What are those 10 different spectroscopic techniques

Spectroscopy is a combination of two words i.e., spectro and scopy. Spectro comes from the Latin spectrum which means an image while scopy means to study or to observe in Greek terminology. Thus, spectroscopy is the study of the interaction of matter with different regions of the electromagnetic spectrum. The primary goal of applying a  spectroscopic technique is the structural elucidation of the compound under study. There are different types of analytical spectroscopic techniques, each can be applied for a specific task.

In this article, we have tried to summarize all the main spectroscopic techniques; their instrumentation, and, working principles. So, without any further delay, let’s start reading!

What is spectroscopy

Spectroscopy is the study of the interaction of electromagnetic radiations with the matter. When irradiated with a light source, the chemical atoms or molecules present in the sample mixture interact differently by absorbing the distinct packets of energy called photons from radiant light. As a result, the sample molecules undergo excitation followed by de-excitation, transmitting, emitting, or scattering radiant energy. Based on the different types of interaction and the intensity of each interaction, the sample constituents can be identified and quantified.

What are electromagnetic radiations

Electromagnetic radiations behave both as a wave and a particle. These consist of sinusoidally moving electric (E) and magnetic (B) fields. The electric and magnetic fields of an electromagnetic radiation oscillate perpendicular to each other and to the direction of propagation.

All the different radiations together constitute the entire electromagnetic spectrum. Moving across the spectrum from gamma rays to radiowaves, the frequency/energy of the radiations decreases while their wavelength increases according to formula 1.

f=v/ λ…………. Formula 1

where f= frequency, v= speed of the electromagnetic wave, and λ= wavelength.

The frequency is related to energy by formula 2.

E=hf…… Formula 2

where E denotes the energy of the electromagnetic radiation while h stands for Planck’s constant (6.626 x 10^-34 m² kg/s)

Now, let us discuss the many different spectroscopic techniques one by one.

What are the main types of spectroscopic techniques

1. UV-Vis spectroscopy

• The ultraviolet-visible (UV-Vis) spectroscopy is a type of absorption spectroscopy.
• It is based on the electronic excitation of chemical molecules as a result of the absorption of electromagnetic radiations in the ultraviolet (200-400) nm and/or visible (400-800)nm region.
• A hydrogen discharge lamp is used for supplying ultraviolet radiation while a tungsten filament lamp is used for irradiating the sample with visible radiation.
• The sample is held inside the spectrophotometer using a quartz cuvette.
• The monochromator (a prism or a diffraction grating) splits radiant energy into its constituent wavelengths and selects a specific wavelength to pass through the sample at a time.
• Conjugated organic molecules in the sample strongly absorb a specific wavelength based on their chemical structures. The wavelength absorbed corresponds to the energy difference (∆E) between the two electronic energy levels.
• The electrons present in these molecules undergo the transition from a lower electronic energy level to a higher level.
• This excitation is followed by a de-excitation of the same electrons from the higher to the lower energy level which generates an electrical signal. The electrical signal is consequently sent to the detector.

• The detector responds to the electrical signal by plotting a UV-Vis spectrum. The UV-Vis spectrum is a plot of absorbance versus wavelength (nm).
• The highest intensity peak in the spectrum corresponds to the wavelength at which maximum absorption occurs. This is marked as the λmax value. The λmax value helps identify the specific chemical constituent under study.

• Different concentrations of the pure reference compound are then prepared and the absorbance of each is determined at the λmax to plot a calibration curve.
• This calibration curve is a straight line that is used to determine the concentration of the specific chemical compound identified in the sample mixture.
• Thus, UV-Vis spectroscopy allows both qualitative and quantitative analysis. Its main application lies in determining the chemical composition of unknown sample mixtures in the chemistry laboratory.

2. IR spectroscopy

• The infrared (IR) spectroscopy is another example of absorption spectroscopy.
• It is based on the vibrational excitation of electrons present in the sample molecules.
• Electronic energy levels present in a molecule’s structure are subdivided into vibrational and rotational energy levels. Vibrational energy change refers to the change in bond length and bond angle of the molecules.

