For students and curious beginners: A full mass spectrum tells you which ions are present. MS/MS goes one step further: it isolates an ion, activates it and measures the resulting fragments.
See the big picture
| 🔵 1. Choose precursor
Select one m/z range as the starting ion. |
🟣 2. Add energy
Collision gas transfers energy to selected ions. |
🟡 3. Make fragments
Some ions split into charged product ions. |
🟢 4. Read products
A new spectrum shows fragment masses. |
The LEGO model of mass spectrometry
Imagine a LEGO model with several connected parts. Weighing the whole model helps identify its size. Breaking it apart gently and weighing some resulting pieces reveals how it might have been assembled. This is an analogy for tandem mass spectrometry (MS/MS), but real fragmentation follows molecular bonding and gas-phase chemistry, not simply breaking at the weakest visual connection.
What is a precursor ion?
First, an ion source creates ions from the analyte. A mass filter or precursor-isolation step selects ions around a chosen mass-to-charge value; these are called precursor ions. Their neutral molecular origin must still be checked, because an adduct or in-source fragment can become a precursor too.
What happens during collision-induced dissociation?
In collision-induced dissociation (CID), ions collide with neutral gas particles and gain internal energy. Some dissociate into smaller charged pieces called product ions. Their relative intensities may depend on collision energy, molecular structure and instrument design.
Example: three ions from one precursor
| Precursor/product | Illustrative m/z | Meaning |
|---|---|---|
| Precursor | 301 | Selected ion before collision |
| Product A | 283 | Potential neutral loss of 18 Da |
| Product B | 151 | A different possible charged fragment |
These made-up m/z values do not identify a real molecule. A difference of 18 Da might suggest loss of water in an appropriate chemical context, but formula and mechanism must be checked.
What are Q1, Q2 and Q3?
A triple quadrupole commonly uses Q1 as a mass filter for the precursor, Q2 as a collision region and Q3 as a filter for product ions. In multiple reaction monitoring (MRM), an instrument monitors selected precursor → product combinations. Other instruments—including ion traps and high-resolution analyzers—can also perform MS/MS by different sequences of isolation and activation.
Why is MS/MS helpful?
- Identification: Fragment patterns provide structural evidence beyond one precursor mass.
- Selectivity: A precursor/product transition can reject some interfering signals.
- Quantification: Selected transitions can support sensitive targeted assays.
Important limitation
Matching one common fragment is not equivalent to a confirmed identity. Multiple unrelated molecules can share a product ion. Use retention time, standards, accurate masses and appropriate identification rules where available.
Related: Common Neutral Losses in LC-MS/MS and How a Quadrupole Works.
Test yourself (answers below)
- Which ion is selected before fragmentation?
- What does CID stand for?
- Does one product ion always prove molecular identity?
Reveal the answers
- The precursor ion.
- Collision-induced dissociation.
- No; it is supporting evidence that must be evaluated with other data.
Learn more: reliable references
Educational note: The images and numerical examples are schematic. Real LC-MS methods depend on analyte, instrument and laboratory conditions.