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How can mass spectrometric data be used for structure analysis?

We have already seen the mass spectrum of a simple molecule, carbon dioxide. A more complex example is the mass spectrum of acetone, C₃H₆O, in figure 9. This mass spectrum shows many fragment ions in addition to the molecular ion at m/z 58. The most intense ions have been labeled with their mass-to-charge ratio. The fragment ions are used by mass spectrometrists to deduce molecular structures. (Sometimes the symbols [or +·, "plus dot"] and [or -·, "minus dot"] are used to indicate radical [odd-electron] ions. This can be useful in understanding ion fragmentation patterns.) For example, the loss of 15 Da from the molecular ion of acetone to give an ion at m/z 43 indicates the presence of a methyl group(CH₃) in the original molecule. A subsequent loss of 28 Da to give an ion at m/z 15 suggests the presence of CO. By rationalizing such losses and drawing reasonable structures for the resulting ions, the structures of the original compounds can often be deduced. Some commonly observed lossed are 18 Da for water, H₂O; 17 DA for ammonia, NH3; and 77 Da for the phenyl group, C₆H₅.

Another aid in determining molecular composition is exact mass measurement. Every isotope of every element (except carbon which is assigned exactly 12.00000 Da) has a unique, non-integer mass. Exact mass measurement thus allows determination of chemical composition. As illustrated in Figure 10, with high resolution it is possible to distinguish between carbon monoxide (CO, m/z 27.995) and nitrogen (N₂, m/z 28.006) by exact mass measurement. The spectrum shown in Figure 10 was recorded using an ultra-high resolution FT-ICR instrument. Notice that, unlike the simple histogram depictions of spectra in Figures 2 and 9, this spectrum is shown as a plot of the acquired data.