Using a fluorimeter for fixed-angle light scattering measurements at long wavelengths give a Rayleigh scattering signal that varies with the sixth power of the hydrodynamic diameter. There is quite a bit of spatial (and dwell-time) averaging that occurs, and the raw signal is the light scattering intensity. Probably this technique is underused; the main problem is getting a sufficiently homogeneous sample so that a single hydrodynamic radius characterizes the phenomenon of interest. (I've used this technique to monitor at complex assembly on uniformly sized vesicles.)

With a DLS instrument, one is looking at the autocorrelation of the scattering signal. It takes relatively more analysis (typically done by the acquisition software) but much more information as to heterogeneity and molecular dimensions can be discerned. In some cases there is sufficient temporal resolution of the scattering signal to deconvolute the ensemble into populations. Here's an introduction to DLS that touches on Rayleigh scattering. http://www3.nd.edu/~rroeder/ame60647/slides/dls.pdf

Most dedicated static light scattering instruments use detection at multiple angles rather then a single detector at 90 degrees. The angular dependence on scattering allows the absolute molecular weight, rather than the hydrodynamic radius, to be measured.

However, i think the Malvern zetasizer is using a different technique, electrophoretic light scattering, that is more similar to dynamic light scattering than static light scattering

http://en.wikipedia.org/wiki/Electrophoretic_light_scattering

Thanks a lot Ryan Hoffman and Jeffrey Brender. So we can call Light scattering analysis using spectrofluorimeter as Static light scattering. I need one small clarification. Is there any effect of sample dilution on light scattering analysis using single detector at 90 degrees.

The light scattering signal intensity is correlate to the molecular mass and the concentration of the molecule. To make it simple if we do not consider angular dependance, a spherical molecule of 10Da at 1mg/ml will give the same intensity than a molecule of 100Da at 0.1mg/ml.

You can perfectly well use a Zetasizer for static light scattering. The latest models work in backscattering at an angle of 178°.

Virender: re. dilution - it makes a big difference. You should be in a regime where signal/concentration gives a constant number. Then, you are truely dilute.

Thanks Alan. I will try with Zetasizer. You are right that dilution makes difference. But I have seen research articles where people follow sample dilution to monitor aggregation kinetics to manage more points before reaching saturation. But i have doubts that dilution sample at initial stages of reaction might not be similar to dilution at later stages.

In principle, the two are similar in concept and physics. The devil lies in the details. If you use short wavelengths in the fluorimeter then you can have issues with absorption by the sample or fluorescence which adversely affect any quantitative data you expect to see. Angular dependence is an issue when the typical molecular radius is > ~ 60-70x the wavelength, so again shorter wavelengths are more prone to error. If you are in that regime where angular dependence arises, you need to measure at multiple angles and fit to an appropriate curve in order to get the correct molar mass and size. How do you know if you are in that range? Either by dynamic light scattering, which the Malvern instrument will give you, or... you measure at multiple angles to check (and of course you need a multi-angle light scattering, or MALS, detector, to know that, so if you have the MALS detector you are covered both in knowing that it's needed, and in having it for good measurements). If you are following aggregation kinetics then quite possibly you will be in the isotropic scattering regime (no angular dependence) early in the process, but then proceed to angular dependence as the aggregates grow.

A proper light scattering detector can be calibrated to give you molecular weight. Fluorimeters are rarely set up to do this.

Often folks looking at aggregation kinetics do not bother with fully quantitative measurements (i.e. molar mass vs time), they only care about time constants. In that case I would go with a 90 degree or lower scattering angle. At the high backscattering angle used in the Zetasizer you will have much more skew due to the angular dependence that increases as the aggregates grow. In brief, as the aggregates grow the total scattering increases, but not nearly as much at the high backscattering angle as at lower angles.

Please see www.wyatt.com/theory for more details, and www.wyatt.com/products for descriptions of multi-angle light scattering instruments that will give you the true picture: quantitative, detailed information on size *and* Mw.