Question

The natural abundance of 13C is 1.1%. What are the relative peak heights of the M+ and M+1 peaks in the mass spectrum of decane? How do you know?

a. 100:11
b. 89:11
c. 100:1.1
d. 1:1.1

Answer 1

a. 100:11

Step-by-step explanation:

M+1 peak is observed due to the presence of an isotope of an atom in a molecule. Decane has the molecular formula C₁₀H₂₂, so the probability of having 13C atom in this molecule increases ten times. This gives the ratio between M+ and M+1 peak 100:11.

It should be noted that the decane molecule also contains hydrogen atoms whose isotope deuterium (2H) can also be present in the molecule and give M+1 peak. But its relative abundance to protium (1H) is low i.e., 0.015, so its effect can be neglected.

A simple formula for the estimation of the relative peak height of M+1 in a molecule that contains C, H, N, O, F, Si, P, and S is:

`[M+1] = (number of C X 1.1) + (number of H X 0.015) + (number of N X 0.37) + (number of O X 0.04) + (number of S X 0.8) + (number of Si X 5.1)`

PS: remove the element from the formula which is not present in the molecule