Principle of mass spectrometry MCQs With Answer

Principle of mass spectrometry MCQs With Answer

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Principle of Mass Spectrometry MCQs With Answer

Mass spectrometry (MS) is a cornerstone analytical technique in modern pharmaceutical analysis, offering unparalleled selectivity, sensitivity, and structural insight. For M. Pharm students, mastering MS principles—ionization mechanisms, mass analyzers, resolution, accuracy, and interpretation—builds the foundation for advanced applications in drug discovery, metabolite profiling, impurity characterization, and bioanalysis. This quiz focuses on core concepts: formation and behavior of ions (m/z), roles of vacuum and detectors, analyzer physics (quadrupole, TOF, Orbitrap), fragmentation rules, isotopic patterns, and MS/MS strategies. Designed to be application-oriented yet conceptually rigorous, these MCQs will help you cement theoretical understanding and connect it to real-world pharmaceutical analytical techniques, ensuring proficiency for research and practice.

Q1. In mass spectrometry, what does m/z represent and how is it interpreted for singly charged ions?

  • Mass number, equal to the number of protons for singly charged ions
  • Mass-to-charge ratio, numerically equal to the ion’s mass in daltons for z = 1
  • Charge-to-mass ratio, numerically equal to the ion’s charge state for z = 1
  • Relative abundance normalized to base peak for z = 1

Correct Answer: Mass-to-charge ratio, numerically equal to the ion’s mass in daltons for z = 1

Q2. The primary reason mass spectrometers operate under high vacuum is to:

  • Increase ion kinetic energy by reducing space charge
  • Prevent analyte oxidation during ionization
  • Allow ions to travel without collisional scattering and maintain well-defined trajectories
  • Cool the ion source for improved desolvation

Correct Answer: Allow ions to travel without collisional scattering and maintain well-defined trajectories

Q3. In electron ionization (EI), the typical electron energy used to generate molecular ions is:

  • 10 eV
  • 25 eV
  • 70 eV
  • 120 eV

Correct Answer: 70 eV

Q4. The base peak in a mass spectrum is best defined as the:

  • Peak with the highest m/z value
  • Peak corresponding to the molecular ion (M+•)
  • Most intense peak, scaled to 100% relative abundance
  • Peak of the internal standard

Correct Answer: Most intense peak, scaled to 100% relative abundance

Q5. Electrospray ionization (ESI) primarily produces ions by which mechanism?

  • Thermal desorption of neutral molecules followed by electron capture
  • Charge transfer from a plasma to vapor-phase molecules
  • Formation of charged droplets, solvent evaporation, Coulomb fission, and ion release
  • Laser ablation of solid analyte embedded in matrix

Correct Answer: Formation of charged droplets, solvent evaporation, Coulomb fission, and ion release

Q6. Which statement regarding soft versus hard ionization is most accurate?

  • EI is soft and typically yields [M+H]+ with minimal fragmentation
  • ESI is hard and often yields extensive radical cation fragmentation
  • MALDI and ESI are soft, favoring even-electron ions with less fragmentation
  • CI is harder than EI and eliminates adduct formation

Correct Answer: MALDI and ESI are soft, favoring even-electron ions with less fragmentation

Q7. Mass resolving power (R) at full width at half maximum (FWHM) is defined as:

  • R = Δm/m at 10% valley
  • R = m/Δm at half-height of the peak
  • R = (m1 + m2)/Δm of adjacent peaks
  • R = 1/Δm regardless of m

Correct Answer: R = m/Δm at half-height of the peak

Q8. In an Orbitrap mass analyzer, ions are measured based on:

  • Time required to traverse a fixed flight tube
  • Stable trajectories through alternating RF/DC fields
  • Image current generated by coherent axial oscillations, converted via Fourier transform
  • Direct ion counting at a Faraday plate

Correct Answer: Image current generated by coherent axial oscillations, converted via Fourier transform

Q9. The nitrogen rule states that for organic compounds containing C, H, N, O, S, X (halogens):

  • An odd nominal mass implies an even number of nitrogens
  • An odd nominal mass implies an odd number of nitrogens
  • An even nominal mass implies an odd number of nitrogens
  • The rule applies only to even-electron ions in ESI and not to EI

Correct Answer: An odd nominal mass implies an odd number of nitrogens

Q10. Which isotopic pattern is most diagnostic for a compound containing one bromine atom?

