PRINCIPLE

The aim is to measure the absorbance over a wavelength range, and to report it on a graph. Because I₀ changes with λ, it must be measured just like I, for every studied wavelength. In practice, every spectrometer allows to keep track of the succession of each value of I₀ for the studied wavelength range: the blank spectrum needs to be done. The measurement of I and drawing of the sought curve is what remains to be done.

Protocol

1 Choosing the cuvette

2 Turning on the spectrometer

Make sure the spectrometer is in “spectrum recording” mode.

3 Selection of the wavelength range

Three parameters need to be set: λ_min, λ_max and the scanning speed. The slower the scan, the better the signal-over-background-noise rate is. When it comes to verifying the identity of a molecule due to a reference spectrum, the best choice is to choose the same spectral wavelength range than that reference. There are no pre-set rules for molecules of unknown structure, but it is useless to study the visible spectrum of colourless solutions, even though they are concentrated.

4. Recording the blank spectrum

5. Recording the spectrum

6. Analysing the spectrum

The most important information are:

• The presence of absorption peaks: wavelength, shape, value of ε (the calculation requires to know the values of L and C).
• The presence of shoulderings.