Gradient techniques several zones

The concentration gradient can be formed by injecting a single sample zone, as is described in this section, or by injection of two or several zones, as is described in the next section. The resulting readout may be— for both experimental approaches—either a single peak, as exploited in most FIA techniques designed to date, or a multiple peak, which yields additional information on sample composition. 

Merging-zone techniques are based on exploitation of the concentration gradients formed when two (or several) zones are injected simultaneously, and then allowed to merge—or penetrate into each other—thus yielding response curves, which either completely or partially overlap each other. The purpose of such an approach is twofold (1) to save sample and/or reagent solutions and (2) to create a composite zone that is information rich and that may yield a multiple readout. 

Gradient Techniques with Injection of Two or Several Zones... 

Since spillover phenomena have been most directly sensed through the use of IR in OH-OD exchange [10] (in addition, in the case of reactions of solids, to phase modification), we used this technique to correlate with the catalytic results. One of the expected results of the action of Hjp is the enhancement of the number of Bronsted sites. FTIR analysis of adsorbed pyridine was then used to determine the relative amounts of the various kinds of acidic sites present. Isotopic exchange (OH-OD) experiments, followed by FTIR measurements, were used to obtain direct evidence of the spillover phenomena. This technique has already been successfully used for this purpose in other systems like Pt mixed or supported on silica, alumina or zeolites [10]. Conner et al. [11] and Roland et al. [12], employed FTIR to follow the deuterium spillover in systems where the source and the acceptor of Hjp were physically distinct phases, separated by a distance of several millimeters. In both cases, a gradient of deuterium concentration as a function of the distance to the source was observed and the zone where deuterium was detected extended with time. If spillover phenomena had not been involved, a gradientless exchange should have been observed. 

In order to overcome the above limitations of the F-CVI technique, alternative techniques using thermal gradients or pressure gradients have been examined for several years [11]. In the thermal gradient process, the core of the fibrous preform is heated in a cold wall reactor. The heat loss by radiation is favorable to get a colder temperature in the external surface. The densification front advances progressively from the internal hot zone toward the cold side of the preform. In the P-CVI process, the source gases are introduced during short pulses [11]. The P-CVI process is appropriate to the deposition of thin films. 

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