PHOTON-COUNTING DEVICE COMPATIBLE WITH CONVENTIONAL FLOW CYTOMETRIC DATA-ACQUISITION ELECTRONICS

We present an electronic scheme that enables us to use a photon-counti ng device (photomultiplier or avalanche photodetector) for measuring e xtremely weak signals in a now cytometer. It can be used as a sole det ector, or in combination with other (conventional) detectors using the data acquisition hardware of a conventional flow cytometer. The essen tial principle is that photon-counting pulses are converted to an anal ogue signal that is continuously proportional to the number of detecte d photons during the last integration time. The integration time shoul d be approximately equal to the time an object is illuminated in the n ow chamber. In this way, the photon burst due to real events is measur ed correctly and discriminated from the background pulses (fluorescenc e and Raman). The use of this scheme for the measurement of single DNA molecules is illustrated. (C) 1998 Wiley-Liss, Inc.