Mass analysers: Orbitrap MCQs With Answer

Mass analysers: Orbitrap MCQs With Answer

This quiz set is designed for M. Pharm students studying Advanced Instrumental Analysis (MPA 201T). It focuses on the Orbitrap mass analyser — its physical design, ion motion principles, detection method, performance parameters (resolution, mass accuracy), practical factors (injection, C‑trap, space‑charge), and data processing (transient, Fourier transform). Questions probe conceptual understanding and application-level knowledge relevant to pharmaceutical analysis, proteomics and small-molecule accurate-mass work. Use these MCQs to test preparation for exams or to reinforce how Orbitrap operation and limitations influence experimental design, data quality, and interpretation in advanced analytical workflows.

Q1. Which statement best describes the fundamental principle by which the Orbitrap determines mass-to-charge (m/z) of ions?

• Ions are separated by time-of-flight and arrival time is converted to m/z
• Ions are deflected by a magnetic field and their radius is used to compute m/z
• Ions oscillate axially in an electrostatic potential; oscillation frequency is measured and converted to m/z
• Ions are fragmented and product ion abundances are correlated to m/z

Correct Answer: Ions oscillate axially in an electrostatic potential; oscillation frequency is measured and converted to m/z

Q2. The axial oscillation frequency (f) of an ion in an ideal Orbitrap depends primarily on which parameter?

• The initial kinetic energy of the ion
• The ion’s mass-to-charge ratio (m/z)
• The ion’s charge distribution within the packet
• The pressure of residual gas in the trap

Correct Answer: The ion’s mass-to-charge ratio (m/z)

Q3. How does resolving power (R) of an Orbitrap change with transient acquisition time (T) for a given m/z?

• R is independent of transient time
• R decreases linearly with increasing T
• R increases roughly linearly with increasing T
• R increases with the square of T

Correct Answer: R increases roughly linearly with increasing T

Q4. Which of the following best describes the relationship between Orbitrap axial frequency and m/z?

• Frequency is directly proportional to m/z
• Frequency is proportional to the square of m/z
• Frequency is inversely proportional to the square root of m/z
• Frequency is independent of m/z

Correct Answer: Frequency is inversely proportional to the square root of m/z

Q5. What is the primary signal that the Orbitrap detector records to generate a mass spectrum?

• Ion current collected on a Faraday cup
• Light emission from excited ions
• Image current induced on the outer electrodes by oscillating ions
• Electron yield from secondary emission

Correct Answer: Image current induced on the outer electrodes by oscillating ions

Q6. The Orbitrap’s electrostatic potential is produced by which electrode geometry?

• A pair of parallel plates and a central wire
• A central spindle electrode surrounded by an outer barrel-shaped electrode
• A quadrupole rod assembly
• A toroidal magnetic coil and an inner ring

Correct Answer: A central spindle electrode surrounded by an outer barrel-shaped electrode

Q7. What role does the C‑trap play in modern Orbitrap-based hybrid instruments?

• It provides the magnetic field for ion cyclotron motion
• It stores, accumulates and injects bunched ion packets into the Orbitrap
• It fragments ions for tandem MS inside the Orbitrap
• It cools the Orbitrap electrodes to cryogenic temperatures

Correct Answer: It stores, accumulates and injects bunched ion packets into the Orbitrap

Q8. Which factor is NOT a primary contributor to degradation of mass accuracy in an Orbitrap?

• Space-charge effects from excessive ion population
• Electronic frequency stability and calibration drift
• Residual gas collisions causing dephasing
• Laser wavelength drift in the ionization source

Correct Answer: Laser wavelength drift in the ionization source

Q9. How does increased ion population in the Orbitrap (space-charge) typically affect the measured frequencies?

• It has no observable effect on frequency
• It shifts frequencies and can broaden peaks, reducing resolution and accuracy
• It always increases frequency leading to higher m/z assignments
• It converts axial motion into radial motion, improving resolution

Correct Answer: It shifts frequencies and can broaden peaks, reducing resolution and accuracy

Q10. Which signal processing step is essential to convert the time-domain transient recorded from the Orbitrap into a mass spectrum?

• Wavelet denoising followed by inverse Laplace transform
• Fast Fourier Transform (FFT) to convert frequency-domain to m/z domain
• Direct centroiding of time-domain peaks without transform
• Fast Fourier Transform (FFT) to convert time-domain frequencies into a frequency spectrum, followed by calibration to m/z

Correct Answer: Fast Fourier Transform (FFT) to convert time-domain frequencies into a frequency spectrum, followed by calibration to m/z

Q11. Which experimental choice increases practical resolving power for high m/z ions on an Orbitrap?

