I apologise if this seems a very simple question, and also if I am overlooking something that is very simple.

For part of my A Level course, we have to be able to interpret mass spectrometer readings for isotopes of elements. Further on in the course, it is extended to molecules. However, quite often it has come up where a question like this arrives: Example

Given that relative abundance implies that all values are relative to each other, how can all of the relative abundances sum to a number that is greater than 100?

  • $\begingroup$ The raw numbers for relative abundance are arbitrary (in this case it looks like the commonest isotope is assigned a value of 100). So they give only the ratio. To get the absolute abundance you need to divide those relative numbers by the total abundance which gets you the absolute proportion of the two isotopes. $\endgroup$ – matt_black Aug 5 '19 at 13:23

2.3:10, 23:100 and (23/123):(100/123) are all correct values for the ratio of the relative abundance of B-10 to B-11. However, only the last value, (23/123):(100/123) or 18.7:81.3 shows the normalized ratio of relative abundance of these isotopes. It is this normalized value that adds up to 100.


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