International collaborative study of the retention reproducibility of basic drugs in high-performance liquid chromatography on a silica column with a methanol-ammonium nitrate eluent

A study of interlaboratory influence on column evaluation

A liquid chromatography method for the characterization of base deactivated columns was investigated in a collaborative study involving six laboratories. This work was carried out on two chromatographic supports (Xterra RP 18 and Symmetry Shield). Different cooling systems, namely water bath and air oven, were tested and it was shown that column thermoregulation did not significantly influence chromatographic data. In order to control the mobile phase composition, the latter was prepared by weight rather than volume. Thanks to the injection of a set of selected neutral compounds, extra-column effects were evaluated in each of the participating laboratories. The results showed that chromatographic supports tested in different laboratories and following the same test protocol could be effectively compared.

Monitoring effective column temperature by using shape selectivity and hydrophobicity and the effects of mobile phase temperature

Relative changes in the hydrophobicity and shape selectivity of standard analytes are shown to be a valuable method to monitor the reproducibility of the effective temperature experienced by the analytes in a chromatographic column in different ovens. Significant differences were observed between ovens with the same nominal temperature and could be a major cause of problems encountered in transferring methods between instruments or laboratories. By using two parameters, changes due to the column temperature can be distinguished from those caused by mobile phase composition differences. In addition marked increases in column efficiency were noted as the mobile phase temperature was reduced below the column temperature in contrast to claims that thermal equilibration is essential.

On-line fully automated determination of clozapine and desmethylclozapine in human serum by solid-phase extraction on exchangeable cartridges and liquid chromatography using a methanol buffer mobile phase on unmodified silica

A fully automated method for determination of clozapine and desmethylclozapine in human serum using high-performance liquid chromatography was developed. On-line solid-phase extraction was performed on an exchangeable cyanopropyl cartridge. The analytes were eluted with a methanol-ammonium acetate buffer mobile phase, separated on a silica column, and measured by ultraviolet detection at 261 nm. The total time for one analysis was 13 min. Inter-day variation was < 6% and < 8% for clozapine and desmethylclozapine, respectively. Detector response was linear in the range of 30-300 ng/ml. Comparison with liquid-liquid extraction showed good agreement. The patients had clozapine serum concentrations in the range 40-1500 ng/ml. Desmethylclozapine concentrations were 25% lower and closely related.

Retention reproducibility of basic drugs in high-performance liquid chromatography on a silica column with a methanol-high-pH buffer eluent. Changes in selectivity with the age of the stationary phase

The long-term reproducibility of the separation of basic drugs by high-performance liquid chromatography using a silica column with a methanol-high-pH eluent was examined. The relative capacity factors of specific tertiary amines changed systematically compared with the other drugs on storage of the columns for 1 year. The changes were enhanced on storing the column in mobile phase and were accelerated by prolonged washing with water. A comparison of freshly packed columns suggested that similar changes also occurred with the dry silica on storage. These effects may be a contributor to previously observed batch-to-batch differences in the silica.

Chromatography of beta-adrenergic blocking agents

The determination of beta-blockers has posed pharmaceutical analysts with a variety of problems arising from the essential characteristics of these compounds as bases and the variability of physicochemical properties of individual drugs. Liquid chromatography has become the favoured method of analysis and to a certain extent there is a standardised approach to analysis based on either solvent or solid-phase extraction and reversed-phase high-performance liquid chromatography coupled to fluorescence detection. The analyst must be aware of interactions occurring during extraction stages. All manipulations should be fully evaluated for individual drugs and metabolites prior to use. Other analytical options are chosen for specific or more demanding applications. The use of unmodified silicas for the liquid chromatography of beta-blockers (and other basic drugs) is an example of a potential alternative mode of chromatography. The stereoselectivity of the pharmacology of beta-blockers has spawned a great deal of literature describing the resolution of enantiomers by chromatographic methods. It is envisaged that this area will achieve greater prominence in the future as drug development pursues optical purity. The demand for the availability of enantiomerically pure pharmaceutical preparations will certainly see developments for preparative-scale separations as well as analytical methods and will surely promote developments in new and established methods of chromatography.