ESI ionization MCQs With Answer

ESI ionization MCQs With Answer

by

Introduction

Electrospray Ionization (ESI) is a cornerstone of modern pharmaceutical analytical techniques, enabling sensitive LC–MS analysis of small molecules, peptides, and proteins under soft ionization conditions. This MCQ set, “ESI ionization MCQs With Answer,” is tailored for M. Pharm students to deepen conceptual clarity and analytical proficiency. You will review droplet formation, the Taylor cone, charge-state generation, desolvation dynamics, and ion transfer into the mass spectrometer. Emphasis is placed on solvent selection, volatile buffers, ion suppression, adduct control, native MS, supercharging, and source optimization in both positive and negative modes. Each question targets practical decision-making—choosing ionization modes, tuning parameters, and interpreting spectra—so you can translate theory into robust, high-quality LC–ESI–MS results.

Q1. Which model most consistently explains the formation of highly multiply charged protein ions in ESI?

  • Charge residue model (CRM)
  • Ion evaporation model (IEM)
  • Thermal desorption
  • Penning ionization

Correct Answer: Charge residue model (CRM)

Q2. Why is ESI particularly suitable for analyzing very high molecular weight biomolecules on mass analyzers with limited m/z range?

  • It produces multiple charges that reduce the observed m/z
  • It heavily fragments molecules, simplifying spectra
  • It requires a crystalline matrix to co-desorb analytes
  • It uses high temperature to vaporize proteins intact

Correct Answer: It produces multiple charges that reduce the observed m/z

Q3. In positive-ion ESI for LC–MS, which mobile phase condition usually maximizes protonation and spray stability?

  • 0.1% formic acid in water/acetonitrile
  • 50 mM phosphate buffer (pH 7)
  • 0.1% trifluoroacetic acid (TFA) in water/acetonitrile
  • 0.1 M ammonium sulfate

Correct Answer: 0.1% formic acid in water/acetonitrile

Q4. Nano-ESI typically operates best at which flow rate range?

  • 20–500 nL/min
  • 1–10 μL/min
  • 50–200 μL/min
  • >1 mL/min

Correct Answer: 20–500 nL/min

Q5. The formation of a stable Taylor cone and primary droplet spray in ESI is primarily governed by which parameter?

  • Capillary (spray) voltage
  • Sampling cone (or orifice) voltage
  • Collision energy in the collision cell
  • RF lens voltage in the ion guide

Correct Answer: Capillary (spray) voltage

Q6. The most common mechanistic cause of ion suppression in ESI during LC–MS is:

  • Increased collision energy in the quadrupole
  • Magnetic field fluctuations in the analyzer
  • Co-eluting nonvolatile salts/surfactants competing for charge
  • Detector dead time at high count rates

Correct Answer: Co-eluting nonvolatile salts/surfactants competing for charge

Q7. To minimize [M+Na]+ adducts for small molecules/peptides, which practice is most effective?

  • Use ammonium acetate buffer and plastic autosampler vials
  • Use sodium phosphate buffer for better conductivity
  • Add sodium chloride (10 mM) to stabilize the spray
  • Increase the capillary temperature drastically

Correct Answer: Use ammonium acetate buffer and plastic autosampler vials

Q8. In an ESI spectrum, what is the expected isotopic peak spacing (in Th) for an ion with charge state z = 5?

  • 1.0
  • 0.5
  • 0.2
  • 5.0

Correct Answer: 0.2

Q9. Which combination most directly improves droplet desolvation and ion transfer efficiency in ESI?

  • Higher nebulizing/sheath gas and optimized source temperature
  • Increased detector gain and longer dwell time
  • Lower RF amplitude in the quadrupole
  • Reduced vacuum in the first pumping stage

Correct Answer: Higher nebulizing/sheath gas and optimized source temperature

Q10. For negative-ion ESI of acidic analytes, which additive commonly enhances deprotonation while remaining volatile?

  • 10 mM ammonium hydroxide
  • 0.1% formic acid
  • 10 mM sodium borate
  • 0.1% TFA

Correct Answer: 10 mM ammonium hydroxide

Q11. The maximum charge a droplet can sustain before undergoing Coulombic fission is defined by the:

  • Rayleigh limit
  • Doppler effect
  • Gibbs free energy minimum
  • Zeldovich nucleation criterion

Correct Answer: Rayleigh limit

Q12. For low-polarity analytes at high LC flow (≈1 mL/min) with high organic content, which ionization technique is generally preferred?

  • ESI
  • APCI
  • MALDI
  • EI

Correct Answer: APCI

Q13. Which statement about TFA in LC–ESI–MS is most accurate?

  • It improves peptide chromatographic peak shape but often suppresses ESI signal via ion-pairing and higher surface tension
  • It strongly enhances ESI sensitivity for most peptides
  • It is non-suppressive because it is fully volatile
  • It is essential for negative-mode sensitivity

Correct Answer: It improves peptide chromatographic peak shape but often suppresses ESI signal via ion-pairing and higher surface tension

Q14. What is the primary purpose of using supercharging agents (e.g., sulfolane, m-nitrobenzyl alcohol) in ESI of proteins?

  • Increase average charge state to lower m/z and improve detection
  • Decrease charge state to simplify isotopic envelopes
  • Prevent all adduct formation
  • Enhance in-source fragmentation for sequencing

Correct Answer: Increase average charge state to lower m/z and improve detection

Q15. Which condition best preserves noncovalent complexes for native ESI–MS?

  • 100 mM ammonium acetate at near-neutral pH with minimal organic solvent
  • 0.1% formic acid in 50% acetonitrile
  • 1 M sodium chloride in water
  • 0.1% SDS in water

Correct Answer: 100 mM ammonium acetate at near-neutral pH with minimal organic solvent

Q16. A commonly used calibrant for wide-range m/z calibration in ESI that forms predictable cluster ions is:

  • Cesium iodide solution producing [Cs]n+ clusters
  • Perfluorotributylamine (PFTBA) in EI source
  • Xenon gas introduced into the analyzer
  • Helium leak at the vacuum chamber

Correct Answer: Cesium iodide solution producing [Cs]n+ clusters

Q17. Increasing the sampling cone (or skimmer) voltage during ESI typically results in:

  • More in-source fragmentation and declustering
  • Lower source temperature
  • Higher resolving power in the analyzer
  • Elimination of multiply charged ions

Correct Answer: More in-source fragmentation and declustering

Q18. Adding a small percentage of isopropanol to an aqueous mobile phase in ESI often:

  • Lowers surface tension and enhances spray/desolvation efficiency
  • Increases viscosity and completely suppresses ionization
  • Prevents ionization of basic analytes
  • Only affects MALDI and not ESI

Correct Answer: Lowers surface tension and enhances spray/desolvation efficiency

Q19. How can corona discharge during negative-mode ESI be mitigated?

  • Reduce spray voltage and increase nebulizing/sheath gas flow
  • Increase spray voltage to stabilize the cone
  • Add nonvolatile salts to dampen the discharge
  • Eliminate dry gas to avoid droplet cooling

Correct Answer: Reduce spray voltage and increase nebulizing/sheath gas flow

Q20. In a typical ESI source, which element first transmits ions from atmospheric pressure into the vacuum system?

  • Sampling orifice/cone (leading to skimmer)
  • Quadrupole mass filter
  • Electron multiplier detector
  • Ion trap analyzer

Correct Answer: Sampling orifice/cone (leading to skimmer)

Leave a Comment