Features of light sources used in spectroscopic analysis

Features of light sources used in spectroscopic analysis

A light source is an essential component of a spectrometer as it provides the required energy for excitation essential for spectral transitions. Different sources are available that suit the needs of different analysis. Broadly speaking light sources are classified under three broad categories:

  • Broadband sources
  • Line sources
  • Laser sources

The features of the three types of sources are described in the article

Broadband Sources

Deuterium lamp (Image Courtesy : http://www.photron.net.au/)

Broadband or continuum sources provide a mix of wavelengths over the required wavelength range. Isolation of the required wavelength is achieved by using a device such as a monochromator. Three common broadband light sources used in UV and Visible regions are :

Deuterium lamp – UV region ( 190 – 400 nm)

Tungsten- halogen lamp- Visible region (320-1100 nm)

Xenon Arc lamp – UV & Visible region (190-1100nm)

A typical broadband source consists of a metal filament heated to a high temperature by passing a current through it. The emitted wavelengths merge together to form a non-discrete spectrum

Another source is arc lamp which consists of gas sealed inside a transparent tube.The gas is excited by an electrical discharge resulting in ionization of gas atoms. Radiation released gets bunched together due to collisions with other atoms and electrons thereby resulting in a continuous emission.

Pulsed Xenon light sources extend the useful life of a lamp but the spectral output is not as stable as deuterium or halogen lamps.

 Line Sources

Line sources emit radiation having discrete well-defined frequencies. Low pressure arc lamps generally serve the purpose. Another example is hollow cathode lamps used in atomic absorption spectroscopy. These are capable of emitting characteristic wavelengths of the elements which are to be determined in the sample.

Laser Sources

Laser is a light source that emits high intensity radiation over a very narrow frequency range. Tunable laser sources provide widest coverage from soft x-rays to near infrared regions. The high degree of monochromaticity and intensity of laser sources coupled with durability over a frequency range has made laser sources a valuable asset in fluorescence spectroscopy studies.

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