Chromatographic coelution absence

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. 

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