Unified atomic mass definition

The atomic mass unit (mu) is also called the dalton (Da) - in honour of John Dalton. In response to the increase in the use of the name dalton for the unified atomic mass unit among chemists, it was suggested by IUPAC that the unified atomic mass unit (u) be renamed the dalton (Da). The definition of the unit would remain unchanged as one-twelfth the mass of a neutral 12C atom in its ground state. The International Union of Pure and Applied Chemistry (IUPAC) proposed that both units, u and Da, should be allowed in official use. 

Resolving Power (RP) A measurement of how effectively a mass analyzer can distinguish between two peaks at different, but similar m/z. Mathematically, the formula M/ AM is used, where M is the m/z value for one of the peaks and AM is the spacing, in unified atomic mass units, between the peaks. Most commonly, AM is the mass resolution, either via the 10% valley or FWHM definitions (see below). (Note that the definition used will affect the resolving power calculated.) Resolving power of 500-1000 approximately corresponds to unit resolution (e.g., at m/z 700 and FWHM resolution of 0.7, RP = 1000). 

One of the most useful pieces of information that can be provided by a standard mass spectrometer is the molecular mass of a compound with an accuracy of 1 dalton (Da) (1 Da= 1 unified atomic mass unit (u)= 1.660538921(73) x 10 kg). Even better, high-resolution mass spectrometry can provide us with accurate molecular masses, which are accurate to about 10 Da, depending on the total resolution of the spectrometer. At this level the atomic masses deviate substantially from multiples of 1 Da, e.g. is 12.0000 Da by definition, but the mass of isotope is 15.9949 Da (not 16), and that of is 14.0031 Da (not 14). This sensitivity can be used to work out the elemental composition or, more critically, the isotopic distribution of atoms in a molecule, which can help to identify unknown compounds. And since the basic experimental procedure involves supplying a particular quantity of energy to a molecule or ion to cause it to fragment, we can use it to deduce compound gas-phase stabilization energies, which can be further substantiated by computational modeling. This last application is rather specialized, so discussion is provided in the on-line supplementary section for Chapter 11 on stabilization. 

For the electronvolt and the dalton (unified atomic mass unit), values are quoted from the 2010 CODATA set of the Fundamental Constants (p. 1-1 of this Handbook). The value given for the astronomical unit is from Resolution B2 of the XXVIII General Assembly of the International Astronomical Union (Captaine, R, Klioner, S., and McCarthy, D. [2012]. The re-definition of the astronomical unit of length reasons and consequences. lAU Joint Discussion 7 Space-Time Reference Systems for Future Research. Bibcode 2012IAUJD 7E 40C). Note the definition of the au in terms of the meter is now exact. 

Non-SI unit of mass (symbol Da) that is identical to the unified atomic mass unit, based since 1961 on the mass definition of 12.00000 g/mol. 

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