Difference Between Absorption & Emission Spectroscopy

With multiple radiations occurring all around you, aren’t you intrigued to study them and their interactions? Spectroscopy is about exploring and studying the spectra produced by matter or the dispersion of light into its constituent colours. Learn in detail about the definition, its uses, types, instruments, and more. 

What is Spectroscopy?

definition: The study and measurement of the spectrum that matter produces when interacting with electromagnetic (EM) radiation are called spectroscopy. It is the study of the interaction between matter and radiation as a function of frequency or wavelength.  

The experiment involves passing electromagnetic radiation of a particular range of wavelengths from a source through a sample with certain compounds. It results in either emission or absorption. 

Absorption vs Emission Spectroscopy

While absorption implies that the light source energy is absorbed by the sample, in emission, the sample emits light of a different wavelength than the original wavelength of the light from the source. 

In absorption spectroscopy, a sample with certain compounds gets excited when it absorbs energy on receiving electromagnetic radiation from a light source. Once the molecules gain energy, they jump from a low-energy ground state to a higher energy excitation state. A detector on the sample’s opposite side keeps a record of the wavelength absorptions and determines their extent. This absorbed spectrum of wavelengths by the sample is called the absorption spectrum. Thus, absorption will help find the concentration of metals absorption. 

In emission spectroscopy, the reverse happens, i.e., the electrons emit electromagnetic radiation and transit from excited to the ground state or lower energy levels. It helps find the concentration of the analyte by emission. 

The following table shows the key differences between the two types :

Characteristics	Absorption	Emission
Principle	Absorption of light by electrons	Emission of light by electrons
EM radiations	Absorbed	Emitted
Transitions	Ground to excited state	Excited to ground state
Dependence	Ground-state atoms	Excited-state atoms
Solid samples	Cannot be analysed	Can be analysed
Spectrum	Coloured	Dark lines

Spectroscopy vs Spectrometry

Before studying various types of spectroscopy, it is crucial to understand the difference between spectroscopy and spectrometry. 

It involves the study of the interaction between energy and matter. It doesn’t create results on its own. However, spectrometry is an application to achieve quantification of the results and assess them.

Types of Spectroscopy

The different types of spectroscopy are as follows:

• Acoustic resonance: A spectroscopy in the acoustic region, mainly the ultrasonic and sonic regions. It is a widely used inexpensive method for identifying and quantifying materials. 
• X-ray photoelectron: It relies on diffraction patterns that X-rays create when they pass through crystalline materials. These patterns help deduce the nature of the crystal structure.
• Circular Dichroism: A form of light absorption spectroscopy that you can use to measure the difference in absorbance of left and right circularly polarised light by a sample. 
• Ultraviolet-Visible (UV/Vis) Spectroscopy: It is suitable for probing the electronic structure of molecules and identifying the compounds present. It helps identify peptide bonds, coenzymes, amino acid side chains, and prosthetic groups.  
• NMR Spectroscopy: Nuclear magnetic resonance spectroscopy allows you to measure the magnetic fields around the nuclei. It uses radio waves for atomic nuclei excitation in a sample. Radio receivers detect the resonating nuclei. NMR spectroscopy is a powerful tool for understanding the nature of monomolecular organic compounds.
• Infrared Spectroscopy: The spectroscopy type concerns the electromagnetic spectrum’s infrared region. Infrared rays have longer wavelengths and lower frequencies, and infrared spectroscopy employs the concept of absorption spectroscopy.
• Raman spectroscopy: A spectroscopy technique for analysing rotational, vibrational, and other low-frequency system modes. In Chemistry, it provides a fingerprint for the identification of molecules. It relies on Raman scattering, i.e., inelastic scattering of monochromatic light. 

Did you know: Raman spectroscopy was introduced by an Indian Physicist, CV Raman. It is a non-invasive optical technique for diagnosing certain diseases. It is a promising alternative to invasive options like biopsy. This spectroscopy is also called an optical biopsy technique owing to its vast application in the field of diagnosis.

Spectroscopy Components

Light Sources: These are important spectroscopy components that depend on analysing the electromagnetic spectrum range. For instance, xenon is the most popular light source for UV-VIS and NIR range spectroscopy. Most spectrometers prefer a halogen lamp as a light source as they are more affordable. However, xenon offers a smoother emission. A deuterium arc lamp is suitable for continuous UV spectrum as it offers high intensity and a long life span. 

Non-dispersive Elements: These elements help filter the non-target wavelengths from the light source. They enable you to restrict stray light and improve the resolution. 

Dispersive elements: A prism has always been the most popular dispersive element. Newton used it in the 1660s to split light into a spectrum. Diffraction gratings are also used as dispersive elements in spectroscopy as they are more efficient than prisms. Also, a prism is known to sometimes absorb light passing through it as it refracts the light. However, diffraction gratings reflect light; as a result, no photons are lost or missed. Gratings also support UV rays, while prisms do not. 

What is Mass Spectroscopy?

Often used by chemists and biologists, mass spectrometry is ideal for measuring the mass-to-charge ratio (m/z) for one or more molecules in a sample. This ratio helps calculate the molecular weights of the components of the solution, and using the information, one can determine the solutions’ molecular composition. Mass spectrometry is an important tool in biological research and is suitable for the following purposes:

• Detecting impurities in a sample 
• Analysing a purified protein
• Characterising biomolecules like proteins, sugars, and oligonucleotides
• Studying the protein of cells

Mass spectrometers employ the following three components for measurements: 

• Ionisation source: Converts molecules to the gas phase through vaporisation.
• Mass analyser: Sorts ions as per their mass-to-charge ratio employing deflection and acceleration.
• Ion detection system: Measures the ions and sends the data to a system where the ratios can be stored. 

What are Spectrometers?  

A spectrometer is a device that takes in light, splits it into its constituent spectral components, digitises the signal as a function of wavelength, and displays it via a computer.

Recently, wireless spectrometers or VIS are gaining popularity because they make spectrometry investigations easier and accessible to students and educators. It has easy-to-use software and analysis tools that are similar to the ones used. Wireless spectrometers reduce the testing time and the time to collect the complete spectrum. Students can efficiently analyse the solution’s absorbance and emission of spectra using interactive displays and automated standard curves. 

Spectroradiometer and Spectrophotometer 

Spectroradiometers measure the spectral energy distribution of light sources that are small in size. It disperses light via prisms or diffraction gratings and records the radiation spectrum to calculate parameters like chromaticity and luminance. Sensitivity, stray light, linearity, and polarisation error have less influence on the spectroradiometer. 

Spectrophotometers measure the absorbed, reflected, and transmitted light through a chemical substance. It evaluates the light intensity as the beam passes through a sample. The device has an aperture through which the light enters, and holographic grating separates it into its component wavelengths. The light rays are then onto a CCD array detector for intensity determination of each wavelength via a pixel of the array. Spectrophotometers use IR light wavelengths of the electromagnetic spectrum and are used by organic chemists as they measure the atoms’ vibrations and allow chemists to determine the sample’s functional groups.

Uses and Applications of Spectroscopy 

It finds usage in Chemistry, Physics, and Biology. It is used in analytical and physical chemistry to detect and determine the molecular and structural composition of a given sample. Since every atom has a characteristic way of reflecting, absorbing, or emitting electromagnetic radiation, spectroscopy uses the characteristics and finds the composition of a sample.