• The sample prepared in an IR transparent solvent (such as CCl₄) is irradiated with a Nernst filament lamp or Globar which supplies IR radiations of (2.5 to 16) μm or (625 to 4000) cm^-1.
• Specific sample molecules absorb a specific IR frequency that can lead to a change in the net dipole moment of the molecules. Apart from the frequency absorbed, all the other irradiated frequencies are transmitted.

• The transmitted frequencies are collected by the detector and an IR spectrum is plotted.
• The IR spectrum is a plot of absorbance or transmittance versus wavenumber. It consists of two main regions i.e., a fundamental group region and a fingerprint region.
• The peaks displayed in the fundamental group region correspond to asymmetric stretching vibrations detected in the molecule. This region helps identify the characteristic functional groups present in the molecule.
• The peaks obtained in the fingerprint region correspond to bending vibrations present in the molecule. This region reveals the structural identity of the molecule.

A modified version of IR spectroscopy is FTIR spectroscopy. It is based on a mathematical operation called Fourier transformation. A Michelson interferometer is used in the FTIR spectrophotometer that increases the speed of the process multifold.

3. Atomic spectroscopy  

• The atomic spectroscopy is based on the excitation and de-excitation of electrons present in metal atoms of a sample.
• There are two main types of atomic spectroscopy i.e., atomic absorption spectroscopy (AAS) and atomic emission spectroscopy (AES).
• AAS is primarily used for the detection of heavy metals (Pb, Cr, Hg) present in a sample.
• AES is mainly applied for the identification and quantification of alkali and alkaline earth metal atoms (Na, Mg, Ca).

• Both techniques employ quite similar instrumentation.
• The sample is introduced into the spectrometer in the form of a solution. It is aspirated and mixed with the fuel. The sample-fuel mixture is sprayed onto the flame.   
• The high flame temperature evaporates the solvent leaving behind solid residue. The solid particles are further atomized i.e., converted into constituent metal atoms.
• In the case of atomic absorption spectroscopy, special hollow cathode lamps are provided that supply characteristic wavelengths to the atomized gaseous sample.
• Electrons present in these atoms absorb a specific wavelength and undergo excitation from a lower to a higher energy level. The amount of wavelength absorbed helps determine the concentration of targeted atoms.

• Contrarily, in the case of atomic emission spectroscopy electronic excitation occurs by thermal energy absorption from the flame source.
• Atomic electrons undergo excitation followed by de-excitation and radiant energy emission. The emitted radiant energy gives a characteristic color to the flame and its wavelength is used to identify and quantify the metal atoms present in the sample.

4. Light scattering spectroscopy

• The light scattering spectroscopy also called Raman spectroscopy is based on radiant energy scattering as a result of its interaction with sample molecules.
• Solid state laser diodes are used to irradiate the sample with monochromatic radiations of wavelength 532 nm, 785 nm, 1064 nm, etc.
• The sample molecules absorb photons from irradiated light. The electrons present in the molecules undergo vibrational and rotational energy changes.
• The incident light gets scattered. The frequency of scattered radiations is reduced in consequence of the absorbed energy.
• The scattered radiations are collected by a notch filter and used to plot the Raman spectrum. The Raman spectrum is a plot of light intensity versus Raman shift.
• The Raman shift refers to the frequency difference between incident and scattered light intensities. It helps identify different chemical constituents present in the examined sample.

5. Luminescence spectroscopy

• Luminescence spectroscopy is a type of molecular emission spectroscopy.
• There are three fundamental types of luminescence i.e., fluorescence, phosphorescence, and chemiluminescence.
• Generally, luminescence is referred to as the radiative emission of a specific wavelength of light as a  result of electronic excitation followed by de-excitation in the sample molecules.

• The sample molecules are usually irradiated with an ultraviolet-visible radiation source in the luminescence spectrometer.
• Two monochromators are used. The monochromators are placed at right angles to each other.
• The excitation monochromator splits the UV-Vis radiations into their characteristic wavelengths and selects a specific wavelength to pass through the sample.
• Luminescence occurs at multiple wavelengths. A specific wavelength is selected by the emission monochromator and passed onto the detector.
• A luminescence spectrum is plotted that helps in identifying the chemical composition of the sample mixture.

