![]() ![]() 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. Of course, propane and acetaldehyde are distinguished by elemental analysis data. So the appearance of a mass spectral peak at m/z=44 does not distinguish between propane and acetaldehyde. The mass spectrum of the compound acetaldehyde (ethanal), with molecular formula C 2H 4O, also shows a molecular ion at m/z=44 for +. Molecules with 2H dont contribute very much to the intensity because of the low natural abundance of deuterium as shown in Table 1. ![]() 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. 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. 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. A very minor contribution to the height of the peak at m/z=45 comes from +. ![]() The mass spectrum will also show a smaller peak at m/z=45 amu for the lower abundance ion, +. This is called the molecular ion and given the symbol M +. The mass spectrum will show a peak for + at a mass to charge ratio (m/z) to the nearest integer of 44 amu. For example, lets look at the simple molecule propane, C 3H 8. 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.īecause 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. 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. One of the most common methods for creating a single positive charge is by driving an electron out of the molecule. The charge is most commonly a single positive charge but can also be a negative charge. In the mass spectrometer a compound of interest is vaporized and given a charge by various techniques to be described in some detail later. Atomic masses and percent natural abundance of isotopes of elements relevant to organic compounds. Isotopes of elements important to the organic chemist together with their natural abundance are summarized in Table 1 (below). 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. The only other stable isotope has atomic mass of 13.00335 amu and is present in nature as 1.11% of the carbon. 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 ratio of these isotopes is a constant. Most elements in nature are present as a mixture of stable isotopes that differ by the number of neutrons in the nucleus. 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. ![]()
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