Mass fragmentation rules MCQs With Answer

Mass fragmentation rules MCQs With Answer

This quiz set focuses on mass spectrometry fragmentation rules essential for M.Pharm students studying Advanced Instrumental Analysis. It presents 20 carefully chosen multiple-choice questions that probe core concepts—electron ionization vs. electrospray fragmentation, the even‑electron rule, alpha cleavage, McLafferty rearrangement, charge-remote and charge-directed processes, characteristic losses (H2O, CO, CO2, NH3), isotopic patterns for Cl/Br, radical cation behavior, metastable ions, and interpretation strategies using high‑resolution data. Each question targets practical interpretation of spectra and mechanistic understanding so you can predict fragment ions, recognize structural markers and improve analytical reasoning for drug molecules and metabolites.

Q1. Which rule predicts that even‑electron ions tend to fragment by losing neutral molecules rather than forming odd‑electron radicals?

• Odd‑electron rule
• Even‑electron rule
• Nitrogen rule
• McLafferty rule

Correct Answer: Even‑electron rule

Q2. What is the principal driving feature of an alpha‑cleavage in mass spectrometry?

• Cleavage at the β‑carbon relative to the radical site
• Cleavage adjacent to a heteroatom producing a resonance‑stabilized ion
• Hydrogen transfer from γ‑position producing neutral loss
• Formation of a dimeric ion by recombination

Correct Answer: Cleavage adjacent to a heteroatom producing a resonance‑stabilized ion

Q3. The McLafferty rearrangement typically requires which structural feature?

• A benzylic hydrogen and no heteroatoms
• A γ‑hydrogen relative to a carbonyl and the ability to form a six‑membered transition state
• A tertiary alcohol contiguous to a double bond
• An aromatic ring with halogen substituents

Correct Answer: A γ‑hydrogen relative to a carbonyl and the ability to form a six‑membered transition state

Q4. Which loss is most commonly observed in EI mass spectra of alcohols?

• Loss of CO
• Loss of H2O
• Loss of NH3
• Loss of HCl

Correct Answer: Loss of H2O

Q5. The nitrogen rule states that an odd‑electron molecular ion has an even nominal mass when the molecule contains how many nitrogen atoms?

• Zero or two nitrogen atoms
• An odd number of nitrogen atoms
• Exactly three nitrogen atoms
• Only even‑electron ions are addressed by the rule

Correct Answer: An odd number of nitrogen atoms

Q6. Which observation strongly indicates the presence of a chlorine atom in an EI mass spectrum?

• An M+2 peak about 1/3 the intensity of M
• An M+2 peak equal intensity to M
• No M+2 peak at all
• A base peak exactly at m/z 79

Correct Answer: An M+2 peak about 1/3 the intensity of M

Q7. In electrospray ionization (ESI) mass spectrometry, which ion type is most commonly observed for neutral small molecules?

• Radical cations (M•+)
• Protonated molecules ([M+H]+)
• Electron adducts ([M−e]−)
• Fragment cations exclusively

Correct Answer: Protonated molecules ([M+H]+)

Q8. What is a metastable ion in mass spectrometry?

• An ion that cannot fragment under any conditions
• An ion that fragments in the field‑free region after acceleration, producing broadened peaks or peaks at non‑integer m/z values in sector instruments
• An ion with zero kinetic energy
• An ion formed by recombination of two radicals

Correct Answer: An ion that fragments in the field‑free region after acceleration, producing broadened peaks or peaks at non‑integer m/z values in sector instruments

Q9. Which fragmentation pathway is characteristic for benzyl or allylic positions?

• Simple alpha‑cleavage without stabilization
• Formation of a resonance‑stabilized benzyl/allyl cation leading to intense fragment peaks
• Exclusive loss of CO2
• McLafferty rearrangement only

Correct Answer: Formation of a resonance‑stabilized benzyl/allyl cation leading to intense fragment peaks

Q10. Charge‑remote fragmentation is best described as which process?

• Fragmentation occurring only at the charge site due to coulombic forces
• Cleavage at a remote site while the charge remains on another part of the molecule, common in long aliphatic chains
• Loss of electrons to form radical anions
• Specific to aromatic heterocycles undergoing McLafferty rearrangement

Correct Answer: Cleavage at a remote site while the charge remains on another part of the molecule, common in long aliphatic chains

Q11. Which diagnostic neutral loss strongly suggests the presence of an ester in EI mass spectra?

