4.1: Detection of chemiluminescence

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The detector of choice for chemiluminescence is the photomultiplier tube, a development of the vacuum phototube that permits considerable amplification of the signal. Figure D1.1 shows how the photomultiplier works. The surface of the cathode supports a photoemissive layer that ejects electrons in direct proportion to the intensity of the incident light; several electrons are emitted for each photon and are attracted towards a positively-charged dynode. When the electron beam meets the dynode several electrons (E in figure D1.1) are ejected for each incident electron and these are attracted to a second dynode at a higher positive potential. This process is repeated along a series of dynodes, the intensity of the electron beam increasing continually until when it reaches the anode (at the greatest positive potential) there are over a million electrons for each photon incident at the cathode. The resulting current can be amplified electronically. In the absence of light, the photomultiplier generates a dark current, chiefly due to thermal emission. Thermal dark currents can be eliminated by cooling to ─30°C.

Principle of the photomultiplier (see text).

Diodes are also used for chemiluminescence detection^[1], especially in low-cost applications. Photographic detection was also used in very early work^[2].