By knowing the natural abundance of minor isotopes, it\'s possible to calculate

Question

By knowing the natural abundance of minor isotopes, it's possible to calculate the relative heights of M+ and M+1 peaks. If natural abundances are 12C - 98.9% and 13C - 1.10%, what are the relative heights, to the nearest 0.1%, of the M+ and M+1 peaks in the mass spectrum of ribose, C5H10O5? Ignore the contributions of isotopes like 2H (deuterium; 0.015% natural abundance) and 17O (0.04% natural abundance) that are small. The relative heights are, in order of increasing mass: 100 to .............

Explanation / Answer

The M+1 peak is due to presence of the 13C isotope in ribose, ignoring the contributions of isotopes of hydrogen and carbon.

Ribose has molecular formula C5H10O5

The percentage abundance 13C - 1.10%

Therefore 5 x 1.1 = 5.5 % molecules will contain 1 13C to every 94.5 (100 - 5.5) 12Carbon

So relative heights are : 94.5 : 5.5 (M : M+1)

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