Name: amines in mass spectra: nitrogen rule
Uploaded: 2024-12-05T18:32:20.000Z
Duration: 55 s
Channel: JoVE

Amines undergo &#9082;-cleavage in the mass spectrometer to yield a resonance-stabilized nitrogen-containing cation and an alkyl radical.
Interestingly for amines, the number of nitrogen atoms in the compound affects the molecular weight of the molecular ion, also called the nitrogen rule of mass spectrometry.
As per the rule, a neutral organic compound containing an odd number of nitrogen atoms will produce a molecular ion with an odd-numbered molecular weight.
For instance, consider the mass spectrum of triethylamine, which has one nitrogen atom.
Here, the molecular ion has an odd mass-to-charge ratio of 101.
Fragmentation of the molecular ion upon &#9082;-cleavage frees a methyl radical and generates a base peak at a mass-to-charge ratio of 86.

massaspectrometrie: aminefragmentatie

Mass Spectrometry: Amine Fragmentation

Aminen kunnen worden geïdentificeerd met behulp van massaspectroscopie op basis van hun karakteristieke fragmentatiepatronen. De moleculaire ionen van aminen ondergaan fragmentatie via ⍺-splitsing. De ⍺-splitsing van de koolstof-koolstofbindingen in aminen genereert een alkylradicaal en een door resonantie gestabiliseerd stikstofbevattend kation.


Bij aminen beïnvloedt het aantal stikstofatomen de massa van het moleculaire ion, wat wordt beschreven door de stikstofregel van massaspectrometrie. Deze regel stelt dat een verbinding die een enkel of een oneven aantal stikstofatomen bevat, een moleculair ion produceert met een oneven moleculair gewicht, terwijl een verbinding met geen of een even aantal stikstofatomen een moleculair ion genereert met een even moleculair gewicht.


Bijvoorbeeld, het massaspectrum van 3-methyl-1-butanamine, dat één stikstofatoom bevat, toont een moleculair ion met een oneven massa-tot-ladingverhouding van 87. Fragmentatie van dit moleculaire ion via ⍺-splitsing produceert een basispiek, die een massasignaal vertoont bij een massa-tot-ladingverhouding van 30, wat overeenkomt met het verlies van het 2-methylpropylradicaal.


Op dezelfde manier genereert de fragmentatie van triethylamine via ⍺-splitsing een kation bij een massa-tot-ladingverhouding van 86. De moleculaire ionpiek toont een oneven massa-tot-ladingverhouding van 101, zoals hieronder weergegeven.


<img alt="Figure1" src="/files/ftp_upload/13048/13048_PT_Figure_1.svg" style="text-align: center; height: 105px;" />

Massaspectrometrie: Aminefragmentatie

Amines can be identified using mass spectroscopy based on their characteristic fragmentation patterns. The molecular ions of amines undergo fragmentation ...

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amines in mass spectra: nitrogen rule

Amines can be identified in mass spectrometry by their fragmentation patterns and by the nitrogen rule. The molecular ions of amines often break apart through β-cleavage, which cuts a carbon-carbon bond next to the nitrogen. This process forms an alkyl radical and a resonance-stabilized nitrogen-containing cation.

The nitrogen rule helps predict the mass of the molecular ion. A compound with one nitrogen atom, or any odd number of nitrogen atoms, gives a molecular ion with an odd molecular weight. A compound with no nitrogen atoms, or an even number of nitrogen atoms, gives a molecular ion with an even molecular weight.

The mass spectrum of 3-methyl-1-butanamine shows this pattern clearly. Because it contains one nitrogen atom, its molecular ion appears at m/z 87, which is odd. Fragmentation by β-cleavage gives a base peak at m/z 30, after loss of the 2-methyl propyl radical.

Triethylamine shows a similar fragmentation pattern. Its molecular ion appears at m/z 101, and β-cleavage produces a cation at m/z 86. These peaks help identify amines by linking molecular ion mass with their characteristic breakdown products.

Amines in Mass Spectra: Nitrogen Rule

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key terms and concepts

Learn how amines fragment in mass spectrometry, how β-cleavage works, and how the nitrogen rule helps predict odd or even molecular ion masses.

Amines in Mass Spectra: Nitrogen Rule | JoVE K12

analytical chemistry

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Amines in Mass Spectra: Nitrogen Rule

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