When you think of gel permeation chromatography / size exclusion chromatography (GPC/SEC) you probably associate the technique with the analysis of macromolecules such as proteins and polymers.  After all, that’s what it does best!  However, as researchers and manufacturers push the boundaries of materials science in all directions, the complete characterization of low molecular weight polymers and oligomers has garnered increased interest.  Malvern Panalytical’s OMNISEC GPC/SEC system was designed with increased sensitivity in its refractive index (RI) and light scattering detectors with this specific challenge in mind.  These features allow OMNISEC to excel and provide reliable data even when macromolecules become not-so-macro-molecules.

Therefore, to test OMNISEC’s ability to handle low molecular weight materials I put it through two challenges:

  • First, analyze a low molecular weight polymer.
  • Second, characterize a mixture of oligomers.

The setup

For these analyses, I used a multi-detector OMNISEC system running a mobile phase of THF at 1 mL/min and equipped with 2 × T3000 columns.  The sample concentrations were about 15 mg/mL, a bit higher than the typical 3-5 mg/mL due to their low molecular weight, and 100 µL were injected at a time.

First challenge

Firstly, obtain absolute molecular weight, intrinsic viscosity (IV), and other multi-detector data on a low molecular weight polymer.

Chromatogram of low molecular weight polymer

Chromatogram of low molecular weight poltmer

As is evident from the chromatogram, all of the detectors provided excellent peaks.  A light scattering response of less than 2 mV is no problem for OMNISEC.

Calculated data for low molecular weight polymer

Table of data collected from three injections of the low molecular weight polymer

The calculated molecular data for three injections of the polymer was very repeatable.  In addition, with a molecular weight of 437 Da, an IV value of about 0.02 dL/g, and a hydrodynamic radius (Rh) of just under 0.5 nm, this polymer sounds like an oligomer at most!  First challenge completed.

Second challenge

Secondly, using the same system and setup, obtain absolute molecular weight, IV, and related advanced detection data to thoroughly characterize a mixture of oligomers.

Chromatogram of oligomers

Chromatogram of mixture of oligomers

The calculation limits present in the chromatogram above identify and indicate seven separate fractions within the sample.  This type of resolution is dependent on the column selection. In addition, this resolution is becoming increasingly common with the combination of UPLC and advanced detection.

Calculated data for oligomers

Table of data for all seven fractions observed in the mixture of oligomers

I was impressed with the calculation of the characterization data for each peak!  Especially that OMNISEC could calculate data for even the smallest fraction, all the way down below 300 Da!  And as a bonus, an additional piece of information is available when setting multiple sets of limits: the % fraction of each peak.  In short, second challenge completed!

Final thoughts

In conclusion, OMNISEC rose to the occasion and successfully completed both challenges analyzing small and oligomeric samples.  So, if you have low molecular weight samples or oligomers in need of characterization, please contact us or reach out to me individually (kyle.williams@malvernpanalytical.com) to put OMNISEC to the test!

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