Atomic mass unit, unified

Accurate Mass An experimentally determined mass of an ion that is used to determine an elemental formula. For ions containing combinations of the elements C, H, N, O, P, S, and the halogens, with mass less than 200 Da, a measurement with 5 ppm uncertainty is sufficient to uniquely determine the elemental composition. See also related entries on average mass dalton molar mass monoisotopic mass nominal mass unified atomic mass unit. 

Atomic Mass Unit See unified atomic mass unit. 

Mass See entries on accurate mass average mass dalton molar mass molecular weight monoisotopic mass nominal mass unified atomic mass unit. 

Unified Atomic Mass Unit (u) A non-SI unit of mass defined as one twelfth of the mass of one atom of 12C in its ground state and 1.66 x 10-27 kg. The term atomic mass unit (amu) is not recommended to use since it is ambiguous. It has been used to denote atomic masses measured relative to a single atom of 160, or to the isotope-averaged mass of an oxygen atom, or to a single atom of 12C. 

In the graphic form the abscissa represents the mass of ions (to be more precise, the mass-to-charge ratio, m/z), while the ordinate represents the relative intensity of these ions peaks. Atomic mass units (unified atomic mass unit) or daltons are used as units to measure masses of ions, while intensity is represented in percent relative to the base peak in the spectmm or to the total abundance of all the ions in the spectra. The atomic mass unit (dalton) is equal to the mass of one-twelvth of the mass of a 12C atom (1,661 x 10-27 g) (see Chapter 1). 

The unified atomic mass unit (u), previously symbolized as AMU or amu, is defined to be 1/12 of the mass of one atom of isotope 12 of carbon. Therefore,... 

A factor Avogadro s constant multiplied by 10 enters these expressions on condition that atomic and electronic masses be expressed, as is customary in spectral analyses, in unified atomic mass unit both Uifi and f/o,i contain mass in their units, despite their values being formally independent of atomic mass. The standard errors associated with values of k and in Table 1 include contributions from errors of pertinent fundamental physical constants [94]. 

Symbol used to denote the dimensionless quantity formed by dividing the mass of an ion in unified atomic mass units by its charge number (regardless of sign), m/z should be written in italic and lower case. The Thomson (Th) is sometimes used as unit but it is not recommended. 

Note 3 The dalton, symbol Da, is an alternative name for the unified atomic mass unit, Wu, symbol u, i.e., Wu = 1 u. 

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. 

Mass Unit The unified atomic mass unit, or u, is the fundamental unit of mass for most mass spectrometrists. The Dalton, or Da, is also generally accepted and is commonly used in descriptions of large, biological molecules. The mass unit is defined as one-twelfth of the mass of carbon-12. Atomic mass unit, or amu, is technically incorrect but still commonly used. The unit Thomson (Th) has been used as a unit of m jz. However, Th is not accepted by most mass spectrometry journals and the International Union of Pure and Applied Chemistry (IUPAC). Therefore, m/z used for labeling the x-axis of mass spectra is unit less. 

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). 

The dalton is not accepted within SI [9], It is perceived as a molecular mass of a specific species. In protocols, molar mass, relative molecular mass or unified atomic mass units should be substituted. The last of these, in conjunction with the SI mass unit, is currently acceptable with an added relative standard uncertainty of 10 6-... 

Analyze the sample by MALDI- or ESI-MS. Each derivatized lysine will increase the peptide mass by 42 u (unified atomic mass unit). 

Mass unified atomic mass unit (6.022X10zTn g u or Da... 

The unified atomic mass unit is equal to V12 of the mass of an atom of the nuclide 12C. 

The values of these units in terms of the corresponding SI units are not exact, since they depend on the values of the physical constants e (for the electronvolt) and NA (for the unified atomic mass unit), which are determined by experiment. See chapter 5. 

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