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11
Flow restrictor FR-902
There are two flow restrictors in the system, each generating a backpressure of
0.2 MPa. One is connected to port 5 on the injection valve to prevent buffer
drainage when the injection valve is in position WASTE.
The second flow restrictor is connected after the
conductivity cell. It prevents air bubbles being
formed after the column when the pressure created
in the column is released. Compare to a cork in a
bottle of champagne.
This must be allowed for when setting the maximum
pressure limit in the methods. For example, when
using a column with a pressure limit of 0.3 MPa, the
pressure limit in the method should be set to 0.5 MPa.
This is described below in detail.
Two types of pressure limits
All chromatography columns have two types of pressure limits, one for the
column hardware and one limit for the gel packing.
The pressure affecting the column hardware depends on the backpressure
generated after the column by, for example flow cells, flow restrictors and tubing.
When the pressure limit for the column hardware is exceeded, the column will
start leaking. Empty chromatography columns from GE Healthcare are all
supplied with data on the column hardware pressure limit.
The pressure affecting the gel packing is dependent on the flow rate and viscosity
of the buffer. When the flow rate is too high and/or a high viscosity buffer is used,
the pressure limit for the packed gel bed can be exceeded (the gel pressure limit
is the maximum allowable pressure drop over the packed gel bed). When the
pressure limit is exceeded, the gel particles are forced to the bottom of the
column and cause the backpressure to increase. This leads to a collapse of the
packed bed and the risk of poor chromatographic performance. The pressure
limit data referring to the gel packing is found in the column instructions delivered
with all prepacked columns from GE Healthcare.
How the flow restrictor affects the pressure limits
The flow restrictor only affects the column hardware pressure limit. The gel
packing pressure limit is always unaffected by a flow restrictor. The figure below
shows an example of how a flow restrictor affects the packed column at a
hypothetical flow rate of 1 ml/min. The flow rate generates a pump pressure
reading of 0.3 MPa. With no restrictor mounted, this pressure equals the pressure
drop over both the gel and the column hardware.
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