[Author © Dr Phil Brown PhD: Doc Brown's advanced level organic chemistry exam revision notes suitable for students of UK A level chemistry courses & US K12 grade 11, grade 12 and AP honors chemistry courses: Molecular spectroscopy - analysing the mass spectra of propanoic acid [spectra page updated April 4th 2026 *]

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Students and teachers please note my explanation of the mass spectrum of propanoic acid is designed for advanced, but pre-university, chemistry courses.

If M represents the propanoic acid molecule, the initial ionisation to give the molecular ion is:

M_(g) + high KE e^-  ==> [M•]⁺_(g) + 2e^- and fragmentation equations assume [M]⁺ is the start of the processes and all species are in a gaseous state.

I've not usually shown an unpaired electron on e.g. an ion or a non-ionised alkyl radical R e.g.

[M•]⁺ ==> [X]⁺  +  R•, but you should be aware this is a more accurate depiction of some processes.

I've used simplified equations to show how some of the ions that might be formed in the fragmentation pattern for the mass spectrum of propanoic acid and only the formation of singly charged positive are considered for the mass spectrum of propanoic acid.

I've included a stick diagram and table of m/z ions for the mass spectrum of propanoic acid and doing the mass spectrum analysis under standard conditions, databases can be compiled based on complex fingerprint patterns, often involving the relative intensities of many fragment ions, and used to identify compounds including propanoic acid.

In selected cases, where two different fragment ions have the same integer m/z value, I've pointed out that modern mass spectrometers can measure relative ion mass to four decimal places. So, using accurate isotopic masses, I've calculated and compared the accurate ion masses if appropriate for propanoic acid. BUT strictly speaking, 0.0005 should be deducted for singly charged ions to account for the loss of the electron in their formation. I have NOT done this for propanoic acid, but the mass spectrometer software does!

Propanoic acid  (propionic acid) C₃H₆O₂        

Interpreting the fragmentation pattern of the mass spectrum of propanoic acid

[M]⁺ is the molecular ion peak (M) with an m/z of 74 corresponding to [C₃H₆O₂]⁺, the original propanoic acid molecule minus an electron, [CH₃CH₂COOH]⁺

This is a relatively large peak, suggesting the parent molecular ion is relatively stable.

The small M+1 peak at m/z 75, corresponds to an ionised propanoic acid molecule with one ¹³C atom in it i.e. an ionised propanoic acid molecule of molecular formula [¹³C¹²C₂H₆O₂]⁺

Carbon-13 only accounts for ~1% of all carbon atoms (¹²C ~99%), but the more carbon atoms in the molecule, the greater the probability of observing this ¹³C M+1 peak.

Propanoic acid has 4 carbon atoms, so on average, ~1 in 25 molecules will contain a ¹³C atom.

The most abundant ion of the molecule under mass spectrometry investigation (propanoic acid) is usually given an arbitrary abundance value of 100, called the base ion peak, and all other abundances ('intensities') are measured against it.

The base peak ion for the mass spectrum of propanoic acid is m/z 28 ion [C₂H₄]⁺

Identifying the species giving the most prominent peaks (apart from M) in the fragmentation pattern of propanoic acid.

Unless otherwise indicated, assume the carbon atoms in propanoic acid are the ¹²C isotope.

The parent molecular ion is m/z 74 [C₃H₆O₂]⁺  or  [CH₃CH₂COOH]⁺

Some of the possible positive ions, [molecular fragment]⁺, formed in the mass spectrometry of propanoic acid.

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of propanoic acid

PLEASE NOTE I have found it difficult to find 'authentic' equations to explain mass spectra fragmentation patterns and it is complex chemistry! I've identified the formulae of the ionised fragments on the mass spectrum diagram, but the equations are from the internet or my conjecture as to how the ions might be formed - please take care in using the information, especially for assignments at university or pre-university level.

Atomic masses: H = 1; C = 12 (~1% 13); O = 16

Bond enthalpies kJ/mol: C-C = 348;  C-H = 412;  C-O = 360

Possible equations to explain the most abundant ion peaks of propanoic acid (tabulated above)

Formation of m/z 73 ion:

[CH₃CH₂COOH]⁺  ===>  [C₃H₅O₂]⁺  +  H

C-H bond scission, loss of hydrogen atom,

mass change 74 - 1 = 73 (M-1 ion peak)

Formation of m/z 57 ion:

[CH₃CH₂COOH]⁺  ===>  [CH₃CH₂C=O]⁺  +  OH

[C₃H₆O₂]⁺  ===>  [C₃H₅O]⁺  +  OH

Scission of C-O bond, loss of hydroxyl group,

mass change 74 - 17 = 57 (M-17 ion peak)

Loss of protons leads to the m/z 56 and 55 ions.

Formation of m/z 45 ion:

[CH₃CH₂COOH]⁺  ===>  [COOH]⁺  +  CH₃CH₂

C-C bond scission in the parent molecular ion, loss of ethyl group,

mass change 74 - 29 = 45 (M-29 ion peak)

Loss of protons leads to the m/z 43 and 42 ions.

Formation of m/z 29 ion:

[CH₃CH₂COOH]⁺  ===>  [CH₃CH₂]⁺  +  COOH

C-C bond scission of the parent molecular ion, loss of carboxyl group,

mass change 74 - 45 = 29 (M-45 ion peak)

Formation of m/z 28 ion:

[?]⁺  ===>  [C₂H₄]⁺  or  [CO]⁺  +  ?

Formation of an ionised ethene molecule (H loss from ethyl fragment)? or an ionised carbon monoxide molecule (COOH fragment split)?

The m/z 28 ion is the base ion peak, the most abundant ionised fragment.

Note that an accurate mass spectrometer can sort them out, it can measure relative fragment ion masses to four decimal places e.g. using

e.g. using accurate relative isotopic masses:

¹H = 1.0078  ¹²C = 12.0000   ¹³C = 13.0034: you can then calculate (predict) that the accurate relative ion masses are:

For m/z 28: [CO]⁺ = 27.9949  and [C₂H₄]⁺ = 28.0312, difference of 0.0363 in relative ion mass.

Formation of m/z 15 ion:

[CH₃CH₂COOH]⁺  ===>  [CH₃]⁺  +  CH₂COOH

C-C bond scission in the parent molecular ion, loss of ionised methyl group,

mass change 74 - 57 = 15 (M-57 ion peak)

A ionised methyl group can be lost from other larger fragments e.g. the m/z 73 ion.

The infrared spectrum of propanoic acid (propionic acid)

The H-1 NMR spectrum of propanoic acid (propionic acid)

The C-13 NMR spectrum of propanoic acid (propionic acid)

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