NIBS-175-degree-setup-scattering-volumeEvery once in a while the issue of “How large is the actual scattering volume?” comes up in dynamic light scattering. To understand the background let’s look at the optical design inside the experimental setup. This design differs between different configurations, and there are two main configurations of the Zetasizer: the Non Invasive Back Scattering or NIBS optics and the traditional 90 degree scattering optics. Since NIBS has become the more popular choice over the last years let’s consider it first.

Scattering volume of the Nano S

The Zetasizer models NanoS, Nano ZS, and Nano ZSP all incorporate NIBs optics.  In this case, the scattering volume can be estimated from the beam width and the overlap section to be approximately

Scattering volume (NIBS) ≈ 50μm x 50μm x 50μm / tan(180°-175°)  ≈ 1. 4 nL

For more details on how the estimate is obtained check this frequently asked question FAQ Scattering volume approx 1.8nL(0.57×0.05×0.05). The increased scattering volume as compared to the traditional 90-degree optics is actually one of the great advantages of the NIBS optics: it provides more signal due to the fact that the actual detection volume is an order of magnitude larger than the setup which had typically been used before.

Scattering volume of the Nano S90

The optical setup in the traditional 90 degree scattering optics, such as in the Zetasizer Nano S90, Nano ZS90, or μV can be approximated as

scattering volume (90°) ≈ 50μm x 50μm x 50μm  ≈ 0.13 nL

Here, the assumption of 1/e size is made, and the optical effect of the Gaussian shape of the beam is neglected i.e. not taken into account. Nevertheless, this FAQ Scattering volume in 90 degrees approx 0.2nL(0.057×0.057×0.057) should  provide a reasonable estimate of the scattering volume in the 90 degree setup.

Scattering volume in NTA

The optical setup in the Nanosight nano particle tracking (NTA) system although not strictly dynamic light scattering is of interest for comparison. The illuminated detection volume in the NS300 can be approximated as

scattering volume (NTA) ≈ 80μm x 100μm x 10μm ≈ 0.08 nL

and turns out to be quite similar to (about two thirds of) the volume of a traditional 90 degree setup.

Previously

If you have any questions, please email me at ulf.nobbmann@malvern.com. Thanks! While opinions expressed are generally those of the author, some parts may have been modified by our editorial team.