What is an Exact Mass?
Molecules are composed of atoms. An ion is a molecule or a fragment of a molecule which has fewer or more electrons than the electrically neutral molecule or fragment. (All ions considered at this web site lack a single electron to provide a positively charged ion.) Mass spectrometers manipulate ions with electric and/or magnetic fields to measure their masses. The exact mass of an ion is the sum of the masses of its atoms. Most mass spectrometers (quadrupole and ion trap based instruments) measure nominal masses, rounded to the nearest whole mass. This limitation results in multiple possible compositions for a given measured mass. For example, a m/z ratio of 28 could be due to ions from three common compounds with dissimilar properties: N2 (nitrogen gas), the majority of earth’s atmosphere; CO (carbon monoxide), a poisonous product of incomplete combustion; or C2H4 (ethylene), a simple hydrocarbon used in chemical syntheses.
