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Flow programming, attempts to achieve the same result as temperature programming which is to accelerate the strongly retained peaks through the column (see Gas Chromatography). Some detectors require no other gas than that used as the carrier gas, other require specific gases to be added to the columns eluent for them to function. In some cases the detector prescribes a certain gas to be used as the carrier gas (e.g., the sensitivity of the katharometer is greater when helium is used as the carrier gas). In addition, if the gas chromatograph is being used for permanent-gas analysis, then helium must be used to differentiate the carrier gas from the other gases being analyzed.   All gas chromatographs are designed to operate over relatively wide ranges of temperature (e.g., -20oC to 400oC). Consequently, to avoid solute condensation in the detector or detector-connecting tubes, the detector should be capable of operating at least 20oC higher than the maximum column

relative rates of upward carrier gas flow and downward stationary phase flow (contained on the falling support) some components were arranged to move upward with the carrier gas, and others move downwards with the stationary phase. Referring to figure 39, if the ordinary chromatogram of the mixture is that depicted at (A), the relative speed of the carrier gas and the stationary phase defines an imaginary line on the chromatogram. Those components to the left of the line, move up with the carrier gas (B) and those components to the right of the line, move down with the stationary phase (C). The components that move down in the stationary phase are stripped out by arranging a portion of the column to be heated and a second stream of gas elutes them through a second port (D). Scott and Maggs designed a three stage moving bed system to extract pure benzene from coal gas. Coal gas contains a range of saturated aliphatic hydrocarbons, alkenes, naphthenes and aromatics (benzene

Subsequent to the IR spectrum being obtained, a small sample of the vapor was drawn from the IR cell into a low-resolution mass spectrometer and the mass spectrum was also taken.   This system was not a tandem system but, in fact, the first triplet instrument to be reported (GC/IR/MS). The layout of the pneumatic system of the triplet instrument is shown in figure 30. The procedure for analyzing a peak was as follows. As the peak started to elute it was sensed by the detector and the exit carrier gas diverted through the IR cell into a packed trap which concentrated the peak onto the front of the trap packing. After peak elution was complete, the flow of carrier gas was stopped and the solute regenerated back into the IR cell by heating the trap in a secondary stream of nitrogen.     Figure 30. Diagram of an Automatic GC/IR Tandem System

the katharometer can be used in most GC analyses that utilize packed columns and where there is no limitation in sample availability. The device is simple, reliable, rugged and relatively inexpensive. An example of the use of a katharometer to monitor the separation of various compounds of hydrogen, deuterium and tritium, employinggas solid chromatography is shown in figure 14. The stationary phase was activated alumina [treated with Fe(OH)2], and the column was 3 m long and 4 mm I.D. The carrier gas was neon, the flow rate 200 ml/min (at atmospheric pressure) and the column temperature was -196oC.   The Simple Gas Density Balance The original gas density balance has already been described. It was complicated, difficult to fabricate and its manufacture was notacommercialsuccess. Intheearlydays of chromatography GOW-MAC developed some elegantly designed filaments for use in the construction of katharometers, which, in due course, were used in many other manufacturer's

or condensed from the gas stream which is also difficult to achieve efficiently. Finally, the efficient packing of large GC columns is difficult. Nevertheless, preparative GC has been successfully used in a number of rather special applications; for example the isolation of significant quantities of the trace components of essential oils for organoleptic assessment. The layout of a preparative gas chromatograph is shown in figure38 Figure 38 Layout of Preparative Gas Chromatograph Air can not normally be used as the mobile phase due to likely oxidation and so either a gas tank or a gas (e.g., nitrogen) generator must be used. As the flow rates can be large, more than one generator operating in parallel will often be necessary. The sample is usually placed on to the column with a syringe pump and rapidly vaporized in a suitable heater. Passing the gas in vapor form onto the column helps evenly distribute the sample radially across the column

After the solutes of interest have been eluted, the valve is rotated, and the connection of the column inlet to the sampling system is now directed to waste. At the same time the column exit is disconnected from the detector and connected to a separate carrier gas supply that forces a backward flow of carrier gas through the column and the strongly retained solutes are eluted to waste. As suggested above, to accelerate the purging process, the column temperature can be raised. When back flushing is complete, the valve is returned to the sampling position and the column temperature brought back to the initial conditions for analysis. The sampling stage of the back flushing technique shown in figure 20 and depicts the sample passing through the valve to the column and from the column back to