Spectroscopy: An introduction, principle, instrumentation with its application.

What is spectroscopy?


Spectroscopy is the process of the interaction of electromagnetic radiation with matter. Electromagnetic radiation is a simple harmonic wave of electric and magnetic fields fluctuating orthogonal to each other.

Introduction to Spectroscopy-

Spectroscopy deals with the study of the interaction between electromagnetic radiation and matter. The matter can be in the form of atoms, molecules, or ions.


Spectrometers are which can be used to measure the presence of a particular compound or particle in a molecule.

Spectrum is a simple process of the plot the amount of light absorbed by a sample versus the wavelength of the light. The amount of light absorbed is called the absorbance.

Principle of Spectroscopy-

The spectroscopy is the process in which the measurement and interpretation of Electro Magnetic Radiation (EMR) which is emitted or absorbed when the molecule, atoms, or ions of a sample move from one state of energy to another state of energy.

The change may be from Ground State to excited state or from the excited state to Ground state. At the ground state, the energy of a molecule is the sum total of rotational, vibrational, and electronic energy.

In other words, spectroscopy measures the changes in rotational, vibrational, and/or electronic energies.


Electro Magnetic Radiation (EMR) is made up of discrete particles called photons. EMR has got both wave characteristics as well as particle characteristics. This means that EMR can travel in a vacuum also.

The different types of EMR are Visible radiation, UV radiation, IR radiation, Microwaves, Radio waves, X-rays, y-rays or Cosmic rays, etc. As these radiations have different wavelength or frequency or energy, they are conveniently named so.

Electromagnetic radiation-

 It is a form of energy and has both electrical and magnetic characteristics is called electromagnetic radiation. Electromagnetic radiation has its origin in atomic and molecular processes.


The EMR field may be represented as both electric and magnetic vectors oscillating in mutually perpendicular planes. A unified theory of electromagnetism is developed by James Clerk Maxwell.

It deals with how electrically charged particles interact with each other and with magnetic fields. EMR is a form of energy and has both electrical and magnetic characteristics.

The electric and magnetic fields in an electromagnetic wave oscillate along with directions perpendicular to the propagation direction of the wave.

The energy of electromagnetic radiation can be given by the following equation- 

E= hv

Where E= Energy of radiation

h=Planck’s constant

v =Frequency of radiation

Frequency(v)= c/λ


The velocity of light in vacuum / Wavelength


3 x 108 m/sec-1 / Wavelength

Where- c=velocity of light in vacuum

λ= wavelength

Hence, E=hv


E= hcv

where h is Planck’s constant with the value of 6.626 × 10-34 m2·kg·s-1. Based on energy, electromagnetic radiation has been divided into different regions.  Human beings can also see the region of the electromagnetic spectrum, for example -visible region or visible spectrum.

Frequency– it is the number of complete wavelength units passing through a given point in unit time. Frequency is measured in Hertz (Hz) or cps (cycles per second). The higher units used as-

I Megahertz or MHz = 106Hz

1 Kilohertz or I KHz- 108Hz

1 Fresnel = 1012 Hz


Wavelength – It is the distance between two successive maxima or minima, and the distance between two successive troughs or peaks.  It can be measured in meters, centimeters (cm or 10-2m), millimeters (mm or 10-3m), micrometers (mm or 10 -6m), nanometers (nm or 10-9m) or Angstrom (A or 10-10m).

Wave Number– It is the number of waves per cm. Wavenumber is expressed in cm or Kayser. Wavenumber is especially used in IR spectroscopy where small wavelength measurements are made, to differentiate the frequency of vibrations in molecules.

You may read-infrared spectroscopy.

Electromagnetic spectrum-

It is the arrangement that is obtained by arranging various types of electromagnetic waves or radiations in order of their increasing wavelength or decreasing frequencies is called the electromagnetic spectrum.

The electromagnetic spectrum is divided into a number of regions, and these are artificial divisions in the sense that they have been defined solely as a result of differences in instrumentation that required at a given frequency range for producing and detecting radiation.

