The use of alternative fluids in on-line supercritical fluid extraction – capillary gas chromatography

A key feature differentiating analytical supercritical fluid extraction (SFE) from conventional liquid extraction is the possibility of varying the solvent strength of a supercritical fluid to achieve selective extractions of specific target compounds, or functional classes of compound, from complex matrices. This can be accomplished by using supercritical fluids other than carbon dioxide, for example, sulfur hexafluoride, nitrous oxide, or sulfur hexafluoride-modified carbon dioxide. The use of these fluids will be demonstrated by the characterization of complex environmental and petroleum matrices by directly coupled SFE – capillary GC. On-line SFE-GC involves the decompression of pressurized extraction fluid directly into the heated, unmodified capillary split injection port of the chromatograph. This paper will also show how, by adjustment of the extraction temperature and pressure, SFE selectivity may be further enhanced.     

Extraction of polychlorinated biphenyl with supercritical carbon dioxide, sulfur hexafluoride and subcritical water

In the extraction of spiked PCB from soil, three extracting fluids were investigated: supercritical carbon dioxide (CO2), supercritical sulfur hexafluoride (SF6) and subcritical water. Among the tested fluids SF6 appeared to be appropriate especially for the extraction of low polar PCB. CO2 and water were found to be suitable for the quantitative extraction of all PCB. Water was judged as the best because of its low price, good availability and environmental safety.     

CO2 laser-induced interaction between sulfur hexafluoride and carbon disulfide

Interaction between carbon disulfide and sulfur hexafluoride is excited by cw CO₂ laser radiation and has been investigated for different ratios over a pressure range from 3.1 to 34.6 kPa. The reaction yields sulfur tetrafluoride, sulfur, carbon, thiocarbonyl fluoride, tetrafluoromethane and hexafluoroethane, the ratio of these latter products is dependent on the partial pressure of sulfur hexafluoride in the initial CS₂-SF₆ mixture. Interaction is considered to include both the SF₆-sensitized decomposition of carbon disulfide and reaction between sulfur hexafluoride and carbon disulfide.     

Extraction of polychlorinated biphenyl with supercritical carbon dioxide, sulfur hexafluoride and subcritical water

In the extraction of spiked PCB from soil, three extracting fluids were investigated: supercritical carbon dioxide (CO₂), supercritical sulfur hexafluoride (SF₆) and subcritical water. Among the tested fluids SF₆ appeared to be appropriate especially for the extraction of low polar PCB. CO₂ and water were found to be suitable for the quantitative extraction of all PCB. Water was judged as the best because of its low price, good availability and environmental safety.     

The effects of modifiers in supercritical fluid chromatography

Investigations have been initiated to examine the basic elements of resolution and how they vary individually and collectively in terms of modifiers used in supercritical fluid chromatography (SFC). Capacity factors (k′) have been determined for a mixture of polynuclear aromatic hydrocarbons from SFC experiments as function of modifier identity and concentration using different stationary phases. Using carbon dioxide as the primary mobile phase, the modifiers investigated included methanol, 2-methoxy ethanol, 1-propanol, tetrahydrofuran, dimethyl sulfoxide, acetonitrile, sulfur hexafluoride, and freon 11.     

Thermodynamic parameters of excess and absolute adsorption of CH4 and SF6 on carbons

Isosteres, isosteric heats, and heat capacities of methane and sulfur hexafluoride on carbons with different porosities were calculated to characterize the excess adsorption and absolute adsorption. The nonlinear character of isosteres of excess adsorption and absolute adsorption of sulfur hexafluoride on FAS carbon with developed mesoporosity was shown.     

Nucleophilic Activation of Sulfur Hexafluoride by N-Heterocyclic Carbenes and N-Heterocyclic Olefins: A Computational Study

Sulfur hexafluoride (SF₆) is considered as a potent greenhouse gas, whose effective degradation is challenging. Here we report a computational study on the nucleophilic activation of sulfur hexafluoride by N-heterocyclic carbenes and N-heterocyclic olefins. The result shows that the activation of SF₆ is both thermodynamically and kinetically favorable at mild condition using NHOs with fluoro-substituted azolium and sulfur pentafluoride anion being formed. The Gibbs free energy barrier during the activation of SF₆ has a linear relationship with the energy of HOMO of substrates, which could be a guideline for applying those compounds that feature higher energy in HOMO to activate SF₆ in high efficiency.     

