Carrier Gas Source

A two stage regulator is used at the outlet of the carrier gas cylinder to set an appropriate cyhnder outlet pressure to the GC system and to monitor the residual pressure in the cylinder. Most GC instruments are equipped with a flow controller and flow meter. The flow controller is used to ensure obtaining a constant flow despite changes in pressm-e and pressure drops through the GC colmnn. The flow meter is used to set a carrier gas flow rate to a desirable level and to monitor the stabihty of the carrier gas flow. 

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Fig. 4.3. Schematic diagram of the reactor for reproducible regeneration of carbonyls from microamounts of DNPHs prior to GC analysis. A = injection port (gas chromatograph) B = injector C = exchangeable PTFE tube D = back-pressure valve E = connection to carrier gas source. (Reproduced from/. Chromatogr.,51 (1971)406 [ 106], by courtesy of H. Halvarson.)...

The gas chromatograph shown schematically in Fig. 40 fulfils the necessary conditions for carrying out the intended substance separations, and enables the analyst to control the separation. The system consists of a carrier-gas source (1), e.g. a pressure cylinder with high-purity nitrogen or helium a pressure-reducing valve and precision adjusting valve ensure that the... 

A solution to this problem is to replace the electronic pressure controller with a low-dead-volume computer-controlled valve and a source of carrier gas at some preset pressure. Normally, the valve is closed, and the column junction-point pressure is the natural pressure that occurs at the column junction point in the absence of any additional connections. When the valve is opened, the junction-point pressure assumes the preset value of the additional carrier gas source. Usually, the valve is open for only a few seconds, to enhance the resolution of a particular component pair. Thus, the carrier gas in the two columns undergoes a pulsed-flow modulation (28,29). A particularly attractive version of pulsed-fiow modulation uses the gas chromatographic inlet pressure as the preset pressure (30,31). Thus, when the valve is opened, both ends of column Ca are at the same pressure, and carrier gas flow in Ca stops (stop-flow operation). Stop-flow operation is used to enhance the resolution of a targeted component pair without significantly changing the elution pattern and resolution of other components in the mixture. The concept is illustrated by the band trajectory plots shown in Figure 4.6 for a pair of components labeled 1 and 2 that are completely separated by the first column but coelute from the column ensemble. The solid-line plots are for the case without a stop-flow pulse, and the dashed-line plots for the case with a 5-s-wide pulse occurring at the time indicated by the vertical lines. 

Figure Cl. 1.1. Schematic of a typical laser vaporization supersonic metal cluster source using a pulsed laser and a pulsed helium carrier gas.

Pulsed spark sources, in which the material to be analyzed is part of one electrode, are used for semiquantitative analyses. The numerous and complex processes involved in spark discharges have been studied in detail by time- and space-resolved spectroscopy (94). The temperature of d-c arcs, into which the analyte is introduced as an aerosol in a flowing carrier gas, eg, argon, is approximately 10,000 K. Numerous experimental and theoretical studies of stabilized plasma arcs are available (79,95). 

Not only the pollutant itself but also the carrier gas, the emitting process, and the operational variables of the process affect the selection of the control system. Table 29-1 illustrates the large number of variables which must be considered in controlling pollution from a source (1-4). 

Because the vacuum in the mass spectrometer and the cleanliness of the ion source, transfer line, GC column, and so forth are not perfect, a mass spectrum will typically have several peaks that are due to background. All GC/MS spectra, if scanned to low enough mass values, will have peaks associated with air, water, and the carrier gas. Other ions that are observed in GC/MS are associated with column bleed and column contamination. 

Air leaks are another source of trouble in the MS. A simple method of leak detection is to squirt a small volume of acetone on flanges and other areas where leaks could occur. Caution is advised not to use this procedure near hot surfaces because of the flammability of acetone. A second way to test for small leaks is to tune the MS to m/z 40 and to use argon to test for leaks. The m/z 40 peak will increase if argon enters the source. Helium m/z 4) is a better choice, except when helium carrier gas is used in conjunction with the GC. A small stream of the gas is aimed at all seals where a leak can occur. If a leak is detected at a seal, it can sometimes be stopped by tightening the seal, but it is better to replace the seal than to overtighten it. 

Jet Separator The jet separator contains two capillary tubes that are aligned with a small space (ca. 1 mm) between them. A vacuum is created between the tubes by using a rotary pump. The GC effluent passes through one capillary tube into the vacuum region. Those molecules that continue in the same direction will enter the second capillary tube and will be directed to the ion source. Enrichment occurs because the less massive carrier gas (He) atoms are more easily collisionally diverted from the linear path than the more massive analyte molecules. 

The spectrometer is fitted with a skimmed c.w. supersonic molecular beam source. Many chiral species of interest are of low volatility, so a heated nozzle-reservoir assembly is used to generate, in a small chamber behind a 70-pm pinhole, a sample vapor pressure that is then seeded in a He carrier gas as it expands through the nozzle [103], Further details of this apparatus are given elsewhere [36, 102, 104],... 

Morristown, NJ) for the ion source. No carrier gas separator was used. For determination of nitrosamines and TBDMS derivatives of hydroxy-nitrosamines, columns and operating conditions were identical to those for GC-TEA analyses For most work, the He flow rate was 15 cc/min and the column effluent was split 1 1 between a flame ionization detector and the mass spectrometer. The stainless steel splitter, solvent vent valve (Carle Instruments, Fullerton, CA), and associated plumbing were... 

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