Instant Notes: Analytical Chemistry

E14 – Mass spectrometry 277

tables for all possible combinations of these elements up to an RMM of

several hundred. Theoretical values for any molecular formula are readily

calculated by multiplying the number of atoms of each element by the

corresponding natural isotopic abundances and summing them to obtain

the intensities of the M+1 and M+2 peaks as percentages of the M peak, e.g.

for C 24 H 22 O 7

C H O Total/%

M+ 100.00

(M+1)+ 24 ¥1.108 22 ¥0.016 7 ¥0.04 27.22

(M+2)+ ––7 ¥0.20 1.40

(ii) To determine the numbers of chlorine and bromine atoms in a compound

from the relative intensities of the M, M+2, M+4, M+6, etc., peaks in a

recorded spectrum by comparisons with tabulated values or graphical

plots. The presence of one or more atoms of either halogen will give isotope

peaks two mass units apart (Table 3).

Mass spectra Mass spectral data can be used to provide the following analytical information:

● an accurate RMMif the molecular ion can be identified;

● an empirical molecular formulabased on isotope peak intensities;

Table 2. Empirical formulae and isotope peak ratios for a nominal RMM value of 70

(M =100%)

M+1 M+2 Exact mass

%%

CN 3 O 2.26 0.22 70.0042

CH 2 N 4 2.62 0.03 70.0280

C 2 NO 2 2.67 0.42 69.9929

C 2 H 2 N 2 O 3.03 0.23 70.0167

C 2 H 4 N 3 3.39 0.04 70.0406

C 3 H 2 O 2 3.44 0.44 70.0054

C 3 H 4 NO 3.80 0.25 70.0293

C 3 H 6 N 2 4.16 0.07 70.0532

C 4 H 6 O 4.57 0.28 70.0419

C 4 H 8 N 4.93 0.09 70.0657

C 5 H 10 5.70 0.13 70.0783

Table 3. Isotope peak ratios for molecules containing up to three chlorine and bromine

atoms (M = 100%)

Halogen atom(s) M+2 M+4 M+6

%%%

Cl 32.6

Cl 2 65.3 10.6

Cl 3 99.8 31.9 3.47

Br 97.7

Br 2 195 95.5

Br 3 293 286 93.4

ClBr 130 31.9