22 Types of Spectroscopy with Definition, Principle, Steps, Uses

Spectroscopy is the study of the interaction between matter and electromagnetic radiation. It is a powerful analytical technique that can provide information about the structure, composition, and properties of different substances. Spectroscopy can be classified into different types based on the nature of the radiation, the region of the electromagnetic spectrum, and the type of interaction with the matter.

There are various instruments used in spectroscopy to generate, measure, and analyze the radiation. Some of the common instruments are:

• Spectrometer: A device that measures the intensity or frequency of radiation as a function of wavelength or energy. It usually consists of a source of radiation, a device to disperse or separate the radiation into different wavelengths (such as a prism or a grating), and a detector to measure the intensity of each wavelength.
• Spectrophotometer: A type of spectrometer that measures the amount of light absorbed or transmitted by a sample at different wavelengths. It usually consists of a light source, a sample holder, a monochromator to select a specific wavelength, and a photodetector to measure the intensity of the transmitted or absorbed light.
• Spectroscope: A device that allows visual observation of the spectrum of light emitted or absorbed by a sample. It usually consists of a slit to narrow the beam of light, a prism or a grating to disperse the light into different colors, and an eyepiece to view the spectrum.
• Spectrograph: A device that records the spectrum of light emitted or absorbed by a sample on a photographic plate or a digital sensor. It usually consists of a slit, a prism or a grating, and a camera or an array of detectors.

These instruments can be used for different types of spectroscopy depending on the region of the electromagnetic spectrum and the type of interaction with the matter. Some examples are:

• Absorption spectroscopy: A technique that measures how much light is absorbed by a sample at different wavelengths. It can be used to identify and quantify the molecules present in the sample based on their characteristic absorption spectra.
• Emission spectroscopy: A technique that measures how much light is emitted by a sample at different wavelengths. It can be used to determine the chemical composition and physical state of the sample based on its characteristic emission spectra.
• Fluorescence spectroscopy: A technique that measures how much light is emitted by a sample when it is excited by another source of light. It can be used to study the structure and dynamics of molecules based on their fluorescence properties.
• Raman spectroscopy: A technique that measures how much light is scattered by a sample when it is irradiated by a laser. It can be used to study the vibrational modes and molecular structure of the sample based on its Raman spectra.
• Nuclear magnetic resonance (NMR) spectroscopy: A technique that measures how much radiofrequency radiation is absorbed or emitted by a sample when it is placed in a magnetic field. It can be used to study the structure and interactions of molecules based on their NMR spectra.
• Mass spectroscopy: A technique that measures how much ions are produced by a sample when it is bombarded by electrons or other particles. It can be used to determine the mass and structure of molecules based on their mass spectra.

These are some examples of spectroscopy and its various instruments. In this article, we will discuss 22 types of spectroscopy in detail with their definition, principle, steps, and uses.

A spectrum is a graphical representation of the intensity of radiation as a function of wavelength, frequency, or energy. In spectroscopy, spectra are used to analyze the interaction of electromagnetic radiation with matter and to obtain information about the physical and chemical properties of the sample.

There are different types of spectra depending on the nature of the interaction and the source of radiation. Some of the common types of spectra are:

• Emission spectrum: This is the spectrum of radiation emitted by a substance when it is excited by an external source of energy, such as heat, electricity, or light. The emission spectrum consists of discrete lines or bands corresponding to the specific wavelengths or frequencies of radiation emitted by the atoms or molecules in the sample. The emission spectrum can be used to identify the elements or compounds present in the sample based on their characteristic emission lines or bands.

• Absorption spectrum: This is the spectrum of radiation transmitted through a substance when it is exposed to a continuous source of radiation, such as white light. The absorption spectrum consists of dark lines or gaps corresponding to the specific wavelengths or frequencies of radiation absorbed by the atoms or molecules in the sample. The absorption spectrum can be used to determine the concentration or amount of a substance in a sample based on the intensity or area of the absorption lines or gaps.

