Optical Spectroscopic Methods of Analysis

Optical spectroscopic methods of analysis are based upon the interaction of electromagnetic radiation with matter. The objective of writing this article is to brief you on the fundamental concepts of commonly used spectroscopic techniques

Electromagnetic Spectrum

Electromagnetic Spectrum

The electromagnetic spectrum provides the spectroscopist a wealth of information. Each region of the electromagnetic spectrum is characterised by a range of frequencies or wavelengths and finds several applications in the hands of chemists and physicists. Wavelengths of the electromagnetic spectrum range from inter- atomic dimensions (high energy gamma rays) to several kilometers (radio waves)

Spectroscopic Techniques

Spectroscopic techniques are based on basically three types of interactions of electromagnetic radiation with matter.

• Emission
• Absorption
• Scattering

Emission Spectroscopy

Emission spectroscopic methods are based on the emission of characteristic wavelengths emitted by the elements constituting the sample when excited by thermal, electrical or radiation energy

• ICP – OES Spectroscopy

A plasma sources at high temperature is used to excite the constituent elements which emit characteristic wavelength radiations which can be used for quantitative estimation of the sample

• Fluorescence Spectroscopy

On absorption of light the absorbing molecule gets excited and certain species which are photoluminiscent re-emit the absorbed light after a time delay Fluorescence refers to re-emission  whereas delayed emission after minutes, hours or even days is referred to as phosphorescence.

Fluorescence intensity is directly proportional to fluorescent species present. Some substances which are not naturally fluorescent can be derivatized with fluorescent moieties to improve the detection limits.

Absorption Spectroscopy

The basis of absorption spectroscopy is measurement of absorption of specific wavelength(s) by specific atoms or molecules in the sample. Absorption measurements can be made at a specific wavelength or over a range of wavelengths for simultaneous determinations.

• UV – visible Spectroscopy

The region 180 – 780nm constitutes the UV – visible region and can be used for atomic, molecular or ionic species determinations. Absorption in this region results from electronic transitions between the electron levels of the absorbing species.

• Infrared Spectroscopy

Absorption in this region takes place from about 25,000 cm-1(near IR) to around 10 cmi-1(far IR) depending on the energy of vibration or rotation of the absorbing molecules. The prerequisite for absorption in this region is change in dipole moment of the absorbing molecule. The key area of application is functional group identification of molecules. FT – IR has completely replaced dispersive IR instruments due to the multitude of advantages offered by the FT IR technique.

• Turbidimetry

Turbidimetry is used for determination of suspensions which are homogeneously dispersed in a liquid medium. The opacity of such suspensions is measured from the intensity of transmitted light. Turbidimetric methods at best give rough estimate of concentration

• X-ray Spectroscopy

High energy x-ray radiation is used to knock out electrons from inner shells of atoms which are replaced by electrons from outer shells. The energy is emitted as a photons which is characteristic for each element
Light scattering spectroscopy

• Nephelometry

Nephelometry is based on the study of scattered light by a homogeneous suspension of particles in a liquid medium

• Raman Spectroscopy

Rama shifts in liquid samples result from excitation to higher vibrational states by incident radiation. Raman effect involves scattering of light accompanied by change of wavelength. Raman and infrared spectroscopy are complimentary techniques but Raman has a major advantage that aqueous samples can be handled directly as water does not interfere in Raman measurements

• X-ray Diffraction

X-ray diffraction is not a chemical identification tool but it serves to characterise the atomic and molecular structure of crystalline materials. By measuring the angles and intensities of diffracted x-rays it is possible to arrive at the densities of electrons within the crystal lattice from which the spatial distribution of atoms within the crystal lattice can be deduced.

In subsequent articles similar analytical technique groups will be discussed.