There are two configurations in the split/splitless injector to control the flow/pressure:

1. Pressure regulated system with needle valve: This configuration can be used both in the split injection mode as well as the splitless injection mode. In split operation, the needle valve is open continuously. In splitless operation the valve is initially closed and is opened at the end of the splitless period.
2. Flow controlled system with back pressure regulator: The flow controller is used to keep the total flow at a constant level. The back pressure regulator maintains a constant carrier gas flow through the capillary column. Such a system requires a special feature to allow its use in splitless operation.

The moment the split flow stops, the back pressure regulator will not work. Since a flow controller produces a constant flow, the total gas flow ( > 100 ml/min) which was adjusted to the splitter, would be introduced completely into the column. The general solution is a 'septum purge' gas flow exit, which allows sufficient flow from the flow controller during the splitless time to enable it to continue functioning .

Splitless flow principle

Comparison split vs splitless injection