is a type of IR spectrophotometry that is used to analyze solid and liquid samples. It involves placing the sample in contact with a crystal, such as diamond or zinc selenide, and measuring the absorption of IR light as it passes through the crystal.
- ATR spectrophotometry is a non-destructive technique that does not require the sample to be prepared in a specific way, making it suitable for analyzing fragile or valuable samples.
3. Transmission spectrophotometry
- This is a type of IR spectrophotometry that is used to analyze liquid and gas samples. It involves measuring the absorption of IR light as it passes through a cell filled with the sample.
- Transmission spectrophotometry is commonly used to study the structure and composition of gases and liquids.
4. Reflectance spectrophotometry
- This is a type of IR spectrophotometry that is used to analyze solid samples. It involves measuring the reflection of IR light from the surface of the sample.
- Reflectance spectrophotometry is often used to study the surface properties of materials, such as their roughness, smoothness, and chemical composition.
Each type of IR spectrophotometry has its own advantages and disadvantages, and the appropriate technique will depend on the sample and the desired level of sensitivity and resolution.
Comparison between FTIR, ATR, Transmission spectrophotometry, Reflectance spectrophotometry
Fourier transform infrared (FTIR) spectrophotometry, attenuated total reflectance (ATR) spectrophotometry, transmission spectrophotometry, and reflectance spectrophotometry are all types of infrared (IR) spectrophotometry that are used to measure the absorption of IR light by molecules and identify and analyze the functional groups present in a wide range of materials, including polymers, pharmaceuticals, and chemicals. Here is a comparison of these techniques:
- FTIR spectrophotometry: FTIR spectrophotometry is based on the Fourier transform, a mathematical technique that allows the spectrum to be analyzed in the frequency domain rather than the time domain. It is highly sensitive and has a wide dynamic range, and can be used to analyze a wide range of materials, including liquids, gases, solids, and complex mixtures. However, it requires sample preparation for certain types of samples, is limited to certain functional groups, and may be affected by interference from water.
- ATR spectrophotometry: ATR spectrophotometry involves placing the sample in contact with a crystal, such as diamond or zinc selenide, and measuring the absorption of IR light as it passes through the crystal. It is a non-destructive technique that does not require sample preparation, making it suitable for analyzing fragile or valuable samples. However, it is limited to solid and liquid samples and may not be suitable for analyzing gases.
- Transmission spectrophotometry: Transmission spectrophotometry involves measuring the absorption of IR light as it passes through a cell filled with the sample. It is commonly used to study the structure and composition of gases and liquids. However, it requires sample preparation and is limited to liquid and gas samples.
- Reflectance spectrophotometry: Reflectance spectrophotometry involves measuring the reflection of IR light from the surface of the sample. It is often used to study the surface properties of materials, such as their roughness, smoothness, and chemical composition. However, it is limited to solid samples and may not be suitable for analyzing liquids or gases.
Each type of IR spectrophotometry has its own advantages and disadvantages, and the appropriate technique will depend on the sample and the desired level of sensitivity and resolution.
Infrared spectroscopy table
Infrared (IR) spectroscopy is a technique that measures the vibrational, rotational, and other low-frequency modes of molecules in the infrared region of the electromagnetic spectrum. It is a powerful tool for analyzing the molecular structure of a wide range of materials, including polymers, pharmaceuticals, and chemicals.
The IR spectrum of a molecule is a plot of the absorption of infrared light as a function of wavenumber (the reciprocal of the wavelength). Each molecule has a unique set of vibrational modes, and the IR spectrum can be used to identify the specific bonds and functional groups present in the molecule.
An IR spectrum is typically presented as a table or graph, with the wavenumber on the x-axis and the absorption intensity on the y-axis. The absorption intensity is usually expressed as a percentage of the incident light absorbed by the sample.
