APPI ionization MCQs With Answer

APPI ionization MCQs With Answer

by

APPI Ionization MCQs With Answer

Atmospheric Pressure Photoionization (APPI) is a powerful LC–MS ionization technique especially effective for nonpolar to moderately polar pharmaceutical molecules, lipids, and environmental contaminants. This quiz is designed for M. Pharm students to deepen understanding of APPI fundamentals, including photon-driven mechanisms, dopant-assisted pathways, ion types formed, source design, solvent and additive effects, and method optimization. You will explore how photon energy, ionization energies, dopant selection, mobile phase composition, and source parameters control sensitivity and selectivity. By working through these MCQs, you will build a working knowledge of when to choose APPI over ESI/APCI, how to tune for radical cations versus protonated ions, and how to mitigate chemical noise and matrix effects for robust quantitative analysis.

Q1. In LC–MS, APPI stands for which ionization technique?

  • Atmospheric Pressure Photoionization
  • Atmospheric Plasma Proton Ionization
  • Applied Photon Protonization Interface
  • Ambient Pressure Photochemical Ion Exchange

Correct Answer: Atmospheric Pressure Photoionization

Q2. What is the primary source of ionizing energy in APPI?

  • Corona discharge needle producing reagent ions
  • Vacuum UV photons from a krypton discharge lamp
  • Thermionic emission from a heated filament
  • Laser desorption at near-IR wavelengths

Correct Answer: Vacuum UV photons from a krypton discharge lamp

Q3. Typical photon energies emitted by the krypton lamp used in APPI are approximately:

  • 6.4 and 7.0 eV
  • 8.0 and 8.5 eV
  • 10.0 and 10.6 eV
  • 12.5 and 13.0 eV

Correct Answer: 10.0 and 10.6 eV

Q4. APPI is particularly suitable for ionizing which class of analytes in LC–MS?

  • Highly polar peptides and small nucleotides
  • Inorganic salts and metal complexes
  • Nonpolar to moderately polar molecules such as PAHs, steroids, and neutral lipids
  • Intact, high-mass proteins above 100 kDa

Correct Answer: Nonpolar to moderately polar molecules such as PAHs, steroids, and neutral lipids

Q5. What is the main purpose of adding a “dopant” (e.g., toluene, acetone) in APPI?

  • To lower the desolvation temperature of the source
  • To prevent corona discharge during operation
  • To generate reagent ions via photoionization that promote charge exchange/proton transfer
  • To increase the viscosity of the mobile phase for stable nebulization

Correct Answer: To generate reagent ions via photoionization that promote charge exchange/proton transfer

Q6. For nonpolar aromatic analytes in positive-mode APPI, the predominant ion type observed is typically:

  • [M+H]+
  • [M−H]−
  • M+• (molecular radical cation)
  • [M+Na]+

Correct Answer: M+• (molecular radical cation)

Q7. For basic, moderately polar analytes in APPI positive mode, which ion is most commonly formed?

  • [M+H]+ via gas-phase proton transfer from dopant/solvent reagent ions
  • [M−H]− via base-catalyzed deprotonation
  • M+• exclusively by direct photoionization
  • [M+Cl]− via chloride attachment

Correct Answer: [M+H]+ via gas-phase proton transfer from dopant/solvent reagent ions

Q8. Why does atmospheric O2 largely remain non-ionized in APPI using a krypton lamp?

  • O2 quenches photons before ionization can occur
  • The lamp emits insufficient photon flux at any wavelength
  • The ionization energy of O2 (~12.1 eV) exceeds the lamp photon energies (10.0/10.6 eV)
  • O2 is completely excluded from the source region

Correct Answer: The ionization energy of O2 (~12.1 eV) exceeds the lamp photon energies (10.0/10.6 eV)

Q9. Which statement about mobile phase additives is most accurate for APPI vs ESI?

  • TFA causes more suppression in APPI than in ESI
  • Only ammonium acetate is compatible with APPI
  • TFA causes severe ion suppression in ESI but has a much smaller effect in APPI
  • No acidic modifiers can be used in APPI

Correct Answer: TFA causes severe ion suppression in ESI but has a much smaller effect in APPI

Q10. APPI typically operates effectively without flow splitting at LC flow rates around:

  • 1–10 µL/min
  • 0.1–1.0 mL/min
  • 2–5 mL/min
  • >10 mL/min

Correct Answer: 0.1–1.0 mL/min

Q11. Which property is most desirable in an APPI dopant for efficient ionization?

