How Mass Spectrometry Works
A complete guide to interpreting electron ionization mass spectra and understanding fragmentation patterns
What Is Mass Spectrometry?
Mass spectrometry (MS) is a technique that measures the mass-to-charge ratio (m/z) of ions. Unlike IR and NMR, which probe bonds and magnetic environments, MS actually breaks molecules apart and weighs the pieces.
In electron ionization (EI) mass spectrometry - the most common type for organic molecules - a beam of high-energy electrons (70 eV) strikes the sample molecules. This knocks out an electron, creating a radical cation (M+), which then fragments into smaller pieces.
Why it matters
MS directly tells you the molecular weight of a compound and provides structural clues through fragmentation patterns. Combined with IR and NMR, it is one of the three pillars of organic structure determination.
The spectra in Symmetria use real experimental data from the NIST Chemistry WebBook, recorded at the standard 70 eV ionization energy.
The Molecular Ion (M+)
When an electron is removed from a molecule without breaking any bonds, the result is the molecular ion (M+). Its m/z value equals the molecular weight of the compound.
The molecular ion is critically important because it directly gives you the molecular weight. However, not all molecules produce a visible M+ peak - some fragment so readily that the molecular ion is too unstable to reach the detector.
Recognizing the molecular ion
- - It appears at the highest m/z value (ignoring isotope peaks)
- - For molecules with only C, H, O, and N: if MW is even, it contains an even number of nitrogen atoms (nitrogen rule)
- - It may be weak or absent for molecules that fragment easily
The Base Peak
The base peak is the tallest peak in the spectrum - the most abundant ion. It is assigned a relative abundance of 100%, and all other peaks are measured relative to it.
The base peak is not always the molecular ion. In fact, for most organic molecules, the base peak is a fragment ion. The base peak represents the most stable fragment that forms from the molecular ion.
Example: Acetone
Acetone (MW = 58) has its molecular ion at m/z 58, but its base peak is at m/z 43. The m/z 43 fragment is the acylium ion CH3CO+, formed by losing a methyl group (15 Da). The acylium ion is very stable due to resonance with the C=O bond.
Fragmentation Patterns
When the molecular ion has enough internal energy, bonds break to produce fragment ions. The key principle is that fragmentation produces a cation (detected) and a neutral (lost). Only the charged fragment is detected by the mass spectrometer.
Alpha-cleavage
The most important fragmentation rule for beginners is alpha-cleavage: breaking the bond adjacent to a heteroatom or functional group. For carbonyls, this means breaking the C-C bond next to C=O.
Alpha-cleavage of a ketone:
R-CO-R' + e- → [R-CO-R']+ → R-CO+ + R'-
The acylium ion (RCO+) is detected; the alkyl radical (R') is lost
Why certain fragments are stable
Fragments are abundant when they are stabilized by:
- -Resonance: Acylium ions (RCO+), allylic cations, and benzylic cations are resonance-stabilized
- -Heteroatom stabilization: Oxygen and nitrogen lone pairs stabilize adjacent cations (e.g., CH2=OH+ from alcohols)
- -Hyperconjugation: More substituted carbocations are more stable (tertiary > secondary > primary > methyl)
Common Neutral Losses
When a fragment ion is detected at a certain m/z, the difference between M+ and that fragment tells you what was lost as a neutral. Recognizing common losses is one of the most powerful tools for interpreting mass spectra.
| Loss (Da) | Fragment Lost | Suggests |
|---|---|---|
| 1 | H | Hydrogen radical (aldehyde, etc.) |
| 15 | CH3 | Methyl group |
| 17 | OH | Hydroxyl group (carboxylic acid) |
| 18 | H2O | Alcohol or carboxylic acid |
| 28 | CO | Carbonyl group (from acylium ion) |
| 29 | CHO | Aldehyde |
| 31 | OCH3 | Methyl ester or methyl ether |
| 44 | CO2 | Carboxylic acid or ester |
| 45 | OC2H5 | Ethyl ester or ethyl ether |
How to use this table
Subtract the fragment m/z from the molecular ion m/z. If the difference matches a common loss, you have a strong clue about which functional group was present. For example, if M+ = 60 and you see m/z = 43, the loss is 17 (OH) - suggesting a carboxylic acid or similar group.
Functional Group Signatures in MS
Different functional groups produce characteristic fragmentation patterns. Here are the most common ones you will encounter:
Alcohols
Primary alcohols show m/z 31 (CH2=OH+). Loss of H2O (18 Da) from M+ is common. The molecular ion is often weak.
Ketones
Alpha-cleavage produces acylium ions (RCO+ at m/z 43 for methyl ketones). Loss of 28 (CO) from the acylium gives the alkyl cation.
Carboxylic Acids
Loss of OH (17 Da) gives an acylium ion. Loss of CO2 (44 Da) is also common. m/z 45 (COOH+) and m/z 60 (McLafferty rearrangement for longer chains).
Aromatic Compounds
Benzene ring gives m/z 77 (C6H5+) and m/z 51 (C4H3+, from loss of C2H2). Toluene gives the characteristic tropylium ion at m/z 91.
Amines
Nitrogen-containing compounds follow the nitrogen rule: odd molecular weight suggests an odd number of nitrogen atoms. Alpha-cleavage next to nitrogen is favored.
Step-by-Step Interpretation
Find the molecular ion
Look at the highest m/z value. Is it consistent with the molecular formula? Apply the nitrogen rule (odd MW = odd number of N atoms).
Identify the base peak
What is the most abundant fragment? Calculate the loss from M+ to the base peak. Does it match a common neutral loss?
Look for key fragment ions
Do you see diagnostic fragments? m/z 43 (acetyl), m/z 77 (phenyl), m/z 91 (tropylium), m/z 31 (CH2OH+)?
Calculate neutral losses
Subtract fragment m/z values from M+ and from each other. Consistent losses of 15, 17, 18, 28, or 44 point to specific functional groups.
Combine with IR and NMR
MS gives you the molecular weight and fragmentation clues. IR tells you the functional groups. NMR reveals the carbon skeleton. Together, they uniquely identify the structure.
Ready to practice?
Try the interactive mass spectrometry interpreter to see fragmentation patterns in 3D, or test your knowledge with practice questions.