Mass fragmentation rules MCQs With Answer

Mass fragmentation rules MCQs With Answer

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Introduction

Welcome to our Mass Fragmentation Rules MCQs with Answers for M. Pharm students. This quiz focuses on the core principles that govern how molecular ions break apart in mass spectrometry, a crucial tool in Modern Pharmaceutical Analytical Techniques. You will test your understanding of the even-electron and nitrogen rules, alpha cleavage next to heteroatoms, McLafferty rearrangement, benzylic and allylic cleavages, isotope patterns, and diagnostic neutral losses in EI, CI, ESI, and MS/MS. The questions are designed to sharpen interpretation skills for structural elucidation, impurity profiling, and metabolite identification. Use this set to reinforce mechanistic reasoning behind fragment patterns and to prepare for exams and research applications in pharmaceutical analysis.

Q1. Which statement best describes the even-electron rule in mass spectrometry?

  • Even-electron ions preferentially fragment to other even-electron ions by loss of neutral molecules rather than radicals
  • Even-electron ions preferentially fragment to odd-electron ions by loss of radicals
  • Even-electron ions never undergo rearrangements
  • Even-electron ions always produce the molecular radical cation

Correct Answer: Even-electron ions preferentially fragment to other even-electron ions by loss of neutral molecules rather than radicals

Q2. According to the nitrogen rule, what is the nominal mass parity of [M+H]+ for an organic compound containing one nitrogen atom?

  • Odd nominal mass
  • Even nominal mass
  • Nominal mass cannot be predicted by the nitrogen rule
  • Depends only on halogen content

Correct Answer: Even nominal mass

Q3. Which prerequisite most accurately defines the McLafferty rearrangement?

  • Presence of a gamma-hydrogen relative to a carbonyl/imine enabling a six‑membered transition state
  • Presence of an alpha-hydrogen to any saturated carbon
  • Requirement of an aromatic ring ortho to the carbonyl group
  • Requirement of two adjacent heteroatoms with no hydrogens nearby

Correct Answer: Presence of a gamma-hydrogen relative to a carbonyl/imine enabling a six‑membered transition state

Q4. What is the characteristic fragmentation of molecules containing a heteroatom (O, N, S) under EI conditions?

  • Beta cleavage producing a radical cation and a neutral alkene
  • Alpha cleavage adjacent to the heteroatom forming a stabilized cation (oxonium/iminium/acylium)
  • Only loss of H2 without bond cleavage
  • Exclusive formation of the molecular ion with no fragmentation

Correct Answer: Alpha cleavage adjacent to the heteroatom forming a stabilized cation (oxonium/iminium/acylium)

Q5. In EI spectra of alkylbenzenes, the prominent ion at m/z 91 is generally assigned to which species?

  • Tropylium cation (C7H7+)
  • Benzoyl cation (C6H5CO+)
  • Phenyl cation (C6H5+)
  • Benzylium cation (C6H5CH2+), non-rearranged

Correct Answer: Tropylium cation (C7H7+)

Q6. For saturated hydrocarbons under EI, which fragmentation pathway typically yields the most intense peaks?

  • Cleavage producing primary carbocations
  • Cleavage producing secondary carbocations
  • Cleavage producing tertiary carbocations
  • Random cleavages with no relation to cation stability

Correct Answer: Cleavage producing tertiary carbocations

Q7. The intense ion at m/z 43 observed for many ketones in EI is most commonly attributed to:

  • Ethyl cation (C2H5+)
  • Acetyl cation/acylium (CH3CO+)
  • Allyl cation (C3H5+)
  • Methoxy cation (CH3O+)

Correct Answer: Acetyl cation/acylium (CH3CO+)

Q8. Which statement about odd-electron ions (e.g., M+• in EI) is most accurate?

  • They preferentially undergo homolytic cleavage to give an even-electron cation and a radical
  • They cannot lose radicals
  • They always produce even-electron neutrals
  • They only fragment via rearrangements

Correct Answer: They preferentially undergo homolytic cleavage to give an even-electron cation and a radical

Q9. In positive ESI/CI, which ions commonly undergo dehydration (loss of 18 Da)?

