Re: Sample adsorption in GPC Analysis

[Melanie919]

Hello all;

I'm new to the forum and hope to get some help on a "sticky" problem that I have.

I'm doing GPC on highly cross-linked surfactants EO/PO that is reacted with other oxy containing moieties in the presence of a cross-linking agent. The products have a number of OH groups on them. I'm using 3% Acetic Acid/THF as a mobile phase. Samples are completely soluble in the mobile phase. We use a bank of two mixed bed columns in this analysis. I have a problem with parts of the high MW region of the chromatogram dissappearing over the course of a run, i.e. multiple injections of the same sample vial during the day. We've tried cleaning with injections of acid in between and this seems to help a bit. I have some questions:

1. Does anyone have a good reference that describes this phenomenon so I can understand what's going on at the surface of the column(s)?

2. Would increasing the amount of acid modifier in our mobile phase help?

3. Any other cleaning regimes we might try in bewteen injections. We route the cleaning agents away from the detector with a switching valve.

[Uwe Neue]

If the high molecular weight region of your analyte is disappearing, there could be other reasons for this as well. The first one is a slow chemical breakdown in solution, the other one could be due to increased shearing and thus mechanical breakdown of the sample. Considering that you have a highly crosslinked sample, how did you get it to dissolve in the first place?

[Melanie919]

Uwe;
Thank you for your reply.
Sample's Mw is about 15,000 - 30,000 Daltons so it is not a very large molecule but we believe it is the cross-linked portion of the surfactant that gives the product its performance.

Gentle rotation at 5 rpm for about 60 minutes does a nice job. No gel is visible nor any particles visible. Samples are filtered before analysis.
We have been able to get much more reproducible chromatograms with the acid cleaning in between injections but were interested in what kind of explanation of column interaction may be out there in the literature or if someone has experienced this before and can shed some light on it for us.

[Uwe Neue]

At this MW, we can exclude mechanical degradation.

You have not told us what the column is. I assume it is a styrene-divinylbenzene type column. It is tough to see why such packings would eat your sample, especially with an increase in time. My opinion at this moment would be a chemical reaction of the polymer with the acetic acid, but I know too little about your polymer. I definitely would expect an esterification with time, but this does not explain a decrease in the MW.

Maybe this gives you some ideas. Otherwise, I think I need more information.