High-pressure phase studies on fluid mixtures of low-volatile organic substances with supercritical carbon dioxide

Two high-pressure phase equilibrium cells working according to the analytical method in the temperature range 300 to 450 K and up to 200 MPa have been developed. In the first cell p,T,x,y equilibrium data can be determined by sampling and GC analysis. With this cell the gas-liquid phase behaviour of the ternary systems carbon dioxide + tridecane + hexadecane and carbon dioxide + tridecane + 1-hexadecanol were determined at 423.4 K and between 8 and 35 MPa. For the second apparatus a spectroscopic high-pressure cell fitted with two pairs of sapphire windows was developed. Molar absorptivities are obtained from calibration runs in the homogeneous region. Gasliquid phase equilibria in the system decane + CO₂ were investigated from 342.8 to 400.2 K and between 10 and 20 MPa; here absorption data were measured in the NIR region.     

Using supercritical fluid chromatography to determine diffusion coefficients of 1,2-diethylbenzene, 1,4-diethylbenzene, 5-tert-butyl-m-xylene and phenylacetylene in supercritical carbon dioxide

The binary diffusion of 1,2-diethylbenzene, 1,4-diethylbenzene, 5-tert-butyl-m-xylene and phenylacetylene at infinite dilution in supercritical carbon dioxide were measured between 15.0 and 35.0 MPa and in the temperature range of 313.16 to 333.16K by the Taylor-Aris chromatographic method. The effect of temperature, pressure, viscosity and density was discussed. In the case of temperature dependence, additional measurements were done for 5-tert-butyl-m-xylene from 308.16 to 398.16K at 35.0 MPa. The measured diffusivities of the four solutes were compared with the calculated ones by several predictive formulas.     

Rapid and high resolution capillary supercritical fluid chromatography (SFC) and SFC/MS of trichothecene mycotoxins

The potential application of capillary column supercritical fluid chromatography (SFC) and SFC/mass spectrometry (SFC/MS) for the separation and analysis of mycotoxins of the trichothecene group was examined. Trichothecenes present significant analytical problems for both gas and liquid chromatography with a major difficulty for the latter being the lack of sufficiently sensitive and selective detectors. Supercritical carbon dioxide mobile phases at temperatures up to 100 degrees C were used with deactivated fused silica columns coated with crosslinked stationary phases. Separations were obtained under pressure ramped conditions using long (15 m) 50-micron i.d. columns for several trichothecenes (diacetoxyscirpenol, deoxynivalenol, and T-2 toxin) and related higher molecular weight macrocyclic (roridin and verrucarin) trichothecenes. In addition, new rapid pressure programming techniques with short (less than 2m) 25- to 50-micron i.d. capillary columns were used to obtain fast separations in as little as 1 min. SFC/MS with ammonia chemical ionization provided high selectivity and sensitive detection (with approximately 1-pg detection limits) for trichothecene mixtures. The extension to complex sample matrices is discussed and the application of selective MS/MS detection is demonstrated.     

An attempt to extend ordinary capillary gas chromatography to supercritical fluid chromatography (SFC)

Since capillary columns with well immobilized stationary phases are expected to withstand contact with supercritical fluids, we wished to study their amenability to SFC. Simultaneously, we wished to learn how far SFC can be accomplished with the ordianary tools of capillary GC. The study demonstrates that truly supercritical, not just relatively high, pressure is required to ensure the typical effects of SFC. Results obtained with sub-and supercritical pressure are compared and discussed. A comprehenshive study of the parameters permitting SFC with capillary GC equipment showed a clear preference for CO₂ as a carrier, FID detection, and oncolumn sampling. While no additional equipment is required, a critical feature is the flow restrictor to be mounted on the end of the column. The production and properties of this restrictor are discussed in detail. It is reasonable to hope that SFC with 0.1 mm id capillary columns can be realized in the pressure range of 100–150 bar, where substances which cannot be eluted from a capillary colum under GC donditions are expected to be analyzed.     

