Separation of pharmaceutical bases from neutral and acidic components by capillary electrochromatography

Capillary electrochromatography (CEC) has proved to be applicable to the separation of neutral and acidic (chromatographed in their ion-suppressed mode) compounds but a major factor holding back its further development has been its perceived inability to analyse basic compounds. In this paper we demonstrate that the addition of a competing base, such as triethylamine (TEA) or triethanolamine (TEOA), to a low pH buffer can achieve excellent CEC analysis of a range of strong pharmaceutical bases. Acceptable peak symmetry and efficiencies of up to 510 000 plates m(-1) were obtained for bases such as procainamide, nortriptyline and diphenhydramine on CEC Hypersil C-18, C-8 and phenyl reversed-phase packing materials. The effect of TEA or TEOA concentration and mobile phase pH on the separation behaviour of the bases is discussed. In addition, the paper reports the first example of the simultaneous CEC analysis of acids, bases and neutral compounds by using a mobile phase containing TEOA phosphate at pH 2.5

Chromatographic retention behaviour of n-alkylbenzenes and pentylbenzene structural isomers on porous graphitic carbon and octadecyl-bonded silica studied using molecular modelling and QSRR

The retention behaviour of a series of 15 n-alkylbenzenes and pentylbenzene structural isomers and benzene were investigated using porous graphitic carbon (PGC) and octadecyl-bonded silica (ODS) stationary phases. Shorter chain n-alkylbenzenes and benzene (n = 0-6), and all the pentylbenzene isomers were more strongly retained on ODS, although the selectivity was greater with PGC. For the pentylbenzene analytes the degree of branching in the alkyl chain at the position adjacent to the aromatic ring affects retention on PGC, with higher retention in less branched molecules. Molecular modelling studies have provided new insights into the geometry of aromatic pi-pi stacking interactions in retention on PGC. For alkylbenzenes with high branching at the position adjacent to the ring, the preferred geometry of association with the surface is with the branched chain directed away from the surface, a geometry not seen in the other alkylbenzenes. The most energetically favoured orientation for interaction between analytes and the PGC surface was found to be cofacial for toluene and ethylbenzene, whereas for other analytes this interaction was in a face-edge orientation. The alternative geometry of association observed with both toluene and ethylbenzene may explain the enhanced retention of these two analytes on PGC compared with their longer chain analogues. Quantitative structure-retention relationships revealed the importance of compactness in analyte structure during retention on PGC, with decreased compactness (associated with longer chain length and reduced chain branching) improving retention. (c) 2010 Elsevier B.V. All rights reserved