36 Types of Spectroscopy with Principle, Uses

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Spectroscopy is a study of interaction of matter with electromagnetic radiation. It is used to study the atoms and molecules when they are exposed to different wavelength of light or energy.

In this process, the atoms and molecules absorb, emit, reflect or scatter the radiation. These radiation are absorbed or emitted only at some specific frequencies because the energy level of atoms and molecules are quantized.

The pattern which is formed is known as spectrum. This spectrum is not same for all substances. So, it is used as a chemical fingerprint for the identification of a substance.

By measuring the spectrum, the nature of substance can be known. It also gives the information about molecular structure, composition and concentration of different materials.

Spectroscopy is an important analytical technique. It is used in chemistry, physics, astronomy and biology for the study and analysis of different substances.

Types of Spectroscopy

Different types of spectroscopy are mentioned below-

1. Absorption Spectroscopy

This spectroscopy is based on absorption of radiation by the sample. Here the sample absorbs some specific wavelength of light.

The types are-

  1. Ultraviolet-Visible (UV-Vis) spectroscopy
  2. Infrared (IR) spectroscopy
  3. Fourier Transform Infrared (FTIR) spectroscopy
  4. Near-Infrared (NIR) spectroscopy
  5. Mid-Infrared (MIR) spectroscopy
  6. Far-Infrared (FIR) spectroscopy
  7. Atomic Absorption Spectroscopy (AAS)
  8. X-ray Absorption Spectroscopy (XAS)
  9. Microwave spectroscopy
  10. Mössbauer spectroscopy

2. Emission Spectroscopy

This spectroscopy is based on emission of radiation from excited atoms or molecules. When the excited particles return to lower energy state, radiation is emitted.

The types are-

  1. Fluorescence spectroscopy
  2. Phosphorescence spectroscopy
  3. Chemiluminescence spectroscopy
  4. Atomic Emission Spectroscopy (AES)
  5. Flame Emission Spectroscopy (FES)
  6. Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES)
  7. X-ray emission spectroscopy
  8. X-ray fluorescence

3. Scattering and Diffraction Spectroscopy

This spectroscopy is based on scattering or diffraction of radiation. Here the light is redirected by molecules, particles or crystal arrangement.

The types are-

  1. Raman spectroscopy
  2. Rayleigh scattering
  3. Dynamic Light Scattering (DLS)
  4. Neutron scattering
  5. Inelastic neutron scattering
  6. Compton scattering
  7. X-ray diffraction
  8. Nephelometry
  9. Turbidimetry

4. Resonance Spectroscopy

This spectroscopy is based on resonance between radiation and quantum states. It is used for studying magnetic or nuclear properties of atoms and molecules.

The types are-

  1. Nuclear Magnetic Resonance (NMR) spectroscopy
  2. Electron Spin Resonance (ESR) spectroscopy
  3. Electron Paramagnetic Resonance (EPR) spectroscopy
  4. Nuclear Quadrupole Resonance

5. Other Special Spectroscopy

Some spectroscopy are used for special analysis of substances. They are based on mass, sound, electron emission, optical rotation or plasma emission.

The types are-

  1. Mass spectrometry
  2. Acoustic resonance spectroscopy
  3. Photoacoustic spectroscopy
  4. Auger electron spectroscopy
  5. Circular dichroism spectroscopy
  6. Dielectric spectroscopy
  7. Impedance spectroscopy
  8. Laser-Induced Breakdown Spectroscopy (LIBS)
  9. Photoemission spectroscopy
  10. X-ray Photoelectron Spectroscopy

1. Infrared (IR) Spectroscopy

  • Infrared (IR) Spectroscopy is an analytical technique which measures the absorption of infrared light by a sample.
  • It is generally measured in the range of 4000 to 400 cm⁻¹.
  • When infrared light passes through the substance, the energy is absorbed by the chemical bonds and it causes vibration of the bonds.
  • The vibration may be stretching, bending, rocking or twisting.
  • A vibration is detected in IR spectrum only when it causes change in the net dipole moment of the molecule.
  • Different bonds and functional groups absorb infrared light at specific frequencies.
  • So the spectrum obtained acts as a chemical fingerprint of that molecule.
  • It is used to identify functional groups, characterize organic and inorganic molecules and observe changes in protein structure.

