The intensity-weighted size distribution is the main results of the size distribution fit applied to the autocorrelation function. In PR.Panta Control, it is output in the form of a graph showing the intensity-weighted particle species sorted by size:

Figure: Illustration of an intensity-weighted size distribution in PR.Panta Control. The blue line is the result of the size distribution fit, and the blue triangle indicates the mean radius of the peak. The purple marker corresponds to the average radius of the cumulant fit (a different fitting model).

A size distribution plot is created for each DLS acquisition of a measurement and is displayed in the acquisition details tab. In addition, PR.Panta Control outputs an averaged size distribution from all the acquisitions of a capillary in a size analysis measurement. 

In the tabulated results, the size distribution model reports the hydrodynamic radii and PDI for up to three discreet peaks (the model can fit more than three peaks, but only the three largest will be summarized in the results table). The respective peaks and their mean radius are highlighted in the size distribution plot (see image above). The size distribution plot will also contain a marker for the average radius of another fitting model, the cumulant fit. This makes it easy to compare the results to determine the state of the sample. Note that only for very homogeneous samples, the mean radius from a size distribution peak and the cumulant radius are expected to align. 

Size distribution "heatmap"

In thermal unfolding measurements with DLS, Prometheus Panta software outputs an additional thermal plot for size distribution data. It shows the intensity size distributions from the DLS acquisitions over temperature rotated by 90° and viewed from the top. The y-axis in the thermal plot corresponds to the x-axis in the size distribution plot of single acquisitions (see illustration). The degree of color saturation reflects the magnitude of the corresponding peak at a given temperature:

Note that Prometheus Panta software does not output an onset parameter for this size distribution data, however, the plot will help to understand the underlying pathway of aggregation. 

Intensity vs. volume vs. number

The intensity distribution is the most direct size distribution result of a DLS experiment. Because in Rayleigh scattering the scattering intensity of a particle is proportional to the sixth power of its radius, DLS is very sensitive to even trace amounts of oligomeric species or aggregates. A 10x larger particle will scatter 10⁶ x more light if present in the same concentration and is therefore overrepresented in the intensity distribution.

It is possible to convert the intensity distribution to a volume distribution, which can help to judge the severity of an aggregate contamination of the sample, as it corrects for some of the over-representation of large particles (about a factor of 10³ assuming sphere-shaped particles). Conversion to volume requires using Mie theory and knowledge of the refractive index of the sample. Furthermore, assumptions are made that a given intensity distribution is correct, all particles are spherical and have the same refractive index. The volume distribution can be further converted into a number distribution. Please keep in mind that all of these transformations are error-prone because the made assumptions do not necessarily reflect the true particle composition of a sample. It is, for example, very unlikely that a protein and any non-proteinaceous contaminant share the same refractive index. 

However, this also means that DLS is not an ideal technique for quantifying the number of particles of a given size (e.g. monomer vs aggregate). Prometheus Panta and DLS provide many powerful ways to assess the stability of a protein or biologic, but you should consider alternative methods such as chromatography or nanoparticle tracking if assessing the %aggregate is the main goal of your experiment.