The choice of suitable pore and particle sizes enabled baseline separation between AAV8 and its aggregates. nSEC fractions (AAV8 and AAV8 aggregates) were collected and infused at low nano flow-rate using capillaryelectrophoresis (CE) with sheath liquid MS interface and ionized by nano electrospray ionization (ESI). CD-MS was used to measure individual particle ions and determine the ratio of empty and filled AAV8-capsids. Importantly, AAV8 capsid was selected as a testbed due to its promising clinical relevance.