A screening strategy for the development of enantiomeric separation methods in capillary electrophoresis

Method development of enantiomeric separations in capillary electrophoresis (CE) is a time-consuming task, since finding the appropriate chiral selector is usually a "trial and error" process. It is impossible to predict the selectivity of a selector towards a certain enantiomer. Therefore, the affinity of all selectors has to be examined one at a time. In order to speed up this process, a strategy is proposed based on simple experimental design methodology. The approach includes first a screening in function of the pH to determine the optimal migration conditions followed by a selection of the right chiral selector by means of Taguchi designs. In the approach several variables, such as the type and concentration of cyclodextrin, the concentration of buffer electrolyte, and the percentage of organic modifier, are varied simultaneously to find initial separation conditions rapidly. The resulting initial separation conditions can be optimized in further steps to be more reproducible. We discuss the results of the approach when applied on a number of selected compounds that are recently in development at Johnson & Johnson--Pharmaceutical Research and Development. Parameters, such as quality of the separation and analysis time, are evaluated to determine initial separation conditions for each compound.     

Branching ratio of the C2H2 + O reaction at 290 K from kinetic modelling of relative methylene concentration versus time profiles in C2H2/O/H systems

In earlier work on the room temperature oxidation of C₂H₂ by O atoms, two distinct sources of methylene radicals have been identified: (i) direct, primary production via channel 1b of the C₂H₂ + O reaction, and (ii) delayed formation via the secondary reaction 3 involving the products HCCO and H of the other primary channel 1a: Presently, it was confirmed by a detailed sensitivity analysis that the precise shapes of the resulting total methylene concentration-versus-time profiles in C₂H₂/O systems depend strongly on the k_1a/k_1b branching ratio. Along that line, the important parameter k_1a/k_1b was determined from relative CH₂ concentration-versus-time profiles measured in a variety of C₂H₂/O/H systems using Discharge Flow-Molecular Beam sampling Mass Spectrometry techniques (DF-MBMS). The data analysis was carried out by deductive kinetic modelling; the method, as applied to profile shapes, is discussed at length. Via this novel, independent approach, the CH₂(³B₁) yield of the two-channel C₂H₂ + O reaction was determined to be k_1b/k₁ = 0.17 ± 0.08. The indicated 2σ error includes possible systematic errors due to uncertainties in the rate constants of other reactions that influence the shapes of the CH₂ profiles. The present result, which translates to an HCCO yield k_1a/k₁ = 0.83 ± 0.08, is in excellent agreement with other recent determinations. The above mechanism, with the subsequent reactions that it initiates, also reproduces the measured absolute [C₂H₂], [O], and [H] profiles with an average accuracy of 5%, thus validating the consistency of the C₂H₂/O/H reaction model put forward here. © 1994 John Wiley & Sons, Inc.     

Mechanism of Thermal Toluene Autoxidation

Aerobic oxidation of toluene (PhCH₃) is investigated by complementary experimental and theoretical methodologies. Whereas the reaction of the chain‐carrying benzylperoxyl radicals with the substrate produces predominantly benzyl hydroperoxide, benzyl alcohol and benzaldehyde originate mainly from subsequent propagation of the hydroperoxide product. Nevertheless, a significant fraction of benzaldehyde is also produced in primary PhCH₃ propagation, presumably via proton rather than hydrogen transfer. An equimolar amount of benzyl alcohol, together with benzoic acid, is additionally produced in the tertiary propagation of PhCHO with benzylperoxyl radicals. The “hot” oxy radicals generated in this step can also abstract aromatic hydrogen atoms from PhCH₃, and this results in production of cresols, known inhibitors of radical‐chain reactions. The very fast benzyl peroxyl‐initiated co‐oxidation of benzyl alcohol generates HO₂^. radicals, along with benzaldehyde. This reaction also causes a decrease in the overall oxidation rate, due to the fast chain‐terminating reaction of HO₂^. with the benzylperoxyl radicals, which causes a loss of chain carriers. Moreover, due to the fast equilibrium PhCH₂OOH+HO₂^.?PhCH₂OO^.+H₂O₂, and the much lower reactivity of H₂O₂ compared to PhCH₂OOH, the fast co‐oxidation of the alcohol means that HO₂^. gradually takes over the role of benzylperoxyl as principal chain carrier. This drastically changes the autoxidation mechanism and, among other things, causes a sharp decrease in the hydroperoxide yield.