2. Fourier Transform Infrared (FTIR) Spectroscopy

  • Fourier Transform Infrared (FTIR) Spectroscopy is the modern standard method of performing IR spectroscopy.
  • It is faster and highly accurate than ordinary IR spectroscopy.
  • FTIR uses Michelson interferometer which allows all wavelengths of infrared light to reach the sample and detector at the same time.
  • It does not measure one light frequency at a time.
  • This process produces a complex raw signal called interferogram.
  • The interferogram records light intensity over time.
  • Fourier transform is used to convert the interferogram into standard frequency spectrum.
  • FTIR gives greater throughput of radiation and more energy reaches the detector.
  • It reduces the time required to collect a spectrum from minutes to seconds.
  • It gives better signal-to-noise ratio.
  • It is used for quality control, failure analysis, identification of unknown materials or contaminants and quantifying components in pharmaceuticals, forensics and plastics.

3. Near-Infrared (NIR) Spectroscopy

  • Near-Infrared (NIR) Spectroscopy is a spectroscopic technique which uses electromagnetic radiation in near infrared region, usually 750 nm to 2500 nm or 12,800 to 4,000 cm⁻¹.
  • It measures the absorption of light by the molecules. The absorption is mainly due to overtones and combination vibrational transitions of molecules, especially organic compounds.
  • These overtones and combination transitions are weak transitions. They are also forbidden by quantum mechanics and occur very slowly. So the absorption of light is very weak.
  • NIR spectroscopy is less sensitive than mid-infrared (MIR) spectroscopy. It is not much useful for exact identification of functional groups.
  • But NIR light can penetrate deeper into the sample than mid-infrared light. So it can be used for thick sample and heterogeneous bulk materials.
  • It is used for non-destructive analysis of sample. It can be used directly without much sample preparation.
  • It is also used for in situ analysis of materials. The sample can be analysed in its original condition.
  • NIR spectroscopy is widely used in food, agriculture, pharmaceutical and textile industries.
  • It is used for rapid analysis of moisture content, fat and protein levels. It is used in grains, medicinal powders and other bulk materials.

4. Mid-Infrared (MIR) Spectroscopy

  • Mid-Infrared (MIR) Spectroscopy is a technique which uses light of mid-infrared region of electromagnetic spectrum. It ranges from 2.5 to 50 micrometers (μm) wavelength and 4,000 to 200 cm⁻¹ wavenumber.
  • It is based on absorption of light by molecules. This absorbed light excites fundamental vibration of chemical bonds like stretching, bending or twisting. Rotational-vibrational transition also takes place.
  • MIR light frequency match with natural vibration frequency of specific chemical bonds. So absorbed light forms a unique spectrum. This spectrum is referred to as chemical fingerprint of a compound.
  • It is used for identification of molecules, determining their structure and locating specific functional groups in a sample.
  • Generally IR spectroscopy means Mid-Infrared (MIR) spectroscopy. It is because MIR region is most useful region for characterization of chemical compounds.

5. Far-Infrared (FIR) Spectroscopy

  • Far-Infrared (FIR) Spectroscopy is an analytical technique which deals with lowest energy region of infrared electromagnetic spectrum. It is used to study the absorption of far-infrared light by a sample.
  • The FIR range generally covers wavelength from 50 to 1,000 μm. It corresponds to wavenumber from about 400 to 5 cm⁻¹, or 200 to 10 cm⁻¹.
  • It is based on absorption of far-infrared light by the sample. This absorbed light excites rotational transitions and low-energy vibrational transitions within the molecules of the sample.
  • FIR spectroscopy is highly useful for observing the rotation of small molecules. It is also used to detect backbone vibrations of large molecules and fundamental vibrations of molecules having heavy atoms, such as inorganic or organometallic compounds.

