Deviations from Nominal Mass

As just demonstrated, exact mass values show some deviation from nominal mass that can be on either side, higher or lower, depending on the isotopes encountered. While the matter itself can be easily understood, existing terminology in this specific field is comparatively vague. 

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 term mass deficiency better describes the fact that the exact mass of an isotope or a complete molecule is lower than the corresponding nominal mass. In case of for example, the isotopic mass is 15.994915 u, being 5.085 mmu deficient as compared to the nominal value. 

Note The use of nominal mass is limited to the low mass range. Above about 500 u the first decimal of isotopic mass can be larger than. 5 causing it to be rounded up to 501 u instead of the expected value of 500 u. This will in turn lead to severe misinterpretation of a mass spectrum (Chap. 6). 

The Kendrick mass scale is based on the definition M(cr = 14.0000 u. [28] Conversion from the lUPAC mass scale to the Kendrick mass scale is achieved by multiplying each mass by 14.00000/14.01565 = 0.99888. The intention of the Kendrick mass scale is to provide an effective means of data reduction, an issue gaining importance again as a result of the steadily increasing resolution and mass accuracy (next paragraphs) of modem mass spectrometers. 

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Fig. 3.12. Deviation from nominal mass for some oligomers as a function of nominal mass. PE polyethylene, PEG polyethyleneglycol, PIPE polytetrafluoroethylene, PVC polyvinylchloride.

Pure isotopes have masses that differ measurably from the integers A (based on a scale on which has a mass defined as 12.00000 atomic mass units or amu). None of the stable nuclides (i.e. those with nuclear half-lives >10 years) deviates from integer mass by more than 0.1 amu. The nominal mass of a molecule is the sum of the A values of its constituent atoms. 

Drops for oral use have to meet the requirement that the separate masses of 10 miits equivalent to the normal dose deviate maximally 10 % from the average mass. The total of 10 masses does not differ by more than 15 % from the nominal mass of 10 doses. 

This separation in m/q of different secondary ions is possible as the exact mass of every element, with the exception of C, deviates slightly from their nominal mass (reasons for this are outlined in Section 2.1.1). Indeed, all ions with Z < have a slightly greater mass than their nominal mass, whereas those... 

The experiments were conducted on a heat exchanger described above, having 17 triangular micro-channels with hydraulic diameter of 157 pm and mass flow rates of 5.7 ml/min. The applied heat flux ranged from up to 35 W/cm. The deviation of q from its nominal value does not exceed 0.5%. The characteristic value of the... 

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