Mass (mass spectrometry)

[1] The unified atomic mass unit (symbol: u) is equivalent to the dalton.

One dalton is approximately the mass of one a single proton or neutron.

[3] The amu without the "unified" prefix is an obsolete unit based on oxygen, which was replaced in 1961.

For example, the average mass of natural water with formula H2O is 1.00794 + 1.00794 + 15.9994 = 18.01528 Da.

[10] For molecules with mass below 200 Da, 5 ppm accuracy is often sufficient to uniquely determine the elemental composition.

The exact mass of heavy water, containing two hydrogen-2 (deuterium or 2H) and one oxygen-16 (16O) is 2.0141 + 2.0141 + 15.9949 = 20.0229 Da.

For typical organic compounds, where the monoisotopic mass is most commonly used, this also results in the lightest isotope being selected.

Isotopomers should not be confused with isotopologues, which are chemical species that differ in the isotopic composition of their molecules or ions.

For example, three isotopologues of the water molecule with different isotopic composition of hydrogen are: HOH, HOD and DOD, where D stands for deuterium (2H).

[16][17] The method of stating mass was suggested in 1963 by the chemist Edward Kendrick.

[28] The nitrogen rule states that organic compounds containing exclusively hydrogen, carbon, nitrogen, oxygen, silicon, phosphorus, sulfur, and the halogens either have an odd nominal mass that indicates an odd number of nitrogen atoms are present or an even nominal mass that indicates an even number of nitrogen atoms are present in the molecular ion.

J. J. Thomson discovered the isotopes of neon using mass spectrometry.
Theoretical isotope distribution for the molecular ion of caffeine
Theoretical isotope distribution for the molecular ion of glucagon (C 153 H 224 N 42 O 50 S)
Francis William Aston won the 1922 Nobel Prize in Chemistry for his discovery, by means of his mass spectrograph , of isotopes, in a large number of non-radioactive elements, and for his enunciation of the whole number rule . [ 24 ] [ 25 ]