6. Atomic Absorption Spectroscopy (AAS)

  • Atomic Absorption Spectroscopy (AAS) is an elemental analysis method which is used to determine exact concentration of specific metals and metalloids in a given sample. It is mainly used for quantitative analysis of elements.
  • It is based on the absorption of light by free and unexcited atoms in the gas phase. These atoms are present in ground state and absorb light of specific wavelength.
  • In this method, the sample is generally dissolved in liquid form. Then it is introduced into high temperature environment such as flame or electrothermal graphite furnace. In this step, the solvent is evaporated and the sample is broken down into free gaseous atoms.
  • A specific wavelength of light is then passed through the atomized sample. This light is produced by hollow cathode lamp, which is made from the same element that is tested.
  • The ground state atoms present in the flame absorb this light. This absorbed light gives exact energy required to promote the electrons to higher excited energy state.
  • The amount of light absorbed is measured after passing through the flame. The missing light indicates the amount of absorbed light. From this, the exact concentration of the element in the sample is calculated by using Beer-Lambert Law.
  • AAS is highly specific and very sensitive method. It can detect trace amount of metals up to parts-per-million (ppm) or even parts-per-billion (ppb) level. It can detect about 70 different elements.
  • The main disadvantage is that the sample should be in homogenous liquid form. So solid samples are first digested carefully with acid. Also, different hollow cathode lamp is generally required for each element which is to be analyzed.
  • It is used in environmental monitoring for detection of lead or mercury in water and soil. It is also used in agriculture, pharmaceuticals, clinical chemistry such as magnesium in blood and food safety testing.

7. Microwave Spectroscopy

  • Microwave Spectroscopy is an analytical technique which measures the absorption of electromagnetic radiation in the microwave region by a sample. It is used to study molecular rotation.
  • Microwave spectroscopy is also referred to as rotational spectroscopy. It is because it mainly studies the collective rotational motion of atomic nuclei within molecules.
  • It uses microwave radiation having wavelength from 1 millimeter to 1 meter. Sometimes the range may be defined down to 25 μm. This corresponds to frequency between 3 × 10⁸ and 3 × 10¹¹ Hz.
  • It is based on absorption of microwave energy by molecules. When molecules absorb this energy, transition takes place between rotational energy levels. In simple words, the molecules start to spin or tumble.
  • Microwave spectroscopy is mainly used to study rotation of polyatomic molecules. It also gives information about electron spin flips within the material.

8. Mössbauer Spectroscopy

  • Mössbauer Spectroscopy is a spectroscopic technique which analyzes resonant absorption of gamma rays (γ-rays) by specific isotopic nuclei. It is used for study of nuclei in different atomic environments.
  • It is based on Mössbauer effect. This effect is the recoil-free resonant absorption of gamma radiation by the nucleus.
  • Mössbauer Spectroscopy works in the gamma-ray region of electromagnetic spectrum. This is the highest-energy portion of the electromagnetic spectrum.
  • In this method, transition takes place between nuclear ground state and higher excited nuclear level. It is commonly used for ⁵⁷Fe isotope, where transition is measured to an excited nuclear level which is 14.4 keV higher in energy.
  • Mössbauer Spectroscopy is used to study the properties and behaviour of specific isotopic nuclei. It also helps to know their condition within different atomic environments.

9. Fluorescence Spectroscopy

  • Fluorescence Spectroscopy is a form of emission spectroscopy. It is a type of photoluminescence. Here light emitted by a substance is measured after it absorbs electromagnetic radiation.
  • In this process, molecules absorb light first. The light is usually ultraviolet (UV) or visible light. The electrons are excited to higher energy state.
  • The excited electrons return to original ground state. During return, the extra energy is released as light or photons.
  • The emitted light is generally redshifted. It means wavelength is longer and energy is lower than absorbed light. This is called Stokes shift.
  • Fluorescence is very fast. The emission takes place in picoseconds to nanoseconds.
  • Fluorometer is used to measure fluorescence. The light source is focused on sample. The emitted fluorescence is collected. The collection lens is placed at 90-degree angle to excitation light path. It reduce background interference from source light.
  • Fluorescence Spectroscopy is highly sensitive and selective. It is used to detect trace amount of compounds.
  • It is used in biomedical and environmental research. It is used to study protein-protein interaction and DNA dynamics. It is also used to detect environmental pollutants and diagnosis of diseases like cancer.

