Aqueous Samples

Aqueous GPC analysis brings a whole new set of challenges to the polymer characterization chemist. Most conventional, high performance packings for aqueous GPC analysis are prepared from hydrophilic methacrylate gels, with residual carboxylate groups, giving the column chemistry an overall anionic charge. When doing GPC analysis on water soluble polymers, one must be cognizant of the fact that there could be a charge interaction between the sample and the packing material, unless certain steps are taken. Theoretically, if the polymer is neutral, you could do the analysis in pure water. If there is any anionic charge to the polymer, it will be excluded by the column and elute at the void volume if pure water is used as the eluent. On the other hand, if the polymer has an overall cationic charge, (and you use pure water as the eluent), the sample will stick to the column and never elute. A lot of these ionic problems can be overcome quite easily with the addition of an electrolyte, such as 0.10M NaNO₃. Even for neutral samples, it is a good idea to use 0.10M NaNO₃ as the eluent. Some of the problems that need to be overcome with the correct eluent (see solvent selection guide for water soluble polymers) are as follows:

1. Intramolecular Electrostatic Interactions - Polyelectrolyte expands due to the charges on the molecule itself.
2. Ion Exclusion - Sample polyelectrolyte and packing material have the same charge (both anionic, for example).
3. Ion Inclusion - Polyelectrolyte charge is opposite to that of the packing; sample will stick to the column, (cationic sample, for example) and not elute. pH may need to be adjusted.
4. Ion exchange - This phenomenon may occur as in the case of ion inclusion, where the packing and sample have opposite charges. An ion exchange reaction occurs, causing the sample to elute late or not at all.
5. Hydrophobic Interactions - The non-ionic portion of the polyelectrolyte sample interacts with the non-polar sites of the packing material. This problem can easily be remedied by adding 20% of an organic modifier (acetonitrile, for example) to the eluent.

There are occasionally other interactions that can occur, such as association effects and memory effects, but the 5 problems shown above are the ones most encountered. The aqueous solvent selection guide shown previously will help you to choose the correct eluent for you particular application. We have chromatograms for nearly all of the applications shown in the guide. Just let us know if you need any help with your particular samples.

Three different water soluble polymers were run on the Alliance system, with refractive index detection. The three polymers analyzed (shown below) were Hydroxyethyl cellulose, Pectin, and Polyalginic Acid. Note the excellent reproducibility of the multiple molecular weight distribution overlays. In all cases a three column Ultrahydrogel set (2 Linear plus a 120) was used. Note that the eluent was 0.10M sodium nitrate, an excellent choice for neutral and anionic hydrophilic polymers.

Narrow polyethylene oxide standards were used to develop the calibration curve, so the molecular weight averages shown are based on PEO's.