Nitrogen Rule
Definition
An organic molecule with an odd-numbered molecular weight contains an odd number of nitrogen atoms. A staple trick for narrowing down a structure from M+.
- Field
- Mass spectrometry, structure elucidation
- Statement
- Odd molecular weight implies an odd number of nitrogens
- Applies to
- The molecular ion (M+), common elements only
Common questions
Does the nitrogen rule apply to every compound analysed by MS?+
It holds for organic molecules made of the common elements (C, H, N, O, halogens and similar) under standard valence rules. It can fail for compounds with certain other elements, or when applied to a fragment ion rather than the molecular ion.
How is the nitrogen rule used in practice?+
An examiner checks whether the molecular ion's mass is odd or even to quickly rule structures with an odd number of nitrogen atoms in or out, narrowing candidate structures before detailed spectral interpretation.
Related terms
- Base Peak
- The most intense peak in a mass spectrum, normalised to 100 percent relative abundance. All other peaks in the spectrum are reported...
- EI (Electron Ionisation)
- Hard ionisation at 70 eV. Produces extensive fragmentation, ideal for library search. Standard ion source in GC-MS.
- ESI (Electrospray Ionisation)
- Soft ionisation at atmospheric pressure for polar and thermally fragile analytes in solution. Standard source in LC-MS for drugs and biomolecules.
- Fragmentation
- Breakdown of the molecular ion into smaller charged and neutral pieces. The fragmentation pattern is the chemical fingerprint that identifies the compound.
- M/z (Mass-to-Charge Ratio)
- The x-axis of every mass spectrum. Ions are separated by their mass divided by the number of charges they carry. For singly-charged...
- MALDI
- Matrix-Assisted Laser Desorption / Ionisation. A soft method that uses a UV-absorbing matrix to ionise large biomolecules. Almost always paired with a...
- Molecular Ion (M+)
- The intact ionised molecule with no fragmentation, formed by removing a single electron from the parent. Its m/z gives the molecular weight...
- TOF (Time-of-Flight)
- Mass analyser that separates ions by their flight time down a field-free drift tube. Theoretically unlimited mass range, very high speed.