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Study Notes: Dual Beam Design

Dual beam spectrophotometers are more expensive than single beam models because of the additional complexity and components. In this type of instrument both the blank and sample cells are placed in the instrument at the same time. The source beam strikes a rotating sector mirror or ‘chopper’ that switches the light alternately along two paths to the detector. One path is for the blank (reference) and the other is for the sample. The chopper rotates at a speed such that the alternate measurements occur several times a second.Skip flash movie

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The amount of light passing through the sample is compared with the amount passing through the blank and the net absorbance is determined and displayed by the instrument. The advantages of this system are that it is:

  • much faster than a single beam - the blank is left in the instrument and only the sample is changed.
  • very useful when a range of wavelengths are to be scanned - as each new wavelength is selected the sample is automatically corrected for the blank absorbance.
  • more stable than single beam instruments with regard to lamp intensity drift.

If used in a uncapped cell, the blank must be replaced frequently since evaporation will change its concentration. A new blank will also be required if a different type of sample is tested.

A design similar to the dual beam spectrophotometer is the split beam spectrophotometer. The difference is that a beam splitter simultaneously sends light from the source along the blank and sample paths to two separate but identical detectors.

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