Stop-flow operation

Fig. 35.1. SPCE configuration for (a) batch mode or (b) stop-flow operation.

True LCxGC is slow, regardless of which operational mode is selected. Realtime operation is feasible, but in that case the flow rate in the column should be kept very low to meet the requirement of four reinjections across a peak. Since diffusion coefficients in liquids are low, this will not result in measurable band broadening [16]. Stop-flow operation is equally slow, but again will not result in additional diffusional band-broadening. Collection in vials is feasible as well. 

A particularly attractive version of pulsed-flow modulation uses the GC inlet pressure as the preset pressure (45,46). 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 signiflcantly changing the elution pattern and resolution of other components in the mixture. The concept is illustrated by the band trajectory plots shown in Figure 5.23 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 broken-line plots for the case with a 5-s-wide pulse occurring at the time indicated by the vertical lines. 

FIGURE 5.24 High-speed temperature-programmed separation of a 20-component pesticide mixture without stop-flow operation (a), with a 2-s-wide stop-flow pulse to separate components 2 and 3 and (c), and with two stop-flow pulses to separate component pairs 2,3 and 10,11. Components are 1, a-BHC 2, P-BHC 3, y-BHC, 4, 8-BHC 5, heptachlor 6, aldrin 7, heptachlor epoxide 8, a-chlordane 9, y-chlordane 10, 4,4 -DDE 12, dield-rin 13, endrin 14, 4,4 -DDD 15, endosulfan II, 16, 4,4 -DDT 17, endrin aldehyde 18, metoxychlor 19, endosulfan sulfate 20, impurity 21 endrin ketone. 

FIGURE 5.25 High-speed isothermal separation of a 13-component mixture without stop-flow operation (a), with a 2-s-wide stop-flow pulse to separate components (1 and 2), (b) and (c) with three stop-flow pulses to separate component pairs (1,2), (10,11), and (12,13). Vacuum outlet GC was used with an outlet pressure of 0.5 atm., and atmospheric-pressure air was used as carrier gas. Components are 1, ethyl acetate 2,2-butanone 3, benzene 4, 1-butanol 5, trichloroethylene 6, n-heptane 7, 2,5-dimethylfuran 8, 2,4-dimethylhexane 9, 3-methyl-l-butanol 10, toluene 11, 2-methylheptane 12, butyl-acetate 13, chlorobenzene. 

FIGURE 5.30 Diagram of autonomous microfabricated GC for air monitoring. Vacuum outlet GC with ambient air as carrier gas is used to eliminate the need for compressed gases. A dual-column ensemble consisting of two 3.0-m-long colnmns with independent temperature control and stop-flow operation is used for selectivity enhancement, and a chemiresistor sensor anay is used for vapor identification. 

Compared to the classic work of the twentieth century, various new technologies are considered for CDI today, such as the inclusion of ion-exchange thin membrane barriers in front of the electrodes, - optimized operational modes such as stop-flow operation during ion release, salt release at reversed voltage, constant-current operation, " energy recovery from the desalination/release cycle, " and flow-through electrodes where the water is directed head-on through the electrodes. ... 

Figure 4.6 Plots of band position versus time for a two-component mixture (1 and 2) illustrating stop-flow operation. Solid-line plots are for a case without a stop-flow pulse, and dashed-line plots are for a case with a 5-s-wide stop-flow pulse, indicated by the vertical lines, (a) Stop-flow pulse applied when both components are in Ca (b) stop-flow pulse applied when component 1 is in Cb and component 2 is in Ca (c) stop-flow pulse applied when both components are in Cb- (From ref. 19. Reprinted with permisson of John Wiley Sons, Inc.)...

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