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smell the eluent from a gas chromatography (GC) column in organoleptic assessment. The detector, as well as being an essential supporting device for the gas chromatograph has also played a critical role in the development of the technique as a whole. There has been a synergistic interaction between column development and detector development. The need to develop higher column� efficiencies has demanded higher detector sensitivities which has provoked the development of more sensitive detectors. In turn, the more sensitive detectors has encouraged the improvement of column performance. In fact, the rapid development of GC in the 1950s was possible because or the swift introduction of high sensitivity linear detectors. Classification of Detectors Detectors can be classified into two types, bulk property detectors and solute property detectors. The bulk property detector measures some bulk physical property of the eluent (such as dielectric constant or refractive index) and

Preparative Gas Chromatography Gas chromatography has not been used extensively for preparative work although its counterpart, liquid chromatography, has been broadly used in the pharmaceutical industry for the isolation and purification of physiologically active substances. There are a number of unique problems associated with preparative gas chromatography. Firstly, it is difficult to recycle the mobile phase and thus large volume of gas are necessary. Secondly, the sample must be fully vaporized onto the column to ensure radial distribution of the sample across the column. Thirdly, the materials of interest are eluted largely in a very dilute form from the column and therefore must be extracted 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

. GC detectors should be insensitive to changes in flow rate but, unfortunately, few detectors have this attribute although some, for example the FID, are virtually insensitive to changes in column flow rate. This allows the use of flow programming development if so desired. 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

GC. It is also often the detector of choice for process monitoring. An example of the separation of the various compounds of hydrogen, deuterium and tritium, employing gas solid chromatography and using a katherometer detector is shown in figure 30. 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 four detectors described are well established. reliable and generally simple to operate. They are also, probably the most popular. The FID, ECD, NPD and the katherometer are employed in over 90% of all GC applications. The FID is the most versatile, sensitive and linear, and probably the most generally useful. For details of other GC detectors see Gas Chromatography Detectors

detector described by Martin and Randall (2) in 1951. These two devices should have evoked a growth in LC development, but, in the early fifties, gas chromatography (GC) was invented which completely eclipsed the development of LC. It was not until the early 1960s that the renaissance of LC took place, initially based on the use of the refractive index of Tiselius and Claesson. Although a significant number of GC detectors were developed over two or three years, the development of LC detectors was much slower, largely due to the fact that low concentrations of solute in a liquid do not change the properties of a liquid nearly as much as they do a gas. In fact, the development of LC detectors was gradual and arduous. In a similar way to the development of GC there has been a continuous interaction between improved detector performance and improved column performance. Initially, separations monitored by detectors with improved sensitivity permitted a precise column theory

Data Acquisition and Processing The data acquisition and processing system has been discussed in Principles and Practice of Chromatography and Gas Chromatography and is dealt with in further detail in Gas Chromatography Detectors and Liquid Chromatography Detectors. However, a general outline of the data system layout is included here and is given in figure 29 in the form of a block diagram that shows the individual steps employed. Figure 29. A Typical Data Acquisition and Processing System