Varian 450-GC, User Manual

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Injectors – 1079 PTV Injector
Page: 186 User Manual 450-GC Varian, Inc.
Temperature programming the column oven results in an increase in carrier gas
viscosity which results in a decrease in column flow rate. This effect can be
offset by applying the appropriate column pressure program.
If Constant Flow Programming is enabled, a pressure program based on the
column parameters and the column oven temperature program will be
automatically built. Any parameter change result in an automatic recalculation
of the pressure program.
When entering a pressure program to maintain constant column flow rate, the
program is based on the column temperature program. If the column oven is
operated isothermally, then constant pressure is maintained to achieve a
desired flow rate.
Pressure Pulse
Once you have selected Constant Flow programming, a checkbox “Pressure
Pulse” becomes accessible.
Selecting Pressure Pulse gives you the option of enabling a pressure pulse,
setting the desired pressure and the duration for which the pressure should last.
Typically, the pressure pulse pressure is held for between 30 and 90 seconds.
The purpose of the pressure pulse is two fold. The higher pressure will prevent
the solvent vapor cloud from becoming excessively large allowing larger
injection volumes and providing more efficient passage of the sample into the
column. The higher pressure also causes higher flow rates into the column
making the transfer quicker and thus preventing excessive residence times in
the Injector with consequent decomposition of labile compounds.
If you use
Pressure Pulse you will need to perform septum purge calibration using
the higher pressure pulse pressure.
•
The GC will go ready based on the pressure as defined in pressure pulse.
•
When a run is started the column pressure as defined in pressure pulse will
remain for the “Pulse Duration” time. After the pulse duration time the
column pressure is calculated from the constant flow conditions.