According to the University of Wisconsin, a short-wavelength means that the frequency will be higher because one cycle can pass in a shorter amount of time. Similarly, a long-wavelength has a low frequency because each cycle takes longer to complete.

Electromagnetic spectrum ranges from very short wavelengths (gamma rays) to very long wavelengths (radio waves). The visible region of the spectrum extends approximately over the wavelength range 400-700nm and the shorter wavelengths being the blue end of the spectrum and the longer wavelength the red.

The wavelength range is between 400 to 200nm that makes up the near-ultraviolet region of the spectrum and the wavelength range is above 700nm to approx 2000nm in the ultraviolet region.

Regions of electromagnetic Spectrum-


Law of spectroscopy –

Beer’s law

It shows that when a ray of monochromatic light passes through an absorbance medium, then the intensity of monochromatic light decreases exponentially as the concentration of the absorbing medium increases, it provided the length of the absorbing medium is constant.

Equation-      I= I0e-kc

Whereas, I = Intensity of transmitted ray

I0  = Intensity of the incident ray

K1 = constant

c = concentration of the absorbing medium

Lambert’s law

It shows that when a ray of monochromatic light passes through an absorbing medium, then the intensity of monochromatic light decreases exponentially as the light of the absorbing medium increases, it provided the concentration of the absorbing medium is constant.


Whereas, I = Intensity of transmitted ray

I0 = Intensity of the incident ray

k2 = constant

I = path length of the absorbing medium

Beers-Lambert Law-

When a ray of monochromatic light of initial intensity 10 passes through a solution or a transparent vessel, some of the light is absorbed so that the intensity of the transmitted light is less than 10.

The relationship between I and 10 depends on the path length I of the absorbing medium and the concentration C of the absorbing solution.

Types of Spectroscopy

Spectroscopy is consisting of the different techniques that show their action as per their requirement on the component. It can be divided into the following types based on-

-On the atomic or molecular level-

Atomic Spectroscopy -The change in energy takes place at the atomic level, where either atomic absorption or atomic emission of radiation is being studied. Example- Atomic absorption spectroscopy, Flame photometry

Molecular Spectroscopy – The changes in energy take place at the molecular level, where the molecular absorption, emission, or vibration is being studied. where the molecular absorption, emission, or vibration is being studied. Example-UV spectroscopy, Colorimetry, Infra-Red Spectroscopy, Fluorimetry

-On absorption or emission of EMR-

Absorption Spectroscopy – where the study of absorption of radiation is being studied. Example-UV spectroscopy, Colorimetry, Infra-Red Spectroscopy, NMR Spectroscopy, Atomic Absorption Spectroscopy.

Emission Spectroscopy – where the study of the emission of radiation is being studied. Example-Flame photometry, Fluorimetry.

-On electronic or magnetic levels-

Electronic Spectroscopy– Where the study is done using electromagnetic radiation only (without the influence of the magnetic field). Example- UV spectroscopy, Colorimetry, Fluorimetry

Magnetic Spectroscopy– Where the study is done using electromagnetic radiation under the influence of the magnetic field. Example- NMR spectroscopy, ESR spectroscopy

The energy of a molecule can be due to electronic, vibrational, or rotational energy. They are in the following ratio-

Rotational energy: Vibrational energy: Electronic energy = 1:100:10,000

Theory of Spectroscopy-

When EMR travels through a medium containing atoms, molecules, or ions, any one of the following may take place.


atoms or molecules

The intensity of emergent light (It) = Intensity of incident light (I0)

Therefore, no absorption- It = I0 

No change in energy takes place and hence no information about the molecule can be derived.

Reflection, Refraction or Scattering, (scattering of light by particles) where some studies like Nephelometry or Turbidimetry are being made. The intensity of emergent light < Intensity of incident light, where there is the absorption of energy. Here some information can be derived.

Excitation Process-Absorption of energy or light followed by a transition from the ground state to an excited state is called an excitation process.

The ground state is nothing but the state of a molecule or an atom that is most stable and has the least energy.

Excited-state is a state which is least stable but contains more energy. The lifetime of excited state is normally 10-8 to 10-9 seconds.

