A flow cytometry evaluation system includes a sample effluent system with an effluent collection vessel with an effluent fluid inlet to receive an effluent of the fluid sample exiting the investigation zone during a flow cytometry evaluation; and an effluent fluid conduction path from the investigation zone to the effluent fluid inlet. A pressurized gas delivery system in fluid communication with the sample effluent system applies pressurized gas to the fluid sample effluent system to impede fluid flow through the effluent fluid conduction path toward the effluent fluid inlet during a flow cytometry investigation