Base Peak
Definition
The most intense peak in a mass spectrum, normalised to 100 percent relative abundance. All other peaks in the spectrum are reported as a percentage of the base peak. It is rarely the molecular ion under EI conditions.
- Definition
- Most intense peak in a mass spectrum, normalized to 100 percent relative abundance
- Convention
- All other peaks reported as a percentage of the base peak
Common questions
What is a base peak in a mass spectrum?+
The base peak is the tallest, most intense peak in a mass spectrum. It's set to 100 percent relative abundance by convention, so all other peaks are measured as percentages of it.
Why is the base peak scaled to 100 percent?+
Using 100 percent as the baseline lets you compare the relative intensities of all ions in the spectrum. Without a reference point, raw intensity numbers would be harder to interpret across different samples or instruments.
Is the base peak always the molecular ion?+
No. Under electron ionization (EI) conditions, the base peak is rarely the molecular ion. The molecular ion may fragment heavily, leaving other ion fragments as the most abundant species in the spectrum.
Related terms
- 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...
- 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 most intense peak in a mass spectrum, normalised to 100 percent relative abundance. All other peaks in the spectrum are reported as a percentage of the bas...
- Mass Spectroscopy: Principles, Instrumentation and Forensic ApplicationsThe most intense peak in the spectrum. By convention it is set to 100 percent relative abundance; every other peak is reported relative to it.