Cells, spectroscopic gases

The collected sample at -196°C was isolated from the flow of the GC s helium gas stream and then the loop was warmed to ambient temperature for GC-mass spectroscopic analyses. The gas cell, which contained the isotopic CO2 and the C2Hg standard in helium at one atmosphere, was placed in the injection helium flow of the GC-mass spectrometer for ten minutes, before the mini-switching valve was turned to inject the vapor contents into the instrument. After three minutes, the CO2 peak eluted. The superimposed peaks were sampled ten times during their elution and the relative isotopic quantities of - C02 C02 and C02 were determined. 

Cells of the second type were initially developed by Tinker and Morris at Monsanto [4] and subsequently by Penninger [5]. In these systems, the reaction solution is circulated from the autoclave through an external IR cell of relatively small volume. This arrangement means that the cell can be isolated from the main reaction vessel relatively easily (for example in the event of window failure) thus protecting the spectrometer. Cells of this sort can, in principle, be fitted to plants or pilot plants to monitor liquid streams. However, the circulation of solution from the main reaction vessel through an external cell introduces some potential problems. A pressure drop in the circulation system can lead to release of dissolved gas, which may accumulate between the cell windows and interfere with the spectroscopic measurement. A change in pressure may also influence the catalyst specia-tion, such that the observed spectra may not be truly representative of the bulk reaction solution. 

As described earlier, high pressure cells have been developed for the use of noble gases as solvents for IR spectroscopic studies, either at low temperature, or at ambient temperature where the supercritical phase exists. A particular focus of this work was the study of reactive complexes containing coordinated noble gas atoms or molecular H2, the latter being particularly relevant to hydrogenation reactions. 

The static gas cell work of Halpern and Jackson (153) showed that the upper electronic state of BrCN is bent, in accord with the predictions made from their spectroscopic observations of Rabalais et al. (155). An example of the observed rotational distribution is given in Figure 7. Included in this figure is the rotational distribution obtained at 10 torr and one that was obtained after a few collisions. This latter distribution is identical to one that had been obtained earlier by Heaven et al. (152) and shows that their distribution was not a nascent distribution, since it could not be obtained in a static gas cell after several collisions. One can conclude from the work at 193 nm that the CN(x2e+) is produced rotationally hot mainly in the v" = 0 and less than 6% of the radicals are formed vibrationally excited. 

I. Spectroscopic Determinations. Gas-phase infrared spectra provide a useful adjunct to vapor pressure measurements in the identification of volatile materials. The cell illustrated in Fig. 9.15 allows the sample to be quantitatively returned to the vacuum line after the spectrum has been obtained, so the process is completely nondestructive. The primary problem with a gas cell is to obtain a vacuum-tight seal between the window material and the cell body this may be accomplished with Glyptal paint or with wax- If the latter is used, it is necessary to warm and cool the alkali halide windows slowly to avoid cracking them due to thermal stress. For this purpose an infrared lamp is handy. The most satisfactory method of attaching windows is O-rings because this allows the easy removal of the windows for cleaning and polishing. 

Other means of manipulating ions trapped in the FTMS cell include photodissociation (70-74), surface induced dissociation (75) and electron impact excitation ("EIEIO")(76) reactions. These processes can also be used to obtain structural information, such as isomeric differentiation. In some cases, the information obtained from these processes gives insight into structure beyond that obtained from collision induced dissociation reactions (74). These and other processes can be used in conjunction with FTMS to study gas phase properties of ions, such as gas phase acidities and basicities, electron affinities, bond energies, reactivities, and spectroscopic parameters. Recent reviews (4, 77) have covered many examples of the application of FTMS and ICR, in general, to these types of processes. These processes can also be used to obtain structural information, such as isomeric differentiation. 

The above standard mixtures contained in cylinders are supplemented by several gas measurement facilities which can provide dynamic calibrations of gas mixtures and of gas monitoring instruments. These include an on-line facility which injects gas dynamically into a passivated multipass optical gas cell, where the gas concentration is certified spectroscopically. Some of the gas mixtures which can be certified by these dynamic blending facilities are given in Table 2. 

Other elements were measured by a Perkin-Elmer SCIEX ELAN 6100 DRCII Inductively Coupled Plasma Mass Spectrometry (ICP-MS) instrument equipped with a cyclonic spray chamber, a concentric nebulizer and a dynamic reaction cell (DRC). In the vented (standard) mode, no reaction gas is present in the cell and the instrument shows the typical characteristics of a quadrupole-based ICP-MS apparatus. When the gas is introduced into the cell an ion-molecule reaction takes place that can be tailored so as to eliminate spectroscopic interferences. Experimental conditions are summarized in Table 10.2. 

In the applications described so far, catalytic data were not acquired along with the spectroscopic data, or the cells were unsuitable for correct measurements of the former. The determination of the catalyst structure and performance in a single experiment is not only of interest for catalysts but for any functional material. For instance, rather similar developments as in the field of catalysis have been reported in the fields of gas sensors and electrochemical devices. Many techniques allow for the simultaneous characterization of electrochemical materials and their performance (Luo and Weaver, 2001 Novak et al., 2000). Conductance cells provide a powerful approach to understanding of the structure-performance relationships at the molecular scale (Loridant et al., 1995). 

---