Junction point pressure, changing

The limitation of tunable and programmable selectivity with tandem capillary columns using electronic pressure control at the column junction point is that changing the junction point pressure results in a change in the ensemble elution pattern for the entire mixture or a subgroup of the mixture, and under optimal conditions, any pressure change used to enhance the resolution of one component pair will degrade the resolution of another component pair. 

Figure 4.5 Portions of chromatograms showing the effects of a change in the junction-point pressure using the apparatus of Figure 4.4. The column ensemble consists of a 5.0-m-long 0.25-mm-i.d. dimethylpolysiloxane column (a) followed by a 5.0-m-long 0.25-mm-i.d. polyethylene glycol column b). (From ref. 19. Reprinted with permisson of John Wiley Sons, Inc.)...

A solution to this problem is to replace the electronic pressure controller with a low-dead-volume computer-controlled valve and a source of carrier gas at some preset pressure. Normally, the valve is closed, and the column junction-point pressure is the natural pressure that occurs at the column junction point in the absence of any additional connections. When the valve is opened, the junction-point pressure assumes the preset value of the additional carrier gas source. Usually, the valve is open for only a few seconds, to enhance the resolution of a particular component pair. Thus, the carrier gas in the two columns undergoes a pulsed-flow modulation (28,29). A particularly attractive version of pulsed-fiow modulation uses the gas chromatographic inlet pressure as the preset pressure (30,31). Thus, when the valve is opened, both ends of column Ca are at the same pressure, and carrier gas flow in Ca stops (stop-flow operation). Stop-flow operation is used to enhance the resolution of a targeted component pair without significantly changing the elution pattern and resolution of other components in the mixture. The concept is illustrated by the band trajectory plots shown in Figure 4.6 for a pair of components labeled 1 and 2 that are completely separated by the first column but coelute from the column ensemble. The solid-line plots are for the case without a stop-flow pulse, and the dashed-line plots for the case with a 5-s-wide pulse occurring at the time indicated by the vertical lines. 

Joining two tubes creates a boundary condition at the junction, and it is this junction which determines how the sound propagates as a whole. In considering behaviour at a junction, we can use an important principle of physics which states that pressure and volume velocity caimot change instantaneously anywhere. So despite the sudden change in cross sectional area, the volume velocity and pressure caimot abruptly change, and it follows from this that at the point of the junction the pressure and volume velocity must be equal. Hence... 

For case (a), the pressure pulse is applied with both components are in Ca-Both bands stop during the pulse, and the peaks in the ensemble chromatogram are shifted to a later time but without significant change in resolution. For case (b), the pulse is applied after the first of the component bands has migrated across the column junction point and is in Cb, but the band for the other component is still in the first column. The band in Ca stops for the duration of the pulse, while the band in Cb migrates more rapidly during the pulse. The result is... 

Another type of osmometer is the vapor pressure osmometer. In reality, osmolality measurement in these instruments is not related directly to a change in vapor pressure (in millimeters of mercury), but to the decrease in the dew point temperature of tlie pure solvent (water) caused by the decrease in vapor pressure of the solvent by the solutes. In this instrument, temperature is measured by means of a thermocouple, which is a device consisting of two dissimilar metals joined so that a voltage difference generated between the points of contact (junctions) is a measure of the temperature difference between the points. 

The plant transient analysis code for the Super FR is called SPRAT-F. It is based on the 1-D node junction model with radial heat transfer and point kinetics models such as SPRAT-DOWN for the Super LWR (see Chaps. 4 and 6). The nodalization is shown in Fig. 7.67. The models used in SPRAT-F are the same as those in SPRAT-DOWN. The turbine control valve regulates the main steam pressure by changing the main steam flow rate as in BWRs and the Super LWR. The relation between its stroke and the steam flow rate is the same as that of the Super LWR (Fig. 4.4). The relation between the core pressure and the feedwater flow rate (with constant pump speed) is also the same as that in the Super LWR (Fig. 4.5). 

---