Molecular Ion (M+)
Definition
The intact ionised molecule with no fragmentation, formed by removing a single electron from the parent. Its m/z gives the molecular weight of the analyte directly. Hard ionisation often destroys it; soft ionisation preserves it.
- Also called
- M+• (M-plus-radical-cation)
- How it forms
- Single electron removal from the intact parent molecule
- What it shows
- Direct molecular weight of the analyte
Common questions
What is a molecular ion in mass spectrometry?+
A molecular ion is the intact molecule after losing a single electron. It appears in the mass spectrum as M+•, where M is the molecular weight. This peak gives you the direct molecular weight of the compound you're analyzing.
Why do hard and soft ionization techniques give different molecular ion peaks?+
Hard ionization methods like electron ionization put too much energy into the molecule, often breaking it apart before it can show up as a peak. Soft ionization (like ESI or APCI) is gentler and keeps the molecule intact, so the M+• peak survives and is clearly visible in your spectrum.
How does the molecular ion help identify an unknown compound?+
The m/z value of the molecular ion peak tells you the exact molecular weight. This is your first clue to the compound's identity. From there, the fragment pattern below it helps narrow down the structure, but the molecular ion itself is irreplaceable for determining mass.
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...
- Accurate Mass
- A mass measured to four or more decimal places, sufficient to compute a unique molecular formula. High-resolution accurate-mass (HRAM) instruments (Q-TOF, Orbitrap,...
- 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.
- MALDI
- Matrix-Assisted Laser Desorption / Ionisation. A soft method that uses a UV-absorbing matrix to ionise large biomolecules. Almost always paired with a...
- Mass-to-Charge Ratio (M/z)
- The fundamental MS measurement: the mass of an ion divided by its charge number. A singly charged ion of mass 250 Da...
- 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...
- Nitrogen Rule
- An organic molecule with an odd-numbered molecular weight contains an odd number of nitrogen atoms. A staple trick for narrowing down a...
- Resolution
- The ability of a mass analyser to distinguish two ions of nearly equal m/z, defined as m/Δm at a given peak. A...
- Tandem MS (MS/MS)
- Two stages of mass selection in series with a fragmentation step in between. The first stage selects a precursor ion, a collision...
- 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.
Explained in these topics
- Mass Spectrometry: Principles, Ionisation and Mass AnalysersThe intact ionised molecule with no fragmentation, formed by removing a single electron from the parent. Its m/z gives the molecular weight of the analyte dire...
- Mass Spectroscopy: Principles, Instrumentation and Forensic ApplicationsThe ion formed when one electron is knocked out of the intact molecule, with no fragmentation. Gives the molecular weight of the analyte.