Magnet Academy: Mass Spectrometry: How to Weigh an Atom

It's hard enough to weigh something as itty bitty as atoms or molecules. Factor in that they're careening by faster than Jeff Gordon on steroids, and you get an idea what scientists are up against. Using comet particles from NASA's Stardust mission as an example, this article explains how scientists measure atoms, and what kind of secrets they can uncover in the process. (Includes Java applet)