Weighing picogram aerosol droplets with an optical balance

Optical traps are used to isolate and manipulate small objects in air and liquids, enabling the thorough characterization of their properties in situ. However, no broadly applicable technique for mass measurements of optically trapped objects is currently available. Here we propose an optical balance for mass measurements of optically trapped aerosol particles. By analyzing light-induced harmonic oscillations of a particle, its mass is determined non-destructively and with high accuracy on a time scale of seconds. Its performance is demonstrated for aqueous salt droplets, where masses as low as 4 pg (4 × 10⁻¹⁵ kg) have been measured with an accuracy of ~100 fg. The balance is straightforward to implement and broadly applicable.