BMS 631 - LECTURE 1 Flow Cytometry: Theory J.Paul Robinson Professor of Immunopharmacology School of Veterinary Medicine, Purdue University

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PMT

•Produce current at their anodes when photons impinge upon their light-sensitive cathodes

•Require external powersource

•Their gain is as high as 107 electrons out per photon in

•Noise can be generated from thermionic emission of electrons - this is called “dark current” •If very low levels of signal are available, PMTs are often cooled to reduce heat effects •Spectral response of PMTs is determined by the composition of the photocathode

•Bi-alkali PMTs have peak sensitivity at 400 nm

•Multialkali PMTs extend to 750 nm

•Gallium Arsenide (GaAs) cathodes operate from 300-850 nm (very costly and have lower gain)