Gas-Solid Chromatography The moving bed process described in 1956 by Freund et al. (10) was developed to isolate pure acetylene from the gaseous product obtained from methane oxidation. The actual feed mixture contained 8-9% of acetylene, 4-5% of carbon dioxide, 4-5% of methane, 25% of carbon monoxide, and about 50% of hydrogen. The apparatus shown in figure 24 produces relatively pure acetylene as a direct product. The upper section is cooled with water to reduce the temperature of the carbon and allow the strong adsorption of acetylene, which takes place in the second section. The lighter gases are eluted at the top of the adsorber, (the top product). The cooled absorbent containing acetylene and carbon dioxide is heated and the carbon dioxide is first desorbed and collected as the upper product. As the adsorbent moves into a hotter part of the stripper section, the acetylene is desorbed and collected as the lower product. The product had a purity of about 98

by the proportion of the pyrolysis products that entered the FID. Excepting synthetic polymers, (which often quantitatively produced monomers) many compounds yielded only a few percent of volatile compounds on pyrolysis. Thus the FID could only sense a very small fraction of the products from the solute deposited on the wire.  If the solutes, were completely combusted in oxygen or air, however, then all the carbon in the solute would be converted to carbon dioxide. Furthermore, if the carbon dioxide was then reduced to methane by mixing it with hydrogen and passing it over a nickel catalyst, the carbon dioxide would be quantitatively converted to methane which could be detected by the FID. This procedure would increase the sensitivity of the detector to those substances that gave poor yields on pyrolysis and, in addition, increase the linear dynamic range and possibly provide a predictable response

. However, the device was found to be no more sensitive than the katharometer and considerably more complex. In 1965 Winefordner et al. (41)  developed the detector further. They employed virtually the same physical system but in this case they used a miniature coaxial type sensor with very small spacing between the internal and external cylindrical conductors (0.005 in.). Significantly improved sensitivities of 10-10 g/ml were reported for the detection of oxygen, nitrogen, hydrogen, carbon dioxide, carbon monoxide, methane, nitrogen dioxide, nitrous oxide and sulfur dioxide. In addition, the authors claimed by employing solid state electronics in place of the thermionic tubes the electronic noise the sensitivity could be increased by one or two orders of magnitude. Subsequently Williams and Winefordner claimed a "nearly" linear response for the detector over a concentration range in excess of 4 orders of magnitude for the gases ethylene, ethane, propane and

provide extra noise or signal distortion. The associated electronics may contain an A/D converter to provide a binary output that can be addressed and acquired by a computer or the analog signal may be passed to a computer that has its own A/D converter. In general the sooner the signal is digitized the better, as digital data is far more  immune to external interference than analog signals.   The Katharometer Detector The katharometer was developed in the late 1940s for measuring carbon dioxide in the flue gasses produced from various types of industrial furnaces. A knowledge of the carbon dioxide content allowed the combustion conditions to be changed to improve burning efficiency. With the introduction of gas chromatography, its use as a possible GC detector was explored by Ray (11). T he sensor is a simple device and is depicted in figure 12. Figure 12. The Katharometer Detector

Another typical example of the use of the GC/IR instrument is for the analysis of the essential oils of basil a chromatogram of which, is shown in figure 37. A sample of the oil was obtained by supercritical fluid extraction. Superfluid extraction is often used to obtaining samples of essential oils from botanical tissue. The technique is popular due to it being chemically 'gentle' and rarely causes thermal degradation of labile materials. The herb basil was extracted with liquid carbon dioxide at 60ûC and at 250 atmospheres pressure. The extract (a solution of the essential oil in liquid carbon dioxide) is decompressed through a length of silica capillary into an appropriate solvent. Alternatively, it could be trapped on a suitable adsorbent contained in a packed tube, and then thermally desorbed into the carrier gas of a gas chromatograph. The column that was used in this example was a macro-bore open tubular column, 50 m long, 0.32 mm in diameter carrying film of a

the droplets in a stream of hot air). Silica is produced with a wide choice of surface areas and porosity's, which can range from about 750 m2/g and a mean pore size of 22 Å, to a material having a surface area of only 100m2/g and a mean pore diameter of 300 Å. It is used for the separation of the lower molecular weight gases and some of the smaller hydrocarbons. In a specially prepared form, silica can be used for the separation of the sulfur gases, hydrogen sulfide, sulfur dioxide and carbon disulfide. Molecular sieves are used for the separation of small molecular weight gases largely by exclusion. The naturally occurring aluminosilicates are called zeolites, the synthetic zeolites are the Linde Molecular Sieves of which there are a number of different types available for specific applications. The zeolites have a crystalline structure which does not collapse when dehydrated. When water is removed from the crystals, channels of uniform dimensions are left within