Exact isotope masses

In Table 8.18, mx, is the exact mass of the isotope of X that differs by i integer mass units from m, and is the corresponding relative abundance. Isotopes of relative abundance lower than 0.001 are neglected. Suchcases are with mn i = 2.014102 Da, j = 0.00015 and with = 35.967079 Da, and = 0.00020. A more extensive list of exact masses and isotope distribution for most elements is available online, e.g. [288], as well as in many standard reference books. The exact isotope masses are weighted by their intensities to calculate the average atom mass of a chemical element  

22 Definition (Average atom mass) For a chemical element X , its average atom mass is 

This mass, however, is not of any use in the field of MS as each isotope is measured individually. In the following we will examine how sets of candidate molecular formulas can be restricted using more precise masses. Additionally, we look for conditions that allow derivation of a molecular formula unambiguously from its exact mass, and how precisely the mass has to be measured for that purpose. 

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Haegele et al. (269) have used exact isotope masses and isotope abundances together in determining the detailed fragmentation patterns of square planar rhodium (I) -diketonate complexes. They found that some species postulated by other workers were in error. High resolution is needed to distinguish the 28 mass units for loss of CO (27.9949) from C2H4 (28.0313) (269) or the 69 mass units for PF2 (68.9906) from CFa (68.9952) (90). 

Use of exact isotopic masses (Table 3.2) gives the accepted value for the atomic mass of chlorine ... 

Although exact isotopic masses are known with great precision for most elements, we use the average mass of an element s atoms in most chemical calculations. Explain. 

Table 8.18. Exact isotope masses and distributions for elements of fin.

Although they are related, the measured accurate mass should be distinguished from the calculated exact mass, which is the mass determined by summing the exact isotope masses of the elements present in a particular ion. For example, the calculated exact mass of (acetone molecular ion) is 58.0419 Da, and... 

Each isotopic mass must be divided by the isotopic mass of12 C, 12 u, an exact number. 

The isotopic mass is the exact mass of an isotope. It is very close to but not equal to the nominal mass of the isotope (Table 3.1). The only exception is the carbon isotope which has an isotopic mass of 12.000000 u. The unified atomic mass... 

As a consequence of these individual non-integer isotopic masses, almost no combination of elements in an empirical formula has the same calculated exact mass, or simply exact mass as it is often referred to, as another one. [25] In other words, at infinite mass accuracy it is possible to identify the empirical formula by mass spectrometry alone. 

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 Csl and Au both bear the advantage of being monoisotopic. This insures the peak top to exactly represent the theoretical isotopic mass of the respective cluster ion, independent of its m/z ratio or actual resolution (Chap. 3.3.5, 3.4). Csl, KI, and other alkali salts providing more narrow-spaced cluster ion series can alternatively be employed as saturated solutions in glycerol. [45-47]... 

Table A. 1 comprises the stable elements from hydrogen to bismuth with the radioactive elements technetium and promethium omitted. Natural variations in isotopic composition of some elements such as carbon or lead do not allow for more accurate values, a fact also reflected in the accuracy of their relative atomic mass. However, exact masses of the isotopes are not affected by varying abundances. The isotopic masses listed may differ up to some 10 u in other publications.

Thus, the mass observed for the molecular ion of CO, for example, is the sum of the exact formula masses of the most abundant isotope of carbon and of oxygen. This differs from a molecular weight of CO based on atomic weights that are the average of weights of all natural isotopes of an element (e.g., C = 12.01, O = 15.999). 

The isotopic composition of the elements (including the exact atomic mass and the abundance of the isotopes), the atomic weights of elements, definitions and abbreviations are summarized in Appendix II. 

Because the mass of an atom s electrons is negligible compared with the mass of its protons and neutrons, defining 1 amu as 1/12 the mass of a atom means that protons and neutrons each have a mass of almost exactly 1 amu (Table 2.1). Thus, the mass of an atom in atomic mass units—called the atom s isotopic mass—is numerically close to the atom s mass number. A jH atom, for instance, has a mass of 1.007 825 amu a 292U atom has a mass of 235.043 924 amu and so forth. 

Although mass spectra usually show the particle masses rounded to the nearest whole number, the masses are not really integral. The 12C nucleus is defined to have a mass of exactly 12 atomic mass units (amu), and all other nuclei have masses based on this standard. For example, a proton has a mass of about 1, but not exactly Its mass is 1.007825 amu. Table 12-3 shows the atomic masses for the most common isotopes found in organic compounds. 

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. 

Even mass spectroscopists use the terms exact mass and accurate mass interchangeably when they probably should not. An exact molecular mass is the mass calculated from the accepted exact atomic masses of the isotopes for a specified empirical formula. It is the value that one would expect to observe if one could measure a molecular mass exactly. An accurate mass is a value measured carefully, with high precision, on an instrument capable of making such precise measurements, typically expressed to at least four decimal places—the nearest 0.1 mDa. An accurate mass measurement is compared to the exact masses of empirical formulae being considered. Sufficiently accurate measurements can be used to assign empirical formulae to peaks in a mass spectrum. Knowing that these two terms are commonly used interchangeably, but that there may be subtle differences in the way practitioners use them, is sufficient for our discussions here. We will comment more on the utility of accurate mass measurements later. 

Determination of exact molecular mass -high-resolution instruments enable the molecular formula of a compound to be determined by summation of the masses of the individual isotopes of atoms, e.g. both ethane and methanal have integral... 

In nature, elements come to us as nonchanging mixtures of isotopes, and each isotope has its own atomic mass. In order to have an atomic mass that is accurate for an element and accounts for the natural mixture of its isotopes, it is necessary to determine what is called a weighted-average atomic mass. This sounds complicated, but it is the best way to get exact atomic masses of the elements. 

The exact molecular masses of some common isotopes are listed in Table 13.3. Some computer programs can determine the molecular formula of a compound from the compound s exact molecular mass. 

We have presented the isotopic masses in (he calculations above with six digits to the right of the decimal point. Although certain types of state-of-the-art mass spectrometers are capable of such resolution (see Section 20C-4), normally, exact masses are quoted to... 

Since the advent of high-resolution mass spectrometers, it is also possible to use very precise mass determinations of molecular ion peaks to determine molecular formulas. When the atomic weights of the elements are determined very precisely, it is found that they do not have exactly integral values. Every isotopic mass is characterized by a small mass defect, which is the amount by which the mass of the isotope differs from a perfectly integral mass number. The mass defect for every isotope of every element is unique. As a result, a precise mass determination can be used to determine the molecular formula of the sample substance, since every combination of atomic weights at a given nominal mass value will be unique when mass defects are considered. For example, each of the substances shown in Table 1.4 has a nominal mass of 44 amu. As can be seen from the table, their exact masses, obtained by adding exact atomic masses, are substantially different when measured to four decimal places. 

This tiny mass is exactly equal to one-twelfth of the mass of one atom of carbon (isotope mass number 12). This means that the mass of one atom of gC is exactly 12 on the atomic mass scale. This is written as m( gC) = 12 u. 

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