Isotopic peaks MCQs With Answer

Isotopic peaks MCQs With Answer

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Isotopic peaks MCQs With Answer (Modern Pharmaceutical Analytical Techniques)

Isotopic peaks are central to interpreting mass spectra in modern pharmaceutical analysis. They arise from naturally occurring isotopes (such as 13C, 37Cl, and 81Br) and produce diagnostic patterns like M, M+1, and M+2 that reveal elemental composition, charge state, and even the number of halogen atoms. For M. Pharm students, mastering isotopic envelopes helps in formula confirmation, halogen detection in APIs, and understanding monoisotopic versus average mass. These MCQs cover quantitative rules of thumb (e.g., 1.1% per carbon for M+1), characteristic chlorine/bromine signatures, sulfur’s distinct M+2, high-resolution requirements for isotopic fine structure, and best practices in isotope dilution quantification. Use these questions to sharpen problem-solving for both qualitative identification and quantitative LC–MS/MS applications.

Q1. In mass spectrometry, “isotopic peaks” primarily refer to:

  • Signals arising from molecules containing naturally occurring isotopes (isotopologues) of the same elemental composition
  • Peaks produced by different adduct ions such as [M+Na]+ or [M+K]+
  • Peaks due to in-source fragmentation pathways
  • External calibrant peaks injected for mass accuracy

Correct Answer: Signals arising from molecules containing naturally occurring isotopes (isotopologues) of the same elemental composition

Q2. The dominant contributor to the M+1 peak in an organic molecule is:

  • 13C (~1.1% natural abundance)
  • 15N (~0.37% natural abundance)
  • 2H (deuterium, ~0.015% natural abundance)
  • 17O (~0.038% natural abundance)

Correct Answer: 13C (~1.1% natural abundance)

Q3. A quick estimate for the relative intensity of M+1 (in %) for a hydrocarbon is:

  • Approximately 1.1 × number of carbon atoms
  • Approximately 0.37 × number of nitrogen atoms
  • Approximately 4.3 × number of sulfur atoms
  • Exactly equal to the number of carbon atoms divided by molecular weight

Correct Answer: Approximately 1.1 × number of carbon atoms

Q4. Which heteroatom yields a characteristic M and M+2 pattern with ~3:1 intensity ratio?

  • Chlorine (35Cl/37Cl)
  • Bromine (79Br/81Br)
  • Sulfur (32S/34S)
  • Silicon (28Si/29Si/30Si)

Correct Answer: Chlorine (35Cl/37Cl)

Q5. A mass spectrum shows two peaks 2 Da apart with nearly equal intensities. This most strongly indicates the presence of:

  • One bromine atom in the molecule
  • One chlorine atom in the molecule
  • Two sulfur atoms in the molecule
  • One silicon atom in the molecule

Correct Answer: One bromine atom in the molecule

Q6. For a molecule containing two chlorine atoms, the approximate M:M+2:M+4 pattern expected is:

  • 9:6:1
  • 1:1:1
  • 1:3:9
  • 1:2:1

Correct Answer: 9:6:1

Q7. An unusually intense single M+2 peak of about 4–5% relative to M (without halogen-like pattern) suggests the presence of:

  • Sulfur (34S at ~4.3%)
  • Bromine
  • Chlorine
  • Phosphorus

Correct Answer: Sulfur (34S at ~4.3%)

Q8. Resolving isotopic fine structure (e.g., distinguishing one 34S from two 13C at M+2) typically requires:

  • FT-ICR MS or high-field Orbitrap operating at resolving power >200,000
  • Single quadrupole MS at unit resolution
  • Low-resolution ion trap in MS2 mode only
  • MALDI-TOF at low acceleration voltage

Correct Answer: FT-ICR MS or high-field Orbitrap operating at resolving power >200,000

Q9. In ESI-MS, the spacing between neighboring isotope peaks in a charge state z is:

  • Approximately 1/z Da
  • Always 1 Da
  • Approximately z Da
  • Independent of charge

