Photon-counting device compatible with conventional flow cytometric data acquisition electronics

We present an electronic scheme that enables us to use a photon-counting device (photomultiplier or avalanche photodetector) for measuring extremely weak signals in a flow cytometer. It can be used as a sole detector, or in combination with other (conventional) detectors using the data acquisition hardware of a conventional flow cytometer. The essential principle is that photon-counting pulses are converted to an analogue signal that is continuously proportional to the number of detected photons during the last integration time. The integration time should be approximately equal to the time an object is illuminated in the flow chamber. In this way, the photon burst due to real events is measured correctly and discriminated from the background pulses (fluorescence and Raman). The use of this scheme for the measurement of single DNA molecules is illustrated.