Exact mass table

Exact Mass Differences. If the exact mass of the parent or fragment ions are ascertained with a high-resolution mass spectrometer, this relationship is often useful for combinations of C, H, N, and O (Table 1.15b) ... 

Table 6.6 presents a list of some of the most commonly encountered atoms in polymer/additive analysis, together with their monoisotopic and average masses. For the same nominal mass, different exact masses (elemental compositions) do exist. Knowledge of the exact mass of an unknown substance allows its atomic composition to be established. The exact mass of an ion proves the presence of a particular species (compound in a mixture). 

If you are lucky, the ion with the highest mass to charge value will be the molecular ion. However, this is often not the case, as textbooks on mass spectrometry make clear. If it is possible to carry out high resolution mass spectrometry on the molecules in question, and the molecular ion is indeed observed, the exact mass can be used in combination with tables to obtain the molecular formula directly. Alternatively, you can use the internet (http //www.sisweb. com/cgi-bin/masslO.pl) to calculate and plot mass distributions for any molecular fragment you think may be present. 

TABLE A.1. Exact Masses and Isotopic Abundances of Selected Elements... 

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... 

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.

With permission from J.H. Beynon, Mass Spectrometry and its Application to Organic Chemistry, Amsterdam, 1960. The columns headed FM contain the formula masses based on the exact mass of the most abundant isotope of each element these masses are based on the most abundant isotope of carbon having a mass of 12.0000. Note that the table includes only C, H, N, and O. 

The elemental composition and exact mass of many common carotenoids is shown in Table F2.4.1. 

Table F2.4.1 Elemental Compositions and Exact Masses" of Common Carotenoids6...

Several highly characteristic peaks are observed in Fig. 2. The compositions of till major peaks are given in Table 2. These are based on their exact mass determination, which is accurate to within 0.01 amu. This is possible in a TOFSIMS instrument because of its higher mass resolution as compared with quadrupole instruments. Therefore, peak identities can be determined with higher certainty than in quadrupole instruments which have only unit mass resolution. 

High-resolution mass speedometers which can measure m/e values to four decimal places are capable of confirming the molecular formula of die molecular ions. These so-called exact mass measurements can be used because die atomic weights of die elements are not exactly whole numbers (except for 12C, which is die standard at 12.0000 amu). The exact masses of some elements and their most abundant isotopes are given in Table 11.2. To find the exact mass of a molecule, die atomic mass of the most abundant isotope for each element is used to calculate the exact mass of the compound. This is compared to die exact mass measured on a high-resolution... 

Table 11.2 Exact Masses of Elements and Their Common Isotopes...

Consider the three compounds CsHi6N2, C9H18N, C9H16O. All would give a molecular ion of m/e = 140 in tire low-resolution mass spectrum. Using the elemental exact masses in Table 11.2, the molecular exact masses are calculated ... 

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

The table below lists some common spectral interferences that are encountered in inductively coupled plasma mass spectrometry (ICP-MS), as well as the resolution that is necessary to analyze them.1 The resolution is presented as a dimensionless ratio. As an example, the relative molecular mass (RMM) of the polyatomic ion 15N160+would be 15.000108 + 15.994915 = 30.995023. This would interfere with 31P at a mass of 30.973762. The required resolution would be RMM/8RMM, or 30.973762/0.021261 = 1457. One should bear in mind that as resolution increases, the sensitivity decreases with subsequent effects on the price of the instrument. Note that small differences exist in the published exact masses of isotopes, but for the calculation of the required resolution, these differences are trivial. Moreover, recent instrumentation has provided rapid, high-resolution mass spectra with an uncertainty of less than 0.01%. 

Using Table 1.4, calculate the exact mass for the above compounds (a-o). 

These values are not the same as the atomic masses in the periodic table because these are the exact masses of individual isotopes.The masses in the periodic table are average masses of the element based on the masses and natural abundances of the isotopes of which it is composed. 

If the HRMS measured the exact mass of this ion as 44.029 mass units, we would conclude that the compound has a molecular formula of C2H40, because the mass corresponding to this formula is closest to the observed value. Published tables of exact masses are available for comparison with values obtained from the HRMS. Depending on the completeness of the tables, they may include sulfur, halogens, or other elements. 

For the bee alarm pheromone, the accurate mass turns out to be 114.1039. Table 3.4 compares possible atomic compositions, and the result is conclusive. The exact masses to three places of decimals fit the observed exact mass only for the composition C7H140. You may not think the fit is very good when you look at the two numbers, but notice the difference in the error expressed as parts per million. One answer stands out from the rest. Note that even two places of decimals would be enough to distinguish these four compositions. 

Table 3.4 Exact mass determination for the bee alarm pheromone ...

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