Metastable ions and isotopic peaks MCQs With Answer

Introduction: This quiz collection on Metastable ions and isotopic peaks is designed specifically for M.Pharm students studying Advanced Instrumental Analysis (MPA 201T). It focuses on the principles and practical implications of metastable ion formation and isotopic pattern recognition in mass spectrometry, emphasizing interpretation, calculation, and instrument-related behavior. The questions progress from foundational definitions to applied problems such as estimating M+1 and M+2 abundances, recognising halogen patterns, distinguishing metastable peaks from isotopic peaks, and understanding how resolution and ion fragmentation timing affect spectra. Use these MCQs to test your conceptual clarity and improve spectral interpretation skills essential for pharmaceutical analysis and structure elucidation.

Q1. What best describes a metastable ion in mass spectrometry?

• An ion that fragments immediately within the ion source
• An ion that fragments after acceleration or after leaving the ion source
• An ion that cannot be detected by the mass analyzer
• An ion that gains electrons to become neutral

Correct Answer: An ion that fragments after acceleration or after leaving the ion source

Q2. Which isotopic contribution is the primary source of the M+1 peak for most organic molecules?

• 34S (sulfur-34)
• 15N (nitrogen-15)
• 13C (carbon-13)
• 2H (deuterium)

Correct Answer: 13C (carbon-13)

Q3. A compound contains one chlorine atom. What approximate ratio of M : M+2 would you expect in its electron ionization mass spectrum?

• 1 : 0.5
• 3 : 1
• 1 : 1
• 9 : 1

Correct Answer: 3 : 1

Q4. Brominated organic molecules typically show which distinctive isotopic pattern?

• A single sharp molecular ion peak with no M+2
• Large M+1 peak due to 13C only
• Two peaks of approximately equal intensity separated by 2 mass units (M and M+2)
• Continuous broad hump instead of discrete peaks

Correct Answer: Two peaks of approximately equal intensity separated by 2 mass units (M and M+2)

Q5. Metastable peaks are most commonly observed in which type of mass spectrometer scanning mode?

• Quadrupole in SIM (selected ion monitoring) only
• Magnetic sector instruments using scanning of m/z or kinetic energy
• Time-of-flight with orthogonal pulsing where no fragmentation occurs
• Ion trap in isolation mode only

Correct Answer: Magnetic sector instruments using scanning of m/z or kinetic energy

Q6. Which experimental observation helps distinguish a metastable peak from an isotopic peak?

• Metastable peaks always appear at exact integer m/z values
• Isotopic peaks change intensity when ion source temperature changes
• Metastable peaks often appear at non-integer or broadened positions and their intensity depends on flight path length
• Isotopic peaks disappear when using low-resolution instruments

Correct Answer: Metastable peaks often appear at non-integer or broadened positions and their intensity depends on flight path length

Q7. For a molecule C8H10O, estimate the approximate percentage intensity of the M+1 peak relative to M from 13C alone.

• ~0.8%
• ~8.8%
• ~88%
• ~1.1%

Correct Answer: ~8.8%

Q8. Which isotopes contribute significantly to the M+2 peak for molecules that do not contain Cl or Br?

• Only 13C
• 34S and two 13C contributions (13C+13C), and to a smaller extent 18O
• 2H and 15N primarily
• No elements contribute to M+2 except halogens

Correct Answer: 34S and two 13C contributions (13C+13C), and to a smaller extent 18O

Q9. In interpreting an EI mass spectrum, a prominent peak at M+2 with roughly equal intensity to M most likely indicates:

• Presence of two sulfur atoms
• Presence of one chlorine atom
• Presence of one bromine atom
• Presence of heavy oxygen isotopes only

Correct Answer: Presence of one bromine atom

Q10. Why does increasing the resolving power of a mass spectrometer help distinguish isotopic peaks from metastable peaks?

• Higher resolution increases ion fragmentation to eliminate metastables
• Higher resolution separates closely spaced masses, resolving true isotopic mass differences from broadened metastable signals
• Higher resolution suppresses all M+1 isotopes
• Higher resolution converts metastable ions into neutrals

Correct Answer: Higher resolution separates closely spaced masses, resolving true isotopic mass differences from broadened metastable signals

Q11. A molecular ion shows M at 200 and an additional peak at 201. If the molecule has 12 carbon atoms, which statement is most accurate about the 201 peak intensity (M+1)?

