Mass analysers: Time of Flight MCQs With Answer

This blog offers a focused set of Time-of-Flight (TOF) mass analyser multiple-choice questions tailored for M.Pharm students studying Advanced Instrumental Analysis (MPA 201T). The questions cover TOF fundamentals, physics of ion flight, reflectron design, extraction methods, ion optics, detectors, mass calibration, resolution definitions, and practical techniques such as MALDI-TOF and ESI-TOF. Each MCQ targets conceptual clarity and application-level understanding required in postgraduate pharmacy curricula and exam preparation. Answers are provided to enable self-assessment. Use these questions to reinforce key principles, identify knowledge gaps, and prepare for viva or written assessments in mass spectrometry modules.

Q1. What is the fundamental physical principle on which a Time-of-Flight (TOF) mass analyser separates ions?

• Ions are separated according to their velocity which is proportional to the square root of their mass-to-charge ratio, after equal acceleration.
• Ions are separated by magnetic deflection proportional to m/z.
• Ions are separated by differences in ion mobility through a buffer gas at atmospheric pressure.
• Ions are separated by chemical affinity to a stationary phase.

Correct Answer: Ions are separated according to their velocity which is proportional to the square root of their mass-to-charge ratio, after equal acceleration.

Q2. Which expression best relates the flight time (t) to mass (m), charge (z), acceleration potential (V) and flight path length (L) in a simple linear TOF?

• t = L * sqrt(m/(2 z e V))
• t = L * (m/z) / V
• t = (2 V L)/(m z)
• t = V / (L * sqrt(m/z))

Correct Answer: t = L * sqrt(m/(2 z e V))

Q3. What is the primary role of a reflectron (ion mirror) in TOF mass spectrometers?

• To correct for initial kinetic energy spread of ions and improve mass resolution.
• To ionize neutral molecules entering the TOF region.
• To separate ions by charge state using a magnetic field.
• To trap ions for ion mobility analysis.

Correct Answer: To correct for initial kinetic energy spread of ions and improve mass resolution.

Q4. Delayed extraction (or time-lag focusing) in MALDI-TOF is used mainly to:

• Create a correlation between ion position and kinetic energy at extraction to sharpen time focusing and increase resolution.
• Increase ionization efficiency by delaying the laser pulse.
• Reduce detector noise by delaying signal acquisition.
• Separate singly charged from multiply charged ions.

Correct Answer: Create a correlation between ion position and kinetic energy at extraction to sharpen time focusing and increase resolution.

Q5. Which advantage is characteristic of orthogonal acceleration TOF (oa-TOF) compared with axial pulsed TOF?

• It allows continuous ion sources (e.g., ESI) to be converted into pulsed packets and improves duty cycle for LC-MS coupling.
• It eliminates the need for any ion optics or lenses.
• It uses magnetic fields to separate ions rather than electric fields.
• It only works for singly charged ions and is incompatible with multiply charged species.

Correct Answer: It allows continuous ion sources (e.g., ESI) to be converted into pulsed packets and improves duty cycle for LC-MS coupling.

Q6. A key practical difference between MALDI-TOF and ESI-TOF spectra is:

• MALDI predominantly produces singly charged ions whereas ESI commonly produces multiply charged ions for large biomolecules.
• MALDI generates multiply charged ions exclusively, while ESI gives only singly charged ions.
• ESI cannot be coupled to TOF analysers, whereas MALDI is the only compatible source.
• MALDI always gives higher resolution than ESI for the same TOF instrument.

Correct Answer: MALDI predominantly produces singly charged ions whereas ESI commonly produces multiply charged ions for large biomolecules.

Q7. The resolving power R of a TOF instrument is often expressed as R = m/Δm. Which temporal expression relates resolution to flight time t and temporal peak width Δt (FWHM)?

• R ≈ t / (2 Δt)
• R ≈ Δt / t
• R ≈ (t^2) / Δt
• R ≈ 2 Δt / t

Correct Answer: R ≈ t / (2 Δt)

Q8. Which factor does NOT reduce the mass resolution of a TOF analyser?

• High initial kinetic energy spread among ions.
• Large spatial distribution of ion creation events.
• Use of a reflectron tuned to compensate energy spread.
• Space-charge effects at high ion density.

Correct Answer: Use of a reflectron tuned to compensate energy spread.

Q9. In a reflectron, ions with higher initial kinetic energy penetrate deeper into the mirror and therefore:

• Travel a longer path so arrival times are equalized with slower ions, improving resolution.
• Are always lost and thus decrease sensitivity significantly.
• Are converted into neutrals and do not reach the detector.
• Pass through the mirror without change in flight time.

Correct Answer: Travel a longer path so arrival times are equalized with slower ions, improving resolution.

Q10. Post-source decay (PSD) or metastable fragmentation in MALDI-TOF is best detected using which TOF configuration?

• Reflectron TOF, because it separates fragment arrival times from precursor ions and permits detection of PSD fragments.
• Linear TOF, because linear flight path always resolves PSD fragments better than reflectrons.
• Ion trap only, since TOF cannot analyze PSD fragments.
• Quadrupole mass analyser, because PSD occurs only under quadrupole conditions.