The IR spectrum is divided into several regions, each corresponding to a different range of wavenumbers:
- The far-infrared region (10-400 cm-1) corresponds to the lowest energy vibrations, such as lattice modes and rotational modes.
- The mid-infrared region (400-4000 cm-1) corresponds to the vibrations of bonds involving C-H, C-C, N-H, O-H, and other common functional groups.
- The near-infrared region (4000-25000 cm-1) corresponds to vibrations of bonds involving C-O, C=O, N-O, and other functional groups.
An IR spectroscopy table typically lists the wavenumber, intensity, and assignment of each absorption band in the IR spectrum. The assignment refers to the specific bond or functional group responsible for the absorption.
For example, an IR spectroscopy table for an ester might include the following entries:
Wavenumber (cm-1) | Intensity | Assignment |
---|---|---|
2950 | strong | C-H stretch |
1740 | medium | C=O stretch |
1460 | weak | C-O stretch |
This table indicates that the ester has strong absorption at 2950 cm-1 due to C-H stretching, medium absorption at 1740 cm-1 due to C=O stretching, and weak absorption at 1460 cm-1 due to C-O stretching.
Regions of the Infrared spectrum
The infrared (IR) spectrum is the portion of the electromagnetic spectrum that corresponds to wavelengths of light ranging from about 700 nanometers (nm) to 1 millimeter. It is divided into three main regions:
- Far-infrared (FIR) region: This region corresponds to wavelengths from about 700 nm to 30 micrometers (μm) and is also known as the “thermal infrared” region. It is characterized by low energy vibrations, such as lattice modes and rotational modes, and is often used to study the physical properties of solids and liquids.
- Mid-infrared (MIR) region: This region corresponds to wavelengths from about 3 μm to 30 μm and is characterized by the vibrations of chemical bonds involving C-H, C-C, N-H, O-H, and other common functional groups. It is often used to identify and analyze the functional groups present in a molecule.
- Near-infrared (NIR) region: This region corresponds to wavelengths from about 0.7 μm to 3 μm and is characterized by vibrations of bonds involving C-O, C=O, N-O, and other functional groups. It is often used to analyze the composition and structure of materials in the food, pharmaceutical, and chemical industries.

The IR spectrum is typically presented as a plot of the absorption of infrared light as a function of wavenumber (the reciprocal of the wavelength), with the wavenumber on the x-axis and the absorption intensity on the y-axis. Each molecule has a unique set of vibrational modes, and the IR spectrum can be used to identify the specific bonds and functional groups present in the molecule.
Types of Samples used in Infrared Spectroscopy
There are many different types of samples that can be analyzed using infrared (IR) spectroscopy. Some common types of samples include:
- Liquids: IR spectroscopy can be used to analyze the structure and chemical composition of liquids, such as solvents, acids, and bases.
- Gases: IR spectroscopy can be used to study the vibrational modes of gases, such as oxygen, nitrogen, and carbon dioxide.
- Solids: IR spectroscopy can be used to analyze the structure and chemical composition of solids, such as metals, minerals, and polymers.
- Biological samples: IR spectroscopy can be used to study the structure and function of biological molecules, such as proteins, nucleic acids, and lipids.
- Complex mixtures: IR spectroscopy can be used to identify and quantify the individual components of complex mixtures, such as fuels, food products, and pharmaceuticals.
There are several different techniques for preparing samples for IR spectroscopy, depending on the type of sample and the desired level of sensitivity. Some common techniques include:
- KBr pellet method: This method is commonly used for solid samples, and involves grinding the sample with potassium bromide (KBr) and pressing the mixture into a pellet. The pellet is then placed in the IR spectrometer and analyzed.
- ATR method: This method is used for solid and liquid samples and involves placing the sample in contact with a crystal, such as diamond or zinc selenide, and measuring the absorption of IR light as it passes through the crystal.
- Transmission method: This method is used for liquid and gas samples and involves measuring the absorption of IR light as it passes through a cell filled with the sample.
