The technique of
mass spectrometry is also used to analyse samples that consist of
molecular species.
In these cases the
cation arising from loss of a single electron in the ionisation chamber is a
molecule and is referred to as the molecular ion.
Thus mass spectrometry may be used to determine the molecular mass of an unknown compound.
Mass spectrometry is also used in this way to analyse samples in drug testing for athletes and race horses.
The mass spectrum may also reveal more details about the composition of the molecule because
bond-breaking may cause the
cation to decompose into
other cations.
mass spec N2
The mass spectrum of N
2 molecules shows that both N
2+ (
m/
z 28 - the molecular ion) and N
+ (
m/
z 14) cations are produced in the ionisation chamber.
Although from this spectrum it appears that nitrogen has a single isotope (a single peak for N atoms at m/z 14, in fact 15N is present to 0.4%.
For
diatomic elements where the atoms exist as
more than one isotope in signficant %relative abundance, there will be
more than one molecular ion.
Thus, in principle, molecular nitrogen (N2) can consist of 14N –14N (m/z 28), 15N –14N (m/z 29), 15N –15N (m/z 30). Thus one would expect three molecular ions. In practice the relative abundance of the two containing 15N is very small.
For some atoms where the %natural abundance of the isotopes are more comparable than for the nitrogen isotopes, three molecular ions are seen in the mass spectrum. Examples of this are Br
2 and Cl
2.