Correlation of diffusion coefficients for naphthalene and dimethylnaphthalene isomers in supercritical carbon dioxide

An equation proposed by Darken, including the thermodynamic factor and tracer diffusion coefficients of solvent and solute, was adopted to correlate the diffusion coefficients for naphthalene and dimethylnaphthalene (DMN) isomers in supercritical carbon dioxide and the correlated results were compared with the experimental data. IML equation of state with mixing rules and combining rules containing two adjustable interaction parameters were used for calculation of the thermodynamic factor. By using the interaction parameters adjusted to the solubility data, the concentration dependence of diffusion coefficients and their anomaly near the critical point of carbon dioxide can be quantitatively represented. In order to improve the reliability of experimental results, some re-measured diffusion coefficient data for naphthalene, 2,6- and 2,7-DMN at 308.2 K, and new data for naphthalene at 318.2 K and for 2,3-DMN at 308.2 K are presented.     

Direct coupling of capillary supercritical fluid chromatography with supercritical fluid extraction using modified carbon dioxide

Capillary supercritical fluid chromatography has been directly coupled with supercritical fluid extraction using modified carbon dioxide. The mixed fluids were prepared with a single pump on-line mixing system. The most important step in the SFE-SFC interface was the elimination of the modifier solvent. This was achieved by use of a coupled trap, 0.1 mm i.d. and 0.53 mm i.d. capillary tubing connected in series, with the collected solutes refocused on the second (0.53 mm i.d.) trap before transfer into the separation column. This enabled complete elimination of various modifier solvents and high efficiency collection of the solutes. The effect of the modifier on trapping efficiency was investigated using methanol, ethanol, dichloromethane, hexane, and toluene at a variety of concentrations. n-Eicosane was, for example, trapped quantitatively by modified carbon dioxide containing up to 13 % (w/w) methanol. The use of the technique has been demonstrated by selective extraction of n-paraffins, fatty acid methyl esters, and alcohols from a silica matrix; the effect of different modifiers on the extraction of a mixture of pesticides from soil has also been investigated.     

Resolution studies on counter-current chromatography using supercritical fluid carbon dioxide

Counter-current chromatography (CCC) is a form of liquid–liquid partition chromatography. It requires two immiscible solvent phases; the stationary phase is retained in the separation column, generally by centrifugal force, while the mobile phase is eluted. We recently replaced the mobile phase with supercritical fluid carbon dioxide (SF CO₂). Since the solvent strength of SF CO₂ can be varied by changing the temperature and pressure of the system, separation adjustments are thus more versatile. We investigated the pressure and temperature effects on resolution using water and low-carbon alcohol mixtures as the stationary phases. It was demonstrated that these special properties of SF CO₂ were indeed beneficial to the optimization of separations. In addition, the phase retention ratio was examined in terms of separation resolution. The results appeared very similar to those obtained from conventional traditional CCC. This study should be helpful for the future development of SF applications in CCC.     

Solubility of chelating agents and metal-containing compounds in supercritical fluid carbon dioxide

The solubility of 49 metal-containing compounds and 15 "free" ligands in supercritical fluid carbon dioxide is reviewed. Solubilities were found to range over eight orders of magnitude, with the highest value of solubility being 56 g l(-1). Metals complexed with fluorine substituted ligands were found to be the most soluble, and metals complexed with phenyl-substituted ligands the least soluble. A general trend for increasing solubility with increasing oxidation state is observed and this is correlated with the increased number of coordinating ligands protecting the metal center from interaction with the supercritical fluid and also due to the increased number of solvation interactions with the increased number of ligand groups.     

Dissolution of uranium dioxide in supercritical fluid carbon dioxide

Uranium dioxide can be dissolved in supercritical CO2 with a CO2-philic TBP-HNO3 complexant to form a highly soluble UO2(NO3)(2).2TBP complex; this new method of dissolving UO2 that requires no water or organic solvent may have important applications for reprocessing of spent nuclear fuels and for treatment of nuclear wastes.     

Anomalous effects of helium head pressure carbon dioxide in supercritical fluid chromatography and extraction

Helium head pressure carbon dioxide cylinders are commonly used to facilitate the delivery of liquid CO2 to supercritical fluid extraction and chromatographic pumps. It is usually tacitly assumed that the helium used to increase the delivery pressure of the CO2 cylinders is completely insoluble in liquid CO2 and thus remains isolated in the head space of the delivery cylinder. This assumption is invalid because up to 5 mol% helium can be entrained in the liquid CO2 delivered from helium head pressure cylinders. Significantly, contamination of liquid CO2 with even small amounts of helium can cause many unforeseen and usually deleterious effects in supercritical fluid chromatography and extraction schemes. The observed anomalies include decreased density of the fluid phase, irreproducible extraction and retention, ghost peaks, and even phase separation within the column or extraction vessel.     

