vCal™ MESF Calibrator nanoBeads are surface dyed beads calibrated against fluorescence of solutions of known concentration of the same fluorophore. Each product contains 3 populations of beads with increasing amounts of the specified fluorophore. MESF beads are necessary to achieve quantitative immunofluorescence of EV surface cargo which should be performed to ensure EV cargo data is comparable across time and labs and also to understand limit of detection for an instrument in a particular channel to interpret negative events when running samples. The 800nm size is suitable for detection on flow cytometers with lower scatter sensitivity allowing assessment of an instrument’s sensitivity in a given channel independent of scatter performance. Fluorescence intensities these beads are in a relevant range for vesicle immunostaining and consequently much dimmer than other commercial products.