[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 methylamine (methanamine) [spectra page updated April 4th 2026 *]

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

If M represents the methanamine (methylamine) 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 methanamine (methylamine) and only the formation of singly charged positive are considered for the mass spectrum of methanamine (methylamine).

I've included a stick diagram and table of m/z ions for the mass spectrum of methanamine (methylamine) 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 methanamine (methylamine).

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 methylamine. 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 methylamine, but the mass spectrometer software does!

Methylamine  CH₅N           

Interpreting the fragmentation pattern of the mass spectrum of methylamine

[M]⁺ is the molecular ion peak with an m/z of 31 corresponding to [CH₅N]⁺, the original methylamine molecule minus an electron, [CH₃NH₂]⁺

The tiny (M+1) peak at /z 32 may be due to the 1% of methylamine molecules with a ¹³C atom instead of a ₁₂C atom [¹³CH₃NH₂]⁺

The most abundant ion of the molecule under mass spectrometry investigation (methylamine) 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 ion peak for the mass spectrum of methylamine is the m/z 30 ion [CH₄N]⁺

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

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

The parent molecular ion peak for the mass spectrum of methylamine is the m/z 31 ion corresponding to [CH₅N]⁺  or  [CH₃NH₂]⁺

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

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

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; 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 methylamine (tabulated above)

Formation of m/z 30 ion:

[CH₃NH₂]⁺  ===>  [CH₄N]⁺  +  H

Hydrogen atom loss, due to C-H or C-N bond scission,

mass change 31 - 1 = 30 (M-1 ion peak)

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

This ion can lose hydrogen atoms to give the lighter ions of m/z 29, 28 and 27, in fact there is a tiny peak at m/z 26 corresponding to the [CN]⁺ ion.

Formation of m/z 16 ion:

[CH₃NH₂]⁺  ===>  [NH₂]⁺  +  CH₃

C-N bond scission, mass change 31 - 15 = 16 (M-15 ion peak)

Formation of m/z 15 ion:

[CH₃NH₂]⁺  ===>  [CH₃]⁺  +  NH₂

C-N bond scission, mass change 31 - 16 = 15 (M-16 ion peak)

Note the ionisation of both fragments from the C-N bond scission of methylamine, either can ionise, but only one at a time.

The infrared NMR spectrum of methylamine

The H-1 NMR spectrum of methylamine

The C-13 NMR spectrum of methylamine

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Website content © Dr Phil Brown 2000+. All copyrights reserved on revision notes, images, quizzes, worksheets etc. Copying of Doc Brown's pre-university advanced level chemistry website material is NOT permitted. Exam revision summaries & references to science course specifications are unofficial. These organic chemistry revision notes on spectroscopy (mass spectra of methylamine, methanamine) are suitable for use of pre-university students studying AQA advanced level chemistry, Edexcel advanced level chemistry, OCR advanced level chemistry, IB advanced level chemistry, WJEC (Eduqas) advanced level chemistry, CIE advanced level chemistry, CCEA advanced level chemistry, US grade 11-12 AP honors chemistry courses and they will also prove useful to 1st year undergraduate students of chemistry.