Relaxation Process-As the excited state is not the stable form, an atom or molecule loses energy and returns to the ground state. This process is called a relaxation process.The absorbed energy can be lost by any one or more of the following ways-

-Production of heat (collisional deactivation)

-Decomposition into a new species (Photochemical reaction)

-Emission of radiation of

The specific wavelength, characteristic of excited species (as in Flame photometry).

Longer wavelength immediately (as in Fluorescence).

Longer wavelength after a short time lag (as in Phosphorescence),

The excitation process, emission process, ground state, excited state, and series of events that take place when an EMR is passed on a molecule or atom can be represented by the following scheme.

Instrumentation of spectroscopy-

In the instrumentation of spectroscopy, the instruments are changed as per the requirement like in some cases only fitters are used or in some are monochromators and in many, both are used as per the requirement and also in light sources. So, the different components are used in different spectroscopy are as described-

Light source

It provides a sufficient light which is suitable for marking a measurement. It is typically yielding a high output of polychromatic light over a wide range of a spectrum. The common source of light for spectroscopy are like- Tungsten lamp, Carbon arc lamp, hydrogen/deuterium lamps, xenon flash lamps, etc. are used.

These light sources are providing an adequate intensity of radiation over the entire wavelength region with continuous radiation. 

Filters and Monochromator

The light source gives the polychromatic light of several wavelengths. The spectroscopy required only the monochromatic light of a single wavelength. So, the filter or monochromator is used to accepts polychromatic input light from a lamp and outputs monochromatic light.

Filters that are made up of glass, coated with pigments or dyed gelatin are absorption filters and interference filters are made up of dielectric spacer film.

Monochromator is more effective than filers to convert polychromatic light into monochromatic light. Prism (Dispersive and Littrow Types) and grating (diffraction and transmission) are used as the monochromator. Mainly monochromators are consisting of these parts are-

-Entrance Slit

-Collimator (Collimating Lens and mirror)

-Grating and prism (Dispersion element)

-Collimator (Focusing lens or mirror)

– Exit slit


The detector converts light into an electrical signal. When radiation is passed through a sample cell, part of it is absorbed by the sample solution and the rest is being transmitted. The transmitted radiation falls on the detector, then the intensity of absorbed radiation can be determined or displayed on the detector. the photovoltaic cell and phototubes are producing current proportional to the intensity of the light striking them. The most common detectors are-

-Barrier Layer cell or photovoltaic cell

-Photo tubes or photo emissive cells

-Photomultiplier tubes

Readout device

It collects the data from a detector that is displayed by a display device, such as an analog meter, a light beam reflected on a scale, or a digital display, or LCD, and the output can also be transmitted to a computer or printer.

You may read- Mass spectroscopy.

Single beam and double beam instruments


Application of spectroscopy-

Measurement of Concentration-

Prepare samples for the measurement of concentration. Then make a series of standard solutions of known concentrations. Set a spectrophotometer to the λ of maximum light absorption. Measure the absorption of the unknown, from the standard plot, and read the related concentration.

Detection of impurities-

For determination of impurities in organic molecules UV absorption spectroscopy is one of the best methods in spectroscopy.  In the detection process, additional peaks can be observed due to impurities in the sample and it can be compared with that of standard raw material.

Structure elucidation of the Organic Compounds-

For structure elucidation of the Organic Compounds from the location of peaks and a combination of peaks, the spectroscopy elucidates the structure of organic molecules-

– the presence or absence of unsaturation,

– the presence of hetero atoms

 Chemical Kinetics-

Chemical kinetics of reaction can also be studied by using spectroscopy. In spectroscopy the UV radiation is passed through the reaction cell and the absorbance changes can be observed.

Detection of Functional Groups-

The determination of the functional group of a compound in spectroscopy is due to the absence of a band at a particular wavelength regarded as evidence for the absence of a particular group.

Molecular weight determination-

The molecular weights of compounds can be measured spectrophotometrically by preparing the suitable derivatives of given compounds. For example- if you want to determine the molecular weight of amine then it is converted into amine picrate.

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