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What are the characteristics of a mass spectrum?

      A mass spectrum is a graph of ion intensity as a function of mass-to-charge ratio. Mass spectra are often depicted as simple histograms as shown in Figure 2. This record of ions and their intensities serve to establish the molecular weight and structure of the compound being mass analyzed. For example, Figure 2 shows a mass spectrum of the simple molecule carbon dioxide, CO₂.

In this example, all the ions are positively charged. (It is possible to generate and detect negative ions as well.) The ionized CO₂ molecule (or molecular ion) appears at m/z 44¹. Since the ionization process breaks up or fragments some of the CO₂ molecules, a fraction of the ions appear in the spectrum at m/z values less than the m/z value that corresponds to the molecular mass of CO₂. Cleavage of a carbon-oxygen bond in the molecular ion to produce ionized carbon monoxide or ionized atomic oxygen result in the fragment ions at m/z 28 and 16; loss of two neutral oxygen atoms results in an additional fragment at m/z 12 for carbon. The molecular ion is designated as M⁺ or CO₂⁺ and the fragment ions are designated as CO⁺, O⁺ and C⁺.

¹ The ion is singly charged and the "nominal ion mass" is 44 Da: carbon=12 and oxygen=16 (in calculating nominal ion mass, atomic masses are rounded to the nearest integer).