Isotopic mass definition

Deep state experiments measure carrier capture or emission rates, processes that are not sensitive to the microscopic structure (such as chemical composition, symmetry, or spin) of the defect. Therefore, the various techniques for analysis of deep states can at best only show a correlation with a particular impurity when used in conjunction with doping experiments. A definitive, unambiguous assignment is impossible without the aid of other experiments, such as high-resolution absorption or luminescence spectroscopy, or electron paramagnetic resonance (EPR). Unfortunately, these techniques are usually inapplicable to most deep levels. However, when absorption or luminescence lines are detectable and sharp, the symmetry of a defect can be deduced from Zeeman or stress experiments (see, for example, Ozeki et al. 1979b). In certain cases the energy of a transition is sensitive to the isotopic mass of an impurity, and use of isotopically enriched dopants can yield a positive chemical identification of a level. 

One further interesting development took place in 1932, when Urey discovered isotopes of hydrogen. In these cases, the masses of the isotopes differ from each other by as much as 100% (Urey Grieff, 1935). The properties of these isotopes are definitely not the same. For example water (mostly protium oxide) has a boiling point of 100°C, whereas the value for deuterium oxide is 104°C. Strictly speaking, Fajans had been correct to doubt replaceability, but the problem is only significant in isotopes with very low masses, and Urey s discovery did not change the chemical definition of an element in any way. 

Use the fractional abundances and the atomic masses of fhe isotopes to compute the atomic mass according to the atomic mass definition given earlier. 

Let us briefly repeat i) the isotopic mass is also the exact mass of an isotope it) the isotopic mass is very close but not equal to the nominal mass of that isotope Hi) accordingly, the calculated exact mass of a molecule or of a mono-isotopic ion equals its monoisotopic mass iv) due to the definition of our mass scale, the isotope represents the only exception from non-integer isotopic mass. As a consequence of these individual non-integer isotopic masses, almost no combination of elements in a molecular or ionic formula has the same calculated exact mass, or simply exact mass as it is often referred to, as any other one [36]. 

Ihe calculated reference masses are based on the following values for isotopic masses 1,0078250321 Da, 12.0000000000 Da, 13.0033548378 Da, 15.9949146221 Da, Br 78,9183376 Da and Br 80.9162910Da. All listed masses refer to singly positively charged ions. A fosses for isotope peaks have been calculated for a resolving power of 10000 (10% valley definition). The mass of the electron (0.000548579911 Da) was taken into account for the calculation of the ionic masses. 

The absolute mass of an isotope can be computed by summing the individual masses of the subatomic particles (see Table 2.1) and reporting the total number of grams that an isotope must weigh however, it is more convenient to quote masses in relative terms using the atomic mass unit (amu). By definition, the most abundant isotope of carbon is exactly 12.000 amu. All other isotope masses are reported relative to carbon-12. Appendix 1 lists the average atomic mass (i.e, the average of the abundances of the stable isotopes) of each element in atomic mass units. 

Two further expressions are used in discussions on isotope ratios. These are the atom% and the atom% excess, which are defined in Figure 48.6 and are related to abundance ratios R. It has been recommended that these definitions and some similar ones should be used routinely so as to conform with the system of international units (SI). While these proposals will almost certainly be accepted by mass spectrometrists, their adoption will still leave important data in the present format. Therefore, in this chapter, the current widely used methods for comparison of isotope ratios are fully described. The recommended Sl-compatible units such as atom% excess are introduced where necessary. 

Cohen A, Hertz HS, Mandel J, Paule RC, Schaffer R, Sniegoski LT, Sun T, Welch MJ, and White E V (1980) Total serum cholesterol by isotope dilution mass spectrometry A candidate definitive method. Chn Chem 26 854-860. 

Ellerbe P, Cohen A, Welch MJ, and White V E (1990) Determination of serum uric acid by isotope dilution mass spectrometry as a candidate definitive method. Anal. Chem 62 2173-2177. 

Ellerbe PM, Sniegoski LT, and Welch MJ (1995) Isotope dilution mass spectrometry as a candidate definitive method for determining total glycerides and triglycerides in serum. Clin Chem 41 397-404. 

Ellerbe, P, Meiselman S, Sniegoski LT, Welch MJ, and White E V (1989) Determination of serum cholesterol by a modification of the isotope dilution mass spectrometric definitive method. Anal Chem 61 1718-1723. 

SlEKMANN L (1979) Determination of steroid hormones by the use of isotope dilution mass spectrometry a definitive method in clinical chemistry. J Steroid Biochem 11 117-123. 

Calcium exists in the human body as Ca(II) protein-bound and free Ca (II) ions (Dilana et al. 1994). For total extracellular Ca in plasma, serum and urine a definitive isotope dilution-mass spectrometry (ID-MS) method exist. Free Ca(II) in plasma/serum can be determined with PISE, but no definitive and reference methods exist. For Ca in faeces, tissue and blood flame atomic absorption (FAAS) is used widely. 

The atomic mass of an element is the weighted average mass of the isotopes of that element. Based on this definition, which of these does NOT show the correct atomic mass for an element ... 

Certification using one definitive method. This option is employed when a highly established, internationally accepted scientific primary method is available. The method must be shown to have negligible systematic errors and to provide sufficient measurement accuracy. An example of such a method is isotope dilution mass spectrometry. 

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