Five commercial silicas [Nova‐Pak, μPorasil (Waters, Milford, MA), Kromasil (Eka Nobel, Bohus, Sweden) Spherisorb (Sigma, St. Louis, MO) and Select‐KP (Biotage, Charlottesville, VA)] were tested for their efficiency in separating vitamin E and γ‐oryzanol components from saponified rice bran extracts, with the perspective of future large‐scale purification of the various compounds. The effects of mobile phase composition, eluent flow rate, column temperature and the physical properties of the silica on the selectivity and analysis time were studied. All silicas except Select‐KP gave baseline separation of the vitamin E components with isooctane/ethyl acetate (97.5∶2.5) at room temperature. However, only the 4‐μm Nova‐Pak spherical silica gave adequate resolution of the oryzanols into two distinct fractions with an analysis time ofcirca 20 min. Other analytical conditions, i.e., the content of the polar modifier in the mobile phase, temperature and flow rate, can be used to optimize the separation once the silica had been chosen. Speed of separation is enhanced by increasing the polarity of the eluent, temperature and flow rate, while the selectivity is reduced only slightly. The Nova‐Pak silica appears resistant to deactivation from both residual water in the mobile phase and polar components left over after sample clean‐up and is suitable for preparative separation of the target compounds.
Heat capacity measurements were performed on chromatographically purified C,, from 120 to 560 K by DSC. The experimental values were linked to the vibrational heat capacity, calculated based on normalmode vibration frequencies available from the literature. Good agreement is found, as prior for C6w Major orientational disorder is introduced in two closely spaced transitions, reflecting the change to the phase that displays anisotropic rotation of the rugby-ball shaped molecules. The total entropy of the disordering transitions is 22.4 J/(Kmol), slightly lower than that of C,,, but additional entropy is gradually acquired between 120K and the transition temperature, making the total entropy gain higher than that of C,,, as expected for a less symmetric molecule. The Debye temperature for the six lattice vibrations of C,, was calculated to be 45 K, compared to 53 K for C6w No melting temperature could be detected for C,, up to 950 K, but the entropy of fusion is estimated to be 12 J/(K mol). Enthalpies, Gibbs energies, and entropies have been calculated from 0 to lOOOK
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