  • M and M+2 peaks in approximately 3:1 intensity
  • M and M+2 peaks in approximately 1:1 intensity
  • Prominent M+1 due to 13C with no M+2 peak
  • M+4 dominant over M

Correct Answer: M and M+2 peaks in approximately 1:1 intensity

Q11. In ESI positive mode for small to medium polarity drugs in acidic aqueous mobile phases, the most common primary ion species is:

  • [M–H]−
  • [M+H]+
  • [M+Cl]−
  • M+•

Correct Answer: [M+H]+

Q12. A key advantage of TOF analyzers is that they:

  • Require scanning RF/DC, limiting speed
  • Have virtually unlimited m/z range with very fast acquisition
  • Cannot be pulsed and therefore are incompatible with MALDI
  • Provide the highest trapping capacity among analyzers

Correct Answer: Have virtually unlimited m/z range with very fast acquisition

Q13. In a quadrupole mass filter, mass selection occurs because:

  • Ions are separated by their time-of-flight in a pulsed field
  • Only ions with stable trajectories under specific RF/DC fields reach the detector
  • Ions are trapped and ejected by resonance excitation at their cyclotron frequency
  • Ions are separated by differing kinetic energies through a magnetic sector

Correct Answer: Only ions with stable trajectories under specific RF/DC fields reach the detector

Q14. Collision-induced dissociation (CID) in MS/MS primarily works by:

  • Electron capture to form radical anions that spontaneously fragment
  • Absorption of photons to cause direct bond cleavage (photodissociation)
  • Multiple low-energy collisions with inert gas increasing internal energy leading to fragmentation
  • Thermal decomposition on a heated surface before ionization

Correct Answer: Multiple low-energy collisions with inert gas increasing internal energy leading to fragmentation

Q15. Mass accuracy is typically expressed in parts-per-million (ppm). Which formula is correct?

  • ppm = (measured m/z − theoretical m/z) × 10
  • ppm = [(measured m/z − theoretical m/z) / theoretical m/z] × 106
  • ppm = (theoretical m/z / measured m/z) × 100
  • ppm = [(theoretical m/z − measured m/z) / measured m/z] × 103

Correct Answer: ppm = [(measured m/z − theoretical m/z) / theoretical m/z] × 106

Q16. Which detector principle is correctly matched to common mass spectrometers?

  • Electron multipliers: convert ion impacts into cascades of secondary electrons for amplified current
  • Photomultiplier tubes: detect ion fluorescence after recombination
  • Bolometers: measure ion heating to infer abundance
  • Thermocouples: directly convert ion kinetic energy to voltage

Correct Answer: Electron multipliers: convert ion impacts into cascades of secondary electrons for amplified current

Q17. High-resolution accurate-mass (HRAM) instruments help resolve isobaric interferences because they:

  • Increase ion transmission by using wider slits
  • Provide higher resolving power to separate species with the same nominal mass but different exact masses
  • Eliminate chemical noise by using EI at 70 eV
  • Only detect singly charged ions, simplifying spectra

Correct Answer: Provide higher resolving power to separate species with the same nominal mass but different exact masses

Q18. In MALDI, which statement best captures the ionization principle?

  • Corona discharge ionizes analytes directly in the gas phase after nebulization
  • A UV/IR laser excites the matrix, which assists desorption and ionization, yielding mainly singly charged ions
  • Electron beam ionizes molecules in the solid phase generating radical cations
  • Thermal evaporation creates even-electron ions without a matrix

Correct Answer: A UV/IR laser excites the matrix, which assists desorption and ionization, yielding mainly singly charged ions

Q19. Charge state determination in ESI for proteins can be made from isotope spacing because:

  • The m/z spacing between adjacent isotopic peaks equals z
  • The m/z spacing between adjacent isotopic peaks equals 1/z
  • The number of isotopic peaks equals z
  • Isotopic spacing is independent of charge state

Correct Answer: The m/z spacing between adjacent isotopic peaks equals 1/z

Q20. A “lock mass” in HRMS is used primarily to:

  • Increase ionization efficiency of the analyte
  • Continuously correct for mass drift and improve mass accuracy during acquisition
  • Calibrate detector gain for linear dynamic range
  • Suppress matrix effects by competitive ionization

Correct Answer: Continuously correct for mass drift and improve mass accuracy during acquisition

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