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Guide to molecular weight – with the Zetasizer

2 November 2017 No Comment

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Molecular weight measurements with the Zetasizer software?

First a few words of caution: molecular weight or Mw measurements in general are not easy, require time, good sample preparation skills as well as patience. If you are new to the Malvern Zetasizer it might be best to familiarize yourself first with the rapid, user-friendly, and quite forgiving technique of dynamic light scattering (DLS).

OK, with that out of the way, the software can handle static light scattering with the relatively simple concentration series method. All that’s required is

  • a scattering standard (which is usually defaulted to Toluene)
  • the dispersant, or solvent (where the sample material is dispersed in)
  • a series of concentrations in the dispersant (5 or more different ones would be a good number)

We recommend quality glass or quartz cuvettes, in order to avoid any deviations due to scratches, wall thickness variations, manufacturing imperfections that could alter the absolute scattering intensity.

The concentration calculator in the software (free download) can provide guidance on an appropriate concentration range for molecular weight measurements. This can be accessed from Tools – Calculators – SLS-concentration-calcualtor-for-molecular-weightConcentration Utilities. For example, with a sample of expected molecular weight of 75,000 a range of concentrations between 0.2 to 2.0 g/L would be suitable. (This is just an estimated guidance recommendation, somewhat outside that range should also work as well).

For the setup of an SOP (standard operating procedure or measurement method), most of the settings of the ‘Measurement Type Molecular Weight’ are self-explanatory. It is highly recommended to also perform size measurements and check the setting to save partial results (i.e. unsuitable correlation functions) anyway.

With size measurements performed during the molecular weight measurement method, it will be much easier to help troubleshoot any unexpected results.

Why is the molecular weight totally off?

Several potential reasons may combine to lead to an unexpected result.

  1. Sample preparation. The scattered intensity is the sum of all contributions in the sample which then leads to an average molecular weight. And aggregates, dust, debris all contribute to that average. Often filtration, centrifugation, or chromatography may be helpful to “clean up” the sample. Perform DLS size measurements to confirm.
  2. Dispersant not clean. Similar to the first point, specifically if the measurement of the dispersant or solvent is off, this will influence the overall result.
  3. Unsuitable scattering standard. Toluene can degrade, may need filtration, avoid contact with water as it can lead to micelles, cuvette may not be clean. A target count rate for toluene should be 150-350 kcps in a Zetasizer Nano ZSĀ  (400-700 kcps in a ZSP). Perform DLS measurement to confirm the absence of particles.
  4. Incorrect dn/dc value. It is part of the equation and enters with its square, 20% off in dn/dc means 40% off in molecular weight.
  5. Incorrect sample concentration values. Sample material not completely dissolved, material lost on filtration

If none of those are a concern, send us the data.

For routine molecular weight measurement use GPC!

Static light scattering can be cumbersome. If this information is routinely needed, it may be better to consider light scattering in conjunction with a separation technique (often known under the acronym SEC-MALS) like chromatography (for example an OMNISEC), where the sample is always automatically “cleaned up” due to the separation mechanism.

Hope the above can facilitate some improved molecular weight measurements on the Zetasizer for you.

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If you have any questions, please email me at ulf.nobbmann@malvern.com. Thanks! While opinions expressed are generally those of the author, some parts may have been modified by our editorial team.