10. Phosphorescence Spectroscopy

  • Phosphorescence Spectroscopy is a type of photoluminescence spectroscopy. It is an emission spectroscopy. Here light emitted by a material is measured after it absorbs photons.
  • In this process, molecule absorb light first. An electron is excited to higher energy singlet state. In phosphorescence, this electron undergoes intersystem crossing into excited triplet spin state.
  • The electron return from triplet state to ground state and emits photon. This direct return is quantum mechanically forbidden. But still it occurs due to spin-orbit coupling.
  • The excited triplet state acts as metastable state. It means the energy is trapped for some time before the electron relax.
  • Phosphorescence is weaker and slower than fluorescence. It is because relaxation process is delayed. The emission of light may take from microseconds (μs) to several minutes or even hours after the light source is removed.
  • The emitted light is redshifted. It means it has longer wavelength and lower energy than the light which excited the sample.
  • Phosphorescence Spectroscopy is used for characterizing next-generation materials. It is used for nanomaterials, low-dimensional materials and novel semiconductors such as gallium nitride nanostructures.
  • It is also used to study materials made for solar cells, photovoltaics and LED technologies.

11. Chemiluminescence Spectroscopy

  • Chemiluminescence Spectroscopy is a type of emission spectroscopy. It mainly works in ultraviolet and visible (UV/Vis) region of electromagnetic spectrum.
  • In this method, heat or light is not used to excite the sample. Chemiluminescence depends on exothermic chemical reaction. This reaction gives energy required to take the analyte to higher excited energy state.
  • After the atom or molecule reaches excited state, it quickly comes back to lower energy ground state. During this process, excess energy is released as photon or light. This emitted light is measured.
  • When this light emitting chemical reaction takes place naturally by biological or enzymatic process, it is called bioluminescence.
  • The glow of commercial light sticks is an example of chemiluminescence. The flash of firefly is an example of bioluminescence.

12. Atomic Emission Spectroscopy (AES)

  • Atomic Emission Spectroscopy (AES) is a spectroscopic technique. It is used to determine exact elemental composition and quantity of a sample. Here light emitted by atoms of the sample is measured.
  • In this method, the sample is introduced to high-energy source. The source may be heat from flame, plasma, electric arc or spark. This thermal energy excites the atoms and electrons jump to higher energy levels.
  • The excited atoms quickly return back to original ground state. During this return, excess energy is released as light. This emitted light has specific and characteristic wavelengths.
  • Each element emits its own unique pattern of wavelengths. So emitted light is analyzed to identify the elements present in the sample. The intensity of emitted light is used to determine the quantity or concentration of elements.
  • Atomic Emission Spectroscopy is used for elemental analysis. It is mainly used for measuring concentration of metals such as sodium, potassium and calcium. It is used in environmental, industrial and pharmaceutical testing.

13. Flame Emission Spectroscopy (FES)

  • Flame Emission Spectroscopy (FES) is a common analytical technique. It is used to detect specific metals in a sample. Here light emitted by the sample is measured when it is heated in a flame.
  • In this method, sample is introduced into a flame. The flame causes the material to become atomized and excited. The excited atoms or molecules return to ground state and emit light.
  • The emitted light has characteristic wavelength. This wavelength corresponds to energy difference between excited state and ground state of specific atoms. Thus exact elements present in the sample can be identified.
  • FES is particularly sensitive for detecting elements such as Aluminum, Barium, Calcium, Potassium, Indium and Lanthanides. Atomic Absorption Spectroscopy (AAS) is generally better for heavy metals.
  • Flame Emission Spectroscopy is mainly used to determine concentration of metal ions in aqueous solutions.
  • It is also used in medical testing. Blood or urine is tested to determine sodium or potassium levels of patient.

14. Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES)

  • Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) is a highly sensitive atomic emission spectroscopy. It is used to determine exact elemental composition of a sample.
  • In this method, flame is not used. Plasma is used as energy source. Plasma is a high-temperature ionized gas. It is generally Argon and temperature reaches 6,000 to 10,000 K.
  • The sample is introduced into very hot plasma. The atoms are excited by thermal energy. Then light is emitted from the excited atoms. This emitted light is analysed to know the sample makeup.
  • The plasma has intense energy. So many elements are excited and measured at same time. It gives broader dynamic range than Atomic Absorption Spectroscopy (AAS).
  • ICP-OES is used in materials science, environmental testing and food safety analysis.
  • It is used for detecting and analysing trace elements in water, soil and food products.

15. X-ray Emission Spectroscopy and X-ray Fluorescence

  • X-ray Emission Spectroscopy is a form of emission spectroscopy. It is a type of photoluminescence. Here X-rays emitted by a material are analysed.
  • In this process, high-energy X-rays interact with sample. It gives enough energy to remove inner-shell electrons from the atom.
  • When inner-shell electron is removed, a vacancy is formed. This makes the atom unstable. Then electron from higher energy outer shell comes down to fill the empty space.
  • During this movement, excess energy is released as X-ray fluorescence. This emitted X-ray fluorescence is measured by detector.
  • The emitted X-rays are analysed to get information about local structure. It also gives information about electronic environment of specific elements in the sample.
  • It is used for elemental analysis. It is mainly used in materials science and geology.
  • It is used for identifying and quantifying exact elements present in rocks, minerals and different metals.

16. Raman Spectroscopy

  • Raman Spectroscopy is a highly specific analytical technique. It is based on inelastic scattering of light. It is used to study molecular vibrations and rotations of a material.
  • In this method, monochromatic light such as laser is passed on the sample. Most of the light comes back with same energy. This is called elastic scattering or Rayleigh scattering.
  • A very small part of photons interact with molecular vibrations. About 1 in 10 million photons scatter inelastically. In this scattering, photons may lose or gain energy.
  • The photons which lose energy form Stokes lines. The photons which gain energy form anti-Stokes lines. These energy shifts are related with vibrational or rotational energy levels of the molecule.
  • For molecular vibration to be seen in Raman spectrum, the vibration must change the polarizability of molecule. Polarizability is the easiness by which electron cloud can be distorted.
  • The energy shifts are plotted to get a unique molecular fingerprint. It is used for identifying unknown chemicals, determining chemical composition and studying molecular structure and bonding.
  • Raman spectral bands are reported in wavenumbers (cm⁻¹). These are reported relative to energy of excitation light source and not as absolute wavelengths.
  • Raman scattering is a weak effect. So special methods like Surface-Enhanced Raman Spectroscopy (SERS) are used to increase sensitivity of signals.
  • Raman Spectroscopy is non-destructive technique. It complements infrared (IR) spectroscopy. It needs minimum or no sample preparation and can be used for solids, liquids and gases.

17. Rayleigh Scattering

  • Rayleigh Scattering is a type of scattering spectroscopy. It measures the elastic scattering of light.
  • It occurs when light interacts with particles or molecules. These particles are much smaller than the wavelength of incoming light.
  • In this process, scattered light has same frequency and same energy as original incident light. This is different from Raman scattering.
  • When monochromatic light such as laser falls on a sample, Rayleigh scattering is the most common scattering event. Most of the light scatters in this way.
  • Rayleigh Scattering is used to study concentration, size and shape of very small particles.
  • It is used to determine particle size distribution in colloidal systems, suspensions, aerosols and nanomaterials.

18. Dynamic Light Scattering (DLS)

  • Dynamic Light Scattering (DLS) is a type of scattering spectroscopy. It analyses light which is redirected after interacting with particles or molecules in a sample.
  • It is based on measurement of rapid fluctuation in the intensity of scattered light. These fluctuations occur because particles naturally move around in a liquid.
  • The light fluctuations are analysed to determine exact size of particles. It also gives the size distribution of particles in the solution.
  • Dynamic Light Scattering is used for characterization of nanoparticles, colloids and biomolecules. These are suspended in a liquid.
  • It is used for measuring size of proteins, polymers or different nanoparticles in a solution.