  • Protonated alcohols and diols ([M+H]+)
  • Neutral alkanes
  • Metal-adducted aromatic hydrocarbons
  • Radical cations of carbonyls

Correct Answer: Protonated alcohols and diols ([M+H]+)

Q10. Retro-Diels–Alder fragmentation is most characteristic for which type of systems in MS/MS?

  • Cyclic 1,3-dienes and conjugated enone rings, producing two complementary fragments
  • Fully saturated six-membered rings
  • Inorganic salts only
  • Only acyclic alkanes

Correct Answer: Cyclic 1,3-dienes and conjugated enone rings, producing two complementary fragments

Q11. Which neutral loss from an even-electron ion would violate the even-electron rule?

  • H2O (18 Da)
  • NH3 (17 Da)
  • CO2 (44 Da)
  • CH3• (15 Da radical)

Correct Answer: CH3• (15 Da radical)

Q12. The “ortho effect” in EI mass spectra of o‑substituted aromatic acids/esters typically involves:

  • Intramolecular hydrogen bonding that drives loss of H2O/ROH via a six‑membered transition state
  • Exclusive formation of the molecular ion with no fragmentation
  • Spontaneous addition of the ortho substituent to the ring
  • Mandatory formation of tropylium ion at m/z 91

Correct Answer: Intramolecular hydrogen bonding that drives loss of H2O/ROH via a six‑membered transition state

Q13. Which diagnostic ions arise from alpha cleavage of amines in EI?

  • Acylium ions (RCO+)
  • Iminium ions (R2N=CH2+ or related)
  • Phenoxy radical cations
  • Carbanions

Correct Answer: Iminium ions (R2N=CH2+ or related)

Q14. Why are benzylic and allylic cleavages particularly favored in EI fragmentation?

  • They form high-energy, unstable ions that rapidly recombine
  • They are induced by external magnetic fields only
  • They generate resonance-stabilized cations, enhancing ion abundance
  • They exclusively form radical anions

Correct Answer: They generate resonance-stabilized cations, enhancing ion abundance

Q15. Chlorine-containing fragments are recognized by what characteristic isotope pattern at M and M+2?

  • 1:1 intensity (M:M+2)
  • 3:1 intensity (M:M+2) due to 35Cl and 37Cl
  • 9:1 intensity (M:M+2)
  • No discernible M+2 peak

Correct Answer: 3:1 intensity (M:M+2) due to 35Cl and 37Cl

Q16. In low-energy CID of protonated peptides, which product ion series predominates due to charge-directed amide bond cleavage?

  • a and x ions
  • b and y ions
  • c and z• ions
  • Molecular radical cations only

Correct Answer: b and y ions

Q17. Which statement best defines charge-remote fragmentation (CRF) observed for long-chain lipids in high-energy MS/MS?

  • Cleavages occur exclusively at the charge site
  • Cleavages occur at positions remote from the charge, mapping chain features like double bonds
  • Only heteroatom-centered bonds can break
  • CRF is not observed in lipids

Correct Answer: Cleavages occur at positions remote from the charge, mapping chain features like double bonds

Q18. What is the most common fragmentation pathway for ethers in EI mass spectrometry?

  • Beta scission forming carbanions
  • Alpha cleavage adjacent to oxygen yielding oxonium ions
  • Exclusive loss of water
  • Pericyclic ring expansion

Correct Answer: Alpha cleavage adjacent to oxygen yielding oxonium ions

Q19. Aromatic nitro compounds under EI frequently show which diagnostic neutral loss?

  • Loss of NO2 (46 Da)
  • Loss of CH4 (16 Da)
  • Loss of H2S (34 Da)
  • Loss of HCl (36 Da)

Correct Answer: Loss of NO2 (46 Da)

Q20. Why do fragments that retain the positive charge on a heteroatom (e.g., O or N) often exhibit high intensity?

  • Because the ion source selectively detects heteroatom-containing ions
  • Due to inductive and resonance stabilization of the charge by the heteroatom
  • Because such ions are always formed via secondary electron capture
  • They are neutral species mistakenly recorded as ions

Correct Answer: Due to inductive and resonance stabilization of the charge by the heteroatom

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