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

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

If M represents the N-methylethanamine (ethylmethylamine) 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 N-methylethanamine (ethylmethylamine) and only the formation of singly charged positive are considered for the mass spectrum of N-methylethanamine (ethylmethylamine).

I've included a stick diagram and table of m/z ions for the mass spectrum of N-methylethanamine (ethylmethylamine) 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 N-methylethanamine (ethylmethylamine).

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 this secondary amine. 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 this secondary amine, but the mass spectrometer software does!

N-methylethanamine, N-methylethylamine, ethylmethylamine, ,

Interpreting the fragmentation pattern of the mass spectrum of N-methylethanamine (ethylmethylamine)

[M]⁺ is the molecular ion peak with an m/z of 59 corresponding to [C₃H₉N]⁺, the original N-methylethanamine (ethylmethylamine) molecule minus an electron, [CH₃NHCH₂CH₃]⁺

The small M+1 peak at m/z 60, corresponds to an ionised N-methylethanamine (ethylmethylamine) molecule with one ¹³C atom in it i.e. an ionised N-methylethanamine (ethylmethylamine) molecule of 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-methylethanamine (ethylmethylamine) has 3 carbon atoms, so on average, ~1 in 33 molecules will contain a ¹³C atom.

The most abundant ion of the molecule under mass spectrometry investigation (N-methylethanamine (ethylmethylamine)) 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 ethylmethylamine is m/z 44 ion [C₂H₆N]⁺

Identifying the species giving the most prominent peaks (apart from M) in the fragmentation pattern of N-methylethanamine (ethylmethylamine).

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

The parent molecular ion is the m/z 59 ion corresponding to  [C₃H₉N]⁺  or  [CH₃NHCH₂CH₃]⁺

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

Analysing and explaining the principal ions in the fragmentation pattern of the mass spectrum of N-methylethanamine (ethylmethylamine)

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 N-methylethanamine (ethylmethylamine) (tabulated above)

Formation of m/z 58 ion:

[CH₃NHCH₂CH₃]⁺  ===>  [C₃H₈N]⁺  +  H

C-H or N-H bond scission, loss of hydrogen atom from the parent molecular ion,

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

Further hydrogen atom loss will give the m/z 57 and 56 ions.

Formation of m/z 44 and 45 ions:

[CH₃NHCH₂CH₃]⁺  ===>  [C₂H₆N]⁺  +  CH₃

C-N or C-C bond scission, methyl group lost from parent molecular ion,

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

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

The m/z 44 ion is the base peak ion, the most abundant and 'stable' ion fragment.

The m/z 44 ion can further lose hydrogen atoms to give m/z ions 43, 42, 41 and 40.

The m/z 44 ion is unlikely to be [C₃H₈]⁺

The m/z 45 ion is likely to be formed in the same way but containing a 13C atom.

That is [¹³C¹²CH₆N]⁺ rather than [C₂H₇N]⁺ ?

An accurate mass spectrometer sorts this out, measuring relative fragment ion masses to four decimal places e.g. using very accurate relative 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:

[¹³C¹²CH₆N]⁺ = 45.0533, [C₂H₇N]⁺ = 45.0577, a difference of 0.0044 in relative ion mass.

The m/z 44 ion can lose hydrogen atoms to give the m/z 43 down to 40 ions.

Formation of m/z 30 ion:

[CH₃NHCH₂CH₃]⁺  ===>  [CH₄N]⁺  +  CH₂CH₃

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

mass change 59 - 29 = 30 (M-29 ion peak)

Formation of m/z 29 ion:

[CH₃NHCH₂CH₃]⁺  ===>  [CH₂CH₃]⁺  +  NHCH₃

Most likely origin from C-N bond scission in the parent molecular ion,

mass change 59 - 30 = 29 (M-30 ion peak)

The m/z 29 ion can lose a hydrogen atoms/molecule to give the m/z 27 ion [C₂H₃]⁺

Theoretically the m/z 29 could be [CH₃N]⁺ and the m/z 27 ion could be [CHN]⁺?

An accurate mass spectrometer sorts this out, measuring relative fragment ion masses to four decimal places e.g. using very accurate relative isotopic masses,

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

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

For m/z 29: [C₂H₅]⁺ = 29.0390 and [CH₃N]⁺ = 29.0265 relative mass difference of 0.0125.

The infrared spectra of N-methylethylamine (N-methylethanamine, ethylmethylamine)

The H-1 NMR spectra of N-methylethylamine (N-methylethanamine, ethylmethylamine)

The C-13 NMR spectra of N-methylethylamine (N-methylethanamine, ethylmethylamine)

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