Organic Chemistry Semester 1 LABORATORY MANUAL - Moravian ...

Mass Spectrometry Appendix D2 Part V. Techniques and Theory
Analysis of Mass Spectra.
Since all of the peaks in the mass spectra on the previous page occur at masses ≥ the
molecular mass of the compound, it seems reasonable to explore the possibility that the lower
mass peaks may indicate the after being struck by a high energy electron, these two
molecules break up and the peaks give us information on pieces of the original molecule.
This “fragmentation process” can be represented by the equation below. After the molecule
has been struck by the high energy electron and lost an electron it has a single + charge. So
if the molecule fragments, one of the resulting pieces will have the + charge (A + ) and the
other piece will have no charge (B.).
M + . > A + + B.
Mass spectrometers are designed to detect only positively charged particles. So, the mass of
A + can be determined from a peak in the mass spectrum of M, but the mass of B.is not
detected by the mass spectrometer. However, the masses of A + + B. = the mass of M + , so
the mass of B. can be determined by subtracting the mass of the detected peak for A + from
the mass of the molecular ion (M + ). Thus, the peak at m/z = 31 in the spectrum above arises
from the splitting of the molecular ion into an + ion of mass 31 and a neutral radical of mass
57 (M-31). Often the radical is smaller than the ion and can be identified in the molecular
structure more easily. Table 2-77 on p. 103 in Padías gives masses of many common
neutral radicals and cations formed from molecular ion fragmentations.
Exploration of Mass Spectral Fragmentation Patterns.
1. Look carefully at each of the two spectra on D1 and the 7 additional spectra below and on
pp. D3 → D4. Considering the structure of the compound, the brief introduction to mass
spectrometry on pp. D1 & D2 and in Padías & CGWW, try to identify the ions whose
masses are indicated with a number in each mass spectrum and the bond that broke to
form each ion. Assume that the most ions are produced by breakage of only one bond
in the original structure. For large mass ion peaks, you may need to use the [M + - (ion
peak mass)] value to identify the radical first. You will note that there are other peaks
"clustered" around the designated peaks. These peaks often arise from loss of hydrogen
atoms or other small particles from the major ion.
a.
Intensity
b.
m/z
Intensity
O
m/z