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Mass Spectrometry

01Mass Spectrometry

Molecular ion

This section covers formation of the molecular ion and how its m/z value relates to the molecular mass.

Forming the molecular ion

Mass spectrometry can detect an ion made from the intact organic molecule as well as smaller ions formed when the molecule fragments.

Removing one electron from an intact molecule produces the molecular ion. Because an electron has been removed without pairing the remaining electron, the species is both positively charged and a radical.

M→MX+∙+eX−\ce{M -> M^{+\bullet} + e^-}

Using the molecular-ion peak

Most molecular ions considered here have a charge of +1+1. Their mass-to-charge ratio is therefore:

m/z=m1=mm/z=\frac{m}{1}=m

The M peak consequently gives the molecular mass represented by the intact, singly charged ion.

For butane, CX4HX10\ce{C4H10} has a molecular mass of 58, so its molecular-ion peak occurs at m/z=58m/z=58.

m=zrelative abundance294358molecular ionbutane

A small M+1 peak may appear one mass unit above M because some molecules contain the naturally occurring carbon-13 isotope. As the number of carbon atoms increases, this isotope peak becomes larger.

  • Use the M peak to obtain the molecular mass of the main molecular ion.
  • Do not mistake a small M+1 isotope peak for M.

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