Vented septum injectors

VENTED SEPTUM INJECTORS. A more radical injector modification is available. It prevents gas which has had contact with the septum from entering into the column. Figure 6.11 illustrates such a system in which the gas contacting the septum is vented by a bypass pipe. The carrier gas stream is split, the main portion going to the column while a small portion moves past the septum and... 

Figure 6.11. Vented septum injector. Any gas coming in contact with the septum is vented out the septum purge line.

For GC, the injector is most frequently a small heated space attached to the start of the column. A sample of the mixture to be analyzed is injected into this space by use of a syringe, which pierces a rubber septum. The injector needs to be hot enough to immediately vaporize the sample, which is then swept onto the head of the column by the mobile gas phase. Generally, the injector is kept at a temperature 50 C higher than is the column oven. Variants on this principle are in use, in particular the split/splitless injector. This injector can be used in a splitless mode, in which the entire injected sample goes onto the column, or in a split mode, in which only part of the sample goes onto the column, the remainder vented to atmosphere. For other less usual forms of injector, a specialist book on GC should be consulted. 

Figure 1 Chromatograph diagrams (L, lab P, process). LI, column compartment P2, column and valving compartment L2, autosampler P2, sample handling valve(s) L3, pneumatic connections (rear) P3, pneumatic connections (side) and regulator compartment L4, user keypad and electronics P4, electronics compartment L5, data and remote control connections (rear) P5, process computer and local or remote control connection (side) L6, splii/splitless injector with septum P6, process sample input to inject valve and return outlet L7, detector and vent P7, detectors (interior).

S/SL (Split/Splitless) injector a sample is introduced into a heated small chamber via a syringe through a septum - the heat facilitates volatilization of the sample and sample matrix. The carrier gas then either sweeps the entirety splitless mode) or a portion split mode) of the sample into the column. In split mode, a part of the sample/carrier gas mixture in the injection chamber is exhausted through the split vent. Split injection is preferred when working with samples with high analyte concentrations (>0.1%) whereas splitless injection is best suited for trace analysis with low amounts of analytes. (<0.01%)... 

The carrier gas swept across the injector internal septum face to a separate vent so that material emitted from the septum does not enter the inlet. 

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