Carrier pressure regulation

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...

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. 

If the perturbations are in the form of spikes of an irregular nature, the problem is likely to be detector contamination. Such spikes are especially observed when dust particles have settled into the FID flame orifice. Of course, the problem may also be due to interference from electrical pulses from some other source nearby. Regular spikes can be due to condensation in the flow lines causing the carrier, or hydrogen (FID), to pulse, or they can be due to a bubble flow meter attached to the outlet of the TCD, as well as the electrical pulses referred to above. Baseline perturbations can also be caused by pulses in the carrier flow due to a faulty flow valve or pressure regulator. 

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. 

Carrier gases are normally obtained in bottled form at about 2500 psi (150-160 atm). A two stage regulator is recommended at the cylinder. The second stage pressure regulator is usually set at 40-100 psi. After the gas leaves the cylinder it should pass through a molecular sieve trap (grade 13X). The trap will remove... 

The essential parts of a gas chromatograph as shown in Figure 8.1 are carrier gas, flow or pressure regulator, injection port or valve, column, and detector. Usually there are three heated zones, each separately controlled, for the inlet area, the column, and the detector. Connections between these heated zones must also be kept hot enough to prevent condensation of analytes in them. Chromatographs designed for OT columns usually have additional features a more elaborate injection port that allows for sample splitting and a provision for some additional make-up gas for the detector. Information about commercial instruments can be found in the review by Bayer.3... 

In this study, we used the modified Wicke-Kallenbach cell which is tubular membrane cell type. Permeation measurements were performed in the 293K-373K, Oatm-Satm range for H2, N2, CO2 and CH4. Feed gas and retentate gas were controlled by MFC(Mass Flow Controller, Tylan Co.) and BPR(Back Pressure Regulator). Permeate gas flux was measured by soap bubble flow meter, MFM (Mass Flow Meter, Teledyne Co.) and wet gas meter. Especially, MFM was used to measure kinetics of membrane permeation. Separated and retentate gas composition was analyzed by on-line GC(HP 5890 II, TCD type). Helium was used as carrier gas and sweeping gas. Temperature was detected by RTD(Hanyoung. Co.) at inlet, inner cell and skin of cell. Pressures were detected by pressure transducers(Deco Co.) at inlet and permeate part. 

When coupling a low-pressure detector such as the ELSD with SFC, detection takes place at atmospheric pressure, usually downstream of the back-pressure regulator [2]. Figure la shows a common SFC-ELSD interface with downstream pressure control. Factors affecting ELSD response in this configuration include nebulizer design, evaporation conditions, carrier gas flow rate, and the use of makeup fluid. 

The small volume thermal conductivity detector used In this study was a Y-type [ 10,11,12] gas flow pattern 30 Cow-Mac Model 10-955 cell (Gow-Mac Instrument Company, Madison, New Jersey). This cell was compared with the standard Varlan TCD (Gow-Mac Model 10-952) In a Varlan series 3700 gas chromatograph. The standard electronics were modified for operation with the 30 pi TCD. All measurements were made In the constant mean temperature mode. The carrier gas was He. The flow rates were regulated by two 0-60 ml/mln mass flow controllers (model 1000, Porter Instrument Company, Hatfield, Fa.) or by a pressure regulator (Model 8601, Brooks Instrument Division, Emerson Electric Company, Hatfield, Pa.). The capillary column was attached to a modified 1/16 to 1/16 Swagelok union which In turn was connected to the appropriate TCD. All make-up flows were regulated so that the total flow through both the reference and the sample sides were matched. 

Figure 8.2. Pyrolysis block and accessory apparatus as described by Rogers et al. 30)./. Pyrolysis chamber 2. nickel plug J. carrier gas inlet 4. carrier gas outlet 5. cartridge heater wells 12) 6, helical threads cut in inner body of block 7. outer shell of block 8, cooling jacket inlet 9, cooling jacket outlet. A. carrier gas supply B. pressure regulator C, flow-control needle valve D. reference thermal conductivity E. pyrolysis chamber F, combustion tube C. active cell H. manometer 1, pressure-control needle valve J. rotameter.

Fig. 3.2. System for double-column chromatography with intermediate trapping and re-injection, suitable also for direct injection of aqueous solutions. 1, carrier gas 2, pressure regulator 3, flow controller 4, vent for back-flushing S, injection port for heart-cut and back-flushing 6, precolumn (packed) 7, injection port for aqueous solutions 8, control flame ionization detector for pre-separation 9, vent for cutting 10. leak for make-up gas 11, trap 12, outlet of splitter 13, glass capillary column 14, flame ionization detector for main separation. Reproduced from [35].

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