Electron impact ionization MCQs With Answer

Electron impact ionization (EI) MCQs With Answer

This set of 20 MCQs is designed for M.Pharm students studying Advanced Instrumental Analysis (MPA 201T). The quiz focuses on electron impact ionization as used in mass spectrometry — covering fundamental principles, instrument parameters, fragmentation behavior, source design, and comparison with other ionization techniques. Questions emphasize practical implications for pharmaceutical analysis such as appearance of molecular ions, typical electron energies, vacuum requirements, effect on thermally labile compounds, and interpretation of EI spectra. Answers are provided to help self-assess understanding and prepare for exams, method development and spectral interpretation in GC-MS and other EI-based workflows.

Q1. What is the primary ion formed in electron impact (EI) ionization of an organic molecule?

• Protonated molecule [M+H]+
• Molecular radical cation M+•
• Deprotonated molecule [M−H]−
• Adduct ion [M+Na]+

Correct Answer: Molecular radical cation M+•

Q2. What electron energy is most commonly used in EI for producing reproducible mass spectra and enabling library matching?

• 5 eV
• 20 eV
• 70 eV
• 500 eV

Correct Answer: 70 eV

Q3. Which statement best describes why EI often generates extensive fragmentation?

• Electrons transfer protons to the analyte, stabilizing it
• High internal energy deposited into the molecular ion causes bond cleavages
• Low-energy collisions only remove weakly bound adducts
• EI uses chemical reagent gases that react with analytes

Correct Answer: High internal energy deposited into the molecular ion causes bond cleavages

Q4. In EI mass spectrometry, what is the typical order of magnitude for ionization efficiency (fraction of molecules that become ions)?

• ~100%
• ~10%
• ~1% or less
• ~50%

Correct Answer: ~1% or less

Q5. Which source pressure range is commonly maintained in an EI ion source to minimize collisions after ionization?

• Atmospheric pressure (~760 Torr)
• ~1 Torr
• 10^-3 to 10^-5 Torr
• 10^-9 Torr

Correct Answer: 10^-3 to 10^-5 Torr

Q6. Which of the following observations indicates the molecular ion peak (M+•) has survived EI and is present in the spectrum?

• A base peak at low m/z unrelated to molecular weight
• A peak corresponding to the nominal molecular weight of the compound with odd electron character
• Only fragment peaks below 50 m/z
• Multiple adduct peaks with sodium

Correct Answer: A peak corresponding to the nominal molecular weight of the compound with odd electron character

Q7. Which factor most commonly causes the absence or low intensity of the molecular ion in EI spectra for some pharmaceutical compounds?

• High volatility of the analyte
• Thermal lability and rapid fragmentation during ionization
• Use of low-resolution mass analyzers
• Presence of halogens only

Correct Answer: Thermal lability and rapid fragmentation during ionization

Q8. What does the term “appearance energy” refer to in EI mass spectrometry?

• The energy needed to evaporate the compound into the gas phase
• The minimum electron energy required to form a particular ion from a neutral
• The kinetic energy of ions entering the detector
• The activation energy for chromatographic retention

Correct Answer: The minimum electron energy required to form a particular ion from a neutral

Q9. Which of the following is a typical advantage of EI when analyzing volatile drug substances by GC-MS?

• Soft ionization that preserves intact molecular ions exclusively
• Highly reproducible fragmentation enabling spectral library matching
• Compatibility with large nonvolatile biomolecules without derivatization
• No requirement for vacuum systems

Correct Answer: Highly reproducible fragmentation enabling spectral library matching

Q10. How does chemical ionization (CI) primarily differ from EI?

• CI uses electrons at 70 eV while EI uses reagent gases
• CI produces mostly protonated or adduct ions and is softer than EI
• CI requires atmospheric pressure whereas EI requires high vacuum
• CI is used only for negative ion mode

Correct Answer: CI produces mostly protonated or adduct ions and is softer than EI

Q11. Which design element in an EI source helps control the electron beam and enhance ionization reproducibility?

• Capillary column length
• Electron emitter/filament and accelerating electrode geometry
• Mass analyzer magnetic field strength
• Detector ion-to-electron conversion plate

Correct Answer: Electron emitter/filament and accelerating electrode geometry

Q12. Which material is commonly used as a filament in EI sources because of its high melting point and stability?

• Silver
• Tungsten (W)
• Polymethylsiloxane
• Graphene

Correct Answer: Tungsten (W)

Q13. What effect does increasing the source temperature in EI generally have on gas-phase ions for GC-MS analysis?

• Decreases volatility and causes condensation
• Reduces signal by freezing analytes
• Helps desolvate and reduce clustering, improving signal for some analytes
• Makes EI become a negative-ion technique

Correct Answer: Helps desolvate and reduce clustering, improving signal for some analytes

Q14. Which spectral feature is most useful for identifying halogenated pharmaceutical compounds in EI spectra?

• Characteristic isotopic patterns (e.g., M+2 intensity) due to chlorine or bromine
• A single intense peak at 18 m/z
• Complete absence of fragmentation
• Only odd-electron fragment peaks below 30 m/z

Correct Answer: Characteristic isotopic patterns (e.g., M+2 intensity) due to chlorine or bromine

Q15. In EI, what are metastable ions and how are they detected?

• Ions that never fragment, detected as broad peaks in the ion source
• Excited neutral molecules detected by UV absorption
• Ions that fragment after leaving the source and produce peaks at non-integer m/z differences in sector instruments or broadened peaks in TOF
• Only an artifact of chemical ionization and not relevant to EI

Correct Answer: Ions that fragment after leaving the source and produce peaks at non-integer m/z differences in sector instruments or broadened peaks in TOF

Q16. Which parameter would you adjust to reduce extensive fragmentation in EI if the molecular ion is weak?

• Increase electron energy to >200 eV
• Lower electron energy below 70 eV cautiously
• Increase source pressure to atmospheric levels
• Change detector gain only

Correct Answer: Lower electron energy below 70 eV cautiously

Q17. Why are EI spectra highly reproducible across instruments when using standard conditions?

• Because EI uses a reagent gas that standardizes fragmentation
• Standardized 70 eV electron energy and similar source geometries lead to predictable fragmentation pathways
• All mass spectrometers use identical detectors
• Reproducibility is low; EI spectra cannot be compared

Correct Answer: Standardized 70 eV electron energy and similar source geometries lead to predictable fragmentation pathways

Q18. Which of the following is a common application of EI-MS in pharmaceutical analysis?

• Direct analysis of intact proteins without fragmentation
• Qualitative identification of volatile drug impurities and confirmation using spectral libraries
• Quantitative analysis of highly polar drugs in plasma without derivatization
• Measuring macromolecular weight distributions of polymers in solution

Correct Answer: Qualitative identification of volatile drug impurities and confirmation using spectral libraries

Q19. How does matrix complexity (co-eluting components) typically affect EI spectra in GC-MS analysis of pharmaceuticals?

• It has no effect because EI is matrix-blind
• It can suppress or enhance ion signals and complicate spectral interpretation
• It converts EI into CI automatically
• It only affects retention time but not mass spectra

Correct Answer: It can suppress or enhance ion signals and complicate spectral interpretation

Q20. Which strategy is often used when EI fails to give a molecular ion for a thermo-labile pharmaceutical compound?

• Increase carrier gas flow to atmospheric pressure
• Use a softer ionization method such as chemical ionization (CI) or electrospray ionization (ESI)
• Raise the electron energy beyond 200 eV to force molecular ion formation
• Reduce the mass analyzer resolving power

Correct Answer: Use a softer ionization method such as chemical ionization (CI) or electrospray ionization (ESI)

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