Photomultiplier operating conditions

Concentration At high concentrations fluorescence emission l ecomes non-linear due to self-absorption by the sample itself or complete absorption of the excitation energy before it reaches the cell center. High fluorescence Intensity may overload the photomultiplier tube which returns slowly to its normal operating conditions and misrepresents the actual fluorescence signal until restabilized. 

Like chemical actinometers, photocells have to be calibrated against a thermopile-galvanometer system this has to be done frequently as there tends to be some variation with time. Under these conditions, they can be used to measure the absolute intensity of monochromatic light. The cell best suited for photochemical studies is the photoemissive type, which operates via the photoelectric emission of electrons from an irradiated surface. The metallic cathode is mounted either in a vacuum or in a small pressure of one of the inert gases. The cell may involve a single phototube or a multielement photomultiplier. An amplification of about 10 is achieved with the latter. 

Secondary electron multiplier (SEM) detectors replaced Faraday cup detectors for scanning mass spectrometers. The electron multiplier is based on the concept of a photomultiplier except that there is no glass membrane, so ions (electrons) can enter the amplification region of the detector. Because electron multipliers are not sealed and are open to the atmosphere, they must be operated under vacuum conditions and therefore cannot be used directly in atmospheric pressure IMS. 

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