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Mass Spectrometry
Mass Spec Analyzer
Identify fragment losses, common ions, and isotope patterns. Predict M/M+1/M+2 ratios and check the nitrogen rule for unknown identification.
Fragment Loss Identifier
Enter the parent ion m/z and a fragment ion m/z to identify the neutral or radical species lost.
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Ion Identifier
Enter a fragment ion m/z value to identify common ions at that mass.
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Isotope Pattern Predictor
Enter atom counts for a molecular formula to predict M, M+1, and M+2 relative intensities.
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Nitrogen Rule Checker
Enter a molecular weight to determine whether the compound likely contains nitrogen.
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McLafferty Rearrangement
A common mass spectrometric fragmentation involving transfer of a γ-hydrogen (gamma-hydrogen) to the carbonyl oxygen through a six-membered cyclic transition state, resulting in loss of a neutral alkene and formation of an enol radical cation.
Requirements
- ✓A carbonyl group (C=O) or analogous unsaturated system (α,β-unsaturated)
- ✓A γ-hydrogen (hydrogen on the carbon three bonds away from the carbonyl)
- ✓Ability to form a six-membered cyclic transition state
Applicable To
- •Aldehydes (loss of alkene from α-position)
- •Ketones (loss of alkene)
- •Carboxylic acids (loss of alkene)
- •Esters (loss of alkene)
- •Amides (loss of alkene)
- •Carbonates and carbamates
Mechanism Steps
- The γ-hydrogen migrates to the carbonyl oxygen via a six-membered ring transition state.
- The bond between the α-carbon and β-carbon cleaves homolytically.
- A neutral alkene (typically ethylene, C₂H₄) is expelled.
- The charge is retained on the enol radical cation fragment.
Worked Example: 2-Hexanone (methyl butyl ketone)
MW = 100 → γ-H transfer from C-5 to carbonyl oxygen, loss of propene (C₃H₆, 42 Da)
Product ion: m/z 58 (CH₂=C(OH)CH₃⁺·) · Lost: CH₃CH=CH₂ (propene, 42 Da)
Highly diagnostic for carbonyl-containing compounds. The rearrangement produces an even-electron (EE+) ion, which is often the base peak. The mass of the McLafferty product ion directly indicates the portion of the molecule attached to the carbonyl.
Common Fragment Losses Reference
Complete table of common neutral and radical losses in EI mass spectrometry.
| Mass Lost | Fragment | Interpretation | Type |
|---|---|---|---|
| 1 | H· | Hydrogen radical loss | radical |
| 15 | CH₃· | Methyl radical loss | radical |
| 17 | OH· | Hydroxyl radical loss | radical |
| 18 | H₂O | Dehydration | neutral |
| 27 | HCN | Loss of hydrogen cyanide | neutral |
| 28 | CO | Carbon monoxide loss | neutral |
| 28 | C₂H₄ | Ethylene loss (McLafferty rearrangement) | neutral |
| 29 | CHO· | Formyl radical loss | radical |
| 29 | C₂H₅· | Ethyl radical loss | radical |
| 30 | CH₂O | Formaldehyde loss | neutral |
| 31 | OCH₃· | Methoxy radical loss | radical |
| 32 | CH₃OH | Methanol loss | neutral |
| 33 | SH· | Thiol radical loss | radical |
| 35 | Cl· | Chlorine radical loss (³⁵Cl) | radical |
| 37 | Cl· | Chlorine radical loss (³⁷Cl isotope) | radical |
| 36 | HCl | Hydrogen chloride loss | neutral |
| 43 | C₃H₇· | Propyl radical loss | radical |
| 43 | CH₃CO· | Acetyl radical loss | radical |
| 44 | CO₂ | Decarboxylation | neutral |
| 45 | OC₂H₅· | Ethoxy radical loss | radical |
| 46 | NO₂ | Nitro group loss | neutral |
| 57 | C₄H₉· | Butyl radical loss | radical |
| 77 | C₆H₅· | Phenyl radical loss | radical |
| 79 | Br· | Bromine radical loss (⁹⁰Br) | radical |
| 81 | Br· | Bromine radical loss (⁸¹Br isotope) | radical |
| 91 | C₇H₇⁺ | Tropylium cation loss (from benzyl cleavage) | cation |
Isotope Pattern Reference
M+1 and M+2 contributions per atom for common elements.
| Element | Symbol | M+1 (%) | M+2 (%) |
|---|---|---|---|
| Carbon | C | 1.10 | 0.00 |
| Hydrogen | H | 0.01 | 0.00 |
| Nitrogen | N | 0.37 | 0.00 |
| Oxygen | O | 0.04 | 0.20 |
| Chlorine | Cl | 0.00 | 32.50 |
| Bromine | Br | 0.00 | 97.30 |
| Sulfur | S | 0.79 | 4.40 |
| Silicon | Si | 3.40 | 5.10 |
| Fluorine | F | 0.00 | 0.00 |
| Iodine | I | 0.00 | 0.00 |
Common Fragment Ions
Frequently observed cations in electron ionization (EI) mass spectra.
| m/z | Formula | Name | Category |
|---|---|---|---|
| 15 | CH₃⁺ | Methyl cation | alkyl |
| 29 | CHO⁺ | Formyl cation | carbonyl |
| 29 | C₂H₅⁺ | Ethyl cation | alkyl |
| 30 | NO⁺ | Nitrosyl cation | nitrogen |
| 39 | C₃H₃⁺ | Cyclopropenyl cation | unsaturated |
| 41 | C₃H₅⁺ | Allyl cation | unsaturated |
| 43 | CH₃CO⁺ | Acylium (acetyl) cation | carbonyl |
| 43 | C₃H₇⁺ | Propyl cation | alkyl |
| 45 | CO₂H⁺ | Carboxyl cation | carbonyl |
| 51 | C₄H₃⁺ | Butatrienyl cation | aromatic |
| 57 | C₄H₉⁺ | tert-Butyl cation | alkyl |
| 65 | C₅H₅⁺ | Cyclopentadienyl cation | aromatic |
| 69 | C₅H₉⁺ | Pentyl / neopentyl cation | alkyl |
| 77 | C₆H₅⁺ | Phenyl cation | aromatic |
| 80 | C₅H₆N⁺ | Pyridinium cation | nitrogen |
| 91 | C₇H₇⁺ | Tropylium cation | aromatic |
| 105 | C₆H₅CO⁺ | Benzoyl cation | aromatic |
| 105 | C₈H₉⁺ | Xylyl / methyltropylium cation | aromatic |
| 128 | C₁₀H₈⁺ | Naphthalenyl cation | aromatic |