Biochemical Techniques and Instrumentation

1.2.6. Detectors

Detectors used in spectrophotometers are either photocells or photomultiplier cells. Photocells convert quanta of radiation into electrical energy, which is then amplified, detected and recorded. Photons impinging on metal surface in a vacuum cause emission of electrons in proportion to the intensity of radiation or light. A positive electrode attracts these emitted electrons and causes a current flow and this gives a potential difference across a resistor. This potential is electronically amplified and the light absorbed is measured. Photocells are sensitive to light with a wavelength of about 400 nm and insensitive to wavelengths >550 nm. Theoretically accuracy of photocells is 1±0.003.

Photomultiplier tube (PMT) is more sensitive than photocells. It consists of a photocathode and dynodes in an evacuated chamber. Photons that strike the photo emissive cathode emit electrons due to the photoelectric effect. Electrons are accelerated towards a series of additional electrodes called dynodes. Additional electrons are generated at each dynode. This cascading effect creates 10⁵ to 10⁷ electrons for each photon. This amplified signal is finally collected at the anode and measured. The illustration of PMT can be viewed here.

It is better to have one detector instead of two detectors to measure the intensity of light from the control and test samples, simultaneously. This helps to eliminate the potential variations between the two detectors.