• Reflection spectrum: This is the spectrum of radiation reflected by a substance when it is illuminated by a source of radiation, such as sunlight. The reflection spectrum consists of peaks or valleys corresponding to the specific wavelengths or frequencies of radiation reflected by the atoms or molecules in the sample. The reflection spectrum can be used to study the surface properties or characteristics of a substance based on the shape or position of the reflection peaks or valleys.

• Scattering spectrum: This is the spectrum of radiation scattered by a substance when it is irradiated by a source of radiation, such as laser. The scattering spectrum consists of broad bands or features corresponding to the range of wavelengths or frequencies of radiation scattered by the atoms or molecules in the sample. The scattering spectrum can be used to investigate the structure or interactions of a substance based on the width or intensity of the scattering bands or features.

Spectra are significant in spectroscopy because they provide qualitative and quantitative information about the sample under investigation. By comparing the spectra obtained from different samples or different sources of radiation, one can infer various aspects such as:

• The identity or composition of a substance based on its characteristic spectral lines or bands.
• The amount or concentration of a substance based on its spectral intensity or area.
• The physical state or phase of a substance based on its spectral shape or position.
• The chemical structure or bonding of a substance based on its spectral splitting or shifting.
• The chemical reaction or transformation of a substance based on its spectral changes or variations.

Spectra are also significant in spectroscopy because they enable various applications in different fields such as:

• Analytical chemistry: Spectroscopy can be used for qualitative and quantitative analysis of various substances in different samples such as biological, environmental, pharmaceutical, forensic, etc.
• Physical chemistry: Spectroscopy can be used for studying various phenomena such as molecular vibrations, electronic transitions, nuclear spins, etc.
• Material science: Spectroscopy can be used for characterizing various materials such as metals, alloys, ceramics, polymers, nanomaterials, etc.
• Astronomy: Spectroscopy can be used for exploring various celestial objects such as stars, planets, galaxies, etc.
• Medicine: Spectroscopy can be used for diagnosing various diseases or disorders based on their spectral biomarkers.

• Absorption spectroscopy: A technique that measures the frequency or wavelength of absorbed light by a sample.
• Astronomical spectroscopy: A technique that studies the electromagnetic spectrum of astronomical objects such as stars and galaxies.
• Atomic absorption spectroscopy: A technique that measures the absorption of light by free atoms in a gas.
• Circular dichroism spectroscopy: A technique that measures the difference in absorbance of left and right circularly polarized light by a sample.
• Electrochemical impedance spectroscopy: A technique that measures the impedance of a system by applying different AC potential frequencies.
• Electron spin resonance spectroscopy: A technique that measures the absorption of microwave radiation by paramagnetic substances with unpaired electrons.
• Emission spectroscopy: A technique that measures the wavelengths of photons emitted by atoms or molecules during transitions from high to low energy states.
• Energy dispersive spectroscopy: A technique that measures the energy and number of X-rays emitted or absorbed by a sample.
• Fluorescence spectroscopy: A technique that measures the emission of light by a sample after excitation by a higher energy light source.
• Fourier-transform infrared spectroscopy: A technique that measures the infrared spectrum of absorption or emission by a sample using a mathematical process called Fourier transform.
• Gamma-ray spectroscopy: A technique that measures the energy spectrum of gamma rays emitted or absorbed by radioactive substances in a sample.
• Infrared spectroscopy/ Vibrational spectroscopy: A technique that measures the absorption of infrared radiation by a sample, which causes vibrational transitions in the molecules.
• Magnetic resonance spectroscopy: A technique that measures the magnetic resonance of nuclei in a sample, which provides information about their chemical structure and environment.
• Mass spectroscopy: A technique that measures the mass to charge ratio of ions produced by ionizing a sample with electrons or other particles.
• Molecular spectroscopy: A technique that measures the interaction between molecules and electromagnetic radiation, which provides information about their structure and composition.
• Mossbauer spectroscopy: A technique that measures the absorption or emission of nuclear gamma rays in solid particles, which provides information about their nuclear environment and structure.
• Nuclear magnetic resonance spectroscopy: A technique that measures the magnetic resonance of nuclei in a sample, which provides information about their chemical structure and environment.
• Photoelectron spectroscopy: A technique that measures the kinetic energy and number of electrons emitted by a sample after irradiation with X-rays or other high-energy photons.
• Raman spectroscopy: A technique that measures the scattering of light by a sample, which provides information about its vibrational and rotational modes.
• UV spectroscopy: A technique that measures the absorption of UV radiation by a sample, which causes electronic transitions in the molecules.
• Ultraviolet and visible (UV/Vis) spectroscopy: A technique that measures the absorption of UV and visible radiation by a sample, which provides information about its electronic structure and composition.
• X-ray photoelectron spectroscopy: A technique that measures the binding energy and number of electrons in different subshells of atoms in a sample after irradiation with X-rays.
  
