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Baseline shifts
Baseline perturbations are caused by changes in
column flow as the valve is rotated and as the
sample loop equilibrates to system pressure.
Slow valve rotation momentarily stops carrier
gas flow and, when the valve stops rotating, a
sudden increase in flow occurs which slowly
returns to the set point. Check actuator pressure
(usually 40 to 75 psi), valve rotor tension, and
valve temperature to ensure that the valve
rotates as quickly as possible. A restrictor or
backpressure regulator can be added to the
sample vent line to maintain the sample loop at
system pressure. This will reduce the time it
takes for the flow to stabilize after the valve is
switched.
Released: April 2004
Baseline upsets
Frequently, baseline upsets may be seen on chro-
matograms when valves are switched. These
upsets are caused by pressure changes within
the system, injections of large volume samples,
or by changing the amount of restriction in the
flow path. These upsets will become more of a
problem when high sensitivity is required. Addi-
tion of a fixed restriction downstream from the
valve may help minimize the upset. Changes in
column length may also help reduce the upsets.
Fixed restrictors are used immediately before
flame detectors to prevent flameout and are used
in some instances to prevent pressure surges
from damaging TCD filaments. An adjustable
restrictor (needle valve) can also be used where a
matched restriction is desired but not for
preventing pressure or flow surges.
6850 Series Gas Chromatograph Service Procedures
336
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