• Loss of NH3 (17 Da)
• Loss of CH3 (15 Da)
• Loss of alcohol (R’OH) or alkoxy radical leading to loss of 31, 45, etc., depending on ester substituent
• Loss of HCl (36.5 Da)

Correct Answer: Loss of alcohol (R’OH) or alkoxy radical leading to loss of 31, 45, etc., depending on ester substituent

Q12. Which fragment ion often serves as the base peak for alkyl benzenes in EI spectra?

• The molecular ion M•+
• The benzyl cation (C7H7+, m/z 91)
• A fragment at m/z 28 (CO)
• McLafferty rearrangement fragment only at m/z 72

Correct Answer: The benzyl cation (C7H7+, m/z 91)

Q13. How does high‑resolution mass spectrometry help in fragmentation interpretation?

• By increasing fragment abundance regardless of mass accuracy
• By providing exact mass to determine elemental compositions of fragment ions and distinguish isobaric losses
• By preventing fragmentation entirely
• By only indicating isotopic ratios without mass accuracy

Correct Answer: By providing exact mass to determine elemental compositions of fragment ions and distinguish isobaric losses

Q14. Which fragmentation is common for peptides under tandem MS with collision‑induced dissociation (CID)?

• Alpha‑cleavage only producing radicals
• Formation of b‑ and y‑type ions from peptide backbone cleavage
• Metastable dimer formation ignoring backbone
• Exclusive McLafferty rearrangement at side chains

Correct Answer: Formation of b‑ and y‑type ions from peptide backbone cleavage

Q15. What does a prominent peak corresponding to [M+Na]+ in the spectrum indicate?

• Sodium adduction common in ESI or MALDI, indicating formation of sodium adduct rather than protonation
• That the compound is a radical cation
• Exclusive fragmentation to sodium ion only
• Contamination by chlorine atoms

Correct Answer: Sodium adduction common in ESI or MALDI, indicating formation of sodium adduct rather than protonation

Q16. Which statement best describes heterolytic vs homolytic bond cleavage in mass spectrometry?

• Heterolytic cleavage yields two radicals and homolytic yields ions only
• Homolytic cleavage produces radical pairs; heterolytic cleavage produces ionic species (cation + neutral or anion + neutral), with heterolytic more common in even‑electron ions
• Both processes always produce identical fragments
• Only homolytic cleavage is observed in ESI‑MS

Correct Answer: Homolytic cleavage produces radical pairs; heterolytic cleavage produces ionic species (cation + neutral or anion + neutral), with heterolytic more common in even‑electron ions

Q17. The presence of both M and M+2 peaks with almost equal intensity suggests what heteroatom?

• Chlorine
• Bromine
• Oxygen
• Nitrogen

Correct Answer: Bromine

Q18. Retro‑Diels–Alder (RDA) fragmentation is commonly observed in which type of compounds?

• Linear alkanes with no unsaturation
• Cyclic conjugated systems such as cyclohexenes or substituted cyclic dienes leading to characteristic pair fragments
• Simple alcohols with no ring structures
• Peptides exclusively

Correct Answer: Cyclic conjugated systems such as cyclohexenes or substituted cyclic dienes leading to characteristic pair fragments

Q19. Which factor most strongly stabilizes a fragment ion, increasing its likelihood to be observed as an intense peak?

• High gas‑phase basicity only
• Resonance delocalization, aromaticity, or formation of a stable cationic center
• Exclusively a large mass without stabilization
• Presence of heavy isotopes only

Correct Answer: Resonance delocalization, aromaticity, or formation of a stable cationic center

Q20. When interpreting EI spectra of a drug molecule, which combined approach yields the most reliable structural assignments?

• Relying solely on the base peak identity
• Combining fragmentation rules (alpha‑cleavage, McLafferty, rearrangements), isotopic pattern analysis, and high‑resolution exact mass of fragments
• Ignoring isotopic patterns and focusing only on nominal masses
• Assuming all peaks arise from the molecular ion only

Correct Answer: Combining fragmentation rules (alpha‑cleavage, McLafferty, rearrangements), isotopic pattern analysis, and high‑resolution exact mass of fragments

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