Chapter 14: Mass Spectrometry
Table of Contents:
A Mass Spectrometer
Mass spectrometry is a laboratory technique used to help determine the structure of an unknown molecule and accurate molecular masses.
In an ionization chamber, the unknown molecules in the sample are bombarded by electrons. The collisions between molecules and electrons cause the loss of electrons from the sample and formation of positively charged ions. When a molecule loses one electron, the resulting structure is called Molecular Ion, M+. The peak for molecular ion is located on the right most side of the mass spectrum and tells us the mass of the molecule (but sometimes is barely observed).
Molecular ion disintegrates to multiple fragments .
Molecular ion and fragments will be seen on the mass spectrum.
A mass spectrum shows m/z (mass to charge ratio) on the x axis and % abundance on the y axis. Since the charge is +1, m/z tell the mass of each fragment.
Features of a Mass Spectrum
Isotopes are atoms of the same element with different masses.
We can recognize the presence of certain elements in the molecule on the mass spectrum based on their % abundance.
Recognizing the presence of chlorine and bromine in the molecule
Chlorine has two isotopes, chlorine-35 and chlorine-37. Chlorine-37 is two atomic mass units heavier than chlorine-35. If chlorine is present in the molecule, we would see M and M+2 peak in 3:1 ratio.
Bromine has two isotopes, bromine-79 and bromine-81. Bromine-81 is two atomic mass units heavier than bromine-79. If bromine is present in the molecule, we would see M and M+2 peak in 1:1 ratio.
Interpreting Mass Spectra
As mentioned above, molecular ion disintegrates into different fragments, whose masses we observe on the mass spectrum. For example, for a hexane, we would observe molecular ion around 86 m/z (mass of hexane). Loss of methyl radical(15), means a fragment of 86-15 = 71. Loss of ethyl radical (29) would result in a 86-21=57 fragment and so on.
Alkenes show a strong molecular ion signal. Cyclohexenes undergo fragmentation to give a 1,3-diene.
Alkynes show a strong molecular ion signal.
Alcohols have a low intensity for their molecular ion. Usually, alcohols can be distinguished by the presence of a peak corresponding to the loss of OH group (18). Mass of molecule -18.
Aldehydes and Ketones
Cleaving one of the bonds to the C=O can serve as a strong hint for the presence of ketones or aldehyde. For example, the mass of 2-octanone is 128m/z (molecular ion). Cleaving the ketone group (CH3CHO=43m/z) will reasult in a fragment (128-43=113m/z).
The most common fragment results from the alpha cleavage of the carboxyl group to give the ion [COOH] at m/z 45.
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