  • High proton affinity but ionization energy above 12 eV
  • Very high polarity and hydrogen-bonding ability
  • Ionization energy below 10.6 eV and below the analyte’s ionization energy
  • Strong fluorescence between 300–500 nm

Correct Answer: Ionization energy below 10.6 eV and below the analyte’s ionization energy

Q12. Which window material is commonly used with APPI VUV lamps to transmit ~10 eV photons?

  • Fused silica (quartz)
  • Borosilicate glass
  • Magnesium fluoride (MgF2)
  • Sapphire (Al2O3)

Correct Answer: Magnesium fluoride (MgF2)

Q13. Which mobile phase system is especially compatible with APPI and difficult for ESI?

  • Pure water containing 0.5% SDS
  • Hexane/toluene normal-phase eluents
  • 200 mM phosphate buffer at pH 7.4
  • Ionic liquid-based eluents

Correct Answer: Hexane/toluene normal-phase eluents

Q14. Which sequence correctly describes dopant-assisted APPI in positive mode?

  • M + hν → [M−H]−; then [M−H]− + D → D− + M
  • D + hν → D+• + e−; then D+• + M → M+• + D
  • M + hν → [M+Na]+; then [M+Na]+ → M+• + Na
  • D + hν → D−; then D− + M → [M+H]+

Correct Answer: D + hν → D+• + e−; then D+• + M → M+• + D

Q15. In negative-ion APPI, the most common ionization pathway involves:

  • Proton transfer from dopant to analyte
  • Thermal electron attachment to electrophilic analytes via dopant-generated electrons
  • Sodium adduction to form [M+Na]+
  • Direct photodissociation of the analyte into fragments

Correct Answer: Thermal electron attachment to electrophilic analytes via dopant-generated electrons

Q16. Which analyte class typically shows the greatest sensitivity gains with APPI?

  • Short acidic peptides analyzed with 0.1% TFA
  • Quaternary ammonium surfactants in water
  • Polycyclic aromatic hydrocarbons in a nonpolar matrix
  • Highly glycosylated intact proteins

Correct Answer: Polycyclic aromatic hydrocarbons in a nonpolar matrix

Q17. Which change would increase the fraction of radical cations (M+•) relative to [M+H]+ in APPI?

  • Increase water content and add formic acid to the mobile phase
  • Use a non‑protic mobile phase and an aromatic dopant (e.g., toluene) to favor charge exchange
  • Raise source humidity by decreasing desolvation gas flow
  • Add excess ammonium acetate to promote proton transfer

Correct Answer: Use a non‑protic mobile phase and an aromatic dopant (e.g., toluene) to favor charge exchange

Q18. Why is nitrogen typically used as nebulizer/curtain gas in APPI?

  • It selectively scavenges protons to reduce [M+H]+ formation
  • It has high electron affinity to promote negative ions
  • Its ionization energy (~15.6 eV) exceeds the lamp photon energies, minimizing background ions
  • It cools the lamp window by resonant energy transfer

Correct Answer: Its ionization energy (~15.6 eV) exceeds the lamp photon energies, minimizing background ions

Q19. What is a likely consequence of setting the dopant flow excessively high in APPI?

  • Complete loss of radical cation formation
  • Increased chemical noise and ion suppression due to cluster/adduct formation
  • Permanent lamp deactivation by photon quenching
  • Elimination of matrix effects

Correct Answer: Increased chemical noise and ion suppression due to cluster/adduct formation

Q20. What is a key advantage of hybrid APPI/APCI sources offered by some instruments?

  • They operate without any nebulizing gas
  • They enable both photon- and corona-induced chemistry, improving coverage of nonpolar and more polar analytes in one run
  • They require no dopant under any conditions
  • They function only at nano-flow LC rates

Correct Answer: They enable both photon- and corona-induced chemistry, improving coverage of nonpolar and more polar analytes in one run

Leave a Comment