Electron impact ionization MCQs With Answer

Electron impact ionization MCQs With Answer

by

Electron impact ionization (EI) is a cornerstone technique in Modern Pharmaceutical Analytical Techniques, especially in GC–MS. For M. Pharm students, mastering EI helps in understanding how electron–molecule interactions generate molecular radical cations and diagnostic fragments, enabling structural elucidation, impurity profiling, and library matching. This quiz focuses on practical and theoretical aspects of EI: instrument design (filament, repeller, electron trap), operating conditions (electron energy, vacuum), spectral interpretation (molecular ion, base peak, isotopic patterns), and performance nuances (appearance energy, ionization efficiency curves, metastable ions). These MCQs are crafted to reinforce conceptual clarity and analytical reasoning for pharmaceutical applications, including method development, troubleshooting, and data interpretation in routine and research laboratories.

Q1. What is the most commonly used electron energy in electron impact ionization for GC–MS?

  • 70 eV
  • 7 eV
  • 700 eV
  • 15 keV

Correct Answer: 70 eV

Q2. In EI, the initial ion typically formed from a neutral molecule is:

  • A molecular radical cation (M+·)
  • A protonated molecule ([M+H]+)
  • A deprotonated molecule ([M−H]−)
  • A sodium-adduct cation ([M+Na]+)

Correct Answer: A molecular radical cation (M+·)

Q3. EI is often described as a “hard” ionization technique because it:

  • It imparts high internal energy to molecules causing extensive fragmentation
  • It gently protonates analytes via ion–molecule reactions
  • It requires a cryogenic matrix to stabilize ions
  • It solely produces intact molecular ions with minimal fragments

Correct Answer: It imparts high internal energy to molecules causing extensive fragmentation

Q4. Which statement about the base peak in an EI mass spectrum is correct?

  • It is the most abundant ion assigned 100% relative intensity
  • It always corresponds to the molecular ion
  • It is the ion with the highest m/z value
  • It originates from background gas only

Correct Answer: It is the most abundant ion assigned 100% relative intensity

Q5. Which of the following is commonly used as a filament material in an EI ion source?

  • Tungsten
  • Silver
  • Aluminum
  • Graphite

Correct Answer: Tungsten

Q6. The EI ion source typically operates under which vacuum conditions?

  • Approximately 10−5 to 10−6 torr in the ion source
  • Approximately 1 atmosphere in the ion source
  • Approximately 1 torr in the analyzer region
  • Approximately 10−2 torr in the ion source

Correct Answer: Approximately 10−5 to 10−6 torr in the ion source

Q7. EI is best suited for which class of analytes?

  • Volatile, thermally stable small molecules
  • High-mass intact proteins in aqueous solution
  • Inorganic salts in water
  • Non-volatile, thermally labile polymers without derivatization

Correct Answer: Volatile, thermally stable small molecules

Q8. A key analytical advantage of EI is that it:

  • Produces highly reproducible fragmentation enabling library searching
  • Requires no vacuum and minimal maintenance
  • Provides universal detection with no selectivity
  • Yields exclusively molecular ions with negligible fragmentation

Correct Answer: Produces highly reproducible fragmentation enabling library searching

Q9. Why is 70 eV commonly chosen as the electron energy in EI?

  • It maximizes ionization cross-section and yields comparable spectra across instruments
  • It minimizes fragmentation to preserve the molecular ion
  • It matches the ionization potential of nitrogen exactly
  • It prevents electron–ion recombination at high pressures

Correct Answer: It maximizes ionization cross-section and yields comparable spectra across instruments

Q10. Which component in an EI source helps push newly formed positive ions out of the ionization region?

  • Repeller electrode
  • Electron filament
  • Electron trap/collector
  • Diffusion pump

Correct Answer: Repeller electrode

Q11. Which fragmentation pathway is particularly characteristic of EI spectra, especially for heteroatom-containing compounds?

  • Alpha-cleavage adjacent to heteroatoms
  • Sodium adduction of neutral molecules
  • Solvent cluster formation
  • Charge-remote fragmentation induced by lasers

Correct Answer: Alpha-cleavage adjacent to heteroatoms

Q12. EI is most commonly integrated into which hyphenated analytical technique?

  • Gas chromatography–mass spectrometry (GC–MS)
  • Liquid chromatography–electrospray mass spectrometry (LC–ESI–MS)
  • Matrix-assisted laser desorption ionization (MALDI–MS)
  • Capillary electrophoresis–atmospheric pressure chemical ionization (CE–APCI–MS)

Correct Answer: Gas chromatography–mass spectrometry (GC–MS)

Q13. In EI studies, the term “appearance energy” refers to:

  • The minimum electron energy at which a specific ion first appears
  • The energy required to desorb analytes from a matrix
  • The kinetic energy of ions entering the detector
  • The energy difference between base peak and molecular ion

Correct Answer: The minimum electron energy at which a specific ion first appears

Q14. An ionization efficiency curve (IEC) in EI is:

  • A plot of ion signal vs electron energy used to determine appearance potentials
  • A calibration curve of detector response vs concentration
  • A plot of retention time vs oven temperature
  • A plot of resolving power vs acceleration voltage

Correct Answer: A plot of ion signal vs electron energy used to determine appearance potentials

Q15. Metastable peaks observed with sector or TOF analyzers in EI arise due to:

  • From unimolecular decomposition of excited ions between source and analyzer
  • From ions that collide with background gas in the detector
  • From isotopic variants of chlorine and bromine
  • From chemical ionization by reagent gases

Correct Answer: From unimolecular decomposition of excited ions between source and analyzer

Q16. An EI spectrum shows M and M+2 peaks with approximately 1:1 intensity. This strongly suggests the presence of:

  • Presence of one bromine atom
  • Presence of one chlorine atom
  • Presence of two sulfur atoms
  • Presence of three oxygen atoms

Correct Answer: Presence of one bromine atom

Q17. If the molecular ion is weak or absent in EI, which adjustment is most likely to help recover it?

  • Lower the electron energy below 70 eV
  • Increase source pressure to near atmospheric
  • Switch to negative-ion EI by reversing polarity
  • Decrease the vacuum level to increase collisions

Correct Answer: Lower the electron energy below 70 eV

Q18. Which compound is routinely used to tune and calibrate quadrupole GC–MS systems operating in EI mode?

  • Perfluorotributylamine (PFTBA)
  • Caffeine
  • Sodium formate
  • Polyethylene glycol 600 (PEG-600)

Correct Answer: Perfluorotributylamine (PFTBA)

Q19. For solid, non-volatile samples that cannot be chromatographed, EI analysis is facilitated by using a:

  • Direct insertion probe (DIP)
  • Electrospray nebulizer
  • MALDI target plate
  • Atmospheric pressure inlet

Correct Answer: Direct insertion probe (DIP)

Q20. In an EI source, the primary function of the electron trap (collector) is to:

  • Captures excess electrons to prevent them entering the analyzer and causing noise
  • Focuses positive ions into the quadrupole by electrostatic lenses
  • Heats the sample to aid volatilization
  • Measures ion kinetic energy in the detector

Correct Answer: Captures excess electrons to prevent them entering the analyzer and causing noise

Leave a Comment