Nominal mass

Proposed Entry

The mass of an ion in atomic mass units rounded to the nearest integer.

M = n Da = round(m)

The concept of nominal mass is useful because the mass of (small) molecules is always very close to their nominal mass. Many types of mass spectrometers do not have sufficient resolving power to distinguish molecules with the same nominal mass. For example CO and N2 both have the same nominal mass M = 28 and cannot be resolved by many mass spectrometers even though the accurate mass m is slightly different for the two molecules.

See also Accurate Mass, Average Mass, Atomic Mass Unit, Dalton.

Other Definitiions

Base Peak Glossary

The mass of an ion containing the most abundant isotopes, calculated with nominal atomic weights.??????????????

Little Encyclopedia of MS

The nominal mass of an ion is calculated by simply adding the integer masses of the lightest isotopes of all elements contributing to the molecule, e.g. to calculate the nominal mass of CH₃Br you have to use the nominal masses of ¹²C, ¹H, and ⁷⁹Br to obtain M_nomi = 94 amu

Shimadzu Mass Spectrometry Glossary

Same meaning as mass number (Nearest integer when expressing the mass of an atom in units that define ¹²C as 12. The number is approximately the sum of protons and neutrons in the atomic nucleus. In mass analysis the mass number is the sum of the masses of the atoms making up an ion or other particle (¹²C = 12, ¹H = 1, ¹⁶O = 16, ¹⁴N = 14...))