20.1: Molecular Mass Spectra

Figure $\PageIndex{1}$ shows a mass spectrum for p-nitrophenol, C₆H₅NO₃, which has a nominal (integer) mass of 139 daltons. If we send a beam of energetic electrons through a gas phase sample of p-nitrophenol, it loses an electron, which we write as the reaction

$$\ce{C6H5NO3} + e^{-} \rightarrow \ce{C6H5NO3^{+•}} + 2e^{-} \label{pnp1}$$

where the product is a radical cation that has a charge of $+1$ and that retains the nominal mass of 139 daltons. We call this the molecular ion—highlighted here in green—and it has a mass-to-charge ratio (m/z) of 139.

Note

Some of the terminology in this chapter was covered earlier in Chapter 11 on atomic mass spectrometry. See the first section of that chapter for a discussion of atomic mass units (amu) and daltons (Da), and of mass-to-charge ratios.

If reaction $\ref{pnp1}$ is all that happens when p-nitrophenol interacts with an energetic electron, then it would not provide much in the way of useful information. The radical cation $\ce{C6H5NO3^{+•}}$, however, retains sufficient excess energy from the initial electron-molecule collision that it is in an excited state. In returning to its ground state, the molecular ion undergoes a series of fragmentations that result in the formation of ions—called daughter ions—with different mass-to-charge ratios. A plot that shows the relative intensity of these ions as a function of their mass-to-charge ratios is called a mass spectrum. The most abundant fragment in the spectrum—shown here in red and which is called the base peak—is assigned a relative intensity of 100; the intensity of all other ions is reported relative to the base perk.

Figure $\PageIndex{1}$: The mass spectrum for p-nitrophenol showing the molecular ion at a m/z of 139 (in green) and the daughter ions (in blue) formed through fragmentation. The most abundant fragment is the base peak at a m/z of 65 (in red). The original data is available here. This particular spectrum was collected using an electron impact source; more on that in Section 20.2.

A molecule's fragmentation patterns provides rich information about its structure. Figure $\PageIndex{2}$ compares the mass spectra for o-nitrophenol, m-nitrophenol, and p-nitrophenol. All three molecules have clusters of fragment ions at similar mass-to-charge ratios, but the relative abundance of the ions in these clusters varies quite a bit from molecule-to-molecule. For example, the pink rectangle in each spectrum highlights peaks with mass-to-charge ratios from approximately 104 m/z to 115 m/z. All three molecules share the property of producing fragment ions with these mass-to-charge ratios; the relative abundance of the fragment ions, however, varies substantially between the three molecules with o-nitrophenol and p-nitrophenol having a major peak at 109 m/z, but m-nitrophenol showing no more than a trace peak at 109 m/z. We will return to the use of mass spectrometry for determining structure information later in this chapter.

Figure $\PageIndex{2}$: The mass spectra for o-nitrophenol (top), m-nitrophenol (middle), and p-nitrophenol (bottom). All three molecules show the same pattern of the mass-to-charge ratios where clusters of fragment ions appear; however, the relative abundance of each cluster varies from molecule-to-molecule. The pink rectangle highlights the m/z values of 104–115. The original data for these spectra are here, here, and here from top-to-bottom.