Ionization techniques – Electron impact ionization MCQs With Answer

Ionization techniques – Electron impact ionization MCQs With Answer

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Ionization techniques are central to mass spectrometry, and Electron Impact (EI) ionization remains a foundational method for organic and pharmaceutical analysis. This concise guide focuses on Ionization techniques – Electron impact ionization MCQs With Answer tailored for B. Pharm students, covering EI principles, ion source design, electron energy effects, molecular ion formation, fragmentation pathways, spectral interpretation, and analytical applications in drug characterization and impurity profiling. Emphasis is on practical considerations such as sample volatility, derivatization, mass-to-charge interpretation, and limitations compared to soft ionization methods. Clear explanations and targeted MCQs will reinforce mechanistic understanding and problem-solving skills. Now let’s test your knowledge with 30 MCQs on this topic.

Q1. What is the primary process in electron impact (EI) ionization?

  • Analyte molecules gain protons from reagent gas
  • High-energy electrons remove an electron producing radical cations
  • Molecules are ionized by photon absorption producing positive ions
  • Analyte forms adducts with alkali metals

Correct Answer: High-energy electrons remove an electron producing radical cations

Q2. What is the standard electron energy commonly used in EI for reproducible spectra?

  • 5 eV
  • 20 eV
  • 70 eV
  • 500 eV

Correct Answer: 70 eV

Q3. How is EI usually classified among ionization techniques?

  • Soft ionization
  • Hard ionization
  • Atmospheric pressure ionization
  • Electrospray ionization

Correct Answer: Hard ionization

Q4. Which ion species is typically formed directly after EI of a neutral molecule M?

  • Protonated molecule [M+H]+
  • Deprotonated molecule [M-H]-
  • Molecular radical cation (M+•)
  • Sodium adduct [M+Na]+

Correct Answer: Molecular radical cation (M+•)

Q5. What does the term “base peak” mean in an EI mass spectrum?

  • The highest m/z peak observed
  • The peak corresponding to the molecular ion
  • The most intense peak in the spectrum set to 100% intensity
  • The first peak in the chromatogram

Correct Answer: The most intense peak in the spectrum set to 100% intensity

Q6. The McLafferty rearrangement is common in EI spectra of which functional group?

  • Alkyl halides without functional groups
  • Carbonyl-containing compounds (e.g., ketones, esters)
  • Simple alkanes only
  • Aromatic nitro compounds exclusively

Correct Answer: Carbonyl-containing compounds (e.g., ketones, esters)

Q7. For thermolabile or non-volatile pharmaceutical compounds, EI ionization is generally:

  • Ideal without modification
  • Not suitable unless the compound is derivatized or a softer ionization is used
  • Preferred over electrospray for large biomolecules
  • Only used in negative ion mode

Correct Answer: Not suitable unless the compound is derivatized or a softer ionization is used

Q8. EI is most commonly coupled with which separation technique in pharmaceutical analysis?

  • Liquid chromatography (LC)
  • Capillary electrophoresis (CE)
  • Gas chromatography (GC)
  • Thin-layer chromatography (TLC)

Correct Answer: Gas chromatography (GC)

Q9. What type of ion species predominates in EI mass spectra?

  • Even-electron cations ([M+H]+)
  • Radical cations (odd-electron species)
  • Negative molecular anions
  • Multiply charged peptides

Correct Answer: Radical cations (odd-electron species)

Q10. What is the “appearance potential” in the context of EI?

  • The voltage applied to the detector
  • The minimum electron energy required to form a specific ion
  • The time at which a compound elutes from GC
  • The temperature at which a compound vaporizes

Correct Answer: The minimum electron energy required to form a specific ion

Q11. Fragmentation patterns in EI spectra are mainly governed by:

  • The polarity of the solvent used
  • Bond dissociation energies and stability of resulting ions
  • The detector type only
  • The color of the sample

Correct Answer: Bond dissociation energies and stability of resulting ions

Q12. Which halogen produces two molecular isotope peaks of roughly equal intensity (M and M+2) in EI spectra?

