124 BD LSR II User’s Guide
Fluidics
The fluidics system in the BD LSR II flow cytometer is pressure driven—a built-in
air pump provides a sheath pressure of 5.5 psi. After passing through the sheath
filter, sheath fluid is introduced into the lower chamber of the quartz flow cell.
The sample to be analyzed arrives in a separate pressurized stream. When a
sample tube is placed on the sample injection port (SIP), the sample is forced up
and injected into the lower chamber of the flow cell by a slight overpressure
relative to the sheath fluid. The conical shape of the lower chamber creates a
laminar sheath flow that carries the sample core upward through the center of
the flow cell, where the particles to be measured are intercepted by the laser beam
(Figure A-1 on page 125). This process is known as hydrodynamic focusing.
The objective in flow cytometric analysis is to have at most one cell or particle
moving through a laser beam at a given time. The difference in pressure between
the sample stream and sheath fluid stream can be used to vary the diameter of the
sample core. Increasing the sample pressure increases the core diameter and
therefore the flow rate (Figure A-1 on page 125).
• A higher flow rate is generally used for qualitative measurements such as
immunophenotyping. The data is less resolved but is acquired more
quickly.
• A lower flow rate is generally used in applications where greater resolution
and quantitative measurements are critical, such as DNA analysis.
Proper operation of fluidic components is critical for particles to intercept the
laser beam properly. Always ensure that the fluidics system is free of air bubbles
and debris and is properly pressurized.
LSR2.book Page 124 Tuesday, April 25, 2006 3:34 PM
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