[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 ethylamine [spectra page updated April 3rd 2026 *]

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

If M represents the ethylamine 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 ethylamine and only the formation of singly charged positive are considered for the mass spectrum of ethylamine.

I've included a stick diagram and table of m/z ions for the mass spectrum of ethylamine 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 ethylamine.

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 ethylamine. 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 ethylamine, but the mass spectrometer software does!

Interpreting the fragmentation pattern of the mass spectrum of ethylamine

[M]⁺ is the molecular ion peak with an m/z of 45 corresponding to [C₂H₇N]⁺, the original ethylamine molecule minus an electron, [CH₃CH₂NH₂]⁺

The tiny M+1 peak at m/z 46, corresponds to an ionised ethylamine molecule with one ¹³C atom in it i.e. an ionised ethylamine molecule of formula [¹³C¹²CH₇N]⁺

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.

Ethylamine has 2 carbon atoms, so on average, ~1 in 50 molecules will contain a ¹³C atom.

The most abundant ion of the molecule under mass spectrometry investigation (ethylamine) 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 ethene is the m/z 30 ion [CH₄N]⁺

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

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

The parent molecular ion is m/z of 45 corresponding to [C₂H₇N]⁺  or  [CH₃CH₂NH₂]⁺

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

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

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);  N = 14

Bond enthalpies kJ/mol: C-C = 348;  C-H = 412;  C-N = 305;  N-H = 391

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

Formation of m/z 44 ion:

[CH₃CH₂NH₂]⁺  ===>  [C₂H₆N]⁺  +  H

N-H or C-H bond scission to lose a hydrogen atom, mass change 45 - 1 = 44.

The m/z 44 ion can lose hydrogen atoms to give m/z ions of 43 down to 40 still containing a nitrogen atom (see data table of ions for ethylamine above).

Formation of m/z 30 ion:

[CH₃CH₂NH₂]⁺  ===>  [CH₄N]⁺  +  CH₃

C-C bond scission, loss of methyl group,

mass change 45 - 15 = 30 (M-15 ion peak)

The m/z 30 ion is the base peak ion, the most abundant and 'stable' ion fragment.

This is typical for the base peak of alkyl amines.

The m/z 30 ion can lose hydrogen atoms to give the m/z 29 down to 26 ions still containing a nitrogen atom (see data table of ions for ethylamine above).

Formation of m/z 29 ion:

[CH₃CH₂NH₂]⁺  ===>  [CH₃CH₂]⁺  +  NH₂

C-N bond scission and ionisation of the ethyl group,

mass change 45 - 16 = 29 (M-16 ion peak)

The m/z 29 ion [C₂H₅]⁺ can lose hydrogen atoms to give m/z ions of 28, 27 and 26 (see data table of ions for ethylamine above).

I don't know if the ions of similar m/z vales of 29 to 26, [CH₃N]⁺  to  [CN]⁺, are formed, but this would also involve hydrogen atom loss from the [CH₄N]⁺ ion.

Sorting out ions with the same integer m/z value

I'm not sure which fragment species predominates for these pairs, which might be formed by other fragmentation reactions, BUT an accurate mass spectrometer can sort them out - it can measure relative fragment ion masses to four decimal places e.g. using accurate isotopic masses

¹H = 1.0078    ¹²C = 12.0000    ¹³C = 13.0034   ¹⁴N = 14.0031, you can then calculate (predict) that the accurate relative ion masses are:

For m/z 26: [C₂H₂]⁺ = 26.0156 and [CN]⁺ = 26.0031, a difference of 0.0125 in relative ion mass.

For m/z 27: [C₂H₃]⁺ = 27.0234 and [CHN]⁺ = 27.0109, a relative ion mass difference of 0.0125

For m/z 28: [C₂H₄]⁺ = 28.0312 and [CH₂N]⁺ = 28.0187, a relative ion mass difference of 0.0125

For m/z 29: [C₂H₅]⁺ = 29.0390, [CH₃N]⁺ = 29.0265, a ion relative mass difference of 0.0125

The infrared spectrum of ethylamine

The H-1 NMR spectrum of ethylamine

The C-13 NMR spectrum of ethylamine

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