Mass (mass spectrometry)

The mass recorded by a mass spectrometer can refer to different physical quantities depending on the characteristics of the instrument and the manner in which the mass spectrum is displayed.

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Accurate mass

The accurate mass (more appropriately, the measured accurate mass^[1]) is an experimentally determined mass that allows the elemental composition to be determined.^[2] For molecules with mass below 200 u, a 5 ppm accuracy is sufficient to uniquely determine the elemental composition.

Average mass

The average mass of a molecule is obtained by summing the average atomic masses of the constituent elements. For example, the average mass of natural water with formula H₂O is 1.00794 + 1.00794 + 15.9994 = 18.01528.

Exact mass

The exact mass (more appropriately, the calculated exact mass^[1]) is obtained by summing the masses of the individual isotopes of the molecule. For example, the exact mass of water containing two hydrogen-1 (¹H) and one oxygen-16 (¹⁶O) is 1.0078 + 1.0078 + 15.9994 = 18.0106. The exact mass of heavy water, containing two hydrogen-2 (deuterium or ²H) and one oxygen-16 (¹⁶O) is 2.014 + 2.014 + 15.9994 = 20.027.

Isotopomer

Main articles: Isotopomer and Isotopologue

An isotopomers (isotopic isomers) is an isomers having the same number of each isotopic atom but differing in their positions.^[3] For example, CH₃CHDCH₃ and CH₃CH₂CH₂D are a pair of constitutional isotopomers. An isotopomer should not be confused with an isotopologue, which are chemical species that differ only in the isotopic composition of their molecules or ions. An example is water, where three of its hydrogen-related isotopologues are: HOH, HOD and DOD, where D stands for deuterium (²H).

Mass number

Main article: Mass number

The mass number, also called the nucleon number, is the number of protons and neutrons in an atomic nucleus. The mass number is unique for each isotope of an element and is written either after the element name or as a superscript to the left of an element's symbol. For example, carbon-12 (¹²C) has 6 protons and 6 neutrons.

Molecular mass

Main article: Molecular mass

  The molecular mass (abbreviated M_r) of a substance, formerly also called molecular weight and abbreviated as MW, is the mass of one molecule of that substance, relative to the unified atomic mass unit u (equal to 1/12 the mass of one atom of ¹²C). Due to this relativity, the molecular mass of a substance is commonly referred to as the relative molecular mass, and abbreviated to M_r.

Monoisotopic mass

Main article: monoisotopic mass

The monoisotopic mass is the sum of the masses of the atoms in a molecule using the unbound, ground-state, rest mass of the principle (most abundant) isotope for each element instead of the isotopic average mass.^[4] For typical organic compounds, where the monoisotopic mass is most commonly used, this also results in the lightest isotope being selected. For some heavier atoms such as iron and argon the principle isotope is not the lightest isotope. The term is designed for measurements in mass spectrometry primarily with smaller molecules. It is not typically useful as a concept in physics or general chemistry. Monoisotopic mass is typically expressed in unified atomic mass units (u), also called daltons (Da).s

Most abundant mass

  The mass of the molecule with the most highly represented isotope distribution, based on the natural abundance of the isotopes.^[5]

Nominal mass

Main article: Nominal mass

The nominal mass of an ion or molecule is calculated using the integer mass (ignoring the mass defect) of the most abundant isotope of each element.^[6] This is the equivalent of suming the mass numbers of all constituent atoms. For example H = 1, C = 12, O = 16, etc. The nominal mass of H₂O is 18, for example.

See also

• Francis William Aston
• Atomic mass unit
• Gram-molecular weight
• List of elements by atomic mass
• Nitrogen rule (mass spectrometry)

References