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Study Notes: AAS 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 samples are measured in the instrument at the same time. The source beam strikes a ‘beam splitter’ device that splits the beam along two paths to the detector. One path is for the blank (reference) and the other is for the sample. The beam splitter consists of a partially aluminised quartz plate.

Double-beam system

In this example the ‘chopper C’ alternatively interrupts the sample beam to direct the reference beam into the monochromator at high frequency.

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 reading 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.

Of course, a new blank will be required if a different type of sample is tested.

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