Module
3-angle MALS module
Absolute molar mass from scattered light, independent of column standards.
Molar mass from scattered light, independent of column standards.
About the 3-angle MALS detector
What static light scattering measures
In static light scattering, a coherent laser light source is aligned through the flow cell. The intensity of the scattered light is proportional to the mass, M, the concentration, c, the solvent refractive index, and the specific refractive index increment, dn/dc, squared.
Of the equation's terms, three points matter here. Mw is proportional to the excess Rayleigh ratio. dn/dc is a second-order term. And the radius of gyration, Rg, is a component of the angular dependence term.
Absolute molecular weight
Conventional calibration uses a series of narrow molecular weight standards and a curve of molecular weight against retention volume. An absolute measurement is calibrated differently. It uses a single narrow molecular weight standard to find the detector response and the instrument constants.
The concentration is then taken from the RI signal and the molecular weight from the light scattering signal.
Why more than one angle
The light scattering response has an angular dependence. Measuring at multiple angles, comparing the response factors, and calculating from them gives the radius of gyration, Rg.
In a partial Zimm plot, Kc/R(θ) is plotted against sin²(θ/2). The y-axis intercept is 1/Mw, and the slope at the intercept provides Rg. Lower angles give more certainty.
A low angle detector makes a “zero-angle” assumption. For samples smaller than about 10 to 15 nm, one angle is all you need. Above that, the angular dependence has to be measured.
Where MALS fits among the detectors
Light scattering needs a concentration detector alongside it. The detectors used for each type of calibration are:
- Conventional: RI or UV
- Absolute: RI and light scattering
- Universal: RI or UV, and a viscometer
- Triple: RI, light scattering and a viscometer
- Quad: RI, UV, light scattering and a viscometer
The dn/dc of the polymer and solvent pair also limits what light scattering can do. At 0.00 to 0.009 neither RI nor light scattering works. From 0.01 to 0.05 light scattering is impractical without high concentration and molecular weight. From 0.05 to 0.10 it is possible with good concentration or higher molecular weights, and at 0.11 and higher it is possible for most samples above 500 Da.