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UV/VIS spectroscopy

From Academic Kids

Ultraviolet-Visible spectroscopy or Ultraviolet-Visible spectrophotometry (UV/ VIS) involves the spectroscopy of photons (spectrophotometry). It uses light in the visible and adjacent near ultraviolet (UV) and near infrared (NIR) ranges. In this region of energy space molecules undergo electronic transitions.

Contents

Beer-Lambert Law

The method is used in a quantitative way to determine concentrations of an absorbing species in solution, using the Beer-Lambert law:

<math>A =\ <math> −<math>\log_{10}(I/I_0) = \epsilon\cdot c\cdot L<math>,

where A is the measured absorbance, <math>I_0<math> is the intensity of the incident light at a given wavelength, <math>I<math> is the transmitted intensity, L the pathlength through the sample, and c the concentration of the absorbing species. For each species and wavelength, ε is a constant known as the extinction coefficient.

The absorbance A and extinction ε are sometimes defined in terms of the natural logarithm instead of the base-10 logarithm.

UV/ VIS spectrophotometer

The instrument used in UV/ VIS spectroscopy is called a UV/ VIS spectrophotometer. To obtain absorption information, a sample is placed in the spectrophotometer and ultraviolet and/or visible light at a certain wavelength (or range of wavelengths) is shined through the sample. The spectrophotometer measures how much of the light is absorbed by the sample. The intensity of light before going into a certain sample is symbolized by <math>I_0<math>. The intensity of light remaining after it has gone through the sample is symbolized by <math>I<math>. The fraction of light transmittance is (<math>I/I_0<math>), which is usually expressed as a percent Transmittance (%T). From this information, the absorbance of the sample is determined for that wavelength or as a function for a range of wavelengths. Sophisticated UV/ Vis spectrophotometers often do this automatically.

Although the samples could be solid (or even gaseous), they are usually liquid. A transparent cell, often called a cuvette, is used to hold a liquid sample in the spectrophotometer. The pathlength L through the sample is then the width of the cell through which the light passes through. Simple (economic) spectrophotometers may use cuvettes shaped like cylindrical test tubes, but more sophisticated ones use rectangular cuvettes, commonly 1 cm in width. For just visible spectroscopy, ordinary glass cuvettes may be used, but ultraviolet spectroscopy requires special cuvettes made of a UV-transparent material such as quartz.

Ultraviolet-Visible spectrum

An ultraviolet-visible spectrum is essentially a graph (or plot) of light absorbance vs. wavelength in a range of ultraviolet and/or visible regions. Such a spectrum can often be produced by a more sophisticated spectrophotometer. Similarly, for a given material of species, a standard graph of extiction coefficient ε vs. wavelength may be made or used if one is already available. Such a standard graph would be effectively "concentration-corrected" and thus independent of concentration.

Types

In a single-beam UV/ VIS spectrophotometer the light only passes through the sample. In a double-beam UV/ VIS spectrophotometer the light passes through a beam chopper which alternately directs the beam through the sample or a reference cell several times per second.

Common UV/ VIS spectrophotometers

Following is a list of commonly used spectrophotometers:

  • GeneSys 20
  • HP8452A Diode Array
  • Spectronic 20
  • Elico SL159

History

UV/ VIS is the oldest form of spectroscopy.

External Links

  • The Science of Spectroscopy (http://www.scienceofspectroscopy.info) - supported by NASA, includes OpenSpectrum, a Wiki-based learning tool for spectroscopy that anyone can edit
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