Carrier gas regulation

The purge gas extracts the analytes from the sample and transports them to a internal trap. The analytes are retained in this trap and concentrated while the purge gas passes out through the vent. The desorption gas, which is provided by the carrier gas regulation of the GC, enters in this phase via the 6-port switching valve and transfer line the GC injector and maintains the constant gas flow for the column (Figure 2.22). 

The adaptation of external devices such as headspace, P T or thermal desorption is straightforward, with the pressure sensor installed close to the injector head which is generally the case with commercial forward-regulated GC instruments. If the pressure sensor is installed in the split line of the injector, it has to be moved close to the inlet to provide a short carrier gas regulation loop. Carrier gas flow and pressure in the external device is controlled by the EPC module of the GC. The position of the pressure sensor in the flow path of an inlet is important for accurate measurement of the inlet pressure to get rapid feedback control. 

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 2.141 Column switching schematics with pre-column, MCSS flow switch at the switch point and main separation column (PI, P2 carrier gas regulations, D1 monitor detector and D2 main detector).

The physical state of a pollutant is obviously important a particulate coUector cannot remove vapor. Pollutant concentration and carrier gas quantity ate necessary to estimate coUector si2e and requited efficiency and knowledge of a poUutant s chemistry may suggest alternative approaches to treatment. Emission standards may set coUection efficiency, but specific regulations do not exist for many trace emissions. In such cases emission targets must be set by dose—exposure time relationships obtained from effects on vegetation, animals, and humans. With such information, a Ust of possible treatment methods can be made (see Table 1). 

Figure 8.26(A) is an example of a valve type interface [329]. Helium carrier gas is provided to the headspace saiq)ler and is split into two flow paths. One path is flow-controlled and provides a constant flow of carrier gas which passes from the headspace unit through the heated transfer line to the gas chromatograph. The second flow path is pressure-regulated and, in the standby mode, the seunple loop and seuapling needle are flushed continuously by the helium flow. At a time determined by the operator, the sampling needle pierces the septum and helium pressurizes the headspace vial to any desired pressure. The headspace gas is then allowed to vent through the sample loop. Once filled, the sample loop is placed in series with the normal carrier gas flow and its contents are driv Bbhrough the heated...

The principal function of the gas chromatograidi is to provide those conditions required by the column for achieving a separation without lowering the performance of the column in any way. As can be seen from Figure 3.1 this means providing a regulated flow of carrier gas to the column, an inlet system to vaporize and mix the sample with the carrier gas, a thermostated oven to optimize the temperature for the separation, an on-line... 

Split vent. The sample vapors that do not enter the column are ejected through the split vent. A needle valve on this line regulates the total flow of carrier gas into and from the inlet, generating the split ratio, which determines the portion of sample that enters the column. The split ratio is the ratio of the split vent flow to the column flow and provides a measure of the amount of sample that actually enters the column from the injection. A split ratio of 100 1 indicates that a lpl injection from the syringe results in approximately 10 ml of liquid sample reaching the column. 

Apparent near the bench on which the GC unit sits are pressure-regulated compressed gas cylinders of hydrogen and air (in addition to the carrier gas, helium or nitrogen). Metal tubing, typically ]/8-in. diameter, connect the cylinders to the detector. A needle valve is used for flow control. These valves are located in the instrument for easy access and control by the operator. 

Open the valve on the pressure regulator of the carrier gas bottle and ensure that there is flow through the system. Turn on the instrument. Set the temperatures of the column, injector, and detector, and also set up the temperature program indicated in the introduction above. Allow time for all components to come to the set temperatures. 

Importantly, the operating efficiency of a chromatograph is directly dependent on the maintenance of a highly constant carrier gas-flow-rate. Carrier gas passes from the tank through a toggle value, a flow meter, a few feet of metal capillary restrictors, and a 0-4 m pressure gauze. The flow rate could be adjusted by means of a needle value mounted on the base of the flow meter and is controlled by the capillary restrictors. On the downstream side of the pressure regulator, a tee (T) may split the flow and direct it to the sample and the reference side of the detector. 

All gas chromatography was carried out with pressure regulated helium carrier gas and the following temperature programme 15°C for 2 mins and then 5°C to 250°C. 

Flow controller. A device to regulate the flow of mobile phase (carrier gas) through column. 

Even the sample loop should be controlled to the same pressure as the column side to minimize pressure upsets when switching the valve. Frazer et al. (9) found that pressure and flow regulation on all inlets as well as the appropriate micrometering valves (variable restrictors) used to dynamically balance the carrier gas flow and pressure allowed them to switch a 20 foot by 1/8-inch column in or out of a multivalve, multicolumn system with a resulting change in the baseline signal of less than 5 iV. 

Temperature programming also has demands on other parts of the chromatographic system. These have already been discussed in the areas of carrier gas purity, flow regulation, and injection systems. In addition, columns must be stable at the top temperature of the program ramp and should have a low vapor pressure (low bleed). 

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