Two‐dimensional separation techniques using supercritical fluid chromatography

High‐resolution separation systems are essential for the analysis of complex mixtures in a wide variety of application areas. To increase resolution, multidimensional chromatographic techniques have been one key solution. Supercritical fluid chromatography provides a unique opportunity in these multidimensional separations based on its potential for high solvent compatibility, rapid duty cycles, and orthogonality to other separation modes. This review focuses on two‐dimensional chromatography methods from the past decade that use supercritical fluid chromatography because of these advantages. Valving schemes and modulation strategies used to interface supercritical fluid chromatography with other liquid chromatography and gas chromatography techniques are described. Particular applications of multidimensional separations using supercritical fluid chromatography for the analysis of oils and chiral separations of pharmaceutical compounds are highlighted. Limitations of and a potential trajectory for supercritical fluid chromatography in this field are also discussed.     

Inorganic volatile fluorides obtained from electrical decomposition of sulfur hexafluoride in a quartz tube

Recent investigations on sulfur hexafluoride decomposition have shown the need of a rapid and efficient method for the qualitative and quantitative analysis of the reaction products. An analytical method for characterizing the gas mixture obtained from the decomposition of sulfur hexafluoride in a quartz reactor submitted to an r.f. discharge, is presented. A combination of gas-chromatographic, mass spectrometric and infrared spectrophotometric techniques has shown the presence of SF₆, SO₂F₂, SOF₄, SOF₂, SiF₄ and F₂ in the gas mixtures examined. For quantitative purposes a gas-chromatographic method has been found to be most suitable.     

Supercritical fluid solid chromatography using novel porous glassy carbon. Evaluation of retention mechanisms

The use of a porous glassy carbon (PGC) material as a packed-column SFC stationary phase has been previously demonstrated [1]. The material is further characterized in terms of its retention characteristics. The effects of variations in mobile phase composition, pressure, and temperature conditions are evaluated. Variation of temperature and pressure yielded expected results, specifically, decreased solute capacity factors with increased mobile phase density. The choice of supercritical fluid mobile phase allows the most notable control of solute retention; this was evaluated by adding low percentages of organic modifiers of varying molecular weights to the supercritical carbon dioxide mobile phase. PGC-SFC provides reversed phase characteristics similar to those found for PGC-HPLC. Porous glassy carbon has selectivity characteristics previously unavailable in supercritical fluid chromatography. Use of porous glassy carbon in supercritical fluid chromatography may provide distinct advantages in difficult analytical separations, allowing separations of molecules with only slight structural differences.     

Comparison of ultra high performance supercritical fluid chromatography,ultra high performance liquid chromatography,and gas chromatography for the separation of synthetic cathinones

A comparison of ultra high performance supercritical fluid chromatography, ultra high performance liquid chromatography, and gas chromatography for the separation of synthetic cathinones has been conducted. Nine different mixtures of bath salts were analyzed in this study. The three different chromatographic techniques were examined using a general set of controlled synthetic cathinones as well as a variety of other synthetic cathinones that exist as positional isomers. Overall 35 different synthetic cathinones were analyzed. A variety of column types and chromatographic modes were examined for developing each separation. For the ultra high performance supercritical fluid chromatography separations, analyses were performed using a series of Torus and Trefoil columns with either ammonium formate or ammonium hydroxide as additives, and methanol, ethanol or isopropanol organic solvents as modifiers. Ultra high performance liquid chromatographic separations were performed in both reversed phase and hydrophilic interaction chromatographic modes using SPP C18 and SPP HILIC columns. Gas chromatography separations were performed using an Elite‐5MS capillary column. The orthogonality of ultra high performance supercritical fluid chromatography, ultra high performance liquid chromatography, and gas chromatography was examined using principal component analysis. For the best overall separation of synthetic cathinones, the use of ultra high performance supercritical fluid chromatography in combination with gas chromatography is recommended.     

Monocrystalline Quartz ICP Etching: Road to High-Temperature Dry Etching

Plasma Chemistry and Plasma Processing - In this study, the dry plasma-chemical etching process of monocrystalline SiO2 (quartz) in a fluoride-based (sulfur hexafluoride, SF6) inductively coupled...     

Enantioselective separations by packed column subcritical and supercritical fluid chromatography

Enantioselective separations have been one of the most successful applications of supercritical fluid chromatography (SFC). Although analytical scale separations have dominated the literature, the use of SFC for preparative chiral separations is growing. Both analytical and preparative scale SFC separations seek to take advantage of the high efficiency, high throughput, and rapid method development associated with the technique. This review will cover recent developments in the application of SFC to enantioseparations.Contribution of the National Institute of Standards and Technology. Not subject to copyright.     

