4. Absorption

Absorption photometry (2/6)

Analysing photometer data

If you have processed the previous tasks, you have found that the data obtained from the single beam photometer is not only dependent on the absorbing features of the sample. It is rather influenced by the features of the light source, the monochromator, the cuvette, the lenses and the detector as well. To bring light to the unknown variables, it is favourable to do two measurements:

  • one with the solvent (reference)
  • one with the solvent and the dissolved absorbing substance to be inspected (sample)

All other factors (light source, detector, optical alignment etc.) will be held stable. They are summarily represented by the instrumental function G, so that the two test series will be described through the following equations:

I ref =G⋅ I o e − a ref x
I sample =G⋅ I o e − a sample x

 

By division, the instrumental function G and the starting intensity Io at the entrance of the cuvette cancel out:

I ref / I sample = e −( a ref − a sample )x

Finding the log and converting emerges that:

1 x ln( I ref I sample )= a sample − a ref

The result of this data interpretation thus shows the difference of the absorption coefficients of the sample and the reference.

If the sample - as we assumed - is made with the same solvent and a dissolved substance with the absorption coefficient a, it applies: a sample =a+ a ref , and in consequence:

1 x ln( I ref I sample )=a
We have examined, to what extend a double beam measurement procedure with the sample and the reference (the solvent) allows to eliminate the unknown properties of the photometer from our data. As a result we derive data concerning the absorbing features of the dissolved absorbing substances.