Page 94 - Practical-organic-3

➢ Mass spectrometry (MS) is a technique for measuring the mass, and therefore the

molecular weight (Mwt), of a molecule. In addition, it’s often possible to gain structural

information about a molecule by measuring the masses of the fragments produced when

molecules are broken apart.

➢ The mass spectrometer performs three essential functions:

1. First, it subjects molecules to a high-energy beam of electrons, converting some of the

molecules to ions, which are accelerated in an electric field.

2. Second, the accelerated ions are separated according to their mass-to-charge ratios

(m/z) in a magnetic or electric field.

3. Finally, the charged ions that have a particular mass-to-charge ratio are detected by a

device which can count the number of ions striking it. The detector output is amplified
and fed to a recorder.

➢ The mass spectrum of a compound is typically presented as a bar graph, with masses (m/z

values) on the x axis and intensity, or relative abundance of ions of a given m/z striking the

detector, on the y axis. The tallest peak, assigned an intensity of 100%, is called the base
peak, and the peak with the largest value of (m/z) ratio (corresponds to the unfragmented

+
cation radical) is called the molecular ion (M ) and it corresponds to the molecular weight
(Mwt) of the molecule.

➢ Important notes:

• When a high-energy electron strikes an organic molecule, it dislodges a valence electron
from the molecule, producing a cation radical; cation because the molecule has lost an

electron and now has a positive charge; radical because the molecule now has an odd

number of electrons.

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