a mass spectrum is an intensity versus "m/z" (mass-to-charge ratio) plot that represents a chemical analysis. the mass spectrum of a sample is a pattern representing the distribution of ions by mass (more correctly: mass-to-charge ratio) in a sample. it is a histogram usually made using an instrument called a mass spectrometer. not all mass spectra of a given substance are the same. it can depend on the operating conditions of the instrument. for example, some mass spectrometers break the analyte molecules into many pieces; others observe the intact molecular masses with little fragmentation. a mass spectrum can represent many different types of information based on the type of mass spectrometer and the specific experiment; however, all plots of intensity vs. mass-to-charge are called "mass spectra".