  

  For each type of spectroscopy: explanation of its principle, steps involved in the process and its uses in various fields.

1. Acoustic resonance spectroscopy

• Principle: Acoustic resonance spectroscopy is a technique that measures the resonant frequencies of a sample when it is subjected to an acoustic wave. The resonant frequencies depend on the physical properties of the sample, such as its density, elasticity, shape, and size.
• Steps: A sample is placed in a chamber that is filled with a gas or liquid medium. An acoustic wave generator produces a sound wave that excites the sample. A microphone or a piezoelectric sensor detects the sound wave that is reflected or transmitted by the sample. A spectrum analyzer records the frequency and amplitude of the sound wave as a function of time.
• Uses: Acoustic resonance spectroscopy can be used for the characterization of materials, such as polymers, ceramics, metals, composites, and biological tissues. It can also be used for the detection of defects, cracks, porosity, and phase transitions in materials.

2. Time-resolved spectroscopy

• Principle: Time-resolved spectroscopy is a technique that measures the changes in the optical properties of a sample as a function of time after it is excited by a short pulse of light. The changes can include fluorescence, phosphorescence, absorption, emission, reflectance, and scattering. The time scale of the changes can range from femtoseconds to milliseconds.
• Steps: A sample is exposed to a short pulse of light from a laser or a flash lamp. The light can be monochromatic or broadband, depending on the type of spectroscopy. A detector or a spectrometer measures the intensity or the spectrum of the light that is emitted or transmitted by the sample at different time intervals after the excitation. A data acquisition system records and analyzes the data as a function of time.
• Uses: Time-resolved spectroscopy can be used for the study of fast chemical reactions, molecular dynamics, energy transfer, electron transfer, photophysical processes, and photochemical processes. It can also be used for the identification and characterization of transient species, such as radicals, ions, excited states, and intermediates.

3. Photoemission spectroscopy

• Principle: Photoemission spectroscopy is a technique that measures the kinetic energy and angular distribution of electrons that are emitted from a sample when it is irradiated by photons. The kinetic energy and angular distribution of the electrons depend on the binding energy and orbital symmetry of the electrons in the sample.
• Steps: A sample is placed in a vacuum chamber and exposed to a beam of photons from a light source, such as an X-ray tube or a synchrotron. The photons cause some of the electrons in the sample to overcome their binding energy and escape from the surface. An electron analyzer collects and analyzes the electrons according to their kinetic energy and angle of emission. A spectrum of intensity versus kinetic energy or binding energy is obtained.
• Uses: Photoemission spectroscopy can be used for the determination of the electronic structure, chemical composition, and chemical state of materials, especially solids and surfaces. It can also be used for the study of band structure, density of states, Fermi level, work function, valence band, core level, and surface states.

4. X-ray photoelectron spectroscopy

• Principle: X-ray photoelectron spectroscopy is a type of photoemission spectroscopy that uses X-rays as the photon source. The X-rays have enough energy to ionize the core electrons of atoms in the sample. The kinetic energy and angular distribution of the emitted photoelectrons depend on the binding energy and orbital symmetry of the core electrons in the sample.
• Steps: A sample is placed in a vacuum chamber and exposed to a beam of X-rays from an X-ray tube or a synchrotron. The X-rays cause some of the core electrons in the sample to overcome their binding energy and escape from the surface. An electron analyzer collects and analyzes the electrons according to their kinetic energy and angle of emission. A spectrum of intensity versus kinetic energy or binding energy is obtained.
• Uses: X-ray photoelectron spectroscopy can be used for the determination of the elemental composition, chemical state, and electronic structure of materials, especially solids and surfaces. It can also be used for the study of oxidation states, chemical bonding, surface contamination, thin films, catalysts, corrosion, and adsorption.

