1. In GPC, shift in retention time will have a greater impact in determining Mw distribution. If we have two polymer solutions one in a pure solvent and the other in the same solvent plus samll amount of other components eg. water, another solvent, say upto 10%, would both these solutions give the same retention time or would there be a retntion time shift due to differences in the composition of the sample matrix (althought the plymer conc. is kept the same)

2. Another related topic what is the effect of concentration on retention time, assuming that no column overload.
3. I read that sonication is not recommended in dissolving polymer and analyze by GPC. Is it true and why?

Thank you in advance for your comments.

Shelton

1. Unless you have something unusual going on, the presence of another solvent should not be a factor. However, you have to keep in mind that GPC requires that the polymer is in a randon coil configuration. If the addition of the other solvent causes a significant change in this, it could have an impact. In my opinion, you would see solubility problems at the same time.
2. Concentration should have no effect on retention, if there is no overload. However, in GPC you can get into overload quite easily. So be careful.
3. Sonication is a good way to degrade you sample. Most GPC experts are really careful about how they dissolve the sample. The higher the MW, the easier the stuff breaks. I have seen demonstrations where even the speed with which the sample is sucked into the injector can degrade a sample.

Uwe, it's been a while since I worked with GPC but I thought sample concentration could have an effect on retention. If the sample concentration were high enough so you were approaching the solubility limit, wouldn't the polymer start to "ball up" (technical term there) so it was no longer in a random coil orientation? Once it was injected, the injected material would be diluted by the mobile phase. This dilution would allow better solvation of the polymer and it would change to a random coil. Until the polymer reached the "true" random coil orientation, the size could be different for part of the retention on the column, which should change the retention compared to an injection where the size was constant from the start of the injection. Don't know how fast the polymer orientation would change, though. The differences in retention may not be detectable for actual runs.

Yes, at high concetrations you can get strange effects in GPC. This is why a rather dilute solution is usually recommended.

Hi, recently I was analysed metal coodination polymer on GPC using 0.025 M NH4PF6 in DMF as eluent on three column bank consists of styragel columns HT 2, HT 4 and HT 6. Calibration curve was generated by using polystyrene standards and molecular weight estimated as ~30000. Then I started study the effect of additive concentration (NH4PF6)on molecular mass of the polymer. I was obtained strange results. With 0.01M NH4PF6 in DMF, gave molecular mass ~ 19000 and with 0.035 M it was ~41000. In all the cases retention times of stadards were almost same, but sample retention time was drastically changed. Sample preparation was done in eluent with low concentration.
Can anybody help me in conclusion about molecular weight of my polymer?

Let me change the problem slightly. Suppose two polymer solutions gave exactly the same retention time (Mp is same), but one solution gave low Mn and Mw( Mn is ~4% low, Mw is ~ 10% low). Does this mean polymer in one solution (low Mw) has brokendown/degraded? Or solution composition in the sample could create this difference. Both polymer solutions were prepared in DMAC at 75C. Only difference in the two polymer solutions is, one may contain smaller molecualr weight compound ~ 40%, which had higher Mn and Mw.

Thanks for your input.

Shelton

To make sure that the data are correct, the best thing to do is to use calibration standards of the same polymer type as your samples. Today, a well characterized broad molecular weight standard can be used, and narrow standards are not necessary.
One of the things that is often important is the temperature of a GPC separation. Are you using the same temperature as the literature reference that you are following?

Thank you Uwe, because of non availability of metal coordination polymer standards in the molecular weight of my sample, I was used polystyrenes as standards. All the analysis have done at 40 C, as literature reference.

The GPC-column primarily „sees“ the hydrodynamic volume Vh of the molecule. Calibration in conventional GPC is done by foreign standards (PS,Pullulane....) to get the molar relation.
Therefore generally spoken: All effects that alter Vh (e.g. coiling, formation of as-soziates...) have an impact on molar mass. Those effects are especially severe with aqueous GPC (e.g. viscosity).
Shearing of high MW-molecules is also observed on the stat.phase (particle size,flow rate).