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Isotopes
On the previous page we looked at a spectrum of slightly
fragmented NAD and noted that although the main peak is intact, unfragmented NAD occured
at approximately 662.1 amu, there were minor peaks at +1 and +2 from this. To the
right is an expanded view illustrating the relative sizes of these peaks. 
These peaks are caused by the presence of heavy isotopes. In this case the major contributor
to the peak at +1 is likely to be 13C. Only about 1% of naturally occuring carbon
is 13C, but NAD has the formula C21H27N7O14P2,
which means it contains 21 times more 13C than a single-carbon compound such as
methane.
The peak at +2 has more than one source. A lot of it is likely to be caused by 18O, the heavy isotope of oxygen, which is 2 amu heavier than "normal" oxygen. However, there is also a likelihood that some molecules of NAD will contain 2 rather than 1 atom of 13C. These molecules will also appear at +2.
It is possible to calculate the sizes of M+n peaks.
So which elements can we see by their isotopes?
Reference tables of heavy isotopes and their abundances can be found in mass spec text books such as de Hoffmann and Stroobant, "Mass Spectrometry, principles and applications", and in reference books, as well as some of the sites listed in our links. Roughly speaking, the following isotopes are relevant in plant biochemistry:
| Carbon | Approximately 1% +1 Not a high abundance, but because most plant metabolites contain lots of it, a major determinant of the +1 peak. Much more so than hydrogen. |
| Deuterium | Heavy hydrogen at +1 is present at very low abundance, so 13C is more
important (usually). |
| Nitrogen | About 0.4% heavy, at +1. Not irrelevant! |
| Oxygen | Although the world contains a tiny amount of 17O, the major isotope is at
+2, where it accounts for about 0.2% of all oxygen. |
| Sulphur | Beloved of those who study glucosinolates. About 4% of natural sulphur is +2, which
makes the presence of sulphur in an organic compound very easy to detect. It is even
possible to estimate the amount of sulphur, since a single sulphur causes a +2 peak
of about 4% intensity relative to the main peak, and two cause about 8% and so on.
This is a good estimate, since other elements are unlikely to make enough contribution
to mess things up. |
| Chlorine and Bromine | These two halogens have lovely heavy isotopes at huge abundance. Chlorine is roughly
25% 37Cl, which is why its relative atomic mass is often given as 35.5,
and bromine roughly half-and-half heavy and light bromine. |
| Fluorine and Phosphorus | Anoyingly these elements do not have appreciable amounts of heavy isotopes, so no
information can be gleaned about their presence in a molecule from isotope peaks. Pity. |
Chlorine
As an example of what isotopes can look like when they are present at high abundance, the spectrum below is that of trichlorophenol. It also illustrates that isotope peaks are not always perfectly quantified. In this particular example the heavy peak is as large as the light, whereas we'd expect roughly 3 times 25%, i.e. 75%. Note also the +4 peak where two heavy chlorines are present.