19. Neutron Scattering

  • Neutron Scattering is a scattering technique. It uses neutrons instead of light or photons to scatter from a sample.
  • It gives detailed information about atomic and molecular structures.
  • Neutron Scattering is especially useful for studying materials which contain lighter elements. Hydrogen is one important example.
  • It is used in materials science, chemistry and biology. It is used to study complex materials, soft matter and atomic structures of proteins, polymers and crystals.

20. Inelastic Neutron Scattering

  • Inelastic Neutron Scattering is a special type of neutron scattering. It is based on inelastic scattering of neutrons. In this process, neutrons exchange energy with the sample during collision.
  • It is similar to Raman spectroscopy in concept. But Raman spectroscopy uses photons to study vibrations, while inelastic neutron scattering uses neutrons.

21. X-ray Diffraction

  • X-ray Diffraction is an analytical technique. It measures how a sample redirects or scatters high-energy X-ray radiation.
  • It is not like many other spectroscopic techniques. In X-ray diffraction, exchange of energy between light or photons and sample does not takes place. It is based on elastic scattering or diffraction of light.
  • In this method, a beam of X-rays falls on crystalline sample. The regular and repeating lattice structure of crystals scatters the X-rays in specific directions.
  • X-ray Diffraction is used in crystallography. It is used to determine exact 3D arrangement of atoms in solid crystals and complex biological molecules like proteins.

22. Nephelometry and Turbidimetry

  • Nephelometry and Turbidimetry are spectroscopic techniques. These are based on scattering of light by a sample. It does not depend on absorption or emission of light.
  • Both techniques use electromagnetic radiation in ultraviolet and visible region. This region is called UV/Vis region.
  • In these techniques, exchange of energy between photon and sample does not takes place. So it is different from many other spectroscopic methods.
  • Here incoming electromagnetic radiation is scattered by particles present in the sample. Due to scattering, the direction of propagation, amplitude, phase angle or polarization is changed.

23. Nuclear Magnetic Resonance (NMR) Spectroscopy

  • Nuclear Magnetic Resonance (NMR) Spectroscopy is a powerful analytical technique. It uses magnetic properties of certain atomic nuclei. It is used to determine structure, composition and dynamics of molecules.
  • It uses low-energy radio frequency (rf) radiation.
  • Many atomic nuclei have a quantum property called spin. The common nuclei are ¹H and ¹³C. Due to spin, they have angular momentum and behave like tiny magnetic dipoles.
  • When sample is placed in strong and uniform external magnetic field, these small nuclear magnets align themselves. Due to this, their energy levels are split into different states. These may be parallel and anti-parallel to magnetic field.
  • When radio waves of specific frequency is given to the sample, the nuclei absorb this energy. This frequency is called Larmor frequency. Then nuclei show transition between the split energy levels.
  • The resonance frequency of a nucleus depends on its local chemical environment. The electron cloud around nucleus slightly shield it from external magnetic field. This small variation is called chemical shift.
  • Chemical shift is measured in parts per million (ppm), relative to a standard. It is used to identify location and types of atoms in a molecule.
  • The magnetic spin of one nucleus can interact with spin of neighbouring nuclei through shared chemical bonds. This is called spin-spin coupling or J-coupling.
  • Due to this interaction, main spectral signals split into smaller patterns. These patterns may be doublets or triplets. It gives information about how atoms are connected with each other.
  • NMR Spectroscopy is used by chemists and biologists. It is used to know 3D structures of complex organic molecules, polymers and proteins. It is also used to analyze mixtures and study chemical reactions.
  • The basic principle of NMR is also used in Magnetic Resonance Imaging (MRI) scanners in hospitals.

24. Electron Spin Resonance (ESR) or Electron Paramagnetic Resonance (EPR) Spectroscopy

  • Electron Spin Resonance (ESR) Spectroscopy is a spectroscopic technique. It is also known as Electron Paramagnetic Resonance (EPR) Spectroscopy. It is conceptually similar to Nuclear Magnetic Resonance (NMR) spectroscopy.
  • ESR or EPR does not study atomic nuclei like NMR. It measures spin excitement of unpaired electrons present in a sample.
  • It is based on quantum spin property of electrons. In presence of magnetic field, electron spins flip between different energy states. The energy involved in this spin flip is measured.
  • In this technique, exchange of energy takes place between sample and photons. It generally uses radio wave or microwave region of electromagnetic spectrum.
  • Electron paramagnetic resonance was first discovered in 1944 by Jevgeni Konstantinovitch Savoiski.

