Method development for the enantiomeric purity determination of low concentrations of adrenaline in local anaesthetic solutions by capillary electrophoresis.

L-adrenaline is often included in local anaesthetic (LA) solutions for injection to improve the quality of the anaesthetic block. The concentration of the LA is between 2.5 and 20 mg/ml and the concentration of adrenaline is typically < or = 0.1% of the LA concentration. In order to follow the racemization into d-adrenaline, not only is chiral separation needed but also sufficient resolution from the LA and other components of the injection solution. Furthermore, very high sensitivity is needed in order to be able to determine the d-enantiomer at very low concentrations, i.e. down to about 0.1 microg/ml. The development of a chiral capillary electrophoresis method that is able to determine the racemization of adrenaline is described, together with a limited validation. Samples are injected without pretreatment and analysed with a capillary electrophoresis buffer containing 40 mM heptakis(2,6-di-O-methyl)-beta-cyclodextrin, 0.10 M phosphoric acid and 0.05 M triethanolamine. The amounts of d-adrenaline found in the LA products tested were typically < 3% of the l-adrenaline concentration and < 0.003% of the LA concentration.

Sample matrix influence on the choice of background electrolyte for the analysis of bases with capillary zone electrophoresis.

Low levels of impurities need to be determined in drugs. Consequently, if UV detection is used, a large sample amount must be loaded and as narrow peaks as possible obtained. The sample matrix and the stability of the samples as well as the peak resolution should be considered when the electrolyte is chosen. In this study the influence of the sample matrix composition with varying background electrolytes on the peak appearance of model mixtures loaded in large amounts was investigated. A robust electrolyte for analysis of bases in a sample with varying pH was found to consist of a buffering co-ion and a buffering counter-ion (the pH was approximately 4.2 in the electrolyte). If a minor component has higher mobility than the macrocomponent and the co-ion, better peak shape can be obtained if, for instance, enough sodium chloride is added to the sample, i.e., sample self-stacking is exploited. The effect of addition of organic modifiers, isopropanol or acetonitrile, was examined and good linearity and precision have been shown for impurities in the concentration range tested, approximately 0.03 to 5 mol% of the main component, in model mixtures.

Increase of sample load without peak deterioration by careful selection of electrolyte in capillary zone electrophoresis.

In this study it is demonstrated that much higher concentrations of bases dissolved in water can be injected in capillary zone electrophoresis without causing peak deterioration, e.g., peak splitting, if it is the co-ion that buffers instead of the counter-ion. Those findings can be utilised to control peak shapes and in this way an increase in the sample load and indirectly a decrease in the detection limits of impurities in the sample can be obtained. Good results were obtained with 4-aminobutyric and 6-aminocaproic acids as buffering co-ions. Another possibility evaluated successfully was that of using a dibasic acid, malic acid or succinic acid. With an electrolyte containing both succinic acid and 6-aminocaproic acid at pH 4.5, it was possible to load at least 10-20 times more of the test substances imidazole, creatinine or 2-aminopyrimidine dissolved in water than with an electrolyte at the same pH containing acetic acid and tris(hydroxymethyl)aminomethane.