- Reflectance method: This method is used for solid samples and involves measuring the reflection of IR light from the surface of the sample.
What is fourier transform infrared spectroscopy?
- Fourier transform infrared (FTIR) spectroscopy is a type of infrared (IR) spectroscopy that is used to analyze the vibrational, rotational, and other low-frequency modes of molecules. It is a powerful tool for identifying and analyzing the functional groups present in a wide range of materials, including polymers, pharmaceuticals, and chemicals.
- FTIR spectroscopy works by measuring the absorption of IR light as a function of wavenumber (the reciprocal of the wavelength). The sample is placed in the path of an IR beam, and the absorption of the IR light is measured as a function of wavenumber. The resulting spectrum is a plot of the absorption intensity as a function of wavenumber, and can be used to identify the specific bonds and functional groups present in the sample.
- FTIR spectroscopy is based on the Fourier transform, which is a mathematical technique that allows the spectrum to be analyzed in the frequency domain rather than the time domain. This makes it possible to measure the spectrum over a wide range of wavenumbers in a single scan, and allows for more precise and accurate analysis of the sample.
- FTIR spectroscopy is widely used in a variety of fields, including chemistry, biology, material science, and environmental science, and is often used in conjunction with other analytical techniques, such as Raman spectroscopy, to provide a more complete understanding of the sample.
What is near infrared spectroscopy?
Near infrared (NIR) spectroscopy is a type of infrared (IR) spectroscopy that is used to analyze the vibrational modes of molecules in the near-infrared region of the electromagnetic spectrum (wavelengths ranging from about 0.7 micrometers to 3 micrometers). It is a powerful tool for identifying and analyzing the functional groups present in a wide range of materials, including polymers, pharmaceuticals, and chemicals.
NIR spectroscopy works by measuring the absorption of NIR light as a function of wavenumber (the reciprocal of the wavelength). The sample is placed in the path of an NIR beam, and the absorption of the NIR light is measured as a function of wavenumber. The resulting spectrum is a plot of the absorption intensity as a function of wavenumber, and can be used to identify the specific bonds and functional groups present in the sample.
NIR spectra are typically characterized by the vibrations of bonds involving C-O, C=O, N-O, and other functional groups. The NIR region is particularly useful for analyzing the structure and composition of materials in the food, pharmaceutical, and chemical industries, as many functional groups absorb NIR light strongly.
NIR spectroscopy is widely used in a variety of fields, and is often used in conjunction with other analytical techniques, such as Raman spectroscopy, to provide a more complete understanding of the sample. It is a non-destructive technique, meaning that the sample is not altered during analysis, making it suitable for analyzing fragile or valuable samples.
Why do Molecules Only Absorb IR Frequency Resonant to Their Internal Vibrational Frequencies?
Molecules only absorb infrared (IR) frequencies that are resonant with their internal vibrational frequencies because the energy of the absorbed IR light must be equal to the energy difference between two vibrational energy levels of the molecule. This is known as the selection rule for vibrational transitions.
When a molecule absorbs an IR photon, it absorbs energy that is equal to the energy difference between two vibrational energy levels. The vibrational energy levels of a molecule are determined by the bond lengths, bond angles, and other structural parameters of the molecule. Each vibrational mode corresponds to a specific vibrational energy level, and the energy difference between two vibrational energy levels is equal to the energy of the absorbed IR photon.
Therefore, a molecule will only absorb an IR photon if the energy of the IR photon is equal to the energy difference between two vibrational energy levels of the molecule. This means that the IR frequency must be resonant with the internal vibrational frequencies of the molecule.
For example, if the vibrational energy levels of a molecule are separated by an energy of 1000 cm-1, then the molecule will only absorb IR photons with a frequency of 1000 cm-1. This frequency will be absorbed by the molecule, while other frequencies will not be absorbed.