25. Nuclear Quadrupole Resonance

  • Nuclear Quadrupole Resonance is a chemical spectroscopy method. It is closely related to Nuclear Magnetic Resonance (NMR).
  • It is mediated by nuclear magnetic resonance of electric field gradient (EFG) inside the material.
  • Nuclear Quadrupole Resonance is different from standard NMR. Standard NMR requires strong external magnetic field. But Nuclear Quadrupole Resonance works in absence of magnetic field.

26. Mass Spectrometry

  • Mass Spectrometry is an analytical technique. It identifies and quantifies components of a sample by separating them on the basis of mass and electrical charge.
  • It produces a spectrum. This spectrum shows mass-to-charge ratio (m/z) and relative abundance of ions of the sample.
  • In ionization step, sample is vaporized into gas. Then it is bombarded with energy such as high-energy electrons. Due to this, electrons are removed and positively charged gas-phase ions are formed. Mass spectrometers generally analyse only these positive ions.
  • In acceleration step, positive ions are passed through an electric field. This electric field accelerates them into a focused beam. Due to this, all ions have same kinetic energy.
  • In deflection step, ion beam passes through magnetic or electric field. This field bends the path of ions. The amount of bending depends on mass-to-charge ratio (m/z). Lighter ions and ions having higher positive charge are deflected more than heavier or less charged ions.
  • In detection step, detector counts the ions when they arrive. Electron multiplier is one example of detector. The physical impact of ions is converted into electrical signal and then amplified. Neutral uncharged molecules are not affected by magnetic field and are removed by vacuum.
  • In data processing step, the signals are changed into mass spectrum chart. In this chart, relative abundance or intensity of ions is plotted on y-axis and m/z ratio is plotted on x-axis.
  • Base peak is the tallest peak in the chart. It represents the most abundant ion. It is given relative intensity of 100%.
  • Molecular ion peak is also called parent peak. It represents the intact and unfragmented molecule. It is used to determine exact molecular weight of the sample.
  • M+1 and M+2 peaks are smaller secondary peaks. These are slightly heavier than parent peak. It occurs due to natural isotopes such as Carbon-13 or Chlorine-37.
  • Mass Spectrometry is often combined with chromatography techniques. GC/MS and LC/MS are common examples. It is used to separate and analyse complex mixtures.
  • It is used in forensics for detecting drug abuse, fire accelerants and explosives. It is also used in environmental testing for pesticide screening and water testing. It is used in pharmaceutical drug development and clinical diagnosis for analysing disease biomarkers such as proteins and lipids.

27. Acoustic Resonance Spectroscopy

  • Acoustic Resonance Spectroscopy is a spectroscopic technique. It is based on sound waves.
  • It mainly uses sound waves in audible and ultrasonic regions.

28. Photoacoustic Spectroscopy

  • Photoacoustic Spectroscopy is a technique. It measures the effect of absorbed electromagnetic energy on matter.
  • This measurement is done by acoustic detection.

29. Auger Electron Spectroscopy

  • Auger Electron Spectroscopy is a spectroscopic method. It is used to study the surfaces of materials.
  • It works on micro-scale.
  • Auger Electron Spectroscopy is frequently used with electron microscopy.
  • It is classified as a type of electron spectroscopy.

30. Circular Dichroism Spectroscopy

  • Circular Dichroism Spectroscopy is an analytical technique. It measures differential absorption of left-handed and right-handed circularly polarized light.
  • Vibrational Circular Dichroism Spectroscopy is a specific variation of this method.
  • In this method, circularly polarized light is passed through a sample having chiral molecules. These molecules are asymmetric and cannot be superimposed on their mirror image.
  • The left-handed and right-handed circularly polarized light are absorbed at slightly different rates. The instrument measures this small difference in absorbance at different wavelengths.
  • Circular Dichroism spectra gives information about 3D conformation of chiral molecules. Large biological structures like protein backbone are chiral, so their spectra act as sensitive signature of structure.
  • It is used in structural biology and biochemistry. It is used to determine secondary structure of proteins such as alpha-helices and beta-sheets.
  • It is also used to study protein folding and unfolding. It also show change in protein shape due to temperature, pH or drug interaction.

