Instrumentation of mass spectrometer MCQs With Answer

Instrumentation of mass spectrometer MCQs With Answer

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Instrumentation of Mass Spectrometer MCQs With Answer

For M. Pharm students specializing in Modern Pharmaceutical Analytical Techniques, mastering mass spectrometry instrumentation is essential for accurate qualitative and quantitative analysis. This quiz-focused blog presents a curated set of MCQs that explore the architecture and working principles of mass spectrometers, including ion sources (ESI, APCI, EI), mass analyzers (quadrupole, TOF, Orbitrap, ion trap), ion optics, vacuum systems, detectors, and tandem MS components. Each question targets practical understanding—such as pressure regimes, resolution metrics, calibration, and scan strategies—that directly impacts method development, sensitivity, and reliability in pharmaceutical analysis. Use these MCQs to test your grasp of critical instrument parameters, troubleshoot setups, and refine your analytical decision-making in LC–MS/MS and GC–MS workflows.

Q1. What is the primary function of the mass analyzer in a mass spectrometer?

  • To separate ions based on their mass-to-charge ratios (m/z)
  • To ionize neutral molecules entering the instrument
  • To amplify the ion signal produced at the detector
  • To convert ions back to neutral species for detection

Correct Answer: To separate ions based on their mass-to-charge ratios (m/z)

Q2. Why is an electron energy of approximately 70 eV commonly used in electron ionization (EI) sources?

  • It generates reproducible fragmentation patterns suitable for spectral libraries while providing efficient ionization
  • It minimizes ionization thresholds for all analytes to prevent fragmentation
  • It prevents excess fragmentation, ensuring only molecular ions are produced
  • It selectively ionizes pi-electron systems over sigma bonds

Correct Answer: It generates reproducible fragmentation patterns suitable for spectral libraries while providing efficient ionization

Q3. In an ESI interface, what is the primary role of the skimmer (or skimmer cone)?

  • To transmit ions from atmospheric pressure to high vacuum while reducing neutral gas load
  • To focus neutral solvent molecules into the analyzer
  • To increase initial droplet formation at the capillary outlet
  • To measure ion current prior to the mass analyzer

Correct Answer: To transmit ions from atmospheric pressure to high vacuum while reducing neutral gas load

Q4. Stable ion transmission through a quadrupole mass filter is achieved by:

  • Applying specific RF and DC voltages that create stability regions (a–q parameters) for selected m/z
  • Using only a DC potential with no RF component
  • Using only a high-frequency RF potential with no DC component
  • Applying a static magnetic field orthogonal to the ion path

Correct Answer: Applying specific RF and DC voltages that create stability regions (a–q parameters) for selected m/z

Q5. Which statement best describes resolving power in mass spectrometry?

  • R = m/Δm, where Δm is measured at full width at half maximum (FWHM)
  • R = Δm/m, where Δm is measured at 10% valley
  • R is independent of peak width and depends only on detector gain
  • R increases linearly with ion source temperature

Correct Answer: R = m/Δm, where Δm is measured at full width at half maximum (FWHM)

Q6. Which vacuum pump is typically used to maintain high vacuum in the analyzer region of a mass spectrometer?

  • Turbomolecular pump
  • Rotary vane pump
  • Diaphragm pump
  • Peristaltic pump

Correct Answer: Turbomolecular pump

Q7. The reflectron in a TOF mass analyzer is used primarily to:

  • Improve mass resolution by compensating for kinetic energy spread
  • Extend the upper mass range by increasing acceleration voltage
  • Enhance ionization efficiency at the source
  • Increase detector sensitivity via electron multiplication

Correct Answer: Improve mass resolution by compensating for kinetic energy spread

Q8. The Orbitrap measures m/z by:

  • Detecting image current induced by coherent axial oscillations of ions around a central electrode
  • Counting ions that impinge upon an electron multiplier detector
  • Measuring Larmor precession frequency in a strong magnetic field
  • Timing the flight of ions along a field-free drift tube

Correct Answer: Detecting image current induced by coherent axial oscillations of ions around a central electrode

Q9. Why are electron multipliers commonly used as detectors in mass spectrometers?

