Chromatography Forum

I have a problem which is hard to understand.

I analyze an acrylate type ofpolymer with amine functionality. I dissolved the polymer in THF (I tried both 1% and 2,5%). The column is a linear type polystyrene/ divinylbenzene column. 2x 8mm X 300 mm.

I’m quite sure that my polymers aren’t eluting outside the exclusion limit. I inject 100uL.

The problem is the following. If I do to repetitive injections I’ve have good reproducibility. If I wait, two hours the shoulder on the high MW site increases dramatically (the low end site stays the same). As results the Mw value is completely off. What could be the reason of this?

The first thing that jumps out is that you are putting way too much material down the column. If you look at the sheets that will have accompanied your PMMA standards then you will see that they (well PL anyway) inject 20ul of a 0.1% solution.

Matt

I would agree that your column loading is too high. For the 8 mm columns, Polymer Labs has told me that the injection volume can be up to 50 uL per column, so 100 uL may be OK, but it is on the larger side. I would reduce to 50 uL.

I also agree that your sample concentration is too high. Start at 0.1% as suggested.

As to why your MWD changes after storage, there are two possibilities (in addition to what has already been mentioned).

1. Your system is still active, and continues to polymerize while in the vial. THF (if unpreserved) may have peroxides that could be initiating reactions. I don't know your chemistry or sample prep, so I can't say if this is likely or not, but it is possible.

2. You have a solubility issue. The high MW polymers are always slower to dissolve than the smaller molecules. Some may require overnight to completely go into solution. If all injections are from the same vial, then this is less likely.

I tried different injection volumes. This doesn’t seem to matter. The reason why I inject this high concentration is to suppress surface interactions. I dissolve my polymer in just in THF and let it stand for 1 night.

I will try to inject 0.1% but I’m not sure if this will help. My polymers are not that big (below 100.000). The hypothesis of polymerization is interesting. I will not exclude this option. It’s just weird that the peak is not shifting to left or right but increases on the X-axis.

I know that a few people use HFIP for this application. This should have a reason.

You indicate that you inject more to suppress "surface interactions" This is not a good thing.... Surface interactions will falsify your results.

If they are mild enough, you can eliminate them by increasing the temperature. The standard approach with THF is to run it at 40 degrees Celsius.

Otherwise, the most important thing that you can do is to reduce the concentration, as advised above.

Koen, since adsorption phenomena are a matter of equilibrium (mass action law) an increase in concentration increases the adsorbance. Maybe you thought that you can hide this by lowering sensitivity? Normally one gets rid of such interactions by choosing an approbriate mobile phase, stat. phase, or maybe some tricks like Uwe mentioned.

Koen,

Have you the same problem with a polymer not amine-functionalized?

Maybe it's a mobile phase problem, as HW was mentioning in the previous reply.

If I were you I would try few % (1-5) of acetic acid in THF to see if the problem vanish or I would change completely mobile phase to DMF/LiBr 0.1 %.

I will reduce the concentration of sample. In second stage Ì will add some acetic acid.

DMF with LiBr didn't gave me good chromatography, splitted peaks etc.

I'll keep you posted