The mass spectrometer can be used to measure mass of
Answer Details
The mass spectrometer is an instrument used to determine the mass-to-charge ratio of ions. It works by ionizing a sample, accelerating the ions in an electric and/or magnetic field, and then separating the ions based on their mass-to-charge ratio. Therefore, it can be used to measure the mass of an ion, which can represent the mass of a molecule or atom. Therefore, the correct option is "an atom."