Molecular mass

Molecular mass is the mass of a molecule, usually expressed in unified atomic mass units or daltons. It equals the sum of the masses of its constituent atoms.

Unified atomic mass unit: A unit of mass defined as one twelfth the mass of a carbon-12 atom. It is the standard unit used to express the mass of an individual molecule.
Monoisotopic mass: The mass calculated using the most abundant isotope of each element in a molecule. It gives an isotope-specific alternative to mass based on average atomic weights.
Molar mass: The mass of one mole of a substance, usually expressed in grams per mole. It describes a mole of molecules, whereas molecular mass describes one molecule.
Molecular ion: An ion formed from a molecule without fragmentation, retaining its molecular composition. Its measured mass can provide the molecular mass of the neutral compound.
Dalton: A unit of mass equal to one unified atomic mass unit. Molecular masses are commonly reported in daltons.
Average molecular mass: The abundance-weighted average mass of molecules with naturally occurring isotopic compositions. It differs from monoisotopic mass because natural samples contain multiple isotopes.
Relative molecular mass: The ratio of a molecule’s average mass to one twelfth the mass of a carbon-12 atom. It is a dimensionless ratio, unlike molecular mass expressed in mass units.
High-resolution mass spectrometry: Mass spectrometry with sufficient resolving power to distinguish ions with closely spaced mass-to-charge ratios. Accurate mass measurements help distinguish formulas with similar nominal masses.
Atomic mass: The mass of an atom, commonly expressed in unified atomic mass units. A molecule’s mass is calculated from the masses of its atoms.
Mass spectrometry: A technique that measures ions’ mass-to-charge ratios to analyze their composition. It measures molecular ions to determine or constrain molecular mass.

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