Diffusion coefficients of metal acetylacetonates in supercritical carbon dioxide

Binary diffusion coefficients, D₁₂, of the metal acetylacetonates, palladium(II) acetylacetonate and cobalt(III) acetylacetonate, were measured from 308.2 to 343.2 K over the pressure range from 9 to 40 MPa at infinite dilution in supercritical carbon dioxide using the chromatographic impulse response method. The effects of pressure, temperature, density, and viscosity on D₁₂ values were examined. It was observed that the D₁₂ values of palladium(II) acetylacetonate and cobalt(III) acetylacetonate were larger than those of lipids with similar molecular weights, such as arachidonic acid and monoolein, respectively. Furthermore, the measured D₁₂ data of each metal acetylacetonate were well correlated by the hydrodynamic equation D₁₂/T as a function of carbon dioxide viscosity.     

Planarity selectivity in supercritical fluid and liquid chromatography using octadecylsilane-modified silicas with carbon dioxide

Summary The influence of column temperature and pressure on the planarity selectivity of encapsulated and polymeric octadecylsilane-modified silicas was examined using carbon dioxide mobile phase in supercritical fluid and liquid chromatography. The use of liquid carbon dioxide was found to enhance remarkably the molecular planarity recognition capability of the polymeric stationary phase compared with supercritical conditions. The influence of pressure and temperature on selectivity was seen to be significant with the polymeric phase but less with the encapsulated. It seems that pressure and temperature change the morphology of the polymeric phase to a greater extent than the encapsulated one.     

Construction of a new interface for comprehensive supercritical fluid chromatography x reversed phase liquid chromatography (SFC x RPLC)

The design of a new interface to hyphen high efficiency supercritical fluid chromatography (SFC) and fast RPLC in a comprehensive configuration is described. SFC x RPLC is a viable alternative to normal phase (NP) LC x RPLC and is characterized by high orthogonality. Compared to NPLC x RPLC an additional advantage is the expansion of supercritical carbon dioxide (CO(2)) when exposed to atmospheric pressure leading to fractions consisting of solvents that are miscible with the second dimension RPLC mobile phase. The interface consists of a two-position/ten-port switching valve equipped with two packed octadecyl silica (C(18)) loops for effective trapping and focusing of the analytes after elution from the SFC dimension. The addition of a water make-up flow to the SFC effluent prior to entering the loops is of fundamental importance to efficiently focus the solutes on the C(18) material and to reduce interferences of expanded CO(2) gas on the second dimension separation. The features of the system are illustrated with the analysis of a lemon oil sample.     

Selectivity changes towards steroids in packed column supercritical fluid chromatography (SFC) induced by temperature and pressure variation

Summary Pressure/temperature variation in SFC was shown to influence selectivity towards steroids. Major changes were observed for polar column packings compared to non-polar packings and variation of the amount of modifier showed that this is valid over the whole investigated range. The effect is underlined by a fall in resolution in the low density range (low pressure/high temperature) for polar stationary phases and appears to be significantly larger than for non-polar phases. Major selectivity shifts induced by variation of the physical properties of the mobile phase are considered to be due to the greater effect of polar modifiers on the nature of polar stationary phases than on non-polar phases.     

Polyacrylate liquid crystalline stationary phases in supercritical fluid chromatography with carbon dioxide mobile phase

Summary New polyacrylate liquid crystalline compounds were coated onto glass or fused-silica capillary columns as stationary phases and applied to supercritical fluid chromatography. These stationary phases, were very stable: no bleeding was observed at 200°C and up to 200kg/cm² pressures of carbon dioxide mobile phase. The wide working range of the capillary column was extended below the g-n transition temperature. Isomeric compounds such as - and -methoxynaphthalene, anthracene and phenanthrene and several phenolic compounds were separated.