The mass spectrometer is used to measure the mass of particles, not
the average over molar quantities. For a natural sample (ie, one that
is not enriched to favor one particular isotope), we observe different
peaks for the different isotopes present. Hence, the M
ion is a molecule composed entirely of the most abundant isotopes.
Consider an hydrocarbon molecule containing only carbon and hydrogen;
the molecular ion signal is a molecule containing only carbon-12 and
hydrogen-1.
Because a macroscopic compound is made from a sample of carbon atoms that are 98.9% carbon-12 and 1.1% carbon-13, there is a 1.1% chance that any carbon in the molecule is a carbon-13. So, the [M+1] (since carbon-13 is one mass unit more massive than carbon-12) signal should have an intensity of
This can be extended to the [M+2] ion signal for a molecule
containing two carbon-13 atoms, but since this probability is small
(0.0121%), the [M+2] signal from carbon-13 is usually ignored.
Exercise: Examine the tabulated mass spectrum for PFTBA. From the
isotope peaks, determine the number of carbon atoms associated with
the tuning mass peaks at m/z 69, 219, 502 amu, keeping in mind that
the above statements relate to fragments as well as the molecular
ion. Do these predictions corraborate your earlier predictions for
the structure of these PFTBA fragment ions?
Exercise: Compare the molecular mass calculated using the average
masses shown on the periodic table to that calculated using the isotopic
mass of the most abundant isotopes for heptachlor, bromazepam and
diazepam. How might these results affect your assignment of molar
mass or identity confirmation? Is the nitrogen rule affected by the
difference between isotopic atomic masses vs. average masses? How
so?
Exercise: The natural relative abundance of chlorine-37 is about 33%. Use the MS UTILS software to determine how many Cl atoms there are in the molecular ion of alprazolam. How many Cl atoms are there in the m/z 273 ion as indicated by the isotope distribution? What simple cleavage reaction can be proposed that is consistent with these isotopic distributions that explains the formation of the m/z 273 ion?