Correct Answer: Approximately 1/z Da

Q10. The mass difference between M and the first 13C isotopologue is approximately:

  • ≈ 1.003355 Da (13C − 12C)
  • Exactly 1.000000 Da
  • ≈ 0.9995 Da
  • ≈ 1.0078 Da

Correct Answer: ≈ 1.003355 Da (13C − 12C)

Q11. The monoisotopic mass of a molecule is defined as the mass of:

  • The composition using the most abundant isotope of each element
  • The isotope-averaged composition weighted by natural abundances
  • The base peak in the mass spectrum
  • The neutral molecule including solvation

Correct Answer: The composition using the most abundant isotope of each element

Q12. Which statement best describes isotopic envelopes for large biomolecules (e.g., peptides >2 kDa)?

  • The monoisotopic peak is often weak or unobserved; the most intense peak occurs several Daltons above M
  • The monoisotopic peak is always the base peak and easily observed
  • The isotopic spacing becomes larger than 1 Da regardless of charge
  • Isotopic peaks disappear due to rapid fragmentation

Correct Answer: The monoisotopic peak is often weak or unobserved; the most intense peak occurs several Daltons above M

Q13. Given an M+1/M ratio of 13.2% for an organic compound with few heteroatoms, the approximate number of carbons is:

  • About 12 (13.2/1.1)
  • About 6 (13.2/2.2)
  • About 24 (13.2/0.55)
  • About 9 (13.2/1.5)

Correct Answer: About 12 (13.2/1.1)

Q14. A compound containing two bromine atoms will exhibit an M:M+2:M+4 pattern that is approximately:

  • 1:2:1
  • 3:1:0
  • 9:6:1
  • 1:1:1

Correct Answer: 1:2:1

Q15. The key advantage of using a stable isotope-labeled internal standard (SIL-IS) for LC–MS/MS quantification is:

  • It corrects for matrix effects and recovery because it co-elutes and shares ionization efficiency with the analyte
  • It increases the analyte signal by chemical reaction
  • It eliminates the need for calibration curves
  • It prevents in-source fragmentation

Correct Answer: It corrects for matrix effects and recovery because it co-elutes and shares ionization efficiency with the analyte

Q16. For oxygen-containing molecules without halogens or sulfur, the primary contributor to an M+2 peak is:

  • 18O (0.20% natural abundance)
  • 17O (0.038% natural abundance)
  • 15N (0.37% natural abundance)
  • 2H (0.015% natural abundance)

Correct Answer: 18O (0.20% natural abundance)

Q17. Which parameter of a mass spectrometer most directly controls the ability to separate overlapping isotope peaks at high m/z?

  • Resolving power (m/Δm)
  • Ionization voltage
  • Collision energy
  • Source temperature

Correct Answer: Resolving power (m/Δm)

Q18. Estimate the M+1 intensity for C10H14N2O considering 13C and 15N only (use 13C ≈ 1.1% and 15N ≈ 0.37%).

  • Approximately 11.7%
  • Approximately 1.1%
  • Approximately 22.0%
  • Approximately 0.74%

Correct Answer: Approximately 11.7%

Q19. When selecting labels for SIL-IS in quantitative LC–MS/MS, a common best practice is:

  • Prefer 13C/15N labeling over 2H to minimize chromatographic isotope effects
  • Prefer 2H labeling because it is cheaper and always co-elutes identically
  • Use 18O labeling only, as it gives the largest mass shift
  • Avoid labeling; unlabeled analogs work equally well

Correct Answer: Prefer 13C/15N labeling over 2H to minimize chromatographic isotope effects

Q20. To avoid misassigning a neighboring compound’s isotope peak as your analyte, the most robust data-analysis strategy is to:

  • Match the full theoretical isotopic distribution (envelope) and exact masses for the proposed formula
  • Rely only on the base peak m/z
  • Increase collision energy until isotope peaks disappear
  • Use a wider mass window during extraction

Correct Answer: Match the full theoretical isotopic distribution (envelope) and exact masses for the proposed formula

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