• It will be negligible because only halogens give large M+1 peaks
• Approximate intensity from 13C alone will be about 13 × 1.1% ≈ 14.3% of M
• It will be equal intensity to M due to 13C abundance
• It will be ~1.1% irrespective of carbon count

Correct Answer: Approximate intensity from 13C alone will be about 13 × 1.1% ≈ 14.3% of M

Q12. Which practical step can reduce the appearance of metastable peaks in a mass spectrum?

• Using a longer flight path to allow more metastable decay
• Lowering ion source pressure and shortening delay between ionization and analysis
• Increasing source temperature to maximize post-source fragmentation
• Deliberately adding halogenated solvents

Correct Answer: Lowering ion source pressure and shortening delay between ionization and analysis

Q13. The presence of a broad signal or plateau on the baseline between two discrete peaks is most characteristic of:

• An instrumental electronic fault only
• An unresolved cluster of isotopic peaks of carbon only
• Metastable ion decomposition producing a continuum of energies
• A perfectly resolved isotopic pattern

Correct Answer: Metastable ion decomposition producing a continuum of energies

Q14. For formula C6H6Cl2, which isotopic pattern is expected for the molecular ion cluster (considering chlorine isotopes)?

• Single peak at M with no satellites
• Three peaks at M, M+2, M+4 with relative intensities about 9:6:1
• Two peaks only at M and M+2 with 3:1 ratio
• Three peaks at M, M+2, M+4 with relative intensities approximating 9:6:1 (binomial distribution for two Cl atoms)

Correct Answer: Three peaks at M, M+2, M+4 with relative intensities approximating 9:6:1 (binomial distribution for two Cl atoms)

Q15. What is the main reason metastable peaks sometimes appear at non-integer m/z values in sector mass spectrometers?

• Isotopic abundances are fractional
• Fragmentation after acceleration changes ion kinetic energy, producing signals at apparent non-integer mass in energy- or momentum-scanned detection
• The detector reports half masses by design
• Mass calibrations are always off by fractional amounts

Correct Answer: Fragmentation after acceleration changes ion kinetic energy, producing signals at apparent non-integer mass in energy- or momentum-scanned detection

Q16. Which isotopic pair contributes almost equally to M and M+2 peaks, causing a characteristic 1:1 pattern?

• 35Cl and 37Cl
• 79Br and 81Br
• 12C and 13C
• 16O and 18O

Correct Answer: 79Br and 81Br

Q17. When calculating expected M+1 peak height from carbon only, which multiplication is appropriate?

• Number of carbon atoms × 0.011 (1.1%)
• Number of carbon atoms × 0.10 (10%)
• Number of carbon atoms × 0.00011 (0.011%)
• Number of carbon atoms × 0.5 (50%)

Correct Answer: Number of carbon atoms × 0.011 (1.1%)

Q18. Which statement about isotopic fine structure is correct?

• Low-resolution instruments can always resolve isotopic fine structure
• High-resolution accurate-mass instruments can resolve contributions from multiple isotopes (e.g., 13C vs 15N vs 34S) that overlap at nominal masses
• Isotopic fine structure is irrelevant for elemental composition determination
• Isotopic fine structure only occurs for halogenated compounds

Correct Answer: High-resolution accurate-mass instruments can resolve contributions from multiple isotopes (e.g., 13C vs 15N vs 34S) that overlap at nominal masses

Q19. A molecular ion peak is weak but a strong peak appears at a lower m/z that is not an integer difference. Which is the most likely cause?

• An impurity with higher molecular weight
• Metastable decomposition of the molecular ion occurring after acceleration
• Instrument noise, always random
• Only isotopic substitution can produce non-integer differences

Correct Answer: Metastable decomposition of the molecular ion occurring after acceleration

Q20. Which approach is most useful to confirm whether a peak is due to an isotopic variant rather than a metastable fragment?

• Change the ionization energy and observe if the relative intensity of the peak follows isotopic abundance predictions and remains fixed in mass position
• Assume any additional peak is isotopic by default
• Only use visual inspection without calculation
• Replace the sample solvent and re-run without considering isotope calculations

Correct Answer: Change the ionization energy and observe if the relative intensity of the peak follows isotopic abundance predictions and remains fixed in mass position

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