Improved Flow Cytometric Light Scatter Detection of Submicron-Sized Particles by Reduction of Optical Background Signals

Flow cytometry allows multiparameter analysis on a single-cell basis and is currently the method of choice to rapidly assess heterogeneity of cell populations in suspension. With the research field of extracellular vesicles (EV) rapidly expanding, there is an increased demand to address heterogeneity of EV populations in biological samples. Although flow cytometry would be the ideal technique to do so, the available instruments are in general not equipped to optimally detect the dim light scatter signals generated by submicron-sized particles like EV. Although sideward scatter light and fluorescence are currently used as a threshold signal to identify EV within samples, the forward scatter light (FSC) parameter is often neglected due to the lack of resolution to distinguish EV-related signals from noise. However, after optimization of FSC detection by adjusting the size of the obscuration bar, we recently showed that certain EV-subsets could only be identified based on FSC. This observation made us to further study the possibilities to enhance FSC-detection of submicron-sized particles. By testing differently sized obscuration bars and differently sized pinholes in the focal plane behind the FSC detection lens, we generated a matrix that allowed us to determine which combination resulted in the lowest optical background in terms of numbers of events regarding FSC detection of submicron-sized particles. We found that a combination of an 8-mm obscuration bar and a 200-μm pinhole reduced optical background in a reproducible manner to such extent that it allowed a robust separation of 100-nm polystyrene beads from background signals within the FSC channel, and even allowed thresholding on FSC without the interference of massive background signals when both beads and EV were measured. These technical adaptations thus significantly improved FSC detection of submicron-sized particles and provide an important lead for the further development and design of flow cytometers that aid in detection of submicron-sized particles. © 2020 The Authors. Cytometry Part A published by Wiley Periodicals, Inc. on behalf of International Society for Advancement of Cytometry