Science Fair Project Encyclopedia
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. The method is used in a quantitative way to determine concentrations of an absorbing species in solution, using the Beer-Lambert law:
where A is the measured absorbance, I0 is the intensity of the incident light at a given wavelength, I is the transmitted intensity, L the length of the cell, 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.
To the Adminitrator
the formula A=log (I/Io) is incorrect, a "minus" sign is missing if I denotes the transmitted intensity.
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
UV/VIS is the oldest form of spectroscopy.
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