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 lactic acid, C₃H₆O₃?
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 _________

Answer 1

Given data: Natural abundance of C-12 = 98.9 % and of C-13 = 1.1 %. M.F. of lactic acid is C3H6O3. No. of C atoms = 3.

The % intensity of (M+1) peak is given by formula,

%(M+1) = % Abundance of C-13 x number of C-13 atoms

              = 1.1 x 3

               = 3.3 %

Hence relative heights are in order of increasing mass are M : (M+1) : : 100 : 3.3.