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. 

With capillary columns, we have many different flows to measure with extremely large differences in the range of flows. Capillary columns may have flows lower than 1 mL/min, yet have septum purge flows in the 2-5 mL range and splitter vent flows in the 100-300 mL range. It is preferred to measure capillary column flows in terms of linear gas rates. It is acceptable to measure the other flows with electronically controlled flow measuring devices or soap-bubble meters. Since the split ratio for an analysis is determined by dividing the flow of the column into the vent flow volume from the splitter, we normally do both of these flows electronically. Any other flows that are to be measured are not critical and either manually or electronic measurement is acceptable. 

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). 

Figure 14.10 Schematic diagram of the aromatics analyser system BP, back-pressure regulator CF, flow controller CP, pressure controller Inj, splitless injector with septum purge V, tliree-way valve column I, polar capillary column column 2, non-polar capillary column R, restrictor FID I, and FID2, flame-ionization detectors.

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.

To continuously clean the septum via a purge flow with carrier gas... 

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. 

Figure 4.20 Sketch of a split-splitless injector for GC. The fractions of the inlet gas flow that exit through the septum purge outlet and the split outlet are controlled by needle valves.

The term back pressure describes the position of the EPC valve behind the injector in the split exit line, in combination with a mass flow controller in front of the injector (see Figure 2.48). In this widely used carrier gas regulation scheme for split/splitless injectors, the pressure sensor is typically found in the septum purge line close to the injector body to ensure a pressure measurement close to the column head. A filter cartridge may need to be used to protect the regulation unit from any carrier gas contamination. 

Figure 2.48 Injector back pressure regulation. 1. Carrier gas inlet filter, 2. mass flow regulator, 3. electronic pressure sensor, 4. septum purge regulator, 5. solenoid valve, 6. electronic pressure control valve and 7. injector with column installed.

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. 

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