What is Mass Spectrometry?
Mass spectrometry (MS) is an analytical technique that ionizes chemical compounds and separates the ions by their mass-to-charge ratio (m/z). The resulting spectrum reveals the molecular weight and fragmentation pattern, providing structural clues about the compound.
MS vaporizes and ionizes a sample, then uses electric/magnetic fields to separate ion fragments by m/z ratio, creating a mass spectrum that identifies the molecular ion and its fragments.
- 1↓IonizationSample is ionized (electron impact, ESI, MALDI, etc.) to form M⁺ (molecular ion)
- 2↓AccelerationIons are accelerated in an electric field to high velocity
- 3↓Separation (m/z analyzer)Ions separated by m/z using magnetic field, quadrupole, ToF, or other method
- 4↓DetectionDetector counts ions at each m/z, creating intensity vs. m/z plot
- 5Fragmentation patternPeaks show molecular ion and fragment masses, revealing structure
Step-by-step worked examples
A mass spectrum of ethanol (C₂H₅OH, MW 46) shows peaks at m/z = 46, 31, and 29. Explain each peak.
m/z = 46: Molecular ion M⁺ (C₂H₅OH⁺) — not always prominent m/z = 31: [CH₃OH]⁺ loss of CH₃ (46 − 15 = 31) m/z = 29: [CHO]⁺ or [HCO]⁺ from further fragmentation These fragments are characteristic of alcohols.
A mass spectrum shows m/z = 100 as the base peak (most intense). What does this suggest?
The base peak is the most stable cation formed from the sample. If m/z = 100 is very stable, it likely results from loss of a small, stable neutral molecule. This could be M⁺ − H₂O, M⁺ − CH₃, or another characteristic loss. The m/z value provides clues about the molecular structure.
Benzoic acid (C₇H₆O₂, MW 122) has a very weak M⁺ peak. Why?
Carboxylic acids easily lose –OH (17 mass units) forming m/z = 105 [C₆H₅CO]⁺ This acylium ion is very stable and becomes the base peak. Weak M⁺ = easy fragmentation to a stable, charged fragment. This is a characteristic pattern for carboxylic acids and esters.
Flashcards
Quick quiz
Q1.A mass spectrum of an organic compound shows m/z = 58 as the base peak. If the molecular ion is at m/z = 88, what neutral fragment was lost?
Q2.Which would give the highest m/z peak in a mass spectrum?
Q3.Why do carboxylic acids often show a weak M⁺ peak?
Q4.In electron impact (EI) ionization, what removes an electron from the sample?
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Common mistakes
The highest peak (tallest) in a mass spectrum is always the molecular ion. — Correct: The highest peak is the base peak (most stable fragment), which may or may not be the molecular ion.
All molecules show a strong M⁺ peak. — Correct: Some molecules fragment easily; their M⁺ peak can be very weak or absent (e.g. alcohols, carboxylic acids).
m/z directly gives the molecular weight for all ions. — Correct: m/z = mass/charge. For doubly charged ions (M²⁺), m/z = MW/2, not MW.
Fragmentation is random and provides no structural information. — Correct: Fragmentation is predictable — specific functional groups produce characteristic fragments.
FAQ
What is the difference between EI and ESI mass spectrometry?
EI (electron impact) produces high fragmentation and works for volatile compounds; ESI (electrospray ionization) is gentle, produces mostly M⁺, and suits biomolecules and large compounds.
Can mass spectrometry distinguish between isomers?
Not directly — isomers have the same molecular weight. However, their fragmentation patterns often differ, which can help.
What is a McLafferty rearrangement?
A common rearrangement in EI-MS where a hydrogen is transferred to a double bond, followed by cleavage — produces a characteristic m/z = 74 for methyl ketones.
Why is a mass spectrometer under vacuum?
Vacuum prevents ions from colliding with gas molecules, allowing them to reach the detector without interference.