The multitude of chemical species making up particles larger than 10 nm makes it impossible to directly infer composition from particle mass. However, with the advent of high resolution, high precision mass analyzers, for the smallest of particles, it is now possible to determine exact chemical composition from particles composed of up to several tens of molecules, or even larger for particles formed in well-controlled laboratory settings. Filtering of particles by mass can be performed non-destructively, yielding a compositionally pure "sample" of particles for further study. Such unambiguous identification of particle composition leads to detailed study of their chemical transformation from an exactly known starting point, rather than inferring these changes from measurements of a disperse ensembles of particles.