Carbon dioxide Carbon dioxide is a gas at normal temperatures and pressures, has a molecular weight of 44.01, and a critical point at 31C and 73 atmospheres (ca 1050 lbs per sq. in.). In chromatography it is often used under super-critical conditions as an extracting solvent and as the mobile phase in supercritical fluid chromatography. The advantages of super critical carbon dioxide as a solvent is its purity (leaves no residue), the high solubility of organic compounds (particularly flavors and fragrances and other essential oils) and the ease of recovery. After extraction the liquid carbon dioxide can be removed by merely reducing the pressure and allowing the gas to be evolved. This can be carried out at relatively low temperatures and so thermally labile materials (such materials being frequently found in essential oils) are not decomposed. The advantage of using supercritical carbon dioxide as the mobile phase in liquid solid chromatography is that it has the characteristics of both a liquid (strong solvating power) and those of a gas (fast exchange kinetics) and, thus, provide improved elution rates and more efficient columns. Although some of these advantages have been realized, they have not shown sufficient improvement to make the technique competitive with normal liquid chromatography. In many examples, the same results could have been obtained by using conventional liquid chromatography, employing a slightly longer column, slightly smaller particle diameters or a different operating temperature.

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Author: RPW Scott Book:Preparative Chromatography
Section:Preparative   Alternative-Techniques   Moving-Bed-System   Acetylene-GSC-Purification

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

Preparative   Alternative-Techniques   Moving-Bed-System   Acetylene-GSC-Purification

Author: RPW Scott Book:Liquid Chromatography Detectors
Section:HPLC-Detectors   Transport   Modified-Moving-Wire

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

HPLC-Detectors   Transport   Modified-Moving-Wire

Author: RPW Scott Book:Gas Chromatography Detectors
Section:GC-Detectors   Less-Common-Detectors   Dielectric-Constant

. 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

GC-Detectors   Less-Common-Detectors   Dielectric-Constant

Author: RPW Scott Book:Gas Chromatography Detectors
Section:GC-Detectors   Katharometer

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

GC-Detectors   Katharometer

Author: RPW Scott Book:Gas Chromatography - Tandem Techniques
Section:GC-Tandem   Modern-Systems   Light-Pipe-Interfaces

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 60ūC 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

GC-Tandem   Modern-Systems   Light-Pipe-Interfaces

Author: RPW Scott Book:Liquid Chromatography Detectors
Section:HPLC-Detectors   Transport   Modified-Moving-Wire

compounds. They mixed the combustion gases with hydrogen and passed the mixture directly into the NPD. At a column flow rate of 0.37 ml/min., the sensitivity of the detector was stated to be about 3 x 10-7 g/sec, which is equivalent, in concentration units, to about 1.6 x 10-6 g/ml. The moving wire detector has also been modified by Dugger (43) for radioactivity detection (e.g., detection of 14carbon labeled compounds). To detect 14carbon compounds, the solute on the wire was oxidized to carbon dioxide and the radioactive gas passed to a Geiger-Muller tube. To detect tritium, the tritiated water produced on combustion was passed over heated iron to reduce it to hydrogen and tritium, which was then also passed to a Geiger-Muller tube

HPLC-Detectors   Transport   Modified-Moving-Wire