Mass analysers: Quadrupole MCQs With Answer

Mass analysers: Quadrupole MCQs With Answer

This quiz compilation is designed for M.Pharm students studying Advanced Instrumental Analysis (MPA 201T) to deepen understanding of quadrupole mass analysers. It covers operating principles, ion motion under combined RF/DC fields, design aspects of rod electrodes (hyperbolic vs cylindrical), mass filtering, resolution vs transmission trade-offs, stability diagrams (Mathieu equations), scanning modes, low-mass cut-off, and practical considerations in pharmaceutical mass spectrometry. Questions range from conceptual to application-level scenarios encountered in analytical labs, including tandem quadrupole configurations and calibration. Use these MCQs to test knowledge, prepare for exams, or reinforce problem-solving skills relevant to quadrupole-based mass spectrometry in drug analysis.

Q1. What is the primary principle by which a quadrupole mass analyser separates ions?

• Magnetic deflection based on ion momentum
• Time-of-flight differentiation using pulsed acceleration
• Filtering ions by stability of their trajectories under combined RF and DC electric fields
• Resonant ejection of ions from a trapped volume

Correct Answer: Filtering ions by stability of their trajectories under combined RF and DC electric fields

Q2. In a typical linear quadrupole mass filter, how many parallel rods are used to generate the quadrupolar field?

• Two rods
• Three rods
• Four rods
• Six rods

Correct Answer: Four rods

Q3. Which combination of voltages is applied to the opposite rods in a quadrupole mass filter for mass selection?

• Pulsed DC only
• RF only with no DC component
• Superimposed RF and DC voltages on each rod pair, with opposite polarity between pairs
• Constant AC voltage of different frequencies on each rod

Correct Answer: Superimposed RF and DC voltages on each rod pair, with opposite polarity between pairs

Q4. What effect does increasing the RF amplitude at fixed DC have on the mass range transmitted by the quadrupole?

• It shifts the transmitted m/z to lower values only
• It increases the maximum m/z that can be transmitted (extends high-mass transmission)
• It narrows the transmitted m/z peak width without changing the range
• It causes all ions to become unstable and be lost

Correct Answer: It increases the maximum m/z that can be transmitted (extends high-mass transmission)

Q5. The stability of ion motion in a quadrupole is described by which set of equations?

• Schrödinger equations
• Navier–Stokes equations
• Mathieu equations
• Klein–Gordon equations

Correct Answer: Mathieu equations

Q6. Which rod shape provides the ideal quadrupolar electric field without higher-order field distortions?

• Cylindrical rods with large diameter-to-spacing ratio
• Flat plate electrodes
• Hyperbolic rods
• Square rods

Correct Answer: Hyperbolic rods

Q7. Why are cylindrical rods commonly used in commercial quadrupoles despite hyperbolic rods being ideal?

• Cylindrical rods are cheaper, easier to manufacture, and can approximate hyperbolic fields closely with optimized diameter-to-spacing ratios
• Cylindrical rods produce a perfect quadrupole field inherently
• Cylindrical rods allow operation at higher vacuum pressures
• Cylindrical rods reduce ion fragmentation

Correct Answer: Cylindrical rods are cheaper, easier to manufacture, and can approximate hyperbolic fields closely with optimized diameter-to-spacing ratios

Q8. In a quadrupole operated in “scanning mode,” how is mass selection achieved?

• By stepping ion source temperature
• By linearly ramping the RF and DC voltages to sweep the stability window across m/z
• By alternately switching on and off the electron multiplier
• By changing the gas pressure in the ion source

Correct Answer: By linearly ramping the RF and DC voltages to sweep the stability window across m/z

Q9. What is the role of the DC component relative to RF in determining resolution of a quadrupole?

• Higher DC relative to RF increases the mass resolving power by narrowing stability regions
• DC has no impact on resolution; only RF matters
• Lower DC relative to RF increases collision-induced dissociation
• DC only affects ion source currents and not filtering

Correct Answer: Higher DC relative to RF increases the mass resolving power by narrowing stability regions

Q10. The term “low-mass cutoff” (LMCO) in a quadrupole ion trap or certain quadrupole configurations refers to:

• The minimum kinetic energy required for ion injection
• The lowest m/z value that remains stable and is transmitted under given RF/DC conditions
• The m/z above which fragmentation cannot occur
• The mass range affected by space-charge limitations only

Correct Answer: The lowest m/z value that remains stable and is transmitted under given RF/DC conditions

Q11. Which parameter primarily determines the resolution while keeping sensitivity acceptable in a quadrupole?

