TEACHERS’ NOTES
MASS SPECTROSCOPY
Student learning outcomes for mass spectroscopy should include:

Interpret a mass spectrum

Calculate the molar mass of a compound based on the mass spectrum provided

Identify the main peaks (molecular ion peak and base peak)
What does mass spectrometry measure?
John B. Fenn, the originator of electrospray ionization for biomolecules and the 2002 Nobel
Laureate in Chemistry, probably gave the most apt answer to this question:
Mass spectrometry is the art of measuring atoms and molecules to determine their molecular
weight. Such mass or weight information is sometimes sufficient, frequently necessary, and always
useful in determining the identity of a species. To practice this art one puts charge on the molecules
of interest, i.e., the analyte, then measures how the trajectories of the resulting ions respond in
vacuum to various combinations of electric and magnetic fields.
Mass spectrometry is used to find out the molecular mass of a compound and it is also possible to
determine the elemental composition of a substance.
Advantages of mass spectroscopy are that it has high sensitivity and high accuracy.
A mass spectrum is obtained by converting components of a sample into gaseous ions and
separating them due to their mass-to-charge ratios. It is based on the motion of a charged particle,
an ion, in an electric or magnetic field.
A mass spectrum provides information about:

The quality and quantity of the inorganic and organic components in a mixture

The structures of a wide variety of molecular species

The ratio of isotopes in a sample

Structure and composition of solid surfaces
Mass spectrometry enables the mass of individual ions to be determined from compounds in the gas
phase.
How is a mass spectrum produced?

Microgram quantities of the pure compound are vaporised

It is then bombarded with electrons in an ionising chamber

The molecules lose 1 electron to form a positive ion, some of this breaks down to smaller
ions by losing radicals or neutral molecules

The positive ions are filtered out as they are accelerated through an electric field

They are then analysed as they are deflected through a magnetic field

Finally they are detected by an ion collector

A recorder prints the spectrum as a graph of the relative abundances of their ions against
their mass to charge ratio
The mass to charge ratio (m/z): m is the numerical value for the mass of the ion and z is the
numerical value for the charge of the ion.
For example, if an ion had a mass of 28 and a charge of 1+, its mass/charge ratio would be 28. An
ion with a mass of 56 and a charge of 2+ would also have a mass/charge ratio of 28.
The ionised molecule occurs as one electron (usually) is expelled forming a radical cation.
M → M• + + • (e-)
Example of a mass spectrum of pentan-3-one:
www.chemguide.co.uk
The peak at 86 would be the molecular ion peak. This corresponds to the molecular weight of a
singly charged molecule, CH3CH2COCH2CH3• + .
CH3CH2COCH2CH3• + = 5 carbons (5x12) + 10 hydrogens (10 x 1) + 1 oxygen ( 1 x 16) = 86
The peak at 57 would be the base peak (highest peak). This is shown as the most intense m/z
(highest relative abundance). This corresponds to CH3CH2CO+ .
The peak at 29 would correspond to the ethyl ion CH3CH2+.
Note: the more stable an ion the more likely it is to form. eg. In the case of carbocations 3°>2°>1°
Why is mass spectroscopy useful?
By performing mass spectroscopy on substances you could:

Work out the chemical structures of molecules such as carbohydrates, nucleic acids and
peptides

Detect and identify the use of steroids in athletes

Detect dioxins (poisons) in contaminated fish

Determine the isotopes of an element within a sample
1. A useful 8 minute video can be found on Youtube. www.youtube.com/watch?v=J-wao0O0_qM
This is produced by the Royal Society of Chemistry and explains how the spectrometer works and
how mass spectra are produced.
2. www.chemguide.co.uk
Go to Instrumental analysis, mass spectroscopy. This has good explanations of organic compounds
containing halogen atoms.
3. The following website contains ‘A self-guided introduction to mass spectrometery’.
Includes how a mass spectrometer works. Problems for:



Interpreting mass spectra
Drawing mass spectra
Working out fragments shown on mass spectra
www.ccmr.cornell.edu/education/modules/documents/WhatsMassGottoDoWithIt.pdf
4.
The website www.rod.beavon.clara.net/spectra.htm contains a range of mass spectra and
infrared spectra for selected alcohols, aldehydes, ketones, carboxylic acids, amines and an
ester and acid chloride.
‘A mass spectrometrist is someone who figures out what something is by smashing it with a hammer
and looking at the pieces’. www.research.uky.edu
5. The website http://www.vicsco.com.au/downloads/Mass_Spec.pdf contains brief notes on what
mass spectroscopy is used for and how to read a mass spectrum