Effect of mobile phase acidic additives on enantioselectivity for phenylalanine analogs

The use of acidic mobile phase additives allows the chiral separation of underivatized phenylalanine analogs on a common amylosic column. In addition to decreasing retention and band-broadening arising from non-ideal interactions, acidic additives may also increase selectivity. This appears to be due to the minimization of non-selective binding in the recognition site. Effects of the additives are related to additive pKa and size. Ethanesulfonic acid was typically the most effective additive with trifluoroacetic acid being one of the least effective.     

Memory effect of mobile phase additives in chiral separations on a Chiralpak AD column

Using chiral probes shown to be sensitive to the presence of mobile phase additives, a memory effect for these additives by an amylosic column was demonstrated. Exposure to these additives gave prolonged chromatographic performance changes even after their removal from the mobile phase. This finding is consistent with strong binding of the additives to the stationary phase. A procedure to remove bound additives was developed.     

Electrophilic additions involving fluoronium ions. I. Fluoroxy additions to perfluoro-olefinic bonds

The fluoroxy addition of perfluoro-t-butyl hypofluorite to hexafluoro-propene leads predominantly (> 95%) to perfluoro-n-propyl perfluoro-t-butyl ether. The unique structural orientation of this perfluoro-n-propyl product is of interest because the products of fluoride ion addition reactions have previously been reported as perfluoroisopropyl derivatives. The results obtained in this study provide a simple experimental demonstration of electrophilic attack by the O-F group, leading to the addition of the fluoronium ion to the more nucleophilic unsaturated carbon atom.The same mechanism as for a perfluoro-α-olefin applies in fluoroxy addition to a perfluorovinyl ether. The perfluoro-t-butoxy group also adds predominantly (> 95%) to the terminal unsaturated carbon atom.     

Some new perfluoroethers and copolymers from ‘Dewar’ hexafluorobenzene

The thermal and photolytic reactions of hexafluorobicyclo[2.2.0]hexa-2,5-diene and bis(fluoroxy)difluoromethane yielded four-membered fused-ring-containing perfluoroethers and copolymers. The reaction products were analyzed by gas chromatography, gas chromatography-infrared, gas chromatography-mass spectra, field ionization mass spectra and ¹⁹FNMR. The elucidation of the polymer microstructures was assisted by the ¹⁹FNMR spectroscopy of the related compounds, hexafluorobicyclo[2.2.0]-hexanes and -hexene in the volatile fraction.An increase of the concentration of ‘Dewe’ C₆F₆ in hexafluorobenzene for copolymerization with CF₂(OF)₂ decreased the six-membered ring moieties. At about 90% level of the para-bonded C₆F₆, no six-membered ring-containing perfluoroethers were found in the volatile fraction and only a low concentration of the six-membered ring moieties were in the polymeric fractions. The molecular weights of the copolymers ranged from 1000 to over 10,000. The microstructures of the thermal and photopolymers about 1000 range were suggested.     

Empirical relationship between chiral selectivity and mobile phase modifier properties.

An empirical relationship was derived which relates properties of the mobile phase modifier to the chiral selectivity factor for a given analyte/chiral selector combination. Using carbon dioxide and heptane-based mobile phases, the effect of various mobile phase modifiers on Pirkle-type stationary phases may be accurately modeled using a two-parameter equation. Similar results are obtained using cellulosic stationary phases with carbon dioxide-based mobile phases. Modeling separations performed using heptane-based mobile phases with cellulosic stationary phases were not successful. The predictive ability of this modeling approach was demonstrated using novel modifiers and chiral analytes.     

Effect of mobile phase amine additives on enantioselectivity for phenylalanine analogs

Curve fitting seems to be one of the best methods for the evaluation of chromatographic signals. As it is known, in this case mathematical function is fitted to digitized measured points. The most important task is to find the best mathematical function, which corresponds perfectly to the peak shape, and then to determine the parameters of the equation using a computerized least-squares method of approximation. In this work, a new mathematical function was sought for with the purpose of describing different chromatographic signals and it was fitted to the digitized measured points. The fitted curve is suitable for a quick evaluation of chromatographic information, noise filtering and correction of baseline drift. The fitting of gas chromatographic and high-performance liquid chromatographic signals were completed. The mathematical function, the generated chromatographic curves, the application of the function for describing real signals and the fitting process will be demonstrated in this study.