GPC universal calibration and accuracy woes

[chemEZ]

Hello all!

I have some experience working with HPLC, but I have recently inherited a lovely GPC unit to study some polymer systems. I've come across a few issues that I am struggling to understand:

1. Universal calibration - Is there really any difference between using polystyrene or pmma standards? Do I have to match my calibration standards as closely as possible to the samples? (I'm studying acrylic polymers...) I think I understand that universal calibration is better than the conventional/narrow standards because it relates the calibration to the specific polymer you're measuring. If that is the case, for my acrylic system do I need to use pmma standards? Or is it still okay to use the polystyrene?

2. Accuracy - My GPC unit has been sitting idle for a few months, I am fortunate to have a second GPC unit available to me to compare results and verify everything has come back up in working order. However! I am finding a difference between the instruments. Independently both units have stellar calibrations and precision. Depending on the samples I am running I am finding a range of 20,000-200,000 g/mol discrepancy between the instruments. (Note that the secondary unit is only equipped with an RI detector, while the primary unit has a RI and viscometer). Should I be alarmed at this range? I am inclined to be, but my background in polymers is limited.

Any input is very appreciated.

Thank you!

[Gerhard Kratz]

To your question 1) please contact the colleagues from PSS Polymer Standard Service in Mainz. Universal calibration is so tricky that I don't want to say anything about that. It is different depending on the polymer standards you are using.

Question 2, mmh, if you get in contact with PSS please ask them also this question. RI plus differential viscosimeter is getting more precise results than RI alone. Two different instruments from different manufacturers will give different results. Need to optimize the instruments.

Good luck

[chemEZ]

Thanks for the information source!

PSS has a plethora of GPC tips & tricks on their website that I am finding very helpful.

I also wanted to clarify that the primary and secondary GPC units are both from the same manufacturer (Agilent/Polymer Laboratories), but have different ages. I've had a chance to look up some historical data for these two units, and it appears there has always been a difference of varying magnitude - depending on the polymer being measured. Perhaps you are right, and I am just seeing the effect of having an online viscometer + RI, versus just the RI alone...

I'm still digging for a clear answer about the PS versus PMMA standards for universal calibration, but very much appreciate your response. PSS is a great resource.

Thanks!

[Gerhard Kratz]

linda.lloyd@agilent.com
Maybe Linda can help you or
JLeinweber@polymer.de from PSS.
Good luck