INFICON Explorer Portable Gas Chromatograph - Manual (Page 329)

Appendices
327
The injection port is located in the carrier gas stream just before the head of the
column, and contains a septum, which is a silicone disc coated with Teflon. The
sample is collected in the gas tight syringe, then the syringe needle is pushed
through the septum. The syringe plunger is depressed to inject the gas sample into
the carrier gas stream and onto the head of the column for analysis, and finally the
needle is withdrawn.
Precolumn Backflushing
The analysis in a GC is complete once all the sample components have been carried
the length of the column and have flowed through the detector. The analysis time is
therefore determined by the retention time of the most-retained sample component.
If a second sample is introduced onto the GC column before the first analysis is
completed, it is possible for little-retained components in the second sample to flow
through the detector at the same time as more-retained components in the first
sample. This leads to difficulty in interpreting the detector output signal.
To avoid this, a technique known as precolumn backflushing is used in Explorer.
Here, two GC columns, a shorter precolumn and a longer analytical column, are
joined in series with a tee between them. Initially the branch of the tee is closed to
flow and carrier gas is directed along the precolumn, through the run of the tee, and
then through the analytical column to the detector. The sample is introduced onto
the head of the precolumn and carried along the precolumn towards the analytical
column. After an interval of time the carrier gas is switched and introduced at the
branch of the tee between the two columns. Those sample components which have
already passed the tee continue to be carried towards the detector along the
analytical column. Those which are still in the precolumn are flushed backwards
along the precolumn and vented from the head of the precolumn.
Backflushing of the precolumn to clean it for the next sample occurs while the
analysis continues on the analytical column. Once all components have eluted from
both columns, the carrier gas can be switched back to flow through both columns in
series in preparation for the next sample.