Injection septum purge flow

Direct injection rehes on vaporization processes to introduce sample into the column. However, it lacks a purge activation function, a septum purge flow, or secondary cooling (Fig. 5). The direct injection mode is used with packed columns or wide-bore (0.53-mm-I.D.) capillary columns. Wide-bore columns are used as higher-efficiency replacements for packed columns without the need to extensively modify the packed-column injector. 

The septum purge flow is recommended to stay on as well during the injection phase. With properly chosen injection conditions keeping the solvent/sample vapour cloud inside of the insert liner, there will be no loss of sample analytes via the septum purge outlet (see Section 2.2.5.1 Hot Sample Injection). 

In both systems the flow of helium carrier gas through the columns was 0.7-0.8 ml min-1, with a septum purge of 0.5 ml min-1 and a split valve flow of 4-4.5 ml min-1. The injection ports were maintained at 260°C and the detector ovens at 240° C. The detector employed was either a flame ionisation or a nitrogen-specific NPD-40 thermionic detector (Erba Science (UK) Ltd) and the output was recorded on a HP 3390 integrator (Hewlett Packard Ltd, Wokingham, UK). 

Figure 5.2.1. Simplified diagram of a Py-GC system (not to scale). The pyrolyser is schematized as a heated filament type. A piece of a deactivated fused silica line is passed through the injection port of the GC and goes directly into the pyrolyser. This piece of fused silica is connected to the column, which is put in the GC oven. The pneumatic system consists of (1) a mass flow controller, (2) an electronic flow sensor, (3) a solenoid valve, (4) a backpressure regulator, (5) a pressure gauge, and (6) septum purge controller. The connection (7) is closed when working in Py-GC mode, and connection (8) is open. (Connection (7) is open when the system works as a GC only.) Connection (9) is closed and connection (10) is open when the GC works in splitless mode (purge off). Connection (10) is closed and connection (9) is open when the GC works in split mode (purge on). No details on the GC oven or on the detector are given.

Splitless injection uses the same hardware as spUt injection (Fig. 6.9), but the split valve is initially closed. The sample is diluted in a volatile solvent (like hexane or methanol) and 1 to 5 /iL is injected in the heated injection port. The sample is vaporized and slowly (flow rate of about 1 mL/min) carried onto a cold column where both sample and solvent are condensed. After 45 seconds, the split valve is opened (flow rate of about 50 mL/min), and any residual vapors left in the injection port are rapidly swept out of the system. Septum purge is essential with splitless injections. 

With the correct choice of insert, the sample cloud does not reach the septum so that the low purge flow does not have any effect on the injection itself. 

Figure 2.5 Injectors, (a) Above left, injection chamber. The carrier gas enters the chamber and can leave by three routes (when the injector is in split mode). A proportion of carrier gas (1) flows upward and purges the septum, another (2) exits through the split outlet (a needle valve regulates the split) and finally a proportion passes onto the column, (b) Above right, cold injection onto the column, (c) Below, a typical chromatogram obtained in splitless mode. For solvent peaks which are superimposed upon those of the compounds, a selective detector which does not see the solvent is recommended.

Liquid samples are introduced into the vaporization chamber by syringe through a septum or airlock. An auxiliary flow of gas is used to purge septum bleed products and contaminants away from the vaporization chamber. Appropriate column loads are usually achieved by injecting sample volumes of 0.2-2.0 pi with split ratios between 1 10 and 1 1000. 

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