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How to Find the Mass to Charge Ratios

Mass spectroscopy separates chemical molecules and parts of molecules based on their mass to charge ratio. Calculate the mass of a molecule or part of a molecule (fragment) by summing the atomic mass units (amu) of the atoms in the molecule or fragment as found on the periodic table. When the molecule passes through the ionization chamber, it emerges as a positively charged ion along with fragments of the molecules that form from the bombardment of electrons. Each molecule or fragment of a molecule created in the ionization chamber moves through the spectrometer based on the mass to charge ratio.

Things You'll Need

Periodic table
Calculator

Instructions

- 1
  Calculate the atomic weight of the parent molecule. Add the atomic weights of each of the atoms present in the molecule together. The atomic weight of each atom appears on the periodic table. For example, if the sample contains ethanol (CH3CH2---OH), then the parent molecule contains two carbon atoms, six hydrogen atoms and one oxygen atom. The atomic weight of carbon is 12.011, of hydrogen is 1.0079 and of oxygen is 15.999. Sum the values of all the atoms in the molecule, (2 x 12.011) + (6 x 1.0079) + 15.999 = 46.068.
- 2
  Determine from the spectrum of the sample what the mass of the highest peak is. The highest mass value peak on the spectrum is the parent molecule with a single positive charge. If there is a second positive charge on the parent molecule, then you should also find a peak at mass value that is half of the parent molecule. The charge on an electron is -1. If there is a +2 charge on a molecular ion or fragment, then a peak should appear at a mass value of one-half of the mass of the fragment or molecular ion.
- 3
  Divide the mass of the fragment or molecular ion by the number of charges it possesses. For example, using ethanol as the sample, the mass to charge ratio is mass ethanol / charge on the molecular ion, which is 46.068 / 1, or 46.068. If there were two positive charges on the molecular ion, then the mass to charge ratio is 46.068 / 2 = 23.034. A peak will appear at each mass to charge ratio for molecular ions or fragments present. A fragment is determined in the same way. Take CH3- for example; the mass is 12.011 + (3 x 1.0079) = 15.034. The charge ratio is 15.034 / 1 = 15.034.

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