31. Dielectric Spectroscopy

  • Dielectric Spectroscopy is a spectroscopic technique. It measures dielectric properties of a medium as a function of frequency.

32. Impedance Spectroscopy

  • Impedance Spectroscopy is a technique. It measures impedance of a medium.
  • Impedance is the ability of a medium to slow down or oppose the transmittance of energy.
  • In optical application, this ability to impede energy transmittance is shown by index of refraction of the material.

33. Laser-Induced Breakdown Spectroscopy (LIBS)

  • Laser-Induced Breakdown Spectroscopy (LIBS) is a specific type of atomic emission spectroscopy.
  • It is also referred to as laser-induced plasma spectrometry.
  • It works in optical region of electromagnetic spectrum. It uses ultraviolet, visible and near-infrared (UV-Vis-NIR) light.
  • In this method, highly energetic laser pulse is used as excitation source. It induces emission from the sample.

34. Ultraviolet-Visible (UV-Vis) Spectroscopy

  • Ultraviolet-Visible (UV-Vis) Spectroscopy is an analytical technique. It measures absorption or transmission of ultraviolet and visible light by a sample. It generally covers 190 to 800 nm wavelength range.
  • It is based on absorption of UV and visible light by molecules. This light gives energy required for electronic transitions. Valence electrons are promoted from lower energy ground state like HOMO to higher energy excited state like LUMO.
  • UV-Vis Spectroscopy is based on Beer-Lambert Law. According to this law, amount of light absorbed by sample is directly proportional to concentration of absorbing chemical species. So it is used for quantitative analysis.
  • A UV-Vis spectrometer has light source, monochromator and detector. Tungsten or halogen and deuterium lamps are used as light source. Monochromator selects specific wavelength. Detector measures light intensity.
  • For UV region below 350 nm, quartz cuvettes are used. Standard glass absorbs UV light, so glass cuvette is not used in this region.
  • It is used to determine concentration of biomolecules, transition metals and organic compounds.
  • It is used for enzyme kinetics and diagnosis of tissue damage in clinical chemistry. It is also used for dissolution testing of pharmaceutical tablets.
  • It is used for quantifying DNA and proteins in biochemistry. It is also used for monitoring heavy metals in environmental water testing.

35. X-ray Absorption Spectroscopy (XAS)

  • X-ray Absorption Spectroscopy (XAS) is a spectroscopic technique. It measures absorption of high-energy X-rays by a given sample.
  • In this method, sample is irradiated with X-rays. The X-ray radiation is absorbed by the sample. This gives enough energy to excite inner-shell electrons of atoms.
  • The absorption is analysed to get information about local structure. It also gives information about electronic environment of specific elements in the material.
  • XAS gives information about coordination of sample. It also gives oxidation state and overall electronic structure.
  • X-ray Absorption Spectroscopy is used for material analysis. It is used in materials science, geology, chemistry and catalysis.
  • It is used to characterize exact local structure of metal ions. These metal ions may be present in catalysts or other materials.

36. Photoemission Spectroscopy

  • Photoemission Spectroscopy is a spectroscopic technique. It measures energy or spin of electrons emitted from materials.
  • It is based on photoelectric effect. In this process, incident light gives enough energy to remove electrons from a material.
  • X-ray Photoelectron Spectroscopy (XPS)
  • X-ray Photoelectron Spectroscopy (XPS) is a special type of photoemission spectroscopy. It uses high-energy X-ray beam to produce photoemission.
  • XPS is mainly used as surface analysis technique. It is used to determine chemical state and composition of surface atoms.
  • Photoemission spectroscopy can also use ultraviolet (UV) light. This is called Ultraviolet Photoelectron Spectroscopy (UPS).
  • It may also be modified into angle-resolved photoemission spectroscopy and two-photon photoemission spectroscopy.

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