A special type of luminescence spectroscopy is fluorescence spectroscopy.

6. Fluorescence spectroscopy

• Fluorescence is a sub-type of luminescence.
• On absorbing radiant energy, the electrons present in special chemical molecules undergo electronic excitation from the ground state energy level to a vibrational level of a higher electronic state.
• The electrons stay unstable at this higher energy level. Thus, this excitation is followed by de-excitation.
• The de-excitation occurs by thermal energy loss as well as loss of photons of a characteristic wavelength. The de-excitation by photon loss is called fluorescence.
• The instrumentation for fluorescence spectroscopy is the same as we discussed for luminescence spectroscopy. It is only the chemistry within the molecules that differs slightly.
• The sample is irradiated with a UV-Vis light source. The two monochromators perform the task of specific wavelength selection.
• Photomultiplier tubes are used as detectors that detect the emitted wavelength.
• A fluorescence spectrum is finally plotted as a graph of fluorescence intensity against emission or excitation wavelength. This graph helps determine the chemical composition of the sample.
• The sample concentration can be determined using Beer Lambert’s law.

F= QI_a εcl……….. Formula 3

where F denotes fluorescence, Q is constant for a certain chemical species, I_a= intensity of absorbed light, ε= molar extinction constant, and l= path length.

7. Circular dichroism (CD) spectroscopy

• CD spectroscopy is a type of absorption spectroscopy.
• It is based on the differential absorption of right and left circularly plane-polarized light.
• Optically active chiral compounds preferentially absorb light circularly plane-polarized in a specific direction.
• Electromagnetic radiations from the ultraviolet-visible or near-infrared (800-2500 nm) region are chosen as the light source.
• The radiations are linearly plane-polarized by passing them through a polarizer.
• This linearly plane polarized light is then circularly polarized by passing it through a piezoelectric element.
• The incident light beam is split into two beams of equal intensity where one beam is circularly plane polarized in the right direction while the other is circularly polarized towards the left. 
• As these oppositely polarized light beams pass through the sample mixture, a specific chemical compound absorbs radiations polarized towards the left to a larger extent than the right ones, and vice versa.
• The differential absorption is recorded as the circular dichroism data and used to plot a CD spectrum.

• The CD spectrum is a plot of the circular dichroism data (see formula 4) versus wavelength.

∆ A= A_l-A_r……… Formula 4

where ∆ A= differential absorbance, A_l= absorbance of left circularly polarized light, and A_r= absorbance of right circularly polarized light.

• The CD spectrum helps in the structural identification of two similar chemical compounds specifically two optically active isomers called enantiomers.

8. NMR spectroscopy  

• NMR spectroscopy stands for nuclear magnetic resonance spectroscopy.
• It is an extremely powerful atomic absorption spectroscopic technique.
• It is based on the absorption of radiowaves (25 MHz to 1 GHz) by magnetically spinning nuclei.
• There are two main types of NMR spectroscopy namely proton NMR spectroscopy and ¹³C-NMR spectroscopy.
• As their names suggest, the proton NMR spectroscopy is performed with respect to  ¹H nuclei present in a sample while ¹³C-NMR spectroscopy is performed with respect to the ¹³C nuclei.
• Both ¹H and ¹³C nuclei consist of an odd mass number. Therefore, they are continuously spinning about a fixed axis and possess a specific angular momentum (I).
• When an external magnetic field (B₀) is applied in the NMR spectrometer, the nuclei undergo Zeeman splitting i.e., they adopt two spin orientations of varying energy. 
• A majority of nuclei that align with the external field occupy the low-energy spin state. In contrast to that, only a small proportion of nuclei adopt a high-energy spin state and start spinning against the applied field.
• A certain energy gap (∆E) exists between the two spin states.

