Thermodynamics of an odd chiral separation In chiral chromatography, both chiral and nonchiral retention mechanisms contribute to the separation of enantiomers. Georges Guiochon and colleagues at the University of Tennessee-Knoxville and Oak Ridge National Laboratory studied the retention of propranolol enantiomers on immobilized cellobiohydrolase I (CBH I) and illustrated that the chiral and nonchiral interactions must be considered separately to obtain correct thermodynamic values. Propranolol exhibits unusual temperature dependence in enantiomer separations. At pH 5.5, (S)-propranolol was retained on the CBH I stationary phase for longertimesas the temperature increased from 5 °C to 45 °C, but at 55 °C the trend reversed itself and the retention time was less than that at 35 °C. In contrast, ,he retentiontimeof the (R) enantiomer grew progressively shorter as the temperature
Identifying hemoglobin variants
increased, following "normal" retention behavior. The thermodynamic functions could not be correctly derived directly from the van't Hoff plots of retention factors.
Instead, the authors separated the chiral and nonchiral contributions to the retention and modeled them using bi-Langmuir isotherms. One of the Langmuir contributions is the same for both enantiomers and accounts for the nonspecific interactions with the stationary phase. For chiral sites on the stationary phase that selectively interact with the enantiomers the interaction enthalpy and entropy are -1.9 kcal/mol and-2 6cal/(molK) for (©-propranolol and +1 6 kcal/mol and +11 6 cal/(molK) for (S)-Dronranolol at DH 5 5 These results demonstrate why the two enantiomers behave differently at this Elation profiles at different temperatures of the R (shorter r>H (I Am Chem Sor retention times) andS enantiomers of propranolol. Note that the 1QQ7 jiq 1?S4-fi41 S retention times increase at higher temperatures until 55 °C.
using on-line capillary electrophoresis with electrospray ionization and mass More than 700 known structural variants spectrometric detection to analyze variants in the p Hb chain. They report rapid of hemoglobin (Hb) exist, and many inand sensitive identifications, detecting volve single amino acid substitutions in electrophoretic peaks from CE as narrow the a or p chains. Some of these variaas 1 s full width at half-maximum. To detions cause severe diseases and thus are tect these fast-moving peaks, the rean important area of research and clinisearchers used an ion trap storage/refleccal practice. Conventional methods for tron time-of-flight mass spectrometer cadetecting Hb variants rely on electropable of acquiring a single mass spectrum phoresis and isoelectric focusing, which at a speed of > 10 Hz. is typically labor intensive and time consuming. In this study, the a and p chains were separated, and the p chains were then diDavid M. Lubman and co-workers at gested by trypsin and the resulting mixture the University of Michigan investigated separated on aminecoated fused-silica capillaries. A comparison of the total ion electropherograms and mass spectra detects the mutation site. In addition collisioninduced dissociation in the vacuum interface-skimmer region can pinpoint the identity of the site (Rabid Commun Mass St>ectrom 1997 11 The total ion electropherogram of a digest of Hb. (Adapted with 99-108) permission from John Wiley & Sons.)
Pressure drop effects in packedcapillary SFC It is well documented that pressure drops across chromatographic columns have a measurable effect on retention and an inconsequential effect on efficiency and resolution. The situation for packed-column supercritical fluid chromatography is not so cut and dried. Researchers studying conventional and microbore packed columns in SFC have differed on whether the effects are significant, and until now none of the SFC studies have looked at the effects of pressure drops on packed-capillary SFC. Milton L. Lee and colleagues at Brigham Young University reported the relationship between column pressure drop and column parameters such as length, diameter, and particle size. They also determined the effect of pressure drop on column efficiency and resolution. As would be expected, the column pressure drop increased with increasing column length and decreasing particle size. Resolution was affected by the pressure drop, but efficiency losses could not be correlated with pressure drop. (J- Ckromatosr. A 1997 758,117-23)
Analytical Chemistry News & Features, April 1, 1997 225 A