5. Circular Dichroism

• Principle: Circular dichroism is a type of light absorbance spectroscopy that measures the difference in absorbance between left- and right-circularly polarized light by a chiral molecule or structure. The difference in absorbance depends on the molecular structure and conformation of the chiral molecule or structure.
• Steps: A sample containing chiral molecules or structures is placed in a cuvette and exposed to linearly polarized light from a light source. A polarizer converts the linearly polarized light into circularly polarized light by rotating its plane of polarization. A monochromator selects a specific wavelength of light to pass through the sample. A photodetector measures the intensity of light that passes through or is reflected by the sample. The polarizer alternates between left- and right-circularly polarized light at different wavelengths. A spectrum analyzer records the difference in absorbance between left- and right-circularly polarized light at each wavelength.
• Uses: Circular dichroism can be used for the determination of secondary structure, tertiary structure, folding kinetics, conformational changes, protein-ligand interactions, protein-protein interactions, protein-DNA interactions, protein-RNA interactions, enzyme activity, membrane proteins, peptide synthesis, drug discovery, and quality control.

6. IR Spectroscopy (Infrared spectroscopy)

• Principle: IR spectroscopy is a type of vibrational spectroscopy that measures the absorption or emission of infrared radiation by molecules. The absorption or emission depends on the vibrational modes (stretching or bending) of the bonds between atoms in molecules. Different bonds have different vibrational frequencies that correspond to different wavelengths of infrared radiation.

• Steps: A sample containing molecules with bonds that vibrate in response to infrared radiation is placed in a spectrometer that has an infrared light source and a detector. The infrared light passes through or reflects off the sample and reaches the detector. The detector measures the intensity or spectrum of infrared radiation that is absorbed or emitted by the sample at different wavelengths.

• Uses: IR spectroscopy can be used for the identification and characterization of organic compounds based on their functional groups, such as alcohols, carboxylic acids, amines, aldehydes, ketones, and ethers. It can also be used for the analysis of polymers, plastics, oils, paints, coatings, adhesives, explosives, pesticides, pharmaceuticals, foods, and beverages.

7. Raman spectroscopy

• Principle: Raman spectroscopy is a type of vibrational spectroscopy that measures the scattering of monochromatic light by molecules. The scattering occurs when the photons interact with the vibrational modes (stretching or bending) of the bonds between atoms in molecules. Most photons are scattered elastically (Rayleigh scattering), which means they have the same wavelength as the incident light. However, a small fraction of photons are scattered inelastically (Raman scattering), which means they have a different wavelength than the incident light. The difference in wavelength depends on the vibrational frequency of the bond involved in the scattering process.

• Steps: A sample containing molecules with bonds that vibrate in response to monochromatic light is placed in a spectrometer that has a laser as a light source and a detector. The laser beam passes through or reflects off the sample and reaches the detector. The detector measures the intensity or spectrum of scattered light at different wavelengths. The Raman spectrum consists of a strong peak at the wavelength of the incident light (Rayleigh peak) and weaker peaks at different wavelengths (Raman peaks).

• Uses: Raman spectroscopy can be used for the identification and characterization of organic compounds based on their functional groups, such as alkenes, alkynes, aromatics, nitriles, nitro compounds, and sulfides. It can also be used for the analysis of polymorphs, crystallinity, phase transitions, stress/strain, temperature/pressure effects, surface-enhanced Raman scattering (SERS), tip-enhanced Raman scattering (TERS), resonance Raman scattering (RRS), and coherent anti-Stokes Raman scattering (CARS).

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