Page 26 - Instrumental Analysis - Pharm D Clinical- 07-PA403

UV-visible Spectrophotometry

Phototube

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Photomultiplier tube

• The photomultiplier tube (PMT) is similar in construction to the phototube but is
significantly more sensitive. PMT contains a photocathode and an anode similar
to that of the phototube, in addition, PMT has a series of electrodes called
dynodes.

• The electrons emitted from the cathode are accelerated toward the first dynode
that is maintained 90-100 V positive with respect to the cathode.

• Each accelerated electron that strikes the dynode surface produces several
electrons that are then accelerated to dynode 2, which is held 90-100 V more
positive than dynode 1.

• By the time this process has been repeated at each of the dynodes, 106 to 107
electrons have been produced for each incident photon. This cascade of
electrons is finally collected at the anode to provide an amplified current that is
measured.
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