Reactive carrier gas

As the name implies, the sample is introduced into the mass spectrometer as a gas (Nier 1940). There are two types of sources, the classic viscous flow source and the continuous flow source. The viscous flow source typically consists of two identical inlet systems that are coupled to the mass spectrometer by a change-over valve, which allows rapid switching for comparison of isotope ratios measured for sample and standard gases. In the continuous flow source, samples gas is introduced as a bubble in a non-reactive carrier gas stream. 

The selection of proper elements and compounds needs to take into account the very complex situation. The chemical state of the radioisotopes in the reactive carrier gas and the standard sublimation enthalpy of this chemical state must be known. In principle all compounds, which are unstable under the selected conditions, have been rejected as well as compounds, which undergo diffusion processes or irreversible reactions with the stationary phase. The following correlations (Method 8) were obtained experimentally for elements and selected compound classes, see also Figure 8 A-C ... 

The knowledge of the type of the adsorption process is crucial for the determination of the adsorption enthalpy from experimental results. One experimental approach to gain this information is by varying the partial pressure of the reactive carrier gas, which is involved in the mechanism of the adsorption reaction [45,8]. 

Fig. 16. Relative yields in isothermal gas chromatography of l04Mo (O) and l58W ( ) oxide hydroxides in quartz columns using humid oxygen as reactive carrier gas component. Sg was observed at an isothermal temperature of 1300 K.

Thin films of -Ta205 with (111) orientation were prepared by Tominaga and coworkers using Ta(thd)4Cl as CVD precursor (source temperature 215 °C, reactive carrier gas 1 1 argon-oxygen, substrate quartz and Si(lOO) at 600-700°C, pressure 5 Torr, deposition rate 50-150 Amin ) °. The films are smooth but appeared to contain a number of pits caused by columnar growth perpendicular to the substrate. 

Extracted molecules can often be removed from solid particles by heating in a gentle stream of a non-reactive carrier gas. This is a convenient way to transfer the molecules into a gas chromatograph for analysis. 

The selection of proper elements and compounds needs to take into account a very complex situation. The chemical state of the radioisotopes in the reactive carrier gas... 

In Fig. 18, the relative yields of Mo and W oxide hydroxides in open quartz columns using humid O2 as reactive carrier gas component are shown as a function of isothermal temperature. The solid lines are the result of a Monte Carlo model... 

Slagt et al. [134] have stated that because of their thermal instability and reactivity sultones could not be easily analyzed by gas chromatography. They studied the two methods published by Martinsson and Nilsson using a Carlo Erba Fractovap G1 equipped with a flame ionization detector and a glass column (length 0.65 m OD 1/4 in.) filled with 10% OV 1 on Chromosorb W-AW (80-100 mesh). The column temperature was 230°C and the injector/de-tector temperature 275°C. The gas flow rates were N2 25 ml/min (carrier gas), H2 25 ml/min, and air 250 ml/min. One microliter of sample was injected. 

Only lead tetramethyl, carrier gas, hydrogen, and stable hydrocarbons issue from the downstream end of the tube, and none of these will react with lead. Clearly some unstable and reactive intermediate is... 

Nanocrystalline MgO and CaO also allow the destruction of chlorinated benzenes (mono-, di-, and trichlorobenzenes) at lower temperatures (700 to 900°C) than incineration.73 The presence of hydrogen as a carrier gas allows still lower temperatures to be used (e g., 500°C). MgO was found to be more reactive than CaO as the latter induces the formation of more carbon. 

The limitations on the total pressure in the FP-RF cell are far less severe than those for FFDS. The lower end of the pressure range that can be used is determined by the need to minimize diffusion of the reactants out of the viewing zone. The upper end is determined primarily by the need to minimize both the absorption of the flash lamp radiation by the carrier gas and the quenching of the excited species being monitored by RF. In practice, pressures of 5 Torr up to several atmospheres are used. The kinetic analysis is again typically pseudo-first-order with the stable reactant molecule B in great excess over the reactive species as outlined earlier. Table 5.5 gives some typical sources of reactive species used in FP-RF systems. 

All our preliminary CVD experiments were carried out under an atmosphere of N2 gas in order to determine the intrinsic ability of the studied molecules to serve as precursors to ceramic materials. Further studies could be run under a reactive medium such as H2 in order to reduce the C and O content of the films. This was done in the case of compound 19. Cold-wall CVD experiments on this molecule were performed at 973 K and normal pressure under H2 carrier gas. These experiments resulted in the formation of highly pure VC films.38 XPS and EPMA-WDS analyses of these films showed both the free carbon and oxygen contents to be lower than the limits of detection of the techniques. Various factors can account for these results diminution of the C content induced by H2, stabilization of the Cp ligand in the gas phase due to the presence of f-butyl groups, and decomposition mechanism involving a methyl activation leading to the formation of V = CH2 species.38... 

When the carrier gas of TPR was changed from He to H2, more complicated TPR spectra were obtained (Figure 21.8), because now H2 could form water and methane. The position of the water peak moved systematically to higher temperatures as the temperature of the oxygen treatment was raised. Thus, treatment temperature seems to affect not only the amount of oxygen retained in WC, but also its reactivity toward H2. However, the position of the CO peaks remained unchanged as they did in the TPR in He. 

Sugioka and Aomura (133) provide kinetic evidence indicating that the rate-determining step in the hydrocracking of aliphatic sulfur compounds over silica-alumina is catalyzed by Brdnsted acid sites. Conversions of reactants were measured by use of a pulse reactor hydrogen was used as the carrier gas. They found that reactivities of mercaptans are in the following order ... 

How are NMR measurements of adsorbates and reactive species on catalysts made This has been the subject of several recent reviews 6-8). In brief, there are a number of approaches that differ in the extent to which they are intended to reproduce the conditions of an actual catalysis experiment. Heterogeneous catalysis in a microreactor is typically carried out in a carrier gas flow stream at temperatures of 673 K or even higher. Products emerging from the microreactor are most commonly analyzed with an on-line gas chromatograph (GC). The contact time of the reactant on the catalyst typically ranges between several tenths of a second to several tens of seconds. In contrast, the acquisition of a 13C spectrum of an adsorbate on a typical catalyst at 298 K takes several minutes or tens of minutes, even with enriched compounds. As the sample temperature increases to the range commonly used in catalysis, NMR sensitivity rapidly decreases, and the discrepancy in time scale becomes greater. 

Instead of using a cluster-jet for the transportation of the nuclear reaction products to the chemistry apparatus, it is possible to use chemical reactions of the recoils in the carrier gas Either the recoils are stopped in a reactive gas where highly volatile products are formed which are continuously swept out of the stopping chamber, or volatile compounds are formed by thermal synthesis. The volatile species are then... 

Figure 3.90 In contrast to the (non-reactive) Taylor model, the tracer gas speed for a reacting gas is unequal to the carrier gas speed. This fact can be derived from the exponential term in the solution of the concentration of the tracer gas cA at the channel exit [38].

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