Micron-sized iron and copper particles accelerated to 2--20 km/s in a 2 MV van de Graaff electrostatic accelerator were used to test the performance of our recently developed cosmic dust mass spectrometer. This compact in situ dust analyzer, known as the Dustbuster, is designed to determine the elemental composition of cosmic dust particles through impact ionization and subsequent time-of-flight mass spectrometry. Results from 750 laboratory impacts show high mass resolution, typically 150--350 (m/Δm) for projectile components and 300--600 for the target material (tantalum). Peaks corresponding to H, C, O, Na, and K ions are also observed, consistent with previous microparticle impact experiments. Field-induced emission of electrons immediately before impact is a possible cause of ion formation from species with high ionization potentials. The high mass resolution, large sensitive target area, and small size make the Dustbuster an ideal instrument for inclusion on a spacecraft payload.