Talk:Mass (mass spectrometry)

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The nominal mass is the integer mass of the most abundant naturally occurring stable isotope of an element. The nominal mass of a molecule is the sum of the nominal masses of the elements in its empirical formula.[1]

H2O = 18

This is a poor definition, According to this deuterium should not have a nominal mass. So D2O should not have a nominal mass.

Exact Mass[edit]

I have seen numerous cases of articles on chemical compounds in Wikipedia containing the "property" exact mass. I would appreciate it is someone could explain why this is NOT vandalism. Few if any chemical compounds exist as combinations of single isotopes. So the inclusion of "exact mass" if profoundly misleading. If used, the isotopic composition should also be listed. It is just wrong.71.31.153.56 (talk) 23:10, 11 June 2011 (UTC)[reply]

Exact mass is indeed only a property of a specific isotopic composition. This does not necessarily imply that the property is invalid for any chemical compound that is naturally a mixture of "isotopomers" (the compound but a specific isotopic composition thereof). The property is only valid when referring to a specific isotopomer. Frequently, and when not further specified, "exact mass" implicitly refers to the "monoisotopic exact mass". The monoisotopic mass is the mass of the isotopomer that consists exclusively of the most abundant isotope of each element and thus there is only one monoisotopic mass for each elemental composition and thus chemical compound. Note that monoisotopic mass is not implicitly exact and is frequently experimental. Frequently, monoisotopic masses are referred to as "accurate masses" since high resolution is required to resolve isotopes and thus the measurement thereof is frequently accurate, but not exact. Exact mass is exact and a unique fundamental property of each elemental composition (in the most common monoisotopic exact mass case), or of each isotopomer in the more specific isotopic case. The "monoisotopic exact mass" is the only mass property for chemicals that is unique and invariable. That said these issues are fairly uncommon and perhaps inappropriate outside of mass spectrometry and maybe particle physics (where isotopic mass is more common, since usually lone isotopes are used).--Nick Y. (talk) 14:26, 13 June 2011 (UTC)[reply]