An all-solid-state flow cytometer for counting fluorescent microspheres

Particle tracers are of interest in the investigation of fractured media. Because their transport behavior differs from that of solute tracers, they supply complementary information on fracture space. This information is crucial in the risk assessment of underground hazardous waste storage sites. An easy-to-handle particle tracer consists of fluorescent polystyrene spheres with diameters on the order of 1 μm. Up to now, detection and counting of these particles was performed by fluorescence microscopy of filtered samples. To allow faster and on-line continuous monitoring of single particles, a flow cytometer has been developed in the present work. The cytometer is built from solid-state optical components, resulting in a robust and transportable system suitable for field use. To maximize the fluorescence quantum efficiency, a new dye, JA22, was used. Use of an avalanche diode allowed the efficiency of photon detection to be maximized. For complete digital data acquisition and analysis, only a personal computer is required. The detection efficiency of the cytometer is >25%, the probe flow is 20 mL/h, and the present accuracy is <160 particles/mL.