From each DLS acquisition, PR.Panta Control calculates an average scattering intensity (sum of detected signals over time). This intensity is proportional to the size and concentration of the particles in the detection volume, with large particles scattering more light than small particles. If a large particle (dust or aggregate) moves through the detection volume during a Size Analysis measurement, the average scattering intensity will increase for this acquisition compared to the other acquisitions of this capillary. These intensity variations are used in PR.Panta Control to identify acquisitions containing dust particles and automatically mark them as outliers. Another cause of variations in intensity is sedimentation of particles.

Inspect the autocorrelation function of the acquisitions marked as outliers to check if the observed intensity variations are indeed caused by dust or sedimentation. Working with dirt- and dust-free sample solutions is key to determine the correct size of particles. It can therefore be advisable to filter samples using filters with small cutoffs (0.22 µm or 0.02 µm) directly before starting an experiment. Of course, to filter or not to filter depends on your application. If you are, for example, interested in the long-term stability of proteins, you may want to filter the sample at the beginning of the experiment, but then keep it as is for all following measurements to assess if larger particles are forming over time.

An alternative method to remove dirt and dust is spinning down the sample directly before the experiment (e.g. 14,000g, 15 min in a tabletop centrifuge).

An alternative explanation for intensity variations are low sample concentrations resulting in low average scattering intensities or incorrect positioning of the capillary below the DLS optics. Try to increase the sample concentration to improve DLS data quality and make sure no dust/dirt particles are on the outside of the capillary.