I am trying to tag isolated exosomes with PKH67 dye, but every time after doing one round of ultracentrifugation (2,00,000Xg, 90 min) I see some pellets in the control sample with PBS. I am tagging 20ul of the sample, with 1ul of PKH67 with a final volume of 200ul in Diluent C. In the control, I take 20ul of PBS and rest all are kept the same. Does the pellet formation in control signify excess dye remaining? If so then how to get rid of the excess dye and what should be the starting dye concentration?