User guide

242 Advanced User Guide
8 Inlets
A fundamental difficulty with solvent vent mode is the potential
loss of volatile analytes with the solvent. Several solutions are
possible for this situation:
The inlet liner can be packed with a more retentive material,
such as Tenax. This greatly improves volatile analyte
recovery but may impact recovery of higher boiling materials.
Some of the solvent can be left in the liner when sample
transfer begins. The residual solvent acts like a stationary
phase and retains volatile material, but at the expense of a
larger solvent peak.
The inlet temperature can be reduced. This reduces the
vapor pressure of the volatile analytes and permits higher
recoveries.
Solvent removal can be speeded up by:
Reducing pressure in the inlet during sample
introduction—the Vent pressure parameter
Increasing flow through the inlet—the Vent flow parameter
While all these possibilities do complicate use of the PTV, they
provide increased flexibility and new potential to solve difficult
problems.
Sequence of operations
These are the steps in a typical analysis using the solvent vent
mode:
Table 44 The solvent vent process
Step Parameter Value
1 Before injection Flow at split vent Either Purge flow or Saver flow
Inlet pressure Derived from column setpoint
The system is resting, with Purge flow (or Saver flow, if on) through the inlet.
2 Prep Run begins Flow at split vent Vent flow setpoint
Inlet pressure Vent pressure setpoint
Setpoints change to prepare for injection. When GC is ready, the sample is injected. Inlet and oven
temperature program Init times begin. Solvent venting and analyte trapping begin.
3 At Vent end time Flow at split vent None, split valve closed
Inlet pressure Column pressure setpoint
Solvent venting ends, analyte transfer begins as inlet heats up.
4At Purge time Flow at split vent Purge flow setpoint