Pulsed-valve method

The pulse injection method (Fig. 10.24g) uses a fast valve (open only for milliseconds) to inject the solvent with the molecules directly into vacuum onto a sample mounted approximately 100 mm under the opening [136]. The molecules remain uncharged and the pressure in the chamber substantially increases during the deposition. This method is successful even for very large and fragile supramolecular assemblies (see discussion below). 

Transient response method (TRM) mns [11] consisted of stepwise changes of concentration of one species in the feed mixture, realized by using the pulse valves previously described, which assured constant conditions of pressure and global flow. The temperature was normally kept constant during the whole experiment. In particular, the effect of NO/NO2 ratio on the SCR activity was investigated. Space velocities between 90,000 and 210,000 h were used in these experiments. 

Fig. 5.11. Left panel Schematic of the pulsed valve discharge modulation source. A 100 kHz square wave discharge 1 kV, lA) is strongly confined in the 1 mmxSOO /am X 4 cm region behind the slit expansion jaws. This yields slabs of spatially modulated jet-cooled radicals and molecular ions for detection via direct IR laser absorption and lock-in detection methods. Right panel Upper trace direct absorption spectra with conventional discharge modulation in a cw discharge source but without lock-in detection Lower trace spectra with frequency modulated discharge and lock-in detection, revealing substantial elimination of radical precursor and improves the absorption sensitivity to the near shot-noise level.

Step functions, pulses, and square waves can be generated with a low volume, chromatographic-type 4-way valve. We have found that the desired two gas mixtures are best made up and stored in cylinders rather than made continuously by blending two streams. At the time of the switch, there is a momentary stopping of the flow, and this usually results in a change in composition if the mixture is made by the continuous blending of two streams. By this method one or more spurious peaks are added to the desired step function. Naturally these are trivial for slow responses, but important for fast ones. 

Several methods have been used to produce the pulse action required in these columns. A reciprocating, plunger pump, from which the valves have been removed, may be connected to the extractor as at (a), Fig. 12, with a direct liquid connection between the column and the pump piston. Such an arrangement is perhaps mechanically most reliable, al-... 

For the realization of gas pulses, a number of methods can be applied. In gas chromatography, a gas sample is usually collected in a sample loop and injected by an injection valve. However, in order to have some more flexibility concerning the length and the frequency of the pulse, fast switching electromagnetic valves, for example three-way valves of type LFY from Lee Hydraulische Miniaturkomponenten (Figure 3.80) could be used instead. 

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