Zetasizer Red Sensitivity – How much protein do I need for the Zetasizer red?

You have been asking for a while, so here it is! The Zetasizer Red sensitivity calculator! It predicts the minimum concentration in order to measure your protein. There are different values for the size from backscattering, side scattering, or multi-angle MADLS scattering. And as a bonus, the sensitivity calculator also estimates the hydrodynamic size of a typical globular protein. All you have to do is enter the molecular weight.

Why a sensitivity calculator for light scattering?

The intensity of light scattering depends strongly on the size of a molecule or particle: the larger the object, the more light it scatters. In order to acquire good statistics, we then want to ensure enough light arrives at the detector. This is related to the size of the object. We can estimate the size of a protein molecule from its molar mass or molecular weight. In order to get to this estimate, we assume a typical globular shape. The calculator gives you both the size and the minimum concentration for a Zetasizer Advance (Red Label) system, specifically for the Zetasizer Red Ultra/Pro/Lab models.

How do I use the calculator for the Zetasizer red?

The first input box in the sensitivity calculator is for the molecular weight of your protein. Simply enter the expected molar mass of your molecule in kilo Dalton [kDa]. However, if you only know the number of amino acids, you can estimate a typical amino acid is about 0.11kDa. Similarly, if you have the number of base pairs, a typical is about 0.65 kDa. Once you enter a number in the molecular weight box, the estimated radius and sensitivity values automatically update in your browser.

What are the different sensitivity values?

There are basically two kinds of sensitivity:

• DLS or size sensitivity. This is for performing dynamic light scattering or size measurements. There are different values for different single scattering angles (backscattering at 173°, side scatter at 90°, forward scatter at 13° ), and yet another value for performing MADLS (for combining back, side, and forward to obtain higher resolution).
• ELS or zeta sensitivity. This is for performing electrophoretic light scattering or mobility measurements. ELS or zeta measurements are always at a forwarding angle, so there is only a single sensitivity value.

Hope this gets you to the correct minimum concentration for your protein! Thanks for using the Zetasizer red sensitivity and size calculator for proteins.