Natural Abundance Atomic Isotopes

To use the exact molecular mass obtained from high resolution mass spectrometry for obtaining the molecular formula, an understanding of the natural isotopes of elements is required. Most elements in nature are present as a mixture of stable isotopes that differ by the number of neutrons in the nucleus. The ratio of these isotopes is a constant. The major isotope of carbon has an atomic mass of 12.00000 amu and is present in nature as 98.89% of the carbon. The only other stable isotope has atomic mass of 13.00335 amu and is present in nature as 1.11% of the carbon. Examples of elements that have only one stable isotope are fluorine with atomic mass 18.99840 amu, phosphorous with atomic mass 30.97376 amu, and iodine with atomic mass 126.9044 amu. Isotopes of elements important to the organic chemist together with their natural abundance are summarized in Table 1 (below).

Table 1. Atomic masses and percent natural abundance of isotopes of elements relevant to organic compounds.

Element Isotope Mass (abundance) Isotope Mass (abundance) Isotope Mass (abundance)
Carbon 12C 12.00000 (98.89) 13C 13.00335 (1.11) ivory ivory
Hydrogen 1H 1.007825 (99.985) 2H 2.01410 (0.015) ivory ivory
Nitrogen 14N 14.00307 (99.63) 15N 15.00011 (0.37) ivory ivory
Oxygen 16O 15.99491 (99.759) 17O 16.99914 (0.037) 18O 17.99916 (0.204)
Fluorine 19F 18.99840 (100) ivory ivory ivory ivory
Silicon 28Si 27.97693 (92.21) 29Si 28.97649 (4.70) 30Si 29.97376 (3.09)
Phosphorous 31P 30.97376 (100) ivory ivory ivoryivory ivory
Sulfur 32S 31.97207 (95.0) 33S 32.97146 (0.76) 34S 33.96786 (4.22)
Chlorine 35Cl 34.96885 (75.53) 37Cl 36.96590 (24.47) ivory ivory
Bromine 79Br 78.9183 (50.54) 81Br 80.9163 (49.46) ivory ivory
Iodine 127I 126.9044 (100) ivory ivory ivory ivory

In the mass spectrometer a compound of interest is vaporized and given a charge by various techniques to be described in some detail later. The charge is most commonly a single positive charge but can also be a negative charge. One of the most common methods for creating a single positive charge is by driving an electron out of the molecule. The mass spectrometer measures the mass to charge ratio of individual ions commonly written as m/z where m is the mass in amu and z is the charge in units of the charge of a proton.

If the charge is a single positive charge created by removing an electron, z=1, then the mass to charge ratio is equal to the molecular mass since the mass of the electron removed to create the positive charge is negligible.

Because the mass spectrometer measures the mass to charge ratio of individual ions, the molecular masses are measured by the mass spectrometer for all possible isotopic compositions at natural abundance. For example, let’s look at the simple molecule propane, C3H8. The mass spectrum will show a peak for [12C31H8]+ at a mass to charge ratio (m/z) to the nearest integer of 44 amu. This is called the molecular ion and given the symbol M+. The mass spectrum will also show a smaller peak at m/z=45 amu for the lower abundance ion, [13C12C21H8]+. A very minor contribution to the height of the peak at m/z=45 comes from [12C32H1H7]+. The ratio of the height of the peak at m/z=44 to the height of the peak at m/z=45 is 100:3.3 because the natural abundance of 13C is 1.1% and any one of the three carbons can be 13C. The peak at m/z=44 is the molecular ion and the peak at m/z=45 is an isotope peak for the molecular ion. An even smaller isotope peak will appear at m/z=46 representing molecules with two 13C atoms or with one 13C atom and one 2H atom. Molecules with 2H don’t contribute very much to the intensity because of the low natural abundance of deuterium as shown in Table 1.

The mass spectrum of the compound acetaldehyde (ethanal), with molecular formula C2H4O, also shows a molecular ion at m/z=44 for [12C21H416O]+. So the appearance of a mass spectral peak at m/z=44 does not distinguish between propane and acetaldehyde. Of course, propane and acetaldehyde are distinguished by elemental analysis data. Their molecular formulas are also uniquely distinguished by m/z values for their respective molecular ions when measured more precisely than to the nearest integer value, usually to four decimal places. Such a measurement is called an exact mass measurement and is performed with a high resolution mass spectrometer. The exact mass for the M+ ion of propane [12C31H8]+ is 44.0626 amu and of acetaldehyde [12C21H416O]+, 44.0262 amu. In essence, a high resolution measurement of m/z for the molecular ion gives the molecular formula for compounds with molecular masses ranging upwards to a 1000 amu. For compounds with molecular masses above 1000 amu, an exact mass measurement will narrow the range of possible molecular formulas but will not define a unique molecular formula.

Sample Problem

Calculate the exact masses of propane and acetaldehyde.

Exact masses are calculated from the data in Table 1 as follows:

Exact mass of propane = 3 x 12.0000 + 8 x 1.00783 = 44.0626 amu

Exact mass of acetaldehyde = 2 x 12.0000 + 4 x 1.00783 + 1 x 15.9949 = 44.0262 amu


Next section: Exact Molecular Mass versus Molecular Weight

Copyright information: Original content © University of Colorado, Boulder, Chemistry and Biochemistry Department, 2011. The information on these pages is available for academic use without restriction.