Number fluctuations occur if the number of observed particles in the scattering volume vary in the course of a dynamic light scattering measurement, i.e. during a single DLS acquisition. In this situation, intensity fluctuations required to derive particle sizes are not merely attributable to Brownian motion, because the overall scattering intensity will fluctuate as well due to changes in particle numbers, and the resulting DLS data will be erroneous. Number fluctuations are caused by low particle concentration, large particles (see below), sedimentation, or if small numbers of large aggregates/dust particles move through the scattering volume. They can be spotted as sloping/elevated baselines in the autocorrelation function (see image below).

Please be aware that the laser beam in Prometheus Panta is focused into a small area of around 30 x 30 x 60 mm³ to increase sensitivity towards very small particles like proteins and to avoid flare from the capillary wall. This is why number fluctuations are more likely to occur with larger particles or low particle concentrations compared to other DLS instruments. However, the limit of detection for monomeric proteins is typically determined by the low scattering intensity and not by number fluctuations.