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Study Notes: UV-Vis Monochromator

If you wanted to perform quantitative analysis, monochromatic light would need to be passed through the sample otherwise Beer’s Law would not hold true. Alternatively, if you needed to scan a sample to produce a spectrum, monochromatic light at every wavelength to be scanned would also be required.

In effect, a monochromator produces monochromatic light by removing unwanted wavelengths from the source light beam.

A monochromator comprises an entrance slit, a dispersion device and an exit slit.

  • The entrance slit selects a defined beam of (polychromatic) light from the source.
  • The dispersion device causes the different wavelengths of light in the source beam to be dispersed at different angles.
  • The exit slit enables selection of a particular wavelength to produce the required monochromatic light.

Dispersion devices can be prisms but diffraction gratings are used in modern spectrophotometers as they are cheaper, easier to make and provide superior performance.

Below is a simple representation of a monochromator that uses a prism.Skip flash movie

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Ideally a monochromator produces truly monochromatic light but in practice it emits an optically symmetrical band of a certain narrow wavelength range. The width of the band at half its height is called the instrumental bandwidth. This can be as low as 0.1 nm for gratings and 0.5 nm for prisms.

Transmitted light peaks ranging in colour from yellow only, ie a narrow bandwidth, to yellow with some green and orange mixed in, ie a wide bandwidth.

Spectrophotometers will often have a ‘slit width’ adjustment that the analyst will need to check or set depending upon the nature of the work to be done. Adjusting the slit width alters the instrumental bandwidth. The question then is, under what circumstances would the slit width need to be adjusted?

Setting a slit width to narrow values would be necessary where the aim was to uncover fine spectral detail, that is, to resolve narrow absorption bands that would otherwise not be seen at wider settings. However, narrower settings mean that less light is passing through the optics with the result that the noise level increases relative to the signal. Ultimately this may lead to distortion of scans.

For quantitative analysis on the other hand, wider slit width settings (typically 2 to 5 nm) are mostly employed because spectral detail is not important. The analytical wavelength is normally lambdamax where the peak is flat and so a high level of accuracy is not required in setting an exact wavelength.

 

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