The purpose of the Fluidics system is to transport particles in a fluid stream to the laser beam for interrogation. For optimal illumination, the stream transporting the particles should be positioned in the center of the laser beam. At a given moment only one cell or particle should move through the laser beam. The sample is injected into a stream of sheath fluid within the flow chamber. The flow chamber in a benchtop cytometer is called a flow cell and the flow chamber in a stream-in-air cytometer is called a nozzle tip. The design of the flow chamber causes the sample core to be focused in the center of the sheath fluid where the laser beam will then interact with the particles.
Based on principles relating to laminar flow, the sample core remains separate but coaxial within the sheath fluid. The flow of sheath fluid accelerates the particles and restricts them to the center of the sample core. This process is known as hydrodynamic focusing. (11. , 12.)
The sample pressure and the sheath fluid pressure are different from each other. The sample pressure is always greater than the sheath fluid pressure. The difference between the sample and sheath pressure is known as the sample differential. The greater the sample differential the faster the sample will flow in to the flow chamber, this will increase the event rate, or number of cells interrogated in any given time. The speed at which the sample is forced into the flow chamber is known as the flow rate.
A higher flow rate is generally used for qualitative measurements such as immunophenotyping. The data are less resolved, since the cells are less in line in the wider core stream, but are acquired more quickly. A lower flow rate decreases the width of the sample core and restricts the position of the cells to a smaller area. The majority of cells passes through the center of the laser beam; thus the light illuminating the cells and emitted from the cells is more uniform. A lower rate is generally used in applications where greater resolution is critical, such as DNA analysis.
Proper operation of fluidic components is critical for particles to properly intercept the laser beam. Therefore, the operator must always ensure that the fluidics system is free of air bubbles and debris and is properly pressurized at all times.(11. , 12.)
