22.2.3—The molecular mass of an organic molecule

The molecular ion M⁺ is formed when the molecule loses one electron without fragmenting. Its m/e, usually with charge +1, gives the relative molecular mass of the intact molecule.

First identify a plausible high-mass parent peak, then check isotope patterns and fragments. The molecular ion can be weak or absent, so use other evidence if necessary.

If the molecular-ion peak is at m/e 46 for a singly charged molecule, Mr is approximately 46. A peak at 31 may instead be a fragment.

Do not choose the tallest peak automatically or treat a doubly charged ion as if m/e equalled Mr without considering charge.