Size Exclusion Chromatography (SEC) in Aqueous Media

Abstract

<p>This thesis deals with the different aspects of the successful application of size exclusion chromatography (SEC) for the molecular weight distribution (MWD) measurement of water soluble polymers. These aspects include methodology of mobile-phase development, selection of packing pore-sizes and methodology of molecular weight calibration and chromatogram interpretation. Qualitative understanding of ion-exclusion and adsorption, two of the more important and least understood complex phenomena in aqueous SEC was also provided.</p> <p>The polar nature and unique physical properties of water-soluble polymers in solution were found to be critically important in selection of mobile-phases and pore sizes. Due to the active sites present with most porous packing materials adequately suited for aqueous SEC application, adsorption, one of the resulting complications, was reduced preferentially, by addition of non-ionic surfactants such as Tergitol or polyethylene oxide to the mobile-phase. Ion-exclusion was controlled and reduced by addition of varying amounts of salt and/or acid to the mobile-phase. The optimal pH and ionic strength to the mobile-phase depended on the type of polymer being investigated. No common mobile-phase was found for the four polymers investigated (dextran, hydrolysed and non-hydrolysed polyacrylamide, and sodium polystyrene sulfonate).</p> <p>From viscosity data, these polymers were found to cover a very wide range of sizes in solution, with dextran being exceptionally very compact in solution when compared to polyacrylamide of the same molecular weight (MW). For this reason, selection of pore sizes was found to be critically important in achieving minimum peak broadening and maximum separation. Selection of one multi-column SEC system for general application to different water-soluble polymers was found not to be possible.</p> <p>Two powerful methods of molecular weight calibration, where simultaneously the peak broadening correction factors and the true molecular weight calibration curve are obtained, were developed. These methods require the use of multiple polydisperse MW standards, with known (Mn, Mw) or (Mn, [ƞ]). From these methods, a new shape of the instrumental spreading function was found. This was more general symmetric exponential type of spreading function provides a very simple definition of axial dispersion coefficient, which was shown not to be the most important fundamental parameter in SEC. With this shape function, apart from D2, the slope of the true MW calibration curve, the most important fundamental parameter (in the absence of skewing) was found to be the polyplatykurtic coefficient, its important increasing with increasing polydispersity of polymer samples.</p>