Selection of carrier gas

The selection of carrier gas (N2, He, H2) is usually a compromise between several aspects, e.g., resolution and analysis time. Nitrogen results in higher column efticiency than both helium and hydrogen, but its average linear velocity ( ) at elevated temperatures is considerably lower than that of He and H2. Also, the change in efficiency with m is smaller for the latter which thus are the preferred carrier gases. 

Injection in the splitless mode may enhance this effect. The sudden evaporation of the injected volume of liquid hexane (2pL) results in about 0.5mL of vapoin. Ideally, this volume will just fill the glass insert of the injector. However, part of it may escap e and come into contact with hot metal surfaces, potentially inducing reactions p,p -DDT, for instance, may disappear to a large extent. Hus does not happen with on-column injection this mode of injection therefore is reconunended when H2 is used as carrier gas. 

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Caution should be used when selecting the carrier gas so that comparison of the standard and sample are done under the best circumstances. 

A third possibility, illustrated in Figure 9.7(c), is to sweep the permeate side of the membrane with a counter-current flow of carrier gas. In the example shown, the carrier gas is cooled to condense and recover the permeate vapor, and the gas is recirculated. This mode of operation has little to offer compared to temperature-gradient-driven pervaporation, because both require cooling water for the condenser. However, if the permeate has no value and can be discarded without condensation (for example, in the pervaporative dehydration of an organic solvent with an extremely water-selective membrane), this is the preferred mode of operation. In this case, the permeate would contain only water plus a trace of organic solvent and could be discharged or incinerated at low cost. No permeate refrigeration is required [36],... 

In contrast to LC detectors, GC detectors often require a specific gas, either as a reactant gas or as fuel (such as hydrogen gas as fuel for flame ionization). Most GC detectors work best when the total gas flow rate through the detector is 20-40 mL/min. Because packed columns deliver 20-40 mL/min of carrier gas, this requirement is easily met. Capillary columns deliver 0.5-10 mL/min thus, the total flow rate of gas is too low for optimum detector performance. In order to overcome the problem when using capillary columns, an appropriate makeup gas should be supplied at the detector. Some detectors use the reactant gas as the makeup gas, thus eliminating the need for two gases. The type and flow rate of the detector gases are dependent on the detector and can be different even for the same type of detector from different manufacturers. It is often necessary to refer the specific instrument manuals for details to obtain the information on the proper selection of gases and flow rates. All detectors are heated, primarily to keep the... 

After condensation and separation of ozone from the oxygen stream, the condensate is led to an evaporator. As evaporation of ozone by boiling with indirect heat at atmospheric pressure sometimes leads to explosions, the ozone is evaporated by direct contact with a high boiling liquid such as water. The gaseous ozone joins a stream of carrier gas of selected composition and volume. 

The selection of injection mode and detection, options as to the type and length of column, and the choice of carrier gas and carrier gas velocity are experimental parameters to chose for which the experimentalist must exercise judgement. Slight variations in these parameters exist from laboratory to laboratory as do variations in the method of column preparation. Columns prepared by the methods described here have proven to be highly effective for IGC experiments in the authors laboratories. 

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.

Figure 1.10 REMPI-TOF permits selective simultaneous recording of separate spectra for different cluster species or for different isotopomers. The spectra of NiC and NiSi were recorded (by resonant two-photon ionization using an ArF excimer laser to ionize) using laser ablation of a nickel target in a stream of carrier gas containing 3% CH4. No intentional source of silicon was present (but the nickel sample had been roughened using SiC sandpaper) (from Brugh and Morse, 2002, and Lindholm, et al, 2003).

Tosio et al. [70] suggested that in order to identify and determine acid components in mixtures with neutral compounds, a flow of carrier gas should be passed through a reactor (100 X 0.5 cm l.D.) packed with potassium hydroxide on a quartz powder (115 100) after the chromatographic separation of the initial mixture on an analytical column. Selective absorption of the acid components takes place in this reactor. By comparing the chromatogram obtained with the analytical column and that obtained with an alkaline reactor, it is possible to identify and determine the acid and neutral components of the test mixture. As an example, results were presented of the analysis of small amounts of... 

Carrier gas Non-selective concentration of impurity zones as a result of chemical bonding of part of carrier gas 125,126... 

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