Supercritical fluids nitrous oxide

A disadvantage of supercritical fluids for extraction is that most common fluids used for extraction (carbon dioxide, nitrous oxide, sulfur hexafluoride, etc.) are weak solvents, limiting the polarity and molecular weight range of analytes that can be efficiently extracted. Also, for trace analysis the availability of fluids of adeguate ptirity may be a problem. 

Raynie, D. E., Warning concerning the use of nitrous oxide in supercritical fluid extractions, Anal. Chem., 65(21), 3127, 1993. (CA119 187552x)... 

Ashraf-Khorassani M, Taylor LT, Zimmerman P. Nitrous oxide versus carbon dioxide for supercritical fluid extraction and chromatography of amines. Anal. Chem. 1990 62 1177-1180. 

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. 

The first use of supercritical fluid extraction (SFE) as an extraction technique was reported by Zosel [379]. Since then there have been many reports on the use of SFE to extract PCBs, phenols, PAHs, and other organic compounds from particulate matter, soils and sediments [362, 363, 380-389]. The attraction of SFE as an extraction technique is directly related to the unique properties of the supercritical fluid [390]. Supercritical fluids, which have been used, have low viscosities, high diffusion coefficients, and low flammabilities, which are all clearly superior to the organic solvents normally used. Carbon dioxide (C02, [362,363]) is the most common supercritical fluid used for SFE, since it is inexpensive and has a low critical temperature (31.3 °C) and pressure (72.2 bar). Other less commonly used fluids include nitrous oxide (N20), ammonia, fluoro-form, methane, pentane, methanol, ethanol, sulfur hexafluoride (SF6), and dichlorofluoromethane [362, 363, 391]. Most of these fluids are clearly less attractive as solvents in terms of toxicity or as environmentally benign chemicals. Commercial SFE systems are available, but some workers have also made inexpensive modular systems [390]. 

Environmental applications of SFE appear to be the most widespread in the literature. A typical example is the comparison of extraction efficiency for 2,3,7,8 -tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD) from sediment samples using supercritical fluid extraction and five individual mobile phases with Soxhlet extraction was made (101). The mobile phases, carbon dioxide, nitrous oxide, pure and modified with 2% methanol as well as sulfur hexafluoride were examined. Pure nitrous oxide, modified carbon dioxide and modified nitrous oxide systems gave the recoveries in the acceptable range of 80 to 100%. Carbon dioxide and sulfur hexafluoride showed recoveries of less than 50% under identical conditions. Classical Soxhlet recoveries by comparison illustrated the poorest precision with average extraction efficiencies of less than 65%. Mobile phase choice, still as yet a major question in the science of supercritical fluid extraction, seems to be dependent upon several factors polarity of the solute of interest, stearic interactions, as well as those between the matrix and the mobile phase. Physical parameters of the solute of interest, as suggested by King, must also be considered. Presently, the science behind the extraction of analytes of interest from complex matrices is not completely understood. 

When using nitrous oxide, containing organic solutes, as a supercritical fluid for hplc purposes, a 1 ml sample exploded, causing considerable shrapnel damage. It is recommended that it be not used as a supercritical fluid [5]. 

A supercritical fluid is defined as a substance that is above its critical temperature and pressure. It exhibits remarkable liquid-like solvent properties and, therefore, high extraction efficiency. Such common gases as carbon dioxide and nitrous oxide have been successfully employed as supercritical fluids in the extraction of organics from solid matrices. The solid sample is placed in an extraction vessel into which the pressurized supercritical fluid is pumped. The organic analytes dissolve in the supercritical fluid and are swept out of the extraction chamber... 

The liquid-gas equilibrium line terminates at a point known as the critical point. The temperature and pressure that define the critical point are known as the critical temperature and the critical pressure. For example, nitrous oxide has a critical temperature of 36°C and a critical pressure of 72.45 bar (1051 psi). When the temperature and pressure exceed these critical values, the system becomes a supercritical fluid. Supercritical fluids have the flow properties of gases but densities similar to liquids, and supercritical fluids have no surface tension. Therefore, supercritical fluids are terrific solvents. For example, supercritical carbon dioxide is an excellent solvent for extracting caffeine from coffee without resorting to more toxic organic solvents like dichloromethane. 

Karlsson et al. reported the supercritical fluid chromatography of methaqualone, cotinine, and reclopride, among other compounds, using capillary columns of different polarities [19]. Detection was either thermionic nitrogen-phosphorus or flame ionization. Supercritical nitrous oxide was used as the mobile phase. The detection limits obtained were in the range of 2-4 ppm and the precision was in the range of 3-12%. 

Among the works of supercritical fluid separations of PCBs, UV has been the most popular detector. A Microbore Cig column was used to separate individual PCB congeners in Aroclor mixtures. Density and temperature programming was also utilized for separation of PCBs. Both packed (with phenyl and Cig) and capillary (Sphery-5 cyanopropyl) columns were used in this work. Carbon dioxide, nitrous oxide, and sulfur hexafluoride were tested as mobile phases for the separation of PCBs. 

Subra P, Castellani S, Ksibi H, Garrabos Y. Contribution to the determination of the solubility of p-carotene in supercritical carbon dioxide and nitrous oxide experimental data and modeling. Fluid Phase Equilibria 1997 131 269. 

Raynie DE. Warning concerning use of nitrous oxide in supercritical fluid extractions. Anal Chem 1993 65 3127-3128. 

Sauvage E, Rocca JL, Toussaint G. Use of nitrous oxide for supercritical-fluid extraction of pharmaceutical compounds from animal feed. J High Resolut Chromatogr 1993 16 234-238. 

Mathiasson, L., J.A. Jonsson, and L. Karlsson. 1989. Determination of nitrogen compounds by supercritical fluid chromatography using nitrous oxide as the mobile phase and nitrogen-sensitive detection. J. Chromatogr. 467 61-74. 

Supercritical or near-critical fluids can be used both for extraction and chromatography. Many chemicals, primarily organic species, can be separated and analyzed using this approach [6], which is particularly useful in the food industry. Substances that are useful as supercritical fluids include carbon dioxide, water, ethane, ethene, propane, xenon, ammonia, nitrous oxide, and a fluoroform. Carbon dioxide is most commonly used, typically at a pressure near 100 bar. The required operating pressure ranges from about 43 bar for propane to 221 bar for water. Sometimes a solvent modifier is added (also called an entrainer or cosolvent), particularly when carbon dioxide is used. 

A gas supply and pump or flow regulator usually make up the source when a GC-like set-up is being used. The most common mobile phase for SFC is carbon dioxide this is based on its low cost, low interference with chromatographic detectors and good physical properties. Other examples include nitrous oxide and ammonia. Supercritical fluids can... 

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