Mixed-mode electrokinetic chromatography of low molecular weight anions and cations

This work presents a comprehensive study into selectivity control over electrokinetic chromatography (EKC) systems for the determination of small organic anions and cations using various additives. For the separation of anions an electrolyte system comprising a cationic soluble polymer (poly( diallydimethylammonium chloride), PDDAC) and a neutral s-cyclodextrin (s-CD) as pseudo-stationary phases was used. The separation mechanism was a combination of electrophoresis, ion-exchange (IE) interactions with PDDAC, and hydrophobic interactions with s-CD. The extent of each chromatographic interaction was independently variable, allowing for control of the separation selectivity of the system. Various cationic analytes were also examined, including opiate alkaloids, aromatic bases and amino acids. In the case of the opiate alkaloids (morphine, thebaine, 10-hydroxy thebaine, codeine, oripavine and laudanine) a system utilising sulfated-s-cyclodextrin (s-s-C:P) as a pseudo-stationary phase was used. Cation-exchange interactions between the cationic analytes and the anionic s-s-CD (7-11 moles of sulfate groups per mole CD) were found to be the predominant separation mechanism. The separation of a series of aromatic bases was achieved utilising an electrolyte system comprising an anionic soluble polymer (polyvinylsulfonic acid, PVS) and s-CD as pseudo-stationary phases. The separation mechanism was based on a combination of electrophoresis, IE interactions with PVS, and hydrophobic interactions with s-CD. The extent of each chromatographic interaction was independently variable, allowing for control of the separation selectivity of the system. The IE and the hydrophobic ~teractions could be varied by changing the concentrations of PVS and s-CD, respectively. Additionally, mobilities of the bases could be controlled by varying pH, due to their large range of pKa values. Selectivity control of the enantiomeric separation of the three aromatic amino acids (phenylalanine, tyrosine and tryptophan) was demonstrated utilising temperature and s-s-CD and dextran sulfate as pseudo-stationary phases. Two systems were explored using s-s-CD as the chiral selector. In these systems either temperature or the addition of dextran sulfate was used to increase the selectivity control. The possibility of the simultaneous separation of anions and cations was demonstrated using a series of aromatic carboxylic acids, sulfonates and opiates as analytes. Separation was achieved using electrokinetic chromatography employing a mixture of PDDAC and the amphiphilic anion hexanesulfonate as pseudo-stationary phases. In this system, the PDDAC pseudo-stationary phase interacted with the anionic analytes, whereas the hexanesulfonate interacted with the cationic analytes. A further interaction between the combined PDDAC-hexanesulfonate complex and the more hydrophobic analytes was also evident. Mathematical modelling based on physical equilibrium and artificial neural networks was also undertaken, with the models successfully describing each system (r2>0.98 for predicted versus observed migration times). The models were then used to not only optimise each system, but also to allow predictable selectivity control leading to attainment of desired migration orders.