The different types of spectroscopy for chemical analysis (2023)

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Chemicals can be analyzed quantitatively and qualitatively by many different analytical methods, but a large area of ​​analysis is spectroscopy. Spectroscopy investigates the interaction between electromagnetic radiation and matter, with interactions leading to electronic excitations, molecular vibrations or nuclear spin orientations.

Spectroscopic methods can be categorized according to the type of radiation, interaction between energy and material, type of material and the applications for which the technique is used. There are many different types of spectroscopy, but the most common types used for chemical analysis include atomic spectroscopy, ultraviolet and visible spectroscopy, infrared spectroscopy, Raman spectroscopy, and nuclear magnetic resonance.

the classification

Spectroscopy can be defined by the type of radiant energy involved. The intensity and frequency of the radiation allow for a measurable spectrum. Electromagnetic radiation is a common type of radiation and was the first to be used in spectroscopic studies. Both infrared (IR) and near infrared use electromagnetic radiation, as well as terahertz and microwave techniques. Both electrons and neutrons are also a source of radiant energy due to their de Broglie wavelength. Mechanical methods can be applied to solid-state radiation, and acoustic spectroscopy uses radiated pressure waves.

Another way to classify spectroscopy is by the nature of the interaction between energy and material. These interactions include absorption, emission, resonance spectroscopy, elastic and inelastic scattering. The materials used can also define the type of spectroscopy, including atoms, molecules, nuclei and crystals.

Atomic Spectroscopy

Atomic spectroscopy was the first application of spectroscopy to be developed and can be divided into atomic absorption, emission and fluorescence spectroscopy. Atoms of different elements have different spectra, so atomic spectroscopy can quantify and identify the composition of a sample. The main types of atomic spectroscopy include atomic absorption spectroscopy (AAS), atomic emission spectroscopy (AES) and atomic fluorescence spectroscopy (AFS).

In AAS, atoms absorb ultraviolet or visible light to transition to higher energy levels. AAS quantifies the amount of absorption by ground state atoms in the gaseous state. AAS is commonly used in metal detection.

In AES, atoms are excited to emit light by the heat of a flame, plasma, arc or spark. AES used the intensity of emitted light to determine the amount of an element in a sample. Techniques using AES include flame emission spectroscopy, inductively coupled plasma atomic emission spectroscopy, and spark or arc atomic emission spectroscopy.
In AFS, it is a beam of light that excites the analytes, causing them to emit light. The fluorescence of a sample is then analyzed with a fluorometer and is commonly used to analyze organic compounds.

Ultraviolet and visible spectroscopy

Ultraviolet (UV) and visible (Vis) spectroscopy analyzes compounds using the spectrum of electromagnetic radiation from 10 nm to 700 nm. Many atoms can emit or absorb visible light, and this absorption or reflection gives the apparent color of the chemical being analyzed.

Absorption of visible and ultraviolet radiation is associated with the excitation of electrons from a low-energy ground state to a high-energy excited state, and energy can be absorbed by both nonbonding n-electrons and π-electrons within a molecular orbital.

The wavelengths of light are all associated with a specific energy, and only light with the right amount of energy causes it to transition from one plane to another for absorption. Larger gaps between energy levels require more energy to reach the higher energy level, so higher frequencies and shorter wavelengths are absorbed.

UV and visible spectroscopy can be used to measure the concentration of samples using the principles of the Beer-Lambert law, which states that absorbance is proportional to the concentration of the substance in solution and the path length. In addition to measuring the concentration of a sample, UV and visible spectroscopy can be used to identify the presence of free electrons and double bonds within a molecule. A UV/Vis spectrometer is not only an analytical technique that can be used alone, but can also be used as a detector for high performance liquid chromatography.

infrared spectroscopy

Infrared (IR) analyzes compounds using the infrared spectrum, which can be divided into near-infrared, mid-infrared, and far-infrared. Near-infrared has more energy and can penetrate much deeper into a sample than mid-infrared or far-infrared, but it also makes it less sensitive. Infrared spectroscopy is not as sensitive as UV/Vis spectroscopy because the energies involved in the vibration of atoms are smaller than the energies of the transitions.

IR uses the principle that molecules vibrate, with bonds stretching and bending as they absorb infrared radiation. Infrared spectroscopy works by passing a beam of infrared light through a sample, and for an infrared detectable transition, the sample molecules must undergo a change in dipole moment during vibration. When the frequency of the IR is equal to the vibrational frequency of the bonds, absorption takes place and a spectrum can be recorded.

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Different functional groups absorb heat at different frequencies depending on their structure and therefore a vibrational spectrum can be used to determine the functional groups present in a sample. When interpreting data obtained from an IR, the results can be compared to a frequency table to find out which functional groups are present to help determine structure.

Raman spectroscopy

Raman spectroscopy is similar to IR in that it is a vibrational spectroscopy technique but uses inelastic scattering. The Raman spectroscopy spectrum shows a scattered Rayleigh line and Stoke and anti-Stoke lines, which are distinct from irregular IR absorption lines.

Raman spectroscopy works by detecting inelastic scattering, also known as Raman scattering, of monochromatic light from a laser in the visible, near-infrared, or ultraviolet range. For a transition to be Raman active, the polarizability of the molecule must change during vibration and the electron cloud must undergo a change in position.

The technique provides a molecular fingerprint of the chemical composition and structures of samples, but Raman scattering provides inherently weak signals. Techniques such as Surface Enhanced Raman Spectroscopy (SERS) have been developed to improve sensitivity when using Raman spectroscopy.

nuclear magnetic resonance

Nuclear Magnetic Resonance (NMR) uses resonance spectroscopy and nuclear spin states for spectroscopic analysis. All nuclei have a nuclear spin, and the spin behavior of the nucleus of each atom depends on its intramolecular environment and the externally applied field.

When the nuclei of a given element are in different chemical environments within the same molecule, different magnetic field strengths occur due to the shielding and unshielding of nearby electrons, causing different resonance frequencies and defining chemical shift values.

Spin-spin coupling is responsible for the spin states of a nucleus, through intermediate bonds, affecting the magnetic field experienced by neighboring nuclei. Spin-spin coupling causes the absorption peaks of each core group to be split into several components.

There are several types of NMR analysis, namely Hydrogen NMR, Carbon-13 NMR, DEPT-90 and DEPT-135 NMR. The NMR spectrum of a compound shows the resonance signals emitted by the atomic nuclei present in a sample, which can be used to identify the structure of a compound.

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