Detectors for molar mass

Questions about Multi-Angle Light Scattering coupled with Size-Exclusion Chromatography or Field-Flow Fractionation

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Is MALS preferable over LALLS/RALLS detectors for molar mass determination?
Definitely. LALLS (low-angle laser light scattering) uses a single low-angle detector to approximate the scattering at zero angle while avoiding issues of angular dependence of the light scattering, but suffers from excessive noise because any contaminates in the mobile phase or dirt on the optics show up in the low-angle data and compromise the accuracy of the results.

RALLS (right-angle laser light scattering) uses a single detector at 90° scattering where the data are typically much less noisy, but if the molecules are above ~ 10 nm in radius, then one has to correct for the size- and shape-dependent angular asymmetry of light scattering. Hence both RALLS and LALLS are subject to systematic errors.

MALS (multi-angle light scattering) analysis extrapolates from multiple low-noise angles to overcome both the noise and angular asymmetry issues. In addition, MALS can determine the rms radius of the molecule if it is above 10 nm or else deduce that the size is less than 10 nm, both of which are not possible with either RALLS or LALLS.

[answer provided by Steven Trainoff, Ph.D. and Chris Deng, Ph.D., Wyatt Technology]
Daniel Some, Ph.D.
Principal Scientist
Wyatt Technology Corp.
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