Isotopic peaks, metastable ions and ring rule interpretation MCQs With Answer

Introduction: Isotopic peaks, metastable ions and ring rule interpretation are fundamental concepts in advanced mass spectral analysis useful for structure elucidation in pharmaceutical research. This post presents a focused set of 20 multiple-choice questions tailored for M.Pharm students to strengthen understanding of isotopic abundance patterns (13C, 35Cl/37Cl, 79Br/81Br, 34S), calculation of M+1/M+2 peak intensities, identification and origin of metastable ions in electron ionization and sector instruments, and application of the ring rule (degree of unsaturation) for molecular formula interpretation. Each question includes concise answer choices and the correct option for rapid self-assessment and exam preparation in MPC 201T Advanced Spectral Analysis.

Q1. What primarily causes the M+1 peak in an electron ionization mass spectrum?

• Natural abundance of 2H (deuterium)
• Presence of 13C isotopes in the molecule
• Fragmentation of the molecular ion
• Instrumental noise

Correct Answer: Presence of 13C isotopes in the molecule

Q2. Approximately how much relative intensity (percent of M) does each carbon atom contribute to the M+1 peak?

• 0.01% per carbon
• 0.12% per carbon
• 1.1% per carbon
• 10% per carbon

Correct Answer: 1.1% per carbon

Q3. Which isotopic pattern is characteristic of a single chlorine atom in a molecular ion peak?

• M and M+1 peaks in 1:1 ratio
• M and M+2 peaks in 3:1 ratio
• M and M+2 peaks in 1:1 ratio
• Only a single M peak with no satellite

Correct Answer: M and M+2 peaks in 3:1 ratio

Q4. Which halogen gives an approximately 1:1 M and M+2 peak ratio in mass spectra?

• Chlorine (Cl)
• Bromine (Br)
• Fluorine (F)
• Iodine (I)

Correct Answer: Bromine (Br)

Q5. For a molecule containing a single sulfur atom, what is the approximate expected relative intensity of the M+2 peak (due to 34S) compared to M?

• ~0.05% of M
• ~1.1% of M
• ~4.4% of M
• ~50% of M

Correct Answer: ~4.4% of M

Q6. Calculate the approximate M+1 peak intensity for a compound with formula C9H10O (neglecting minor H isotopes).

• ~1.0% of M
• ~9.9% of M
• ~19.8% of M
• ~0.9% of M

Correct Answer: ~9.9% of M

Q7. The ring rule (degree of unsaturation, DBE) formula is DBE = C – H/2 – X/2 + N/2 + 1. What does X represent in this formula?

• Number of oxygen atoms
• Number of halogen atoms (F, Cl, Br, I)
• Number of double bonds
• Number of hydrogen atoms

Correct Answer: Number of halogen atoms (F, Cl, Br, I)

Q8. What is the degree of unsaturation (DBE) for C10H12?

• 2
• 3
• 5
• 7

Correct Answer: 5

Q9. For a molecular formula C7H5NO, what is the DBE (rings + double bonds)?

• 3
• 4
• 5
• 6

Correct Answer: 6

Q10. Which description best defines a metastable ion in mass spectrometry?

• An ion that is stable and does not fragment
• An excited ion that fragments after leaving the ion source, often producing peaks between integer m/z values in sector instruments
• An ion produced only in electrospray ionization
• An ion formed exclusively by collision-induced dissociation

Correct Answer: An excited ion that fragments after leaving the ion source, often producing peaks between integer m/z values in sector instruments

Q11. In which mass spectrometer experiment are metastable ions commonly analyzed by observing the kinetic energy of product ions?

• Tandem quadrupole MS/MS
• PIXE (Particle Induced X-ray Emission)
• MIKE (Mass-Analyzed Ion Kinetic Energy) spectrometry
• UV-Vis spectroscopy

Correct Answer: MIKE (Mass-Analyzed Ion Kinetic Energy) spectrometry

Q12. Which isotopes contribute most significantly to an M+2 peak in organic molecules?

• 13C and 2H
• 35Cl and 79Br
• 37Cl, 81Br and 34S
• 15N and 18O

Correct Answer: 37Cl, 81Br and 34S

Q13. A compound shows major peaks at M, M+2 (1:1) and a minor M+1. Which element is most likely present?

• One chlorine atom
• One sulfur atom
• One bromine atom
• One fluorine atom

Correct Answer: One bromine atom

Q14. Two bromine atoms in a molecule will produce which isotopic peak intensity pattern for M : M+2 : M+4?

• 1 : 2 : 1
• 1 : 1 : 1
• 3 : 1 : 0
• 1 : 0 : 1

Correct Answer: 1 : 2 : 1

Q15. How do metastable ions typically appear in spectra recorded on a magnetic sector instrument?

• As a narrow sharp peak at integer m/z
• As a broad, low-intensity hump or peak between integer m/z positions
• As a baseline noise only
• As a very high intensity base peak

Correct Answer: As a broad, low-intensity hump or peak between integer m/z positions

Q16. Using ring rule, which molecular formula corresponds to a fully saturated acyclic hydrocarbon (no rings, no double bonds)?

• CnH2n+2
• CnH2n
• CnH2n-2
• CnH2n+1

Correct Answer: CnH2n+2

Q17. For chlorobenzene (C6H5Cl), what is the expected approximate intensity ratio of M : M+2, ignoring small M+1 contributions?

• 1 : 0
• 1 : 3
• 3 : 1
• 1 : 1

Correct Answer: 3 : 1

Q18. If a molecule gives an M+1 intensity of ~22% relative to M, approximately how many carbons does it contain? (Use 1.1% per C approximation)

• ~10 carbons
• ~20 carbons
• ~2 carbons
• ~40 carbons

Correct Answer: ~20 carbons

Q19. Metastable ion fragmentation most commonly occurs in which region of a sector mass spectrometer?

• Inside the ion source only
• In the field-free flight region between analyzer stages
• Outside the detector
• During ionization in solution

Correct Answer: In the field-free flight region between analyzer stages

Q20. What is the degree of unsaturation (DBE) for C8H10O?

• 2
• 3
• 4
• 5

Correct Answer: 4

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