Earlier this month Mark Pothecary and Stephen Ball (Product Manager and Product Marketing Manager respectively for Nanoparticle and Molecular Characterization) presented a webinar on our latest development in chromatography detection: the Viscotek SEC-MALS 20. The recorded event contained a review of the 20-angle light scattering system, and a selection of suitable applications for SEC-MALS, which is sometimes also abbreviated as MALLS.
This webinar was very well attended, and can be viewed here. However, due to the number of attendees, not all questions were addressed at the ‘live’ event – so we thought it might be useful to post some of the Questions&Answers we did not get to during the event. For many issues, expanded details are available on our website, or by contacting us directly.
Q: Can Rg determination also be used for proteins so that Rg value can be compared to those obtained from SAXS?
A: The realistic lower limit for Rg measurements for any multi-angle light scattering system is ~10nm. The limit arises essentially from the wave length of laser light used and the inherent noise in the overall setup. Typical proteins are much smaller than 10nm radius, and thus the Rg is not measurable.
Q: Does dn/dc affect the Mw of polymers significantly?
A: Yes, dn/dc has an influence on the Mw obtained from light scattering. For quite a few polymers dn/dc values are known and a general discussion of this parameter can be found here.
Q: What mobile phase was used to measure the analytes?
A: SEC-MALS can be used with organic and aqueous mobile phases. The applications presented were run in aqueous buffer and in THF. Actually, one of the advantages of the optical design of the flow cell is that it will work with any mobile phase, without the requirement to change the flow cell.
Q: Does one need to use dn/dc values to measure molecular weight?
A: Yes, dn/dc is a required parameter to obtain a molecular weight. For many proteins, the value turns out to be 0.185 mL/g on average, and for many other molecules tables of dn/dc values are published.
Q: What’s the volume of the flow cell of MALS-20 detector?
A: The flow cell of the SEC-MALS 20 detector has a volume of only 63 μL.
Q: What is the difference between Rg and Rh?
A: The hydrodynamic radius Rh describes the diffusion of a scattering object. The Rg on the other hand describes the root mean squared mass-average size. For a hypothetical sphere consisting of just an empty thin shell, the two radii would be the same. For a solid sphere the ratio of Rg / Rh is 0.77 and the measured ratio can give an indication of the shape / molecular architecture of the scattering objects.
|Architecture||Rg / Rh|
|Random coil, monodisperse||1.5 – 1.8|
|Random coil, polydisperse||1.7 – 2.1|
|Regular stars||1.3 – 1.1|
|Rigid rod, monodisperse||>2.0|
|Rigid rod, polydisperse||>2.0|
For further details, see the recording of the launch event.