This is why molecules only absorb IR frequencies that are resonant with their internal vibrational frequencies. It is also why the IR spectrum of a molecule is a fingerprint of its vibrational modes and can be used to identify the specific bonds and functional groups present in the molecule.
Types of vibrations
The type of vibrations or vibrational modes refers to the positional variations of the atoms constituting the link. Stretching and bending are the two primary types of vibrational modes.
1. Stretching
This entails modifications to the bond length between atoms. Either the atoms can move closer together, so lengthening the connection, or farther apart, thereby weakening it.
Stretching can be categorised as;
- Symmetric stretching: When two atoms simultaneously move toward or away from a centre atom, this is symmetric stretching.
- Asymmetric stretching: Asymmetric stretching occurs when two atoms attached to a core atom move in opposite directions.
2. Bending
This sort of vibration refers to changes in the ‘angle’ between two bonds and is subdivided as follows:
- Rocking: when two atoms move in opposite directions on the same plane.
- Scissoring: When two atoms move towards or away from one another in the same plane, they are scissoring.
- Twisting: Twisting occurs when two atoms travel out of the plane in opposite directions.
- Wagging: an out-of-plane vibration in which the atoms simultaneously move away from and toward one another in a v-shape.
What are the units of spectroscopy?
Spectroscopy is the study of the interaction of electromagnetic radiation with matter. In spectroscopy, the intensity or intensity ratio of the radiation is typically measured as a function of wavelength, frequency, or energy. The units used to express these measurements depend on the specific type of spectroscopy being used and the nature of the radiation being measured.
In many cases, spectroscopy measurements are expressed in terms of the energy or frequency of the radiation. The energy of electromagnetic radiation is typically expressed in units of electron volts (eV), while the frequency is typically expressed in units of Hertz (Hz).
In other cases, spectroscopy measurements are expressed in terms of the wavelength of the radiation. The wavelength is the distance between two adjacent peaks or troughs of a wave, and it is typically expressed in units of meters (m), nanometers (nm), or angstroms (Å).
In infrared (IR) spectroscopy, the absorption or transmittance of infrared light is often expressed in units of absorbance (A) or transmittance (%T). Absorbance is defined as the logarithm of the ratio of the intensity of the incident light to the intensity of the absorbed light, and it is typically expressed on a scale from 0 to 10. Transmittance is defined as the ratio of the intensity of the transmitted light to the intensity of the incident light, and it is typically expressed as a percentage.
In ultraviolet-visible (UV-Vis) spectroscopy, the absorption or transmittance of ultraviolet or visible light is often expressed in units of absorbance or transmittance, as in IR spectroscopy. In addition, the absorption or transmittance may also be expressed in terms of the molar absorbance coefficient (ε) or the molar extinction coefficient (ε), which are related to the concentration and molar mass of the sample.
In nuclear magnetic resonance (NMR) spectroscopy, the intensity of the magnetic resonance signal is typically expressed in units of parts per million (ppm). The chemical shift of the signal is often expressed in units of ppm relative to a reference compound.
In mass spectrometry, the intensity of the ionized species is typically expressed in units of counts per second (cps) or relative abundance. The mass-to-charge ratio (m/z) of the ions is often expressed in units of atomic mass units (amu).
Infrared vs Raman spectroscopy
Infrared (IR) spectroscopy and Raman spectroscopy are two techniques that are used to identify and analyze the molecular structure of a substance. Both techniques involve the interaction of light with a sample, but they differ in the way that the light is used to probe the sample.
Infrared spectroscopy involves the absorption of infrared light by a sample. Infrared light has a longer wavelength than visible light, and it is absorbed by certain vibrational modes of the bonds between atoms in a molecule. By measuring the absorption of infrared light at different wavelengths, it is possible to determine the vibrational modes of the bonds in the molecule, which can be used to identify the molecular structure of the sample.
