Carbon dioxide liquid chromatography

Quantitative analysis can be carried out by chromatography (in gas or liquid phase) during prolonged electrolysis of methanol. The main product is carbon dioxide,which is the only desirable oxidation product in the DMFC. However, small amounts of formic acid and formaldehyde have been detected, mainly on pure platinum electrodes. The concentrations of partially oxidized products can be lowered by using platinum-based alloy electrocatalysts for instance, the concentration of carbon dioxide increases significantly with R-Ru and Pt-Ru-Sn electrodes, which thus shows a more complete reaction with alloy electrocatalysts. 

In supercritical fluid chromatography (SFC) the mobile phase is a supercritical fluid, such as carbon dioxide [15]. A supercritical fluid can be created either by heating a gas above its critical temperature or compressing a liquid above its critical pressure. Generally, an SFC system typically has chromatographic equipment similar to a HPLC, but uses GC columns. Both GC and LC detectors are used, thus allowing analysis of samples that cannot be vaporized for analysis by GC, yet cannot be detected with the usual LC detectors, to be both separated and detected using SFC. SFC is also in other... 

Gas lasers, 14 681-696 carbon dioxide, 14 693-696 excimer lasers, 14 691-693 helium-neon, 14 681-683 ion lasers, 14 683-688 molecular nitrogen, 14 688-691 Gas lift electrolyte circulation, 9 621 Gas-liquid base stocks, 15 217 Gas-liquid chromatography (glc), 6 374 analysis of sugars via, 23 476 silylation for, 22 692, 697 Gas-liquid contactor, reciprocating jet,... 

Most supercritical fluid chromatographs use carbon dioxide as the supercritical eluent, as it has a convenient critical point of 31.3°C and 72.5 atmospheres. Nitrous oxide, ammonia and n-pentane have also been used. This allows easy control of density between 0.2g ml-1 and 0.8g ml-1 and the utilization of almost any detector from liquid chromatography or gas chromatography. 

Reindt and Hoffler [50] optimized parameters in the supercritical fluid extraction of polyaromatic hydrocarbons from soil. These workers used carbon dioxide -8% methanol for extraction and obtained 88-101% recovery of polyaromatic hydrocarbons in the final high-performance liquid chromatography. 

Fernandez et al. [9] used supercritical fluid extraction combined with ion pair liquid chromatography to determine quaternary ammonium in digested sludges and marine sediments. Carbon dioxide modified with 30% methanol was used as the extractant at an operating pressure of 380atm. Between 0.2 and 3.7g kg-1 surfactant was found in Swiss works effluent sludges, determined with a relative standard deviation of 7%. 

Von Bavel et al. [55] have developed a solid phase carbon trap (PX-21 active carbon) for the simultaneous determination of polychlorodibenzo-p-dioxins and polychlorodibenzofurans also polychlorobiphenyls and chlorinated insecticides in soils using superfluid extraction liquid chromatography for the final determination. Supercritical fluid extraction with carbon dioxide has been applied to the determination of dioxins in soil [114],... 

The method based on immunosorbents coupled on-line with liquid chromatography-atmospheric pressure chemical ionization mass spectrometry [109], discussed in section 9.4.2.1, has been applied to the determination of substituted urea type herbicides. Supercritical fluid extraction with methanol modified carbon dioxide has been applied to the determinants of sulfonyl urea herbicides in soil [261],... 

In packed column SFC, polar solutes such as amines and carboxylic acids often have too much retention or elute with poor peak shapes when neat carbon dioxide is used as a mobile phase [28, 92]. This is mainly due to the weak solvent strength of neat carbon dioxide compared to a liquid solvent. The use of modifiers is often necessary to enhance the solvating power of the mobile phase in SFC. Various alcohols such as methanol and isopropanol are commonly used modifiers in SFC, but other solvents such as acetonitrile was also utilized [92]. The concentrations of modifiers are usually less than 50%. The technique in which the concentrations of modifiers are greater than 50% is often called enhanced-fluidity liquid chromatography [93]. 

Two bioassays are employed to evaluate the effect of samples on terrestrial life forms. For gas samples, the plant stress ethylene test is presently recommended. This test is based on the well-known plant response to environmental stress release of elevated levels of ethylene (under normal conditions plants produce low levels of ethylene). The test is designed to expose plants to various levels of gaseous effluents under controlled conditions. The ethylene released during a set time period is then measured by gas chromatography to determine toxicity of the effluent. For liquid and solid samples, a soil microcosm test is employed. The sample is introduced on the surface of a 5 cm diameter by 5 cm deep plug of soil obtained from a representative ecosystem. Evolution of carbon dioxide, transport of calcium, and dissolved oxygen content of the leachate are the primary quantifying parameters. 

Supercritical fluid chromatography provides increased speed and resolution, relative to liquid chromatography, because of increased diffusion coefficients of solutes in supercritical fluids. (However, speed and resolution are slower than those of gas chromatography.) Unlike gases, supercritical fluids can dissolve nonvolatile solutes. When the pressure on the supercritical solution is released, the solvent turns to gas. leaving the solute in the gas phase for easy detection. Carbon dioxide is the supercritical fluid of choice for chromatography because it is compatible with flame ionization and ultraviolet detectors, it has a low critical temperature. and it is nontoxic. 

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