Theory of mass spectrometry MCQs With Answer

Theory of mass spectrometry MCQs With Answer

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Theory of Mass Spectrometry MCQs With Answer is designed to help M. Pharm students master key fundamentals in Modern Pharmaceutical Analytical Techniques. This quiz set probes deep into ionization mechanisms (EI, CI, ESI, APCI, MALDI), mass analyzers (quadrupole, TOF, ion trap, Orbitrap, FT-ICR), resolution and mass accuracy, isotope patterns, fragmentation rules, and tandem MS workflows (CID/HCD, SIM/MRM). It also touches practical aspects such as adduct formation, lock-mass calibration, vacuum requirements, detector choices, and matrix effects. Each multiple-choice question is framed to test conceptual clarity and analytical reasoning critical for method development, structural elucidation, and quantitative bioanalysis. Use the answers to self-check and consolidate your understanding for exams and research applications.

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

  • To ionize neutral molecules
  • To separate ions according to their mass-to-charge ratio (m/z)
  • To detect ions by converting them into an electrical signal
  • To introduce analytes from the chromatographic system

Correct Answer: To separate ions according to their mass-to-charge ratio (m/z)

Q2. In electron ionization (EI), the most commonly formed molecular ion is:

  • A protonated molecule [M+H]+
  • A radical cation M+•
  • A deprotonated molecule [M−H]−
  • A sodium adduct [M+Na]+

Correct Answer: A radical cation M+•

Q3. A characteristic feature of electrospray ionization (ESI) spectra is:

  • Predominantly singly charged ions with little adduct formation
  • Extensive in-source fragmentation due to high-energy desorption
  • Presence of multiple charge states forming a charge envelope
  • Requirement of a crystalline matrix to assist desorption

Correct Answer: Presence of multiple charge states forming a charge envelope

Q4. The reflectron in time-of-flight (TOF) mass analyzers is primarily used to:

  • Increase the kinetic energy of ions to improve sensitivity
  • Compensate for kinetic energy spread and increase resolution
  • Remove multiply charged ions from the ion beam
  • Enhance ionization efficiency at the source

Correct Answer: Compensate for kinetic energy spread and increase resolution

Q5. Resolving power (R) of a mass spectrometer at mass m is correctly defined as:

  • R = Δm/m (10% valley)
  • R = m/Δm (full width at half maximum, FWHM)
  • R = m × Δm
  • R = √(m/Δm)

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

Q6. According to the nitrogen rule, an odd nominal mass for an organic molecular ion suggests the molecule contains:

  • An even number of oxygen atoms
  • An odd number of nitrogen atoms
  • No halogens
  • At least one sulfur atom

Correct Answer: An odd number of nitrogen atoms

Q7. A molecular ion pattern showing M and M+2 peaks in approximately a 3:1 intensity ratio is indicative of:

  • One bromine atom
  • Two chlorine atoms
  • One chlorine atom
  • Natural abundance of 13C only

Correct Answer: One chlorine atom

Q8. An ion’s theoretical m/z is 300.1204, but the measured value is 300.1234. The mass error is closest to:

  • Approximately 10 ppm
  • Approximately 1 ppm
  • Approximately 100 ppm
  • Approximately 0.1 ppm

Correct Answer: Approximately 10 ppm

Q9. In collision-induced dissociation (CID), fragmentation is driven primarily by:

  • Absorption of UV photons by the precursor ion
  • Multiple low-energy collisions with inert gas increasing internal energy
  • Electron capture by multiply charged cations
  • Thermal decomposition on a heated surface

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

Q10. In a triple quadrupole mass spectrometer, which scan type records all fragment ions from a selected precursor m/z?

  • Precursor ion scan
  • Product ion scan
  • Neutral loss scan
  • Selected ion monitoring (SIM)

Correct Answer: Product ion scan

Q11. Which MS/MS scan detects all precursor ions that produce a specific fragment m/z?

  • Precursor ion scan
  • Product ion scan
  • Full scan
  • Selected reaction monitoring (SRM/MRM)

Correct Answer: Precursor ion scan

Q12. A hallmark of MALDI mass spectra for proteins is:

  • Predominantly singly charged ions
  • Highly multiply charged ion envelopes
  • Requirement of continuous ion beams for analysis
  • Dominance of fragment ions over intact molecular ions

Correct Answer: Predominantly singly charged ions

Q13. The monoisotopic mass of a compound is defined as the mass calculated using:

  • The average isotopic composition of each element
  • The most abundant isotope of each element
  • Only 12C and 1H, ignoring heteroatoms
  • The nominal integer masses of all isotopes

Correct Answer: The most abundant isotope of each element

Q14. Lock-mass calibration in high-resolution MS is used to:

  • Increase ion transmission into the collision cell
  • Correct for mass drift and improve mass accuracy during acquisition
  • Suppress chemical noise via in-source CID
  • Eliminate adduct formation in ESI

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

Q15. In positive-mode ESI during LC-MS with 0.1% formic acid, the most common analyte ion observed for small polar molecules is:

  • [M−H]−
  • [M+Na]+
  • [M+K]+
  • [M+H]+

Correct Answer: [M+H]+

Q16. Excessive ion population in ion traps or Orbitraps can lead to space-charge effects that primarily cause:

  • Improved mass accuracy and narrower peaks
  • Frequency shifts and peak broadening, reducing mass accuracy
  • Increased charge states in ESI
  • Suppression of isotopic peaks

Correct Answer: Frequency shifts and peak broadening, reducing mass accuracy

Q17. High vacuum in the mass analyzer region is essential mainly to:

  • Increase solvent evaporation from droplets
  • Prevent ion–molecule collisions that cause scattering and neutralization
  • Enhance ionization efficiency of the source
  • Reduce detector dark current

Correct Answer: Prevent ion–molecule collisions that cause scattering and neutralization

Q18. Selected ion monitoring (SIM) improves sensitivity because it:

  • Uses higher collision energies to increase signal
  • Monitors only a few targeted m/z values, increasing dwell time
  • Eliminates chemical noise through isotope filtering
  • Forces ions to become singly charged

Correct Answer: Monitors only a few targeted m/z values, increasing dwell time

Q19. For trace-level detection in MS, the most commonly used ion detector is the:

  • Faraday cup
  • Photomultiplier tube
  • Electron multiplier
  • Thermal conductivity detector

Correct Answer: Electron multiplier

Q20. In an Orbitrap mass analyzer, the m/z of ions is determined from the:

  • Time of flight of ions over a fixed path length
  • Cyclotron frequency of ions in a magnetic field
  • Frequency of axial oscillations (image current) in an electrostatic field
  • Collision cross section measured during drift through a buffer gas

Correct Answer: Frequency of axial oscillations (image current) in an electrostatic field

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