Raman spectroscopy, on the other hand, involves the scattering of light by a sample. When light is scattered by a sample, some of the scattered light is shifted to a higher or lower energy level. This energy shift is called the Raman shift, and it is caused by the vibrational and rotational modes of the bonds in the molecule. By measuring the Raman shift of the scattered light at different wavelengths, it is possible to determine the vibrational and rotational modes of the bonds in the molecule, which can also be used to identify the molecular structure of the sample.
Both IR and Raman spectroscopy are widely used in a variety of fields, including chemistry, materials science, and biology. They are powerful tools for identifying and analyzing the molecular structure of a wide range of materials, including organic and inorganic compounds, polymers, and biomolecules.
What is necessary condition for IR spectroscopy?
There are several conditions that are necessary for infrared (IR) spectroscopy to be successful. These include:
- The presence of bonds that absorb infrared light: IR spectroscopy relies on the absorption of infrared light by certain vibrational modes of the bonds between atoms in a molecule. In order for IR spectroscopy to be useful, the sample must contain bonds that absorb infrared light.
- A transparent or translucent sample: IR spectroscopy involves the measurement of the absorption of infrared light by a sample. In order for the measurement to be accurate, the sample must be transparent or translucent, so that the infrared light can pass through it.
- A stable sample: IR spectroscopy involves the measurement of the absorption of infrared light by a sample over a certain period of time. In order for the measurement to be accurate, the sample must be stable and not undergo any changes during the measurement.
- A representative sample: In order for IR spectroscopy to be useful, the sample must be representative of the material being analyzed. This means that the sample must be representative of the average molecular structure of the material, and it must not be contaminated with any other materials.
- A suitable solvent: In many cases, it is necessary to dissolve the sample in a solvent in order to prepare it for IR spectroscopy. The choice of solvent can affect the absorption spectrum of the sample, and it is important to choose a solvent that does not interfere with the measurement of the absorption of the sample.
- Suitable instrumentation: IR spectroscopy requires specialized instrumentation, such as an infrared spectrometer, to measure the absorption of infrared light by the sample. The sensitivity and resolution of the instrumentation can affect the accuracy and precision of the measurement.
Applications of Infrared (IR) Spectroscopy
It has been of major importance to scientists in diverse fields like:
- Protein characterization
- Nanoscale analysis of semiconductors as well as
- Space exploration.
- Analyzing gaseous, liquid or solid samples
- Identification of compounds
- Quantitative analysis
- Information on the functional groups of molecules as well as the structure of molecules could be derived by IR spectrum
- To learn about interactions between molecules
Infrared Spectrophotometry Advantages
- Non-destructive: IR spectrophotometry is a non-destructive technique, meaning that the sample is not altered during analysis. This makes it suitable for analyzing fragile or valuable samples.
- High sensitivity: IR spectrophotometry is highly sensitive and can detect trace amounts of certain functional groups.
- Wide range of applications: IR spectrophotometry has a wide range of applications and can be used to analyze a variety of materials, including liquids, gases, solids, and complex mixtures.
- Versatility: IR spectrophotometry can be used in conjunction with other analytical techniques, such as Raman spectroscopy, to provide a more complete understanding of the sample.
- You can receive both qualitative (structure, presence of functional groups) and quantitative (number of chemicals in the sample) information.
- Different IR regions can be optimised for particular applications, such as the far IR region for the study of inorganic chemicals, the mid-IR region for organic species, and the near IR region for routine quantitative analysis.
- Data interpretation of the regularly employed mid-IF area is straightforward; peak intensities, peak positions, peak widths, and peak shapes can be read readily, and data may be extracted rapidly.
- The FTIR spectrometer can acquire data quickly and with a higher signal-to-noise ratio.
Infrared Spectrophotometry Disadvantages
- Requires sample preparation: Some types of IR spectrophotometry, such as transmission spectrophotometry, require the sample to be prepared in a specific way (e.g., placed in a cell). This can be time-consuming and may require specialized equipment.