Correct Answer: Reflectron TOF, because it separates fragment arrival times from precursor ions and permits detection of PSD fragments.

Q11. Which detector type is most commonly used in TOF mass spectrometers for fast timing and good sensitivity?

• Microchannel plate (MCP) detector.
• Flame ionization detector (FID).
• Thermal conductivity detector (TCD).
• UV-Vis photodiode array.

Correct Answer: Microchannel plate (MCP) detector.

Q12. Space-charge effects in the source or extraction region commonly cause:

• Broadening of peaks, mass shifts and reduced resolution and accuracy at high ion populations.
• Improved resolution by self-focusing ions into narrower packets.
• Conversion of singly charged ions into multiply charged species exclusively.
• Complete elimination of metastable decay.

Correct Answer: Broadening of peaks, mass shifts and reduced resolution and accuracy at high ion populations.

Q13. Which calibration strategy is most appropriate for accurate mass assignment in TOF instruments?

• Use of internal or external calibrants with known masses and fitting using the t^2 versus m relation for calibration.
• Using an arbitrary single-point voltage calibration and assuming linearity of t vs m.
• Calibrating only with zero-mass reference points.
• Relying on detector gain settings without mass calibrants.

Correct Answer: Use of internal or external calibrants with known masses and fitting using the t^2 versus m relation for calibration.

Q14. Two-stage acceleration (double-stage extraction) in TOF design primarily aims to:

• Reduce effective kinetic energy spread of ions and improve time focusing compared to single-stage extraction.
• Create two separate mass ranges that are analysed independently.
• Convert ions to neutrals before they enter the flight tube.
• Enable simultaneous magnetic and electric separation of ions.

Correct Answer: Reduce effective kinetic energy spread of ions and improve time focusing compared to single-stage extraction.

Q15. Which statement about linear vs reflectron TOF modes is correct?

• Linear TOF gives higher sensitivity and is better for high-mass ions unaffected by reflectron fields, while reflectron TOF gives higher resolution and mass accuracy for ions with modest energy spread.
• Reflectron TOF always gives worse resolution than linear TOF.
• Linear TOF cannot detect singly charged ions.
• Reflectron TOF eliminates the need for any ion extraction voltages.

Correct Answer: Linear TOF gives higher sensitivity and is better for high-mass ions unaffected by reflectron fields, while reflectron TOF gives higher resolution and mass accuracy for ions with modest energy spread.

Q16. Delayed extraction time is typically optimized to:

• Allow the initial ion cloud to expand so that faster ions starting nearer the extraction electrode catch up with slower ones, improving focus.
• Increase fragmentation by giving ions more time to collide with residual gas.
• Reduce the ionization efficiency of MALDI to suppress matrix peaks.
• Change the ion charge state distribution from +2 to +1 preferentially.

Correct Answer: Allow the initial ion cloud to expand so that faster ions starting nearer the extraction electrode catch up with slower ones, improving focus.

Q17. Which of the following best describes “duty cycle” in the context of TOF mass spectrometers?

• The fraction of time ions from a continuous source are successfully converted into useful pulsed packets and detected; higher duty cycle improves sensitivity in continuous-source coupling.
• The electrical efficiency of the extraction voltage supply measured as percent energy consumed.
• The fraction of ions that are lost to neutralization in the flight tube.
• The optical efficiency of a MALDI laser system measured as photons per pulse.

Correct Answer: The fraction of time ions from a continuous source are successfully converted into useful pulsed packets and detected; higher duty cycle improves sensitivity in continuous-source coupling.

Q18. In TOF instrumentation, “time-lag focusing” is synonymous with which technique?

• Delayed extraction.
• Collision-induced dissociation.
• Electrospray nebulization.
• Isotope dilution.

Correct Answer: Delayed extraction.

Q19. If you increase the extraction voltage in a TOF analyser, what is the usual immediate effect on flight time and resolution (all else equal)?

• Flight time decreases and, if initial energy spread remains proportionally small, resolution can improve due to reduced relative timing dispersion.
• Flight time increases and resolution always decreases.
• Flight time remains unchanged while resolution doubles automatically.
• Ions stop being detected because higher voltage neutralizes them.

Correct Answer: Flight time decreases and, if initial energy spread remains proportionally small, resolution can improve due to reduced relative timing dispersion.

Q20. In TOF/TOF tandem mass spectrometers, how is the precursor ion typically selected and then fragmented for second-stage analysis?

• Selection by a timed ion gate or pulsed extraction (timing window) followed by collision-induced dissociation in a collision cell, with fragments sent to the second TOF for analysis.
• By using a magnetic sector to deflect the precursor ion into a trap and then using electron capture dissociation only.
• All ions are fragmented simultaneously without any precursor selection and the instrument deconvolutes spectra afterwards.
• Selection by size exclusion chromatography built into the flight tube before fragmentation.

Correct Answer: Selection by a timed ion gate or pulsed extraction (timing window) followed by collision-induced dissociation in a collision cell, with fragments sent to the second TOF for analysis.

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