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e. The total split vent flow is not too high
for the pressure setpoint. In a new,
clean capillary inlet with split compo-
nents installed there will be about
1.5 psi of inlet pressure developed with
400 mL/min of split vent flow (0 psi inlet
pressure setpoint).
There is a diagnostic called the "Vent Test" built
into the control module for the 6850 GC that can
help to determine if there is abnormal split vent
flow path restriction (see the 6850 Series
Control Module user information for more
details). The test sets the inlet total flow to
400 mL/min and the inlet pressure to 0 psi.
Actual inlet pressure is then monitored. The test
passes if the actual pressure is below 1.5 psi.
The example on the following pages will describe
possible causes of test failure.
Released: April 2004
This test can be performed manually on the 6850
with the inlet in split mode by removing the
column and capping the inlet fitting, setting the
inlet pressure to 0.1 psi and the inlet total flow
to 400 mL/min. The actual pressure should be
about 1.1 to 1.5 psi with split liner 5183-4647
installed and about 5 to 6 psi with a splitless
liner installed.
NOTE: This pressure will be significantly higher
using nitrogen carrier instead of helium!
Typical example of the problem
The GC method specifies a 10 meter, 530 µm
internal diameter column, used in split mode. The
inlet pressure needs to be set such that the
helium carrier linear velocity is in the optimum
range (30 to 35 cm/s).
6850 Series Gas Chromatograph Service Procedures
269
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