- Limited to certain functional groups: IR spectrophotometry is limited to the vibrational modes of certain functional groups, and may not be suitable for analyzing all types of molecules.
- Interference from water: Water absorbs IR light strongly and can interfere with the analysis of certain samples. This can be mitigated by using a technique such as attenuated total reflectance (ATR) spectrophotometry, which does not require the sample to be prepared in a specific way.
- Limited to certain sample types: Some types of IR spectrophotometry, such as reflectance spectrophotometry, are limited to solid samples and may not be suitable for analyzing liquids or gases.
- IR spectrophotometry provides no information regarding molecular mass or the relative positions of functional groups.
- A single IR absorption spectra cannot determine if an unidentified material is pure or a combination of components.
- Materials that absorb IR, such as plastic and glass, cannot be utilised in an IR spectrophotometer.
- The dispersive spectrophotometer has low sensitivity and is relatively sluggish.
FAQ
What is the basic principle of infrared spectroscopy?
The basic principle of infrared (IR) spectroscopy is the absorption of infrared light by a sample. Infrared light is a type of electromagnetic radiation with a longer wavelength than visible light. It is absorbed by certain vibrational modes of the bonds between atoms in a molecule.
When infrared light is absorbed by a sample, it causes the bonds in the molecule to vibrate at the same frequency as the absorbed light. The absorbed light is then absorbed as energy by the molecule, and the molecule transitions to an excited state. The molecule will eventually return to its ground state, releasing the absorbed energy as heat or light.
By measuring the absorption of infrared light at different wavelengths, it is possible to determine the vibrational modes of the bonds in the molecule. Each bond in a molecule has a characteristic vibrational frequency, and the absorption of infrared light at these frequencies can be used to identify the molecular structure of the sample.
Infrared spectroscopy is a powerful tool for identifying and analyzing the molecular structure of a wide range of materials, including organic and inorganic compounds, polymers, and biomolecules. It is widely used in a variety of fields, including chemistry, materials science, and biology.
What is infrared spectroscopy used for?
Infrared (IR) spectroscopy is a technique used to identify and analyze the molecular structure of a substance. It involves the absorption of infrared light by a sample, and the measurement of the absorption at different wavelengths. By analyzing the absorption spectrum, it is possible to determine the vibrational modes of the bonds in the molecule, which can be used to identify the molecular structure of the sample.
Infrared spectroscopy is used in a variety of fields, including chemistry, materials science, and biology. It is particularly useful for identifying and analyzing the molecular structure of organic and inorganic compounds, polymers, and biomolecules. It is also used in quality control and research applications, as well as for the analysis of environmental samples and forensic evidence.
What is range of IR spectroscopy?
The range of infrared (IR) spectroscopy is typically defined by the range of wavelengths that can be measured using the instrumentation available. The range of IR spectroscopy is typically between 400 and 4000 cm^-1 (4000 and 400 nm), although some instruments are capable of measuring even lower or higher wavelengths.
The specific range of IR spectroscopy that is used depends on the nature of the sample being analyzed and the vibrational modes of the bonds in the molecule. Different bonds absorb infrared light at different wavelengths, and the absorption spectrum of a molecule can be used to identify the molecular structure of the sample.
For example, the absorption of infrared light by stretching and bending vibrations of bonds in organic molecules is typically in the range of 4000-400 cm^-1 (400-4000 nm). The absorption of infrared light by vibrational modes of inorganic compounds and polymers is typically in the range of 1000-100 cm^-1 (10000-100000 nm). The absorption of infrared light by rotational modes of molecules is typically in the range of 100-10 cm^-1 (100000-1000000 nm).
What instrument is used in IR spectroscopy?
Infrared (IR) spectroscopy is typically performed using a specialized instrument called an infrared spectrometer. An infrared spectrometer consists of three main components: a light source, a sample holder, and a detector.
