Settling and coagulation characteristics of fluorescent particles determined by flow cytometry and fluorometry

Abstract

A new technique for detecting particles in natural waters relies upon analysis of fluorescent emission by flow cytometry. Fluorescent pigment particles ranging in radius from -0.1 to 5 pm are available in sufficient quantity to be useful as model particles. Laboratory coagulation and settling experiments analyzed by fluorometry demonstrate that the efficiency with which the pigment particles coagulate with sewage particles is very low (less than 5 X lo-*). Hence, in field applications these particles provide the limiting case of low-interaction behavior relative to natural particles. Removal from laboratory columns occurs primarily by noninteractive settling while thermal convection currents maintain nearly uniform particle concentration within the columns. Observed decreases in particle number are exponential (first order) for each size class as predicted for settling from well-mixed suspensions. The decrease in total suspended particle mass is higher order as a result of the difference in settling rates among particles. Thus, a system in which noninteractive settling dominates mimics systems in which coagulation processes are important. Flow cytometric analysis of particles removed by noninteractive settling shows the fluorescent emission from individual particles to be proportional to the particle surface area. These results enable flow cytometry to be used to detect, count, and size large numbers of particles rapidly.