Mass Spectrum

A mass spectrum is a bar-graph plot displaying the relative abundance of a molecule's component fragments against their mass-to-charge ratio (m/z). The x-axis represents m/z values, while the y-axis shows relative abundance calculated from signal intensity at the detector. This graphical representation allows chemists to identify fragment ions and understand molecular composition.

The base peak is the most intense signal in the mass spectrum, set to an abundance of 100 percent. All other peak abundances are scaled relative to this reference peak. In some molecules like methane, the base peak and molecular ion peak are identical, while in others like propane, the base peak may be a more stable fragment ion rather than the parent peak.

An electron beam knocks off an electron from the methane molecule, generating a methane radical cation. This molecular ion, detected at m/z = 16, can then undergo fragmentation through loss of hydrogen atoms one at a time, producing progressively lower m/z signals. This process is fundamental to understanding mass spectrometry molecular fragmentation overview.

If the molecular ion is highly unstable, the molecular ion peak may not appear in the mass spectrum. In such cases, the molecule fragments rapidly after ionization, and only the fragment ions are detected. This is common in molecules where the radical cation is energetically unfavorable compared to its fragmentation products.

Only charged fragments are detected in a mass spectrum. For example, when a methane radical cation loses a proton, it produces a neutral methyl radical that cannot be detected without further ionization. In contrast, losing hydrogen atoms generates charged species like methyl cations, which produce detectable signals at lower m/z ratios.

When a molecular ion is more stable than its fragments, the base peak and parent peak coincide, as seen in methane. Conversely, when fragments are more stable than the molecular ion, the base peak appears at a lower m/z value, as observed in propane. Molecular ion stability directly determines whether the parent peak dominates the spectrum.

The m/z ratio represents the mass-to-charge ratio of a fragment ion, where m is the mass of the charged fragment and z is the number of charges it carries. This ratio is plotted on the x-axis of a mass spectrum. For singly charged ions, m/z equals the mass; for multiply charged ions, the ratio is proportionally smaller, allowing detection of larger molecules.