In this case, we can simply use a ruler to measure the heights of the signal. Use the same ruler all throughout to reduce deviations in the measurement.
So for example we have 3 isotopes, so we get 3 signals with heights of (for example):
isotope 1 = 62
isotope 2 = 2
isotope 3 = 8
Therefore the mass of the signals would be:
62 + 2 + 8 = 72
The fractional abundance of each isotope would be:
frac isotope 1 = 62/72 = 0.86
frac isotope 2 = 2/72 = 0.03
frac isotope 3 = 8/72 = 0.11