The light source in an infrared spectrometer is typically a tungsten halogen lamp or a deuterium lamp, which produces a broad spectrum of infrared light. The sample holder is a device that holds the sample and allows infrared light to pass through it. The detector is a device that measures the intensity of the infrared light that is transmitted or absorbed by the sample.
There are several different types of infrared spectrometers, including dispersive spectrometers and Fourier transform spectrometers. Dispersive spectrometers use a prism or grating to separate the different wavelengths of light, while Fourier transform spectrometers use interferometry to measure the spectrum of the sample.
Infrared spectrometers are widely used in a variety of fields, including chemistry, materials science, and biology, to identify and analyze the molecular structure of a wide range of materials, including organic and inorganic compounds, polymers, and biomolecules.
Which type of solvent is used in IR?
Infrared (IR) spectroscopy is a technique that is used to identify and analyze the molecular structure of a substance. In order to prepare a sample for IR spectroscopy, it is often necessary to dissolve the sample in a solvent. The choice of solvent depends on the nature of the sample and the vibrational modes of the bonds in the molecule that are being studied.
In general, it is best to use a solvent that does not absorb infrared light at the wavelengths of interest. This is because the absorption of the solvent can interfere with the measurement of the absorption of the sample.
Common solvents that are used for IR spectroscopy include liquid hydrocarbons (such as hexane, cyclohexane, and toluene), alcohols (such as methanol, ethanol, and isopropanol), and water. In some cases, it may be necessary to use a more specialized solvent, such as dimethylformamide (DMF) or dimethylsulfoxide (DMSO), depending on the nature of the sample.
It is important to note that the choice of solvent can affect the absorption spectrum of the sample. In some cases, the presence of a solvent can alter the vibrational modes of the bonds in the molecule, which can affect the interpretation of the IR spectrum. Therefore, it is important to carefully consider the choice of solvent when preparing a sample for IR spectroscopy.
What is UV and IR range?
The electromagnetic spectrum is the range of all types of electromagnetic radiation, including radio waves, microwaves, infrared (IR) radiation, visible light, ultraviolet (UV) radiation, X-rays, and gamma rays. Each type of electromagnetic radiation has a specific range of wavelengths and frequencies, and is characterized by its unique properties and uses.
The ultraviolet (UV) range is the portion of the electromagnetic spectrum with wavelengths shorter than visible light. UV radiation has wavelengths between 100 and 400 nm, and it is characterized by its ability to cause chemical reactions and damage to living tissues. The UV range is further divided into three regions: UVA (400-315 nm), UVB (315-280 nm), and UVC (280-100 nm).
The infrared (IR) range is the portion of the electromagnetic spectrum with wavelengths longer than visible light. IR radiation has wavelengths between 750 nm and 1 mm, and it is characterized by its ability to be absorbed by certain vibrational modes of the bonds between atoms in a molecule. The IR range is further divided into three regions: near-infrared (NIR, 750-1400 nm), mid-infrared (MIR, 1400-3000 nm), and far-infrared (FIR, 3000-1 mm).
UV and IR radiation have a variety of practical applications, including UV disinfection, UV curing, and IR spectroscopy. They are also used in a variety of scientific and medical instruments, such as UV spectrophotometers and IR cameras.
Can we use water in IR spectroscopy?
Yes, water can be used as a solvent in infrared (IR) spectroscopy. Water is a common solvent for IR spectroscopy because it is widely available, inexpensive, and has a low absorption in the IR region.
However, it is important to note that water can absorb infrared light at certain wavelengths, particularly in the mid-infrared (MIR) region (1400-3000 cm^-1 or 4000-1000 nm). This means that the absorption spectrum of the sample may be affected by the presence of water, particularly if the sample absorbs at the same wavelengths as water.
To minimize the impact of water absorption on the IR spectrum of the sample, it is often necessary to use a deuterated solvent (such as deuterated methanol or deuterated water) or to dry the sample prior to analysis. Deuterated solvents have a lower absorption in the IR region than their non-deuterated counterparts, and they are often used to reduce interference from the solvent in IR spectra. Drying the sample can also help to remove any water that may be present, which can reduce interference from water absorption.
