The Pharmaceutics and Compounding Laboratory
Spectrophotometry as an
Analytical Tool

The Spectrophotometer

To measure the absorbance of light by a sample, we use an instrument called an absorption spectrometer (or spectrophotometer). The basic components of the instrument are shown below. In the visible light region, a common white light bulb (similar to those found in high intensity desk lamps) is used as the light source. In the UV region, a deuterium lamp is used. To isolate the wavelength of interest from all of the others coming out of the bulb, we reflect the light off a grating, which disperses each wavelength of the light beam at a different angle. By controlling the angle of deflection, we can get the proper wavelength to pass through the sample cell into a light detector (phototube). The phototube contains a cathode coated with a substance which will emit electrons when it is struck by photons. The electron current is measured by a meter or a digital readout.

Absorption Spectrophotometer Components

In lab, we will use the Spectronic® Genesys 5 spectrophotometer made by Milton Roy.