Integer mass

Chemical name Formula Integer Mass Accurate Mass... 

A Listing of Elemental Compositions vs. Accurate Mass at Nominal Integer Mass of 58... 

For molecules, the integer molecular mass is obtained by using the molecular formula and adding up the relevant individual masses. Ammonia (NH3) has an integer mass of 17, made up of 1 x 14 for N and 3 X 1 for hydrogen. 

For larger masses, the possibilities increase enormously. At mass 100, there would be literally thousands of possible elemental compositions so that, although integer mass can be measured mass spectromet-rically, attempts to obtain elemental compositions will not lead to a definite answer. 

Nominal ion mass. The mass of an ion with a given empirical formula calculated using the integer mass numbers of the most abundant isotope of each element, e.g., C = 12, H = 1, O = 16. 

Example The nominal mass of an ion is calculated by adding the integer masses of the lightest isotopes of all elements contributing to the molecule, for example, the nominal mass of H20 is (2 1) + 16 Da = 18 Da. 

In order to calculate the approximate mass of a molecule we are used to summing up integer masses of the elements encountered, e.g., for CO2 we calculate the mass by 12 u + 2 X 16 u = 44 u. The result of this simple procedure is not particularly precise but provides acceptable values for simple molecules. This is called nominal mass. [3]... 

The nominal mass is defined as the integer mass of the most abundant naturally occurring stable isotope of an element. [3] The nominal mass of an element is often equal to the integer mass of the lowest mass isotope of that element, e.g., for H, C, N, O, S, Si, P, F, Cl, Br, I (Table 3.1). The nominal mass of an ion is the sum of the nominal masses of the elements in its empirical formula. 

Note Commonly, the term mass defect, defined as the difference between the exact mass and the integer mass, is used to describe this deviation. [3] Application of this concept leads to positive and negative mass defects, respectively. In addition, the association of something being defective with certain isotopic masses can be misleading. 

The difference between the exact mass of an atom molecule, ion and its integer mass in MS. In physics, the mass defect represents the difference between the mass of an atom and the sum of the masses of its unbound constituents. 

The nominal mass of a molecule or ion is the integer mass of the species with the most abundant isotope of each of the constituent atoms. For carbon, hydrogen, and bromine, the most abundant isotopes are l2C, H, and 79Br. Therefore the nominal mass of C2H5Br is (2 X 12) + (5 X 1) + (1 X 79) = 108. 

The inside cover of this book lists the atomic number (the integer mass) of the long-lived isotope of Ra, which is 226. 

Nominal Mass The whole-number (nominal) mass of a molecule (or atom) is calculated from the integer mass of the most abundant, stable isotope of each constituent atom. For example, the nominal mass of protonated dextromethorphan (C18H25NO + H+) is 272 [(18 x 12) + (26 x 1) + (1 x 14) + (1 x 16)]. 

Mass Defect The difference between the exact mass of an ion or molecule and the nominal (integer) mass. The mass defect can be highly characteristic of the constituent atoms and is useful in data handling (see below and Chapters 5 and 6). 

TABLE 4.1. Relative Integer Masses and Exact Masses for Some Elemental Combinations... 

I Atomic mass and mass number are not the same. Atomic mass refers to the naturally occurring mixture of isotopes mass number refers to an individual isotope. Atomic mass is an average and is never an exact integer mass number is a sum (of the number of protons plus the number of neutrons) and is always an integer. Except for the artificial elements, mass numbers are not given in the periodic table. 

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