Illustrated Glossary of Organic Chemistry

High-resolution mass spectrometry: Mass spectrometry in which m/z for each ion is measured to several decimal places (i.e., exact masses are measured, instead of nominal masses). Particularly useful to differentiate between molecular formulas having the same nominal masses.

Example: The nominal mass for M for an unknown was found to be 78. Several formulas fit this data, including C2H6O3, C3H7Cl, C5H4N, and C6H6. To narrow the list, the chemist calculated the exact mass each formula:

Formula
   
Exact mass for M
C2H6O3
(2 x 12.0000) + (6 x 1.007825) + (3 x 15.994915) = 78.0317
C3H7Cl
(3 x 12.0000) + (7 x 1.007825) + (1 x 34.968853) = 78.0236
C5H4N
(5 x 12.0000) + (4 x 1.0007825) + (1 x 14.003074) = 78.0062
C6H6
(6 x 12.0000) + (6 x 1.007825) = 78.0470

In the high-resolution mass spectrum m/z for M was found to be 78.0468, so the best fit is C6H6.