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Exploring the Differences Between Relative and Absolute molecular weight

MHTu

Hello everyone,

I’ve encountered an issue where the number-average molecular weight (Mn) obtained from absolute molecular weight by multi-angle light scattering (MALS) is significantly larger than the Mn derived from relative molecular weight methods by PS standard calibration curve (Relative index). An data example as table below.

Method Mn Mw Mz PDI = Mw/Mn
Relative 1 10 100 10
Absolute 5 10 100 2

I understand that the difference may stem from the fundamental differences in analytical principles, but WHY? and I would like to provide my client with a more detailed explanation rather than simply attributing it to this.

The sample is an acrylic polymer, using THF as the mobile phase.

Could anyone suggest potential reasons for this discrepancy? Or are there any references or resources that can help demonstrate that both testing methods are reliable and that this difference is normal under certain conditions?

Thank you in advance for your insights!

  • 0

    Dear MHTu,

    Different results from different measuring principles are normal. When having calibrated conventionally against polystyrene and then obtaining Mn results for polyacrylic samples, one reason for under/overestimated values is given in the following image:

    The hydrodynamic radius is different for various samples in the same solvent, and so the elution time is different as well. For conventional calibrations it is necessary to specify what was used for the calibration. 

    With light scattering measurements, you obtain true molar masses. The primary information from LS is the Mw. Here, only the LS detector must be calibrated with an isotropic scattering and narrowly distributed sample beforehand. 

    For a better understanding you are welcome to view and read the Agilent eBooks (see the Wiki in the GPC/SEC Community) - especially the basic principles and detection methods!

    Best regards!

  • 0 in reply to akseiffert

    Dear Mr. Akseiffert,

    Thank you very much for your reply.

    In response to your statement, "The hydrodynamic radius is different for various samples in the same solvent":

    1. The data in the table represents the molecular weight of "the same sample" analyzed using two different methods. The relative molecular weight is based on a calibration curve built with PS standards.

    2. If the Mn difference is attributed to the hydrodynamic radius, why is there a significant difference only in Mn for this sample, while Mw and Mz are not notably affected? Alternatively, could the Mn discrepancy between the two methods indicate certain specific properties of the low molecular weight fraction in this sample?

    Again, I sincerely appreciate your feedback.

    Best regards

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