• Shorten transient length to reduce noise
• Increase transient acquisition time and optimize ion optics to reduce space charge
• Increase C‑trap accumulation without limit
• Operate at higher background pressure to damp motion

Correct Answer: Increase transient acquisition time and optimize ion optics to reduce space charge

Q12. The theoretical resolving power R of an Orbitrap at a specific m/z can be approximated as which of the following?

• R ≈ f / Δf and since Δf ≈ 1/T, R ≈ f·T
• R ≈ m/z divided by instrument vacuum pressure
• R ≈ ion kinetic energy × acquisition gain
• R ≈ log(transient length) × f

Correct Answer: R ≈ f / Δf and since Δf ≈ 1/T, R ≈ f·T

Q13. Which calibration approach provides continuous internal calibration to improve mass accuracy during an Orbitrap run?

• External calibration performed before analysis
• Lock‑mass calibration using a known background or spiked reference ion
• Zero-fill calibration during transient acquisition
• Post-run linear regression of m/z values

Correct Answer: Lock‑mass calibration using a known background or spiked reference ion

Q14. Why are Orbitrap instruments particularly valued in high-resolution accurate-mass (HRAM) small-molecule and proteomics workflows?

• Because they inherently fragment ions for sequencing without tandem MS
• Because they provide high resolving power and excellent mass accuracy enabling separation of near-isobaric species
• Because they are the cheapest mass analysers with the highest throughput
• Because they do not require any vacuum system

Correct Answer: Because they provide high resolving power and excellent mass accuracy enabling separation of near-isobaric species

Q15. Which data processing practice can increase apparent digital resolution but does NOT increase true instrumental resolving power?

• Extending the physical transient acquisition time
• Zero-filling the transient before FFT
• Reducing ion injection variability
• Lowering background pressure to reduce collisions

Correct Answer: Zero-filling the transient before FFT

Q16. In practice, why does Orbitrap resolving power typically decline with increasing m/z?

• Higher m/z ions produce stronger image currents and overwhelm electronics
• Axial oscillation frequency decreases with higher m/z (f ∝ 1/√(m/z)), so for fixed transient length f·T (resolution) is lower
• High m/z ions are immune to trapping and leave the instrument faster
• Ionization efficiency increases with m/z causing overcrowding

Correct Answer: Axial oscillation frequency decreases with higher m/z (f ∝ 1/√(m/z)), so for fixed transient length f·T (resolution) is lower

Q17. Which modification can reduce peak broadening due to dephasing in Orbitrap spectra?

• Increasing C‑trap accumulation time without limits
• Operating at higher residual gas pressure
• Minimizing ion population (reduce space-charge) and improving vacuum
• Applying random RF noise to outer electrodes

Correct Answer: Minimizing ion population (reduce space-charge) and improving vacuum

Q18. Which of the following is a limitation specific to standalone Orbitrap analyzers compared with hybrid configurations?

• Inability to measure m/z via frequency detection
• Limited or no true multi-stage MSn fragmentation capability inside the Orbitrap itself
• Poor mass accuracy relative to quadrupole instruments
• Inability to detect image current signals

Correct Answer: Limited or no true multi-stage MSn fragmentation capability inside the Orbitrap itself

Q19. What practical effect does apodization (windowing) of the transient have on Orbitrap data?

• It increases raw frequency resolution without changing transient length
• It reduces spectral noise and sidelobes but can slightly broaden peaks, trading resolution for improved peak shape and S/N
• It doubles the mass accuracy by correcting space-charge effects
• It converts frequency-domain data directly to elemental composition

Correct Answer: It reduces spectral noise and sidelobes but can slightly broaden peaks, trading resolution for improved peak shape and S/N

Q20. For an Orbitrap instrument, which combination of settings would you choose to maximize mass resolution for a high‑mass peptide while maintaining reasonable signal-to-noise?

• Very short transient, maximum ion injection, no apodization
• Long transient acquisition, moderate ion population to avoid space-charge, appropriate apodization and lock-mass calibration
• High C‑trap accumulation without limit and high background pressure
• Disable FFT processing and rely on time-domain centroiding

Correct Answer: Long transient acquisition, moderate ion population to avoid space-charge, appropriate apodization and lock-mass calibration

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