It is also important to carefully consider the nature of the sample when selecting a solvent for IR spectroscopy. In some cases, it may be necessary to use a more specialized solvent, such as dimethylformamide (DMF) or dimethylsulfoxide (DMSO), depending on the nature of the sample. The choice of solvent can affect the absorption spectrum of the sample, and it is important to choose a solvent that does not interfere with the measurement of the absorption of the sample.
How sensitive is IR spectroscopy?
The sensitivity of infrared (IR) spectroscopy depends on a variety of factors, including the nature of the sample, the instrumentation used, and the experimental conditions. In general, IR spectroscopy is sensitive enough to detect small changes in the molecular structure of a sample, and it is often used to identify and quantify trace amounts of compounds in complex mixtures.
One of the main factors that determines the sensitivity of IR spectroscopy is the strength of the vibrational absorption bands in the spectrum of the sample. Strong absorption bands are more likely to be detected than weak absorption bands, and they are generally easier to quantify.
The sensitivity of IR spectroscopy can also be affected by the choice of solvent, the sample preparation techniques, and the instrumentation used. For example, using a deuterated solvent or drying the sample can reduce interference from the solvent, which can improve the sensitivity of the measurement. Using a high-resolution spectrometer or a Fourier transform spectrometer (FTIR) can also improve the sensitivity of the measurement, as these instruments have a higher resolution and can detect smaller changes in the absorption spectrum.
In general, IR spectroscopy is a sensitive and powerful tool for identifying and analyzing the molecular structure of a wide range of materials, including organic and inorganic compounds, polymers, and biomolecules. It is widely used in a variety of fields, including chemistry, materials science, and biology, for a wide range of applications.
Which lamp is used in IR spectroscopy?
Infrared (IR) spectroscopy is typically performed using a specialized instrument called an infrared spectrometer. An infrared spectrometer consists of three main components: a light source, a sample holder, and a detector.
The light source in an infrared spectrometer is typically a tungsten halogen lamp or a deuterium lamp, which produces a broad spectrum of infrared light. The tungsten halogen lamp is a type of incandescent lamp that uses a tungsten filament and a halogen gas to produce light. It has a broad emission spectrum and is capable of producing a wide range of wavelengths in the infrared region.
The deuterium lamp is a specialized type of lamp that uses deuterium, a stable isotope of hydrogen, as the light-emitting element. It produces a spectrum that is rich in ultraviolet and infrared radiation, and it is often used in spectroscopy applications that require a broad spectrum of light.
Both tungsten halogen lamps and deuterium lamps are widely used in IR spectrometry because they produce a broad spectrum of infrared light that is suitable for a wide range of applications. The specific type of lamp used depends on the specific requirements of the measurement, including the wavelength range of interest and the sensitivity of the instrument.
References
- Kamariotis, A.; Boyarkin, O. V.; Mercier, S. R.; Beck, R. D.; Bush, M. F.; Williams, E. R.; Rizzo, T. R. J. Am. Chem. Soc. 2006, 128, 905
- Peter J. Larkin. “Infrared and Raman Spectroscopy. Principles and Spectral Interpretation” Chapters 1 to 6. Radarweg 29, PO Box 211, 1000 AE Amsterdam, The Netherlands. Elsevier 2011
- Marwa El-Azazy. Introductory Chapter: Infrared Spectroscopy – A Synopsis of the Fundamentals and Applications, Infrared Spectroscopy – Principles, Advances, and Applications. 2018. IntechOpen, DOI: 10.5772/intechopen.82210
- Donald L. Pavia, Gary M. Lampman and George S. Kriz. “Introduction to Spectroscopy. A Guide for Students of Organic Chemistry” Chapter 2. Thompson Learning. United States of America 2001.