Ion-exclusion controlled size-exclusion chromatography of methacrylic acid–methyl methacrylate copolymers

Controlled ion-exclusion allows compensation of hydrophobic adsorption in size-exclusion chromatography of negatively charged methacrylic acid–methyl methacrylate (Eudragit) polymers using methanol as a mobile phase. Properly selected low-ionic-strength conditions below 5 mM LiCl provide correct separation in the size-exclusion mode. Possible disturbing effects, mainly related to light scattering, under low-salt conditions are discussed and shown to be negligible if on-line concentration–light scattering detection is used. The absence of these disturbances is checked by a comparison of experiments performed in methanol containing 1.25 mM and 2.5 mM LiCl. Molecular mass averages and distributions identical within the experimental error are obtained.