A mass spectrum is an intensity vs. m/z (mass-to-charge ratio) plot representing a chemical analysis. Hence, 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 acquired using an instrument called a mass spectrometer. Not all mass spectra of a given substance are the same. For example some mass spectrometers break the analyte molecules into fragments; 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 applied; however, all plots of intensity vs. mass-to-charge are referred to as mass spectra. Common fragmentation processes for organic molecules are the McLafferty rearrangement and alpha cleavage. Straight chain alkanes and alkyl groups produce a typical series of peaks: 29 (CH₃CH₂⁺), 43 (CH₃CH₂CH₂⁺), 57 (CH₃CH₂CH₂CH₂⁺), 71 (CH₃CH₂CH₂CH₂CH₂⁺) etc.