Molecular ion

An ion formed simply by the removal from (positive ions) or addition to (negative ions) a molecule of one or more electrons. For molecules containing multi-isotope elements, a number of different isotopologue molecular ions will be formed. The one containing the most abundant naturally occurring isotopes of all atoms that make up the molecule is termed the monoisotopic molecular ion, whereas the others are termed isotopic molecular ions. The most abundant of these isotopologue molecular ions is sometimes termed as the top molecular ion. The mass of the molecular ion is, by definition, calculated for the monoisotopic molecular ion using therefore the masses of the most abundant isotopes with a correction for the masses of the electron(s) that were added or removed. For instance, electron ionization of BF₃ gives two isotopologue ions, ¹⁰B¹⁹F₃^+. of m/z 67 and ¹¹B¹⁹F₃^+. of m/z 68, the heavier one being the (top) monoisotopic molecular ion with a nominal mass of 68 u (Da) since ¹¹B is more abundant than ¹⁰B. The lighter ion of m/z 67¹⁰B¹⁹F₃^+. is an isotopic molecular ion.

Related Terms

Isotopologue Ion Isotope Cluster Ion Mass Units Nominal Mass