A. Abushoffa

Prediction of selectivity for enantiomeric separations of uncharged compounds by capillary electrophoresis involving dual cyclodextrin systems

The single-isomer polyanionic cyclodextrin (CD) derivative heptakis-6-sulfato-beta-cyclodextrin (HSbetaCD) has been tested as chiral additive for the enantioseparation of non-steroidal anti-inflammatory drugs, such as fenoprofen, flurbiprofen, ibuprofen and ketoprofen, in capillary electrophoresis, using a pH 2.5 phosphoric acid-triethanolamine buffer in the reversed polarity mode. In most cases, the enantiomers of these acidic compounds, present in uncharged form at that pH, were only poorly resolved with HSbetaCD alone. However, the use of HSbetaCD in combination with the neutral CD derivative, heptakis-(2,3,6-tri-O-methyl)-beta-cyclodextrin (TMbetaCD), which has a particularly high enantioselectivity towards these compounds, has led to complete enantioresolution in reasonably low migration times in most cases. Affinity constants for the enantiomers with the two cyclodextrins were determined, using linear regression in a two-step approach. Affinity constants with the charged HSbetaCD were first calculated in single systems while those with the neutral TMbetaCD were determined in dual systems. Selectivity for the enantiomeric separation of these compounds in dual CD systems could be predicted using recently developed mathematical models.

Resolution of the enantiomers of oxamniquine by capillary electrophoresis and high-performance liquid chromatography with cyclodextrins and heparin as chiral selectors.

The methods of separation of the enantiomers of the chiral drug oxamniquine are compared, between HPLC with either cyclodextrins and their related derivatives as chiral selectors in the mobile phase or immobilised in a chiral stationary phase (as Cyclobond I and II) and between capillary zone electrophoresis (CZE) where the cyclodextrins are added to the buffer solution. The HPLC experiments, which included structured method optimisation were largely unsuccessful in resolving the enantiomers, with the exception of when a Chiral-AGP protein stationary phase was introduced into the programme. However although this chiral stationary phase provided baseline resolution of the enantiomers the stability of the method was suspect to small changes in the pH (0.2 units). In contrast the CZE method developed for both cyclodextrins and their derivatives gave good resolution of the enantiomers and method stability (R.S.D. < 1%, n = 10 on precision). The basis of the interaction mechanism between selector and selectand was shown as a 1:2 relationship of cyclodextrin to analyte by NMR. In addition the polysaccharide, heparin was investigated as a chiral additive and excellent resolution of the oxamniquine was achieved with 3 mM heparin in 50 mM sodium dihydrogenphosphate (pH 3.0) as buffer in CZE, which also gave a stable procedure. This method allowed the detection of each of the enantiomers in the presence of the other down to 0.23% (m/m). The overall composition of the heparin material from different sources can however be slightly variable and this can result in small differences in resolution capability.

Development and validation of a stability indicating capillary electrophoresis method for the determination of metformin hydrochloride in tablets

A simple and a stability indicating capillary electrophoresis method was developed and validated for the analysis of metformin hydrochloride in tablet formulation. The method employed 40 mM citrate buffer at pH 6.7 as a background electrolyte with an applied voltage of 15 kV (positive polarity). The capillary used was of 68.5 cm length (60 cm to detector) and the detection wavelength was 230 nm. The method was validated in accordance with the ICH requirements, which involved accuracy, precision, linearity, selectivity and both limit of detection and limit of quantitation. The current method was linear over the concentration range of 0.2-2.0mg/ml of metformin hydrochloride. The method was precise as reflected by inter-day and intra-day relative standard deviation (RSD) of less than 2%. The limit of detection and limit of quantitation were 2.0 microg/ml and 8.0 microg/ml, respectively. The stability indicating capability of the method was established by enforced degradation studies combined with peak purity assessment using photodiode array detection.

Determination of trace enantiomeric impurities in chiral compounds by capillary electrophoresis with uncoated, cationic, anionic and neutral surface modified capillaries

Abstract The usefulness of commercially available polyamine, sulphonic acid and neutral coated capillaries is demonstrated for the resolution of chiral drugs. The methods, using cyclodextrins and their derivatives are developed from conventional fused-silica capillary procedures without any changes to the procedures. Different coatings can be used to adjust the order of elution of enantiomers when one of the enantiomers is present as a trace level impurity. Limit of quantitation levels of d - or l -tryptophan in the presence of the other enantiomer with acceptable levels of reproducibility. Similar levels have been detected for warfarin and propranolol, using a polyamine-coated capillary. The coated capillaries were stable to long-term changes in the electrolyte and its pH value and the methods were interchangeable with capillaries obtained from different batches.

Optimization of the separation of β‐blockers by ion‐pair capillary electrophoresis in non‐aqueous media using univariate and multivariate approaches

The separation of four β-Blocking drug substances (atenolol, sotalol, betaxolol, and metoprolol) selected as model basic analytes has been investigated in non-aqueous capillary electrophoresis (NACE) using the principle of ion-pair formation. Camphorsulphonate was selected as the counter-ion in a background electrolyte (BGE) made up of formate buffer in a mixture of acetonitrile/methanol or acetonitrile/ethanol. The influence on resolution of the concentration of the counter-ion, the nature and proportion of the organic solvents, as well as the concentration of the ionic components of the BGE was first studied by a univariate approach. An experimental design was then applied to estimate possible quadratic effects and first-order interactions. To identify the most important factors affecting the separation of the four β-blockers, a two-level fractional factorial design with 16 experimental points was applied as a screening test. A three factor Box-Behnken design with 12 experimental points was used to predict and optimize the selectivity. Finally the optimal conditions obtained by the univariate and the multivariate approaches were compared.

Comparative study on the enantioseparation of glutethimide using dual cyclodextrin systems and cyclodextrin modified MEKC in capillary electrophoresis

In order to critically evaluate the potential of dual cyclodextrin (CD) systems in CE, enantioseparation of the neutral chiral analyte glutethimide (GT) was performed using: (a) charged CDs; (b) combinations of charged and neutral CDs (dual CD systems), and (c) CD-modified micellar electrokinetic chromatography (CD-MECK). The results of this comparative study indicate that properly designed dual CD systems may in certain cases represent a valuable alternative to CD-MEKC and electrokinetic chromatography involving charged CDs for enantioseparation of neutral analytes.