Gas chromatographic analysis of trace gas impurities in tungsten hexafluoride

Highly reactive fluorinated gaseous matrices require special equipment and techniques for the gas chromatographic analysis of trace impurities in these gases. The impurities that were analysed at the low-microg/l levels included oxygen, nitrogen, carbon dioxide, carbon monoxide, sulfur hexafluoride and hydrogen. This paper describes the use of a system utilising backflush column switching to protect the columns and detectors in the analysis of trace gas impurities in tungsten hexafluoride. Two separate channels were used for the analysis of H2, O2, N2, CO, CO2 and SF6 impurities with pulsed discharge helium ionisation detection.     

Enantiomeric resolution of five chiral pesticides on a Chiralpak IB-H column by SFC

The enantiomeric separations of five chiral pesticides, diclofopmethyl, 1; benalaxy, 2; acetofenate, 3; myclobutanil, 4; and difenoconazole, 5, were conducted on a Chiralpak IB-H column by a packed-column supercritical fluid chromatography (p-SFC). All compounds, except difenoconazole and myclobutanil, were well resolved within 10 min. As the mobile phase polarity decreased through changing the percentage and the type of alcohol modifiers in the supercritical carbon dioxide (CO(2)), the retention time, the separation factors, and the resolution increased. However, based on the retention time and the resolution, the optimized separations were obtained with the mobile phase containing 10% 2-propanol for diclofop-methyl 1; benalaxy, 2; myclobutanil, 4; difenoconazole, 5; and containing 3% 2-propanol for acetofenate, 3. The optimized separation temperature was at 35°C under the supercritical fluid condition. The π-π interactions and the hydrogen bonding interactions between Chiralpak IB-H CSP and the analytes might be the main chiral discriminations on enantioseparation of these five pesticides.     

Sulfur hexafluoride's liquid-vapor coexistence curve, interfacial properties, and diffusion coefficients as predicted by a simple rigid model

We present here molecular-dynamics simulation results of the vapor-liquid coexistence curve, surface tension, and self-diffusion coefficients of sulfur hexafluoride. Sulfur hexafluoride is modeled as a rigid molecule, following the model proposed by Pawley [Mol. Phys. 43, 1321 (1981)]. Vapor-liquid coexistence curve and surface tension are obtained through direct molecular-dynamic simulations in the NVT ensemble. Simulation results are able to reproduce the qualitative shape of the vapor-liquid envelope. However, lower densities, a higher critical temperature, and an overestimated surface tension are obtained here. Those deviations are explained on the basis of the rigidity of the molecular model used. Self-diffusion coefficients are calculated from simulations in the NVE ensemble for different gas states at atmospheric pressure. The rigid model performs better for dynamical properties since simulation results provide very good agreement with available experimental data in this case.     

Photochemistry of 1,3-Dioxolane Radical Cations in Sulfur Hexafluoride and Freonic Matrices at 77 K

The elementary stages and efficiency of the photochemical reactions of 1,3-dioxolane radical cations in various low-temperature matrices (sulfur hexafluoride, Freon-11, Freon-113) were determined. The matrix effects in the photochemical processes were observed experimentally. Possible reasons for these effects are discussed.     

Supercritical fluid chromatography and some of its applications: A review

The general topic of supercritical fluid chromatography (SFC) is introduced, and historical aspects of its development are discussed. The physical properties of supercritical fluids, gases and liquids are tabulated. SFC is compared and contrasted with the classical forms of chromatography - gas chromatography (GC) and high performance liquid chromatography (HPLC). The selectivity of SFC, GC, and HPLC are discussed and compared. Instrumentation employed for supercritical fluid chromatography is depicted. A wide variety of SFC applications are introduced. New examples of the use of SFC for analysis of a variety of complex oligomeric mixtures indcluding polypropylene glycol, polysiloxanes, fluorocarbon oligomers (i.e. -3M's fluoro-chemical surfactant Fluorad 171, and Kel-F) and high molecular weight normal alcohols are shown. The use of SFC for separation of mono-, di-, and triglycerides at low operating temperatures is described. Lastly, the use of SFC for separations of complex hydrocarbon mixtures from liquid fuels, polycyclic aromatic hydrocarbons, synthetic alpha-olefins, and petroleum functional group separations are depicted.     

Purification of krypton-xenon mixture from fluorine-containing gases

Active alumina was used to purify krypton-xenon mixtures from fluorine-containing gases (tetrafluoromethane and sulfur hexafluoride). At 580°C, the admixtures are converted into aluminum trifluoride, with their content in the test gas mixture reducing from hundreds of ppm to 0.1 ppm or even below.     

Analysis of commercial dyes by capillary column supercritical fluid chromatography

High resolution separations of selected commercial azo, aniline, and anthraquinone dyes by capillary column supercritical fluid chromatography (SFC) are demonstrated. Supercritical n-pentane was used as a mobile phase and provided efficient separations of multi-functional, polar disperse dyes with molecular weights up to approximately 700 daltons.