• The nuclei spinning at lower energy absorb radio wave frequency equal to ∆E and change their spin orientation. This energy is supplied by a radiofrequency oscillator.
• The nuclei stay unstable at this higher energy state, so they flip back to the lower energy spin state by emitting radiowaves which are consequently recorded by the radiofrequency detector.
• An NMR spectrum is plotted which is the graph of radiofrequency versus chemical shift (symbol δ).
• The chemical shift is calculated with reference to a single strong peak recorded by adding a small amount of tetramethylsilane (TMS) to the sample mixture.

• δ depends on the chemical and electronic environments near spinning nuclei. It is thus used for the structural identification of the compound under study.

9.  X-ray spectroscopy

• X-ray spectroscopy is based on the interaction of high-energy ( 3 x 10¹⁶ Hz)  X-ray photons with chemical compounds.
• An X-ray tube is used as the irradiation source.
• Intense X-ray beams originating from the X-ray tube are aligned in a specific direction using the collimator.
• As the X-rays interact with the sample surface, a part of the energy is absorbed while the rest is emitted back.
• A portion of the X-ray beams is also diffracted away from the sample in a direction different from the direction of the incident beams. The angle (θ) of diffracted X-ray beams is measured using Bragg’s equation (formula 5).

n λ= 2dsinθ……. Formula 5

where n= integer to represent the order of diffraction, λ= irradiated wavelength, and d= distance between atomic layers in a crystal surface.

• A monochromator selects a specific wavelength and directs it onto the detector.
• The detector then plots the X-ray spectrum which is a graph of detector response versus energy (in eV).

• Based on energy absorption, emittance, and diffraction, the three-dimensional structures of crystal surfaces can be determined.
• Thus, the most prominent application of X-ray spectroscopy lies in X-ray crystallography which helps reveal the structural and electronic arrangement of atoms, bond lengths, and bond angles present in a crystal.

10. Mass spectroscopy (MS)

• Mass spectroscopy more popularly known as mass spectrometry is somewhat different from other spectroscopic techniques.
• It is not based on the interaction of electromagnetic radiations with matter. Rather, in mass spectroscopy, an electromagnetic field is applied to determine the relative atomic mass (Ar) of an element. It gives information such as the relative abundance of the different isotopes of an element.
• The sample solution is first vaporized. As the vapors enter the spectrometer, high-speed electrons are bombarded at the gaseous molecules which converts them into gaseous molecular ions.
• The ions are then accelerated by an electric field followed by their deflection under the influence of an applied magnetic field.
• The separated ions finally reach the detector based on their specific mass-to-charge (m/e) ratios.

• A mass spectrum is consequently plotted as a graph of relative abundance versus m/e. This graph helps us determine the different molecular fragments present in the tested sample which are then combined to reveal the identity of the unknown chemical compound.

So, now that we know about the many different spectroscopic techniques available to us in the modern scientific world today the question that arises is which is the best spectroscopic method?

Well, the answer to this question is simple and quite straightforward i.e., it depends on the specific application that one spectroscopic technique proves more useful than the others. Cost, availability, and the quality of results required (in terms of sensitivity, selectivity, and accuracy) also strongly influence the selection of a specific spectroscopic method.

Here are for you the many different applications of analytical spectroscopic techniques in chemistry.

You may also like: What are those 10 different chromatography techniques.

Images source: All the images used in this article are designed by the writer herself (Ammara W.)

References

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3. Błachucki, W., J. Czapla-Masztafiak, J. Sá and J. Szlachetko (2019). “A laboratory-based double X-ray spectrometer for simultaneous X-ray emission and X-ray absorption studies.” Journal of Analytical Atomic Spectrometry 34(7): 1409-1415.

4. Capuano, E. and S. M. van Ruth (2016). Infrared Spectroscopy: Applications. Encyclopedia of Food and Health. B. Caballero, P. M. Finglas and F. Toldrá. Oxford, Academic Press: 424-431.

5. Deshpande, S. S. (2001). “Principles and Applications of Luminescence Spectroscopy.” Critical Reviews in Food Science and Nutrition 41(3): 155-224.

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