4. Absorption

Absorption photometry (1/6)

On the preceding page Lambert's law, which explains the decline in the intensity of light on its way $x$ through an absorbing medium with an absorption coefficient $a$ , has been presented like this:

I (x) = I_{o} e^{- a x}

The single beam photometer

In experiments, a glass cuvette of the length $x$ is filled with a sample and lighted. At the entrance of the cuvette (at $x = 0$ ) the intensity equals $I_{o}$ . The by absorption lowered intensity $I (x)$ is measured at the end of the cuvette.

Schematic structure of a single beam absorption photometer.

The light from the source is divided into its spectral parts with a monochromator. The schematic drawing shows a prism monochromator, in most cases gratings would be used for the spectral decomposition of light. Monochromatic light with a selectable wavelength leaves the exit slit. A converging lens generates a nearly parallel light beam which passes the cuvette. The remaining light is focussed by another converging lens onto a photodetector where it will be converted into an electric signal which is proportional to the intensity. This signal may be further electronically processed.

It is aim of the interpretation to calculate $a (λ)$ or the decadic extinction coefficient $χ (λ)$ from the values $I_{o}$ and $I (x)$ for the different wavelengths and considering the known length $x$ of the cuvette. Spectrophotometers for laboratories would calculate the extinction $E (λ)$ , for which the cuvette length is not needed.