Section 13.4 Determination of Molecular Formulas: High-Resolution Mass Spectrometry 489Mass spectra can show peaks as a result of a contribution from or from
having two heavy isotopes in the same molecule (say, and or two ). Most
of the time, the peak is very small. The presence of a large peak is
evidence of a compound containing either chlorine or bromine, because each of these
elements has a high percentage of a naturally occurring isotope that is two units
heavier than the most abundant isotope. From the natural abundance of the isotopes of
chlorine and bromine in Table 13.2, one can conclude that if the peak is one-
third the height of the molecular ion peak, then the compound contains one chlorine
atom because the natural abundance of is one-third that of If the M and
peaks are about the same height, then the compound contains one bromine
atom because the natural abundances of and are about the same.
In calculating the molecular masses of molecular ions and fragments, the atomic
massof a single isotope of the atom must be used ( or 37, etc.); the atomic
weightsin the periodic table cannot be used because they are the
weighted averagesof all the naturally occurring isotopes for that element, and mass
spectrometry measures the mzvalue of an individualfragment.
PROBLEM 5The mass spectrum of an unknown compound has a molecular ion peak with a relative
intensity of 43.27% and an peak with a relative intensity of 3.81%. How many
carbon atoms are in the compound?13.4 Determination of Molecular Formulas:
High-Resolution Mass Spectrometry
All the mass spectra shown in this text were determined with a low-resolution mass
spectrometer. Such spectrometers give the nominal molecular massof a fragment—
the mass to the nearest whole number. High-resolution mass spectrometers can de-
termine the exact molecular massof a fragment to a precision of 0.0001 amu. If we
know the exact molecular mass of the molecular ion, we can determine the com-
pound’s molecular formula. For example, as the following listing shows, many com-
pounds have a nominal molecular mass of 122 amu, but each of them has a different
exact molecular mass.
Some Compounds with a Nominal Molecular Mass of 122 amu and Their Exact
Molecular Masses
Molecular formula
Exact molecular mass (amu)122.1096 122.0845 122.0732 122.0368 122.0579 122.0225
The exact molecular masses of some common isotopes are listed in Table 13.3. Some
computer programs can determine the molecular formula of a compound from the
compound’s exact molecular mass.
C 9 H 14 C 7 H 10 N 2 C 8 H 10 O C 7 H 6 O 2 C 4 H 10 O 4 C 4 H 10 S 2M+ 1>1 Cl=35.453 2Cl= 35(^79) Br (^81) Br
M+ 2
(^37) Cl (^35) Cl.
M+ 2
M+ 2 M + 2
(^13) C (^2) H, (^13) C’s
M+ 2 18 O
Table 13.3 The Exact Masses of Some Common Isotopes
Isotope Mass Isotope Mass
1.007825 amu 31.9721 amu
13.00000 amu 34.9689 amu
14.0031 amu 78.9183 amu
(^16) O 15.9949 amu
(^14) N (^79) Br
(^12) C (^35) Cl
(^1) H (^32) S