Illustrated Glossary of Organic Chemistry

Exact mass: In mass spectrometry, an m/z measured or calculated to several decimal places (usually four).

In the mass spectrum of nicotine the nominal mass for M = (10 x 12) + (14 x 1) + (2 x 14) = 162 amu, whereas the exact mass is (10 x 12.0000) + (14 x 1.007825) + (2 x 14.003074) = 162.1157 amu.