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Study Notes: UV-Vis Detector

Upon exiting the sample cell the beam of monochromatic light eventually strikes the spectrophotometer’s detector.

Simplified internal layout of spectrophotometer components from light source to detector. White light is emitted from the source and light of different colours are transmitted to the detector.

The role of the detector is to convert a light signal into an electrical signal. The characteristics of an ideal detector are fast response times and a linear response over a wide range of wavelengths with low noise and high sensitivity.

A type of detector found in spectrophotometers is the photomultiplier tube - the principal of operation is the emission of electrons upon exposure to radiation.

The detector contains a photoemissive cathode and a series of dynodes. The number of electrons emitted from the cathode is directly proportional to the intensity of the light beam. Electrons emitted from the cathode are accelerated to the first dynode by a 90 volt potential where the electron impact dislodges several additional electrons which are accelerated to the next dynode by an additional 90V potential. After nine dynodes (each one at +90V with respect to the one before it), the number of electrons finally reaching the anode is in the order of ten million for each incident photon. The current measured at the anode collector is still proportional to the intensity of the light but it has been amplified over a million times.Skip flash movie

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Photomultipliers are extremely sensitive to ultraviolet and visible radiation. They are particularly suited to low intensity applications. They must be kept in a compartment that is free from stray light. Never expose the photomultiplier tube to direct sunlight as this will permanently damage the photoemissive surface of the cathode.

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