Protocol Online logo
Top : Forum Archives: : General Lab Techniques

Spectrophotometer - (May/14/2007 )


i just heard about double beam Spectrophotometer...? can anyone say different it is from the normal Spectrophotometer..and what advantage it has to the usual Spectrophotometer...?

thanks a lot


Double beam specs are common though more expensive than the single instruments. There are two modes; double beam in space and double beam in time. Essentially, using a series of prisms and/or mirrors all components of the single beam are duplicated except the light source. Thus: light source -> monochromator -> sample cuvet -> detector.
The double beam in time actually uses a common detector, though.

The advantages of a double beam system are that light source variations are compensated, as are sensitivity changes in the detector. Physics aside, as far as the user is concerned, a double beam instrument allows you to measure sample and reference standard at the same time. This improves analytical precision and sensitivity. I think the detail of the light source (bandwidth etc) and the filter/monochromator remain the sources of greatest error together with the stray light issue.


Double beam, you need two standardised cuvettes. It will be very troublesome I guess. I have an opinion. I think double beam is not as accurate as single beam since it is using 2 different cuvettes. Is it true?


Doubt it. I think the literature shows double beam is more reproducible and accurate. Have you tried measuring the absorbance on a pair of cuvets twenty times each and seing if there's a statistical difference? (not that I have).


Hmm.... perhaps you are right. And erm.. no.. I didnt measure the absorbance for the cuvettes. Maybe I should one day when I am superbly bored. wink.gif