Molekyljonen

Molekyljonen, also known as the molecular ion, is the ion produced from a molecule by removal of one electron, yielding a radical cation with the formula M+•. In mass spectrometry, it represents the intact molecule in its ionized form, and its m/z value corresponds to the molecule’s molecular mass in electron ionization conditions.

Formation

In electron ionization (EI), a molecule M is exposed to high-energy electrons, producing M+• and a free electron. The stability of the radical cation depends on the molecule’s structure; conjugation, aromaticity, and heteroatoms can help stabilize the ion, while many molecules undergo fragmentation to form smaller ions. Other ionization methods may produce alternative ionic forms of the molecule, including adducts, which are not strictly the bare molecular ion but still provide molecular information.

Observation and interpretation

In a mass spectrum, the molecular ion peak, if present, appears at m/z equal to the molecule’s molecular weight. The peak’s intensity depends on the compound and the ionization conditions. Isotopic patterns (for example, chlorine or bromine giving characteristic pairings) aid identification. High-resolution mass spectrometry can determine the exact mass of the molecular ion, enabling the deduction of a molecular formula when combined with isotope distribution data.

Limitations and variants

Not all compounds form a stable molecular ion; some exhibit rapid fragmentation, making M+• weak or unobservable. Depending on the ionization technique, researchers may instead observe adduct ions such as [M+H]+ in electrospray or [M+Na]+ in MALDI, which provide molecular information but differ from the bare molecular ion.

Examples

Benzene (C6H6) yields M+• at m/z 78; toluene (C7H8) at m/z 92; methane (CH4) at m/z 16. The presence and clarity of M+• assist in determining molecular weight and, with accurate mass, the molecular formula.