[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 propyl methanoate [spectra page updated April 4th 2026 *]

 email doc brown Re-edit mass spectrum of N(CH₃)₃

 Mass spectrometry - spectra index

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

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

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

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 trimethylamine. 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 trimethylamine, but the mass spectrometer software does!

N,N-dimethylmethanamine, N,N-dimethylmethylamine, trimethylamine, ,

Interpreting the fragmentation pattern of the mass spectrum of N,N-dimethylmethanamine (trimethylamine)

[M]⁺ is the molecular ion peak with an m/z of 59 corresponding to [C₃H₉N]⁺, the original N,N-dimethylmethanamine (trimethylamine) molecule minus an electron, [(CH₃)₃N]⁺

The small M+1 peak at m/z 60, corresponds to an ionised N,N-dimethylmethanamine (trimethylamine) molecule with one ¹³C atom in it i.e. an ionised N,N-dimethylmethanamine (trimethylamine) molecule of molecular formula [¹³C¹²C₂H₉N]⁺

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.

N,N-dimethylmethanamine (trimethylamine) has 3 carbon atoms, so on average, ~1 in 33 molecules will contain a ¹³C atom.  The ratio of M:M+1 peaks should be ~33:1.

The most abundant ion of the molecule under mass spectrometry investigation (N,N-dimethylmethanamine (trimethylamine)) 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 trimethylamine is m/z 58 ion [C₃H₈N]⁺

Identifying the species giving the most prominent peaks (apart from M) in the fragmentation pattern of N,N-dimethylmethanamine (trimethylamine).

Unless otherwise indicated, assume the carbon atoms in N,N-dimethylmethanamine (trimethylamine) are the ¹²C isotope.

Some of the possible positive ions, [molecular fragment]⁺, formed in the mass spectrometry of N,N-dimethylmethanamine (trimethylamine).

The parent molecular ion is m/z 59  [C₃H₉N]⁺  or  [(CH₃)₃N]⁺

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of N,N-dimethylmethanamine (trimethylamine)

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-H = 412;  C-N = 305;  N-H = 391

Possible equations to explain the most abundant ion peaks of N,N-dimethylmethanamine (trimethylamine) (tabulated above)

Formation of m/z 58 ion:

[(CH₃)₃N]⁺  ===>  [C₃H₈N]⁺  +  H

C-H bond scission in the parent molecular ion, loss of hydrogen atom,

mass change 59 - 1 = 58 (M-1 ion peak)

The m/z 58 ion is the base peak ion, the most abundant and 'stable' ion fragment.

The m/z 57 and 56 ions will be formed by further hydrogen atom loss from the m/z 58 ion.

Formation of m/z 44 ion:

[(CH₃)₃N]⁺  ===>  [C₂H₆N]⁺  +  CH₃

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

mass change 59 - 15 = 44 (M-15 ion peak)

The m/z 43, 42, 41 and 40 ions can be formed by further hydrogen atom loss.

Formation of m/z 30 ion :

[?]⁺  ===>  [CH₄N]⁺  +  ?

C-N bond scission, loss of methyl group from the m/z 45 ion ?

The m/z 29 and 28 ions will be formed by further hydrogen atom loss.

This is a characteristic ion formed in the mass spectrum of aliphatic amines.

Formation of m/z 15 ion:

[(CH₃)₃N]⁺  ===>  [CH₃]⁺  +  C₂H₆N

C-N bond scission, loss of methyl group from the parent molecular ion, but it carries the positive charge, mass change 59 - 44 = 15.

The positive methyl ion could be formed from other fragments too.

The infrared spectra of N,N-dimethylmethanamine (N,N-dimethylmethylamine, trimethylamine)

The H-1 NMR spectra of N,N-dimethylmethanamine (N,N-dimethylmethylamine, trimethylamine)

The C-13 NMR spectra of N,N-dimethylmethanamine (N,N-dimethylmethylamine, trimethylamine)

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