  • They provide high gain by converting incident ions into cascades of secondary electrons
  • They measure ion current directly without any amplification
  • They are completely insensitive to vacuum fluctuations
  • They are selective only for negative ions and exclude positive ions

Correct Answer: They provide high gain by converting incident ions into cascades of secondary electrons

Q10. In tandem mass spectrometry, the collision cell is typically pressurized with which gas and for what purpose?

  • Argon or nitrogen to induce collision-induced dissociation (CID) for fragmentation
  • Helium to increase ionization efficiency via Penning ionization
  • Oxygen to facilitate charge-transfer ionization
  • Carbon dioxide to maintain atmospheric pressure ionization

Correct Answer: Argon or nitrogen to induce collision-induced dissociation (CID) for fragmentation

Q11. What is the main advantage of Selected Ion Monitoring (SIM) on a single quadrupole?

  • Enhanced sensitivity for target analytes by monitoring only selected m/z values
  • Improved absolute mass accuracy across the entire mass range
  • Higher resolving power than high-resolution instruments
  • Faster polarity switching without compromise

Correct Answer: Enhanced sensitivity for target analytes by monitoring only selected m/z values

Q12. Why is electrospray ionization (ESI) advantageous for large biomolecules?

  • It generates multiply charged ions, bringing high molecular weights into the instrument’s m/z range
  • It eliminates matrix effects and ion suppression
  • It inherently increases gas-phase fragmentation
  • It avoids the need for desolvation gases and heating

Correct Answer: It generates multiply charged ions, bringing high molecular weights into the instrument’s m/z range

Q13. For LC–MS analysis of moderately polar, thermally labile pharmaceutical compounds, which ion source is most appropriate?

  • Electrospray ionization (ESI)
  • Electron ionization (EI)
  • Matrix-assisted laser desorption/ionization (MALDI)
  • Field ionization (FI)

Correct Answer: Electrospray ionization (ESI)

Q14. What is the purpose of a lock-mass in high-resolution MS workflows?

  • To correct mass drift in real time using a known reference ion
  • To increase the resolving power beyond instrument specifications
  • To suppress in-source fragmentation at high source voltages
  • To enable stable negative-ion acquisition

Correct Answer: To correct mass drift in real time using a known reference ion

Q15. Typical pressure in the high-vacuum region of a mass analyzer is:

  • 10⁻⁵ to 10⁻⁷ Torr
  • Approximately 760 Torr (atmospheric)
  • About 1 Torr
  • 10⁻¹ Torr

Correct Answer: 10⁻⁵ to 10⁻⁷ Torr

Q16. Perfluorotributylamine (PFTBA) is commonly used in mass spectrometry for:

  • Tuning and calibrating quadrupole instruments due to its well-defined fragment ions
  • Serving as a MALDI matrix for peptide analysis
  • Acting as a derivatization reagent in GC–MS sample prep
  • Providing a proton source in ESI for positive-mode operation

Correct Answer: Tuning and calibrating quadrupole instruments due to its well-defined fragment ions

Q17. The dynamic range of a mass spectrometric detector refers to:

  • The span of ion intensities over which the response remains linear
  • The total acquisition time per chromatographic run
  • The accessible mass range of the instrument
  • The change in resolving power with scan speed

Correct Answer: The span of ion intensities over which the response remains linear

Q18. What is a typical consequence of space-charge effects in an ion trap?

  • Peak shifts and reduced resolution at high ion populations
  • Improved mass accuracy as ion density increases
  • Lower limits of detection due to enhanced trapping
  • Increased trapping efficiency without spectral distortion

Correct Answer: Peak shifts and reduced resolution at high ion populations

Q19. In an APCI source, the corona discharge needle primarily serves to:

  • Generate reagent ions that ionize analytes via gas-phase reactions at atmospheric pressure
  • Nebulize the liquid effluent into fine droplets
  • Thermally desolvate ions before entry to the mass analyzer
  • Measure ion current for real-time quantitation

Correct Answer: Generate reagent ions that ionize analytes via gas-phase reactions at atmospheric pressure

Q20. Orthogonal acceleration in TOF-MS improves duty cycle by:

  • Periodically pulsing ion packets orthogonally to a continuous ion beam from atmospheric sources
  • Increasing ionization energy to create more ions per pulse
  • Reducing detector noise through active shielding
  • Eliminating the requirement for high vacuum in the flight tube

Correct Answer: Periodically pulsing ion packets orthogonally to a continuous ion beam from atmospheric sources

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