Hello,

I'm analyzing data from an Horiba DLS instrument. The good thing with this instrument is that I can analyze the data with two angles (90 and 173º). So to get a more reliable fit, I fit the two angles G2(tau) correlation curves at the same time with a cumulants expression:

G2=A+B*(exp(-1.*C.*xdata)*(1+0.5*D.*xdata^2)).^2 as described in (Mailer, Clegg and Pusey 2015, I think taken from B Frisken). 

Here A and B are fitting parameters close to 1, C is the decay rate, and D is a parameter related with the polydispersity. According to wikipedia, the polydisperisty index is: D/(C^2)

Once I get the decay rates (parameter C) for the two angles, I obtain the z-weighed diffusion coefficient from the slope of the curve C=d*q^2 where q is the scattering angle. With the diffusion coefficient I get the size of the particles (aprox. spherical).

Seems a lot of work to get parameters that the instrument software gives you, but the fact that with this I can get consistent results for both angles at the same time is really worth it, and I get a lot more confidence in my results. 

Now, the trouble I am having is to have a little more physical insight into the polydispersity index. In other words, how can I relate it (even if just in an approximate way) to the size of my particles?

I have two types of particles. One of about 90 nm and other of about 150 nm. Both have a polydispersity index (D/(C^2)) of 0.28-0.29.

I made a lot of measurements in different samples, and I get consistent results, so I am very confident about these values, and know that at least the samples cannot be too polydisperse. But for this, can I get an idea of the actual width of my size distribution?

(I understand this may be complicated due to bigger scattering power of larger particles).

Any comments are welcome. Just don't bother with the inverse Laplace transform, because I don't want to go through that direction.

Thank you