Critical-mixture curves for 13 CO2-solvent binary mixtures were estimated using the peak-shape method. Mixture critical points were determined within 1 degrees C and 1 atm. The results for CO2-toluene and CO2-methanol were compared to previously reported data from high-pressure view cell studies. No more than a 3% difference was observed in the data generated by the two different techniques. A few abnormalities encountered while using the peak-shape method are also discussed.

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The data so far published on this subject are not very satisfactory [S-lo]. Not only the results but also the dimensions used for expression of enzyme activities differ considerably and are confusing. 5a-reducing enzymes are commonly evaluated by measuring the metabolic conversion rates of a labeled precursor to distinct products followed by calculation of the reaction velocities based on the sum of all conversion rates.We recommend measuring specific radioactivities of all peaks used for calculation of reaction velocities to avoid quantitative errors which may arise from dilution processes caused by endogenous hormone contents. This can occur especially in cases of patients suffering from disorders such as androgen insensitivity syndrome. Radio gas chromatography for qualitative and quantitative analysis of the incubation mixtures is a convenient way to overcome this disadvantage. Simultaneous detection of mass and radioactivity of the chromatogram peaks allows rapid calculation of specific radioactivity of all metabolites of interest. Summary Supercritical fluid chromatography (SFC) sometimes exhibits GC-like behavior and sometimes LC-like behavior, dependingonconditions. However, it is not always clear whether one of these types of behavior, or a combination, operates for a particular set of conditions for every solute in a mixture. For example, some components may be partitioned mostly by theirvapor pressures, while others, in the same mixture, are partitioned predominantly by solvent-like properties of the mobile phase.Plots of retention (as log of the capacity factor) vs. reciprocal temperature at constant pressure reveal a clear change in the character of the separation of well-behaved solutes. A thermodynamicexplanation of the observed behavior is given, based on the assumption that partitioning is controlled by the heats of solutionof solutein themobileandstationaryphases. Amodelof SFC retention as it deviates from pure-GC behavior on the same column is presented. Presented at the Sixth In tern at io n a I Sy mp osiu m on Capillary Chroma tograp hy

Fatty acid esters of sucrose (sucrose polyesters, or SPE's) were successfully separated on an open-tubular column uslng supercrltlcal carbon dloxlde as moblle phase. Room-temperature spllttlng Injection was used wlth an uncoated, fused silica Inlet tube to focus solute on the head of the column, slmllar to the retention gap method In GC. Artlfact-free detectlon was posslble by the use of a robotmade, thin-walled, tapered-caplllary restrlctor as the Interface between the column and a flame Ionization detector. The column temperature was found to have a pronounced effect on the selectlvlty and resolution of the chromatogram. Standards were analyzed to find the effect of carbon number dlstrlbutlon and of unsaturallon on retention tlmes of sucrose octaesters. Expected relative amounts of sucrose octaesters In real samples were calculated based on the random dlstrlbutlon of fatty acids from a starting materlal of known composltlon. A model chromatogram was calculated and plotted and was used to help Interpret the laboratory chromatograms. For sucrose octaesters, carbon number dlstrlbutlon leads to discernible peaks, wlth the peak wldth determined by the dlstrlbutlon of double bonds.Sucrose polyester (SPE) (1) is produced by the catalytic reaction of sucrose and long-chain fatty acid methyl esters, resulting in the esterification of up to all eight of the sucrose hydroxyls. SPE is of interest in both the food and medical science areas because, when eaten, it is neither hydrolyzed nor absorbed from the gastrointestinal tract. Thus, it represents a noncaloric source of edible fat (2). In addition, clinical studies have shown that SPE, taken orally, can lower the level of plasma cholesterol in humans (3,4).Very little work has been published on the analysis of SPE. This is closely tied to two fundamental problems: First, commerical sources of fatty acids used to prepare SPE often have four or more different components at major levels. Second, there is a range in the degree of esterification of the molecules in any SPE sample, with hexa-, hepta-, and octaesters all usually present at major levels. Thus, dozens of (empirical) compounds and thousands of isomers exist in SPE finished product. Only thin-layer chromatography has been capable of determining ester distributions (5) but is incapable of telling additional information such as carbon number distribution within a particular ester family or the extent of unsaturation. Size exclusion chromatography (SEC) has been used to determine SPE in diets and feces, but SEC elutes all SPE molecules in a single peak (5). It is possible that SEC could give, at least, limited data on the range and average degree of esterification of SPE material, but no work on this or any other liquid chromatography approach has been reported. Molecular weights are too high for gas chromatography. Thus, a battery of tests, including SEC, TLC, GC (of residuals), iodine value, and hydroxyl value, are usually run to characterize SPE.We have investigated the use of capillary supercritical-fluid chromatog...

Abstract. Open-tubular supercritical fluid chromatography (SFC) is a useful technique for many analyses, but is somewhat limited, in current practice, with respect to trace analysis of liquid samples. Direct injection of the volumes required for trace analysis with ordinary detectors creates a meter or more length of flooded zone and may lead to unacceptable band broadening. We studied solvent mass transfer on short, uncoated columns for several solvents forming Type I binary mixtures with C02, and found evidence in the solvent peak shapes of distinct liquid and vapor phases, with the liquid present as a dynamically formed film. This study suggests that selecting an injection temperature and pressure to reduce, but not eliminate, the surface tension of the liquidvapor interface, and increasing the mobile phase velocity during injection increase the liquid film thickness and reduce the length of the resulting flooded zone. When a separate uncoated inlet tube (that is, a retention gap) is used to connect the injector with the column, decreasing the inlet tube radius improves the ratio of maximum effective injection volume to inlet tube volume.Just as in GC, dynamically formed films function as pseudo-stationary phases during injection. Without a liquid film, solutes can be transported by injection-solvent-modified mobile phase well beyond the length of a typical film, increasing the difficulty of refocusing the solutes before separation. Refocusing solutes from film-coated flooded zones is easily accomplished using either a solvent effect (specifically, solvent trapping) or phase-ratio focusing.We recommend the use of solvents with phase behavior like toluene and carbon tetrachloride, both of which form films at convenient temperatures and pressures. We discourage the use of solvents with phase behavior like n-pentane which tend to make single-phase, supercritical mixtures with C02 under typical SFC injection conditions.