Effect of mobile phase composition on retention factor in supercritical fluid chromatography

　In supercritical fluid chromatography with a mixture of carbon dioxide and a modifier as the mobile phase, the relationship between retention factor k and modifier mole fraction x was derived as 1/kmix=xCO2/k0CO2 + xmod/k0mod, where kmix, k0CO2, and k0mod are the retention factors for the mixture solvent, pure CO2 and pure modifier, respectively, and xCO2 and xmod are the mole fractions of CO2 and a modifier, respectively. Since k0CO2 and k0mod are often difficult to determine due to too large of a value for k0CO2 and an invalid value of k0mod for the liquid phase, both values were estimated experimentally from the two retention factors available at the lowest and highest modifier mole fractions at each temperature and pressure. The equation was effective for the retention factors of the R- and S-forms of racemic transstilbene oxide measured in the present study by supercritical fluid chromatography using a modifier such as methanol, ethanol or acetonitrile. Moreover, the equation was also valid for the retention data of various enantioselective separations as well as achiral separations reported in the literature

Quantifying rate enhancements for acid catalysis in CO 2 -enriched high-temperature water

Thermodynamic calculations revealed that 10 to 100-fold increases in reaction rate are obtainable with added CO 2 (0.1–1 MPa) for an acid-catalyzed reaction in high-temperature liquid water (HTW) that is first order in H + concentration. These calculations suggest that CO 2 is most effective as a rate-enhancing additive in HTW at lower temperatures (150–200°C). When compared with increased temperature as a competitive option for accelerating acid-catalyzed reactions in HTW, CO 2 addition generally carries a lower pressure penalty (and no temperature penalty) for the model acid-catalyzed reaction with activation energies of up to 35 kcal/mol. An experimental survey revealed that CO 2 addition is effective for achieving increased reaction rates for dibenzyl ether hydrolysis in HTW, but that bisphenol A cleavage, methyl benzoate hydrolysis, and o -phthalic acid decarboxylation were not significantly impacted by added CO 2 . This behavior is consistent with previous results for these reactions wherein mineral acid, rather than CO 2 , was added to lower the pH. A summary of experimental results reported for reactions in CO 2 -enriched HTW revealed that product yields of some reactions can be increased by a factor of 23 with added CO 2 . Taken collectively, these results suggest that CO 2 addition may be a practical technique for making HTW more attractive as a reaction medium for acid-catalyzed organic synthesis. © 2007 American Institute of Chemical Engineers AIChE J, 2008Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/57897/1/11392_ftp.pd