Topic: Sample Flow Rate
Flow cytometers require that particles to be analysed are presented at the sample interogation point as single cells. To achieve this, cells need to be in a single cell suspension prior to injection into the instrument, and this suspension under conditions of laminar flow, forms a core stream within the sheath fluid, normally a buffered saline solution to maintain cell viability.
The sample injection and flow rate is controlled in most flow cytometers by differential pressure. This is achieved by applying pressure (air, nitrogen or other gases) to sample and sheath containers, with each tanks pressure controlled separately by pressure regulators. The amount of pressure applied to the sheath tank controls how fast the sheath fluid passes through the instrument, while the difference in pressure between the sheath and sample containers controls how fast the sample passes through the instrument. This principle is illustrated in this diagram, should you care for more information.