T.M. Jefferies

Ion-pair high-performance liquid chromatography

Abstract The analysis of ionized solutes by high-performance liquid chromatography (HPLC) may be facilitated by the use of ion-pairing agents. This contribution discusses the development of ion-pair HPLC during the past decade, and points to those aspects of the technique which have greatest significance. Mathematical descriptions of various probable ion-pair retention mechanisms are given and related to the practical usage and efficiency of the methods. A comprehensive listing of reported chromatographic systems, pairing ions and solutes studied is given.

Functional group behaviour in ion-pair reversed-phase high-performance liquid chromatography using surface- active pairing ions

Abstract The behaviour of functional groups having widely differing physico-chemical character has been examined in ion-pair reversed-phase high-performance liquid chromatography using surface active pairing ions. The effects of temperature, mobile phase, organic modifier type and percent composition, stationary phase carbon loading and type, ionic strength, and pairing ion structure, charge and concentration on extra-thermodynamic functional group contribution values, have all been determined. Analysis of group behaviour within the framework provided by solvophobic theory is often found to be possible using linear free-energy relationship appraoches, and it is shown that retention behaviour can be described as ion-pair formation in the mobile phase followed by distribution to the stationary phase. In addition, substituent behaviour is found to exhibit linear enthalpy-entropy compensation behaviour, suggesting further that a common retention mechanism can be described for all ionized solutes using these pairing ions.

Chiral discrimination of phenethylamines with β-cyclodextrin and heptakis(2,3-di-O-acetyl)β-cyclodextrin by capillary electrophoresis and NMR spectroscopy

The resolution of nine sympathomimetic phenethylamine racemates by beta-cyclodextrin and heptakis(2,3-di-O-acetyl)beta-cyclodextrin has been investigated by capillary electrophoresis and 1H NMR spectroscopy. The NMR and capillary electrophoresis results showed that beta-cyclodextrin probably formed stronger complexes with the amines than did heptakis(2,3-di-O-acetyl)beta-cyclodextrin but was a poorer chiral discrimination agent in both techniques. The addition of heptakis(2,3-di-O-acetyl)beta-cyclodextrin to the capillary electrophoresis buffer gave baseline resolution of enantiomer peaks for seven of the compounds studied while beta-cyclodextrin resolved only three of the racemates.

Ion-pair high-performance liquid chromatography: the use of low concentrations of long-chain alkylbenzyldimethylammonium chlorides for resolving anionic solutes

The qualitative and quantitative determination of large anionic solutes, including the drug sodium cromoglycate, by an ion-pair high-performance liquid chromatographic technique using alkylbenzyldimethylammonium chlorides as pairing ions is described. The method provides a sensitive and selective assay for the solutes examined in both simple aqueous solutions and in complex biological fluids, and requires no pre-extraction step. Retention mechanisms are discussed.

Effect of β-cyclodextrin acetylation on the resolution of phenethylamines with capillary electrophoresis and nuclear magnetic resonance spectroscopy

The effect of acetylation of β-cyclodextrin (CD) on the chiral discrimination of a series of phenethylamines has been investigated using capillary electrophoresis and nuclear magnetic resonance (NMR) spectroscopy. Therefore, pure, fully derivatized CDs were synthesized. Capillary electrophoresis measurements revealed that the 2,3-diacetylated CD was a better discriminator than the 6-acetylated and the native CD. NMR investigations of the complexes suggest that the phenyl ring of the phenethylamines is inserted in the cavity and the side chain interacts with the hydroxyl rim of the macrocycle. The structures of the complexes indicated by the NMR results were supported by molecular models of the derivatised CDs.

Degradation Pathways of Ampicillin in Alkaline Solutions

Ampicillin trihydrate, sodium salt, in aqueous solution has a pH of about 8. No complete degradation pathway has been proposed to explain the degradation of ampicillin under alkaline conditions and the information available explains the formation of only certain products. The present work was carried out with the aim of providing this information.

Chiral discrimination by NMR spectroscopy of ephedrine and N-methylephedrine induced by β-cyclodextrin, heptakis(2,3-di-O-acetyl)β-cyclodextrin, and heptakis (6-O-acetyl)β-cyclodextrin

NMR spectroxcopy has been used to compare the interaction of ephedrine and N-methylephedrine with β-cyclodextrin, heptakis(2,3-di-O-acetyl)β-cyclodextrin, heptakis(6-O-acetyl)β-cyclodextrin. The stoichiometry of the complexes formed between all three cyclodextrins and N-methylephedrine was found to be 1:1 by UV spectroscopy by means of the Job technique. NMR spectra of the single enantiomers of ephedrine and N-methylephedrine in the presence of all three cyclodextrins gave information about the parts of the ligands which interact differently with the host molecules and may be responsible for the chiral discrimination. To quantify the complex stabilities, binding constants were calculated from the changes in the chemical shifts of the ligand signals upon complexation. Analyses of the coupling constants of both species showed that no significant conformational change occurs upon complexation. ROESY spectra of these optical isomers with all three cyclodextrins provided detailed information about the geometry of the complexes. Different intermolecular cross-peaks between the individual isomers of ephedrine and N-Methylephedrine were found for native β-cyclodextrin and its 2,3-diacetylated derivative but not for 6-acetyl cyclodextrin. Analyses of the intramolecular cross-signals of the ligands confirmed that no significant conformational change occurs upon complexation. Chirality 9:211–219, 1997.

Application of modified cyclodextrins in capillary electrophoresis for enantiomeric resolution of propranolol and analogues

Abstract The enantio-resolution of propranolol and four derivatives was examined by free solution capillary electrophoresis (FSCE) using various cyclodextrin molecules. Of the three modified cyclodextrins investigated, hydroxyethyl-β-cyclodextrin provided the largest chiral resolution values for all five analytes. This may be linked to its extended hydrogen bonding chains on the cyclodextrin rim. Methyl-β-cyclodextrin and heptakis (2,3-di-O-acetyl) β-cyclodextrin gave lower maximum analyte resolutions, probably due to differences in their macrocyclic structure and hydrogen bonding ability. The presence of a bulky, non-polar alkyl grouo on the analytes was found to enhance chiral recognition. Methanol was found to have a varied effect on chiral resolutions, dependent on the type of cyclodextrin and structure of the analyte.

An investigation of high-performance liquid chromatographic methods for the analysis of amphetamines

Three pre-column derivatization reagents, namely o-phthalaldehyde, 4-chloro-7-nitro-benz-2,1,3-oxadiazole, sodium naphthaquinone-4-sulphonate and two ion-pair reagents, namely, naphthalene-2-sulphonate and sodium dodecylsulphate have been investigated for their suitability for the qualitative and quantitative analysis of urine and plasma samples containing amphetamines. The derivatization method employing sodium naphthaquinone-4-sulphonate was found to be selective and sufficiently sensitive for the routine determination of amphetamine and methylamphetamine in urine and plasma samples at the ng/ml level.

Polar organic chiral separation of propranolol and analogues using a ?-cyclodextrin bonded stationary phase

A β-cyclodextrin bonded stationary phase was employed for the enantioresolution of propranolol and several analogues in conjunction with various polar organic mobile phases. The effects of structural alterations in the non-polar regions of the analytes were found to exert profound changes upon chiral resolution and capacity values, indicating that features which cannot hydrogen-bond with the cyclodextrin molecule still play an important role in this chiral recognition process. This was linked to a repulsive steric effect facilitating the necessary conditions for chiral resolution. The degree of ionization of the analytes and the type and concentration of organic modifier used were also seen to influence the analytes1 enantio-selectivity and capacity values.