Jörgen Vessman

Chiral separations of metoprolol and some analogs with carbon dioxide on Chiralcel OD and Chiralpak AD stationary phases. Use of chemometrics

SummaryThe influence of methanol concentration, column temperature, and column back-pressure on the enantios-electivity of the separation of eighteen amino alcohols in supercritical-fluid chromatography has been investigated by use of statistical experimental design. The enantioselective retention of the amino alcohols was studied using Chiralcel OD and Chiralpak AD as the chiral stationary phases and the experimental responses obtained—retention factors (k) and selectivity factors (α)—were evaluated by use of the partial least squares algorithm. The performance of the columns was compared and the enantioselectivity of the Chiralcel OD column was found to be superior. Almost all the racemic amino alcohols tested were separated and separation factors as high as 4.5 were obtained by use of the Chiralcel OD column.Experimental results from a factorial design with three centerpoints resulted in a statistical model based on linear terms only. The first-eluted enantiomers, which, when identified, were found to be theR forms, had similar retention times, whereas the retention times of theS forms were more varied. The column temperature had a greater effect on enantioselectivity than methanol content. Changes in the system back-pressure had no significant influence on enantioselectivity.The results obtained from the factorial design were used to predictk and α. Differences between predicted and experimental data were less than 10%.The effect on enantioselectivity of protolytic mobile phase additives, e. g. dimethyloctylamine, acetic acid, and trifluoroacetic acid, and of mobile-phase flow-rate, were also studied, as was the effect of solute structure. The position of substituents on the aromatic ring, type of alkyl group attached to the nitrogen atom, and the number of methylene groups between the stereogenic center and the nitrogen atom all affected enantioselectivity.The chromatographic system developed could be used to determine enantiomeric purity even if the chiral impurity eluted after the main peak.

Packed-column supercritical fluid chromatography for the purity analysis of clevidipine, a new dihydropyridine drug.

In this paper we describe a packed column supercritical fluid chromatography method that can be used for the analysis of a new dihydropyridine substance. The method is based on methanol-modified carbon dioxide as the mobile phase and Hypersil bare silica as column support at a column temperature of 50 degrees C and 150 bar as back pressure. Using an adjusted methanol gradient the most likely by-products can be separated and detected (240 nm) within 13 min. Occasionally the column needed treatment with 4 mM citric acid in the methanol modifier in order to give a narrow peak of an acidic analogue. The present method can detect analogues at the 0.1% (w/w) level. The precision at this level for one of the analogues was 5.9% RSD. This method shows a higher selectivity than a corresponding reversed-phase liquid chromatographic method.

The use of selectivity in analytical chemistry – some considerations

Selectivity is a central term in analytical chemistry that describes whether the analyte can be measured without interferences. It is, however, often mixed up with specificity. In this review the situation in analytical chemistry is described with representative examples as well as pertinent discussions on the impact of selectivity. In order to generate sufficient selectivity, different interactions have often to be combined either in hyphenated separation methods, in sensors or with multivariate statistical analysis (computational selectivity) of the responses. Reference is also made to the influence from other parts of chemistry as well as how the term is dealt with in scientific journals, in textbooks and among authorities and scientific organizations. Finally recommendation is given to promote the use of selectivity.

Preparative resolution of drug racemates to study the chiroptical properties of their enantiomers

The present work is focused on the resolution of ten racemates, in order to study their chiroptical properties and to test the validity of the requirement specified in the European Pharmacopeia (EP) for demonstrating that a drug entity is a racemate. This work shows that the optical purity of enantiomers and non racemic mixtures of a number of compounds can be determined more accurately by circular dichroic (CD) spectroscopy than by a measurement of the angle of rotation (AoR), the EP requirement. Using only the AoR, some of the racemates could not be distinguished from the enantiomers. CD spectroscopy or chiral chromatography should, therefore, be the technique of choice in the determination of optical purity of a chiral compound, especially for those exhibiting low AoR.

Direct separation of captopril diastereoisomers including their rotational isomers by RP-LC using a teicoplanin column.

A direct reversed-phase liquid chromatography (LC) method has been developed for the separation and analysis of captopril and its 2R,2S diastereoisomer using a teicoplanin stationary phase. The proline containing diastereoisomers, which are known to form conformers in aqueous solution, were also separated from their rotational isomers. The influence of temperature, different organic modifiers and buffer type, concentration and pH were optimised to obtain a working resolution between the two diastereoisomers and their respective rotational isomers. The diastereoisomeric purity of several commercial captopril batches was subsequently evaluated using a 0.05% triethylammonium acetate (TEAA) buffer (pH 3.8) run at 1.0 ml/min with mobile phase reservoir and column temperature controlled at 0 degrees C. Throughout the study online UV diode array and mass spectrometry detection was carried out simultaneously to confirm that peaks eluting from the teicoplanin column were in fact captopril and not its readily converted disulphide dimer. Additionally, as a result of the greater detection sensitivity of mass spectrometry, it also facilitated a more accurate optimisation study where trace amounts of the rotational isomers were found to be present in the baseline at temperatures higher than optimum.

Direct derivatization of drugs in untreated biological samples for gas chromatographic analysis

The possibilities to derivatize an analyte directly in the biological sample are reviewed with examples from our own experiences and from the literature. Techniques, such as extractive acylation, alkylation and benzoylation, are frequently used. Improvement of the extractability of the drug from the matrix is a common feature, especially with hydrophilic compounds, where sometimes cyclizing reactions can be employed. Several analytes are reactive or labile in the sample and can be trapped in derivatization reactions in situ. In many cases, two-phase reactions lead to milder derivatization conditions (e.g. dealkylation of tertiary amines), which is favourable from a clean-up point of view.