• Ion source pressure
• Slit width before detector
• The ratio of DC to RF voltages and the RF frequency
• Detector gain alone

Correct Answer: The ratio of DC to RF voltages and the RF frequency

Q12. In a triple quadrupole (Q1–Q2–Q3) tandem MS system, what is the primary function of Q2?

• Mass filtering incoming ions with high resolution
• Serving as a collision cell where selected precursor ions are fragmented (often RF-only, pressurized)
• Detecting ions with an electron multiplier
• Ionizing the sample

Correct Answer: Serving as a collision cell where selected precursor ions are fragmented (often RF-only, pressurized)

Q13. How does increasing quadrupole RF frequency affect the mass range for a fixed rod geometry?

• It shifts the transmitted masses toward lower m/z values
• It allows transmission of higher m/z values for the same RF amplitude
• It eliminates the need for DC voltage
• It only affects ion detection efficiency and not mass range

Correct Answer: It allows transmission of higher m/z values for the same RF amplitude

Q14. What is the effect of mechanical misalignment or machining imperfections of quadrupole rods on instrument performance?

• Improves mass resolution by breaking symmetry
• Creates field distortions that degrade resolution, cause mass shifts, and reduce transmission
• Only affects ion source performance and not quadrupole behaviour
• Makes the quadrupole selectively transmit only singly charged ions

Correct Answer: Creates field distortions that degrade resolution, cause mass shifts, and reduce transmission

Q15. Which detection or acquisition mode is commonly used with quadrupoles for quantitative pharmaceutical analysis due to its high selectivity and sensitivity?

• Full-scan only
• Selected Ion Monitoring (SIM) or Multiple Reaction Monitoring (MRM) in triple quadrupoles
• Ion imaging mode
• Continuous neutral monitoring

Correct Answer: Selected Ion Monitoring (SIM) or Multiple Reaction Monitoring (MRM) in triple quadrupoles

Q16. The stability diagram (a–q plot) used for quadrupoles maps which two dimensionless parameters that govern ion motion?

• Pressure and temperature
• Dimensionless DC parameter (a) and RF parameter (q)
• Ion current and acceleration voltage
• Magnetic field strength and frequency

Correct Answer: Dimensionless DC parameter (a) and RF parameter (q)

Q17. What advantage does a quadrupole offer compared to a magnetic sector analyser for routine pharmaceutical LC–MS?

• Higher mass resolving power than magnetic sectors
• Compact size, robustness, fast scanning, and ease of coupling with LC
• Ability to analyze neutral molecules directly
• No requirement for vacuum systems

Correct Answer: Compact size, robustness, fast scanning, and ease of coupling with LC

Q18. In single quadrupole MS used for quantitation, what is a common approach to improve specificity in complex matrices?

• Increase source temperature only
• Use SIM (monitoring specific m/z) and optimize collision-induced dissociation
• Decrease desolvation gas
• Operate at atmospheric pressure without ionization

Correct Answer: Use SIM (monitoring specific m/z) and optimize collision-induced dissociation

Q19. Which factor does NOT directly influence the mass resolution of a quadrupole mass analyser?

• RF frequency and amplitude
• DC/RF ratio
• Detector type (e.g., electron multiplier vs Faraday cup)
• Mechanical precision and rod alignment

Correct Answer: Detector type (e.g., electron multiplier vs Faraday cup)

Q20. For accurate mass calibration of a quadrupole instrument used in pharmaceutical analysis, which practice is recommended?

• Calibrate only once at installation and never again
• Use internal or external calibrants that bracket the mass range of interest and perform regular recalibration
• Rely solely on instrument software without experimental calibration
• Calibrate using thermal noise measurements

Correct Answer: Use internal or external calibrants that bracket the mass range of interest and perform regular recalibration

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