  • Chlorine
  • Bromine
  • Fluorine
  • Iodine

Correct Answer: Bromine

Q13. Increasing the electron emission current in an EI source primarily leads to:

  • Reduced ion signal due to space charge
  • Increased ionization efficiency and higher overall signal
  • Conversion from radical cations to protonated molecules
  • Switch to negative ion formation

Correct Answer: Increased ionization efficiency and higher overall signal

Q14. EI mass spectra are particularly valuable for:

  • Direct analysis of non-volatile proteins
  • Structure elucidation through characteristic fragmentation
  • Soft ionization of labile compounds
  • Quantifying salts in aqueous solutions without separation

Correct Answer: Structure elucidation through characteristic fragmentation

Q15. For a singly charged molecular ion observed in EI, the reported m/z value corresponds to:

  • The number of neutrons only
  • The molecular mass (approximately equal to molecular weight)
  • The concentration of the analyte
  • The retention time in GC

Correct Answer: The molecular mass (approximately equal to molecular weight)

Q16. How does chemical ionization (CI) differ from EI?

  • CI uses high-energy electrons to produce radical cations
  • CI is a softer method that often yields protonated molecules [M+H]+
  • CI generates only negative ions
  • CI cannot be used with GC-MS

Correct Answer: CI is a softer method that often yields protonated molecules [M+H]+

Q17. Alpha-cleavage in EI typically occurs adjacent to which group?

  • Aromatic rings exclusively
  • A carbonyl group (cleavage next to the C=O)
  • A saturated hydrocarbon far from functional groups
  • Metal chelation sites only

Correct Answer: A carbonyl group (cleavage next to the C=O)

Q18. The ion source vacuum in an EI instrument is typically on the order of:

  • Atmospheric pressure (1 bar)
  • 10^1 to 10^2 mbar
  • 10^-3 to 10^-6 mbar (high vacuum)
  • Exactly 0 mbar (perfect vacuum)

Correct Answer: 10^-3 to 10^-6 mbar (high vacuum)

Q19. Why might the molecular ion peak be absent or very weak in an EI spectrum?

  • The analyzer was operated in negative mode
  • The molecule is highly volatile and stable
  • Extensive fragmentation or low stability of the molecular radical cation
  • Presence of excess reagent gas that forms adducts

Correct Answer: Extensive fragmentation or low stability of the molecular radical cation

Q20. What is the nominal m/z value of the molecular ion for toluene (C7H8) in EI?

  • 78
  • 92
  • 106
  • 120

Correct Answer: 92

Q21. EI mass spectrometry is most commonly performed in which ion polarity?

  • Negative ion mode
  • Positive ion mode
  • Alternating polarity mode only
  • Neutral detection (no ion mode)

Correct Answer: Positive ion mode

Q22. What is a “metastable ion” in EI mass spectrometry?

  • An ion that forms only at very low temperatures
  • An ion that fragments after leaving the ion source, producing in-flight decomposition signals
  • An ion that never fragments under any conditions
  • An ion detected only in the liquid phase

Correct Answer: An ion that fragments after leaving the ion source, producing in-flight decomposition signals

Q23. To reduce fragmentation in EI and favor observation of the molecular ion, one should:

  • Increase the electron energy well above 200 eV
  • Use a higher source temperature
  • Lower the electron energy from the standard 70 eV
  • Switch to flame ionization detector (FID)

Correct Answer: Lower the electron energy from the standard 70 eV

Q24. Which fragment ion at m/z 91 is commonly observed from aromatic compounds (e.g., alkylbenzenes) in EI?

  • Acetyl cation
  • Benzyl cation (tropylium ion)
  • Oxonium ion
  • Chloronium ion

Correct Answer: Benzyl cation (tropylium ion)

Q25. Is EI suitable for quantitative GC-MS analysis in pharmaceutical labs?

  • No, EI can never be used quantitatively
  • Yes, with appropriate calibration and internal standards it can be quantitative
  • Only for proteins and peptides
  • Only when using negative ion mode

Correct Answer: Yes, with appropriate calibration and internal standards it can be quantitative

Q26. In mass spectrometry terminology, what does “hard ionization” imply?

  • Formation of intact molecular ions without fragmentation
  • Extensive fragmentation of the molecular ion yielding structural information
  • Ionization at atmospheric pressure only
  • Exclusive production of negative ions

Correct Answer: Extensive fragmentation of the molecular ion yielding structural information

Q27. Which experimental change is most likely to increase the intensity of the molecular ion relative to fragments?

  • Increase source electron energy to 200 eV
  • Increase source temperature drastically
  • Lower the electron impact energy below 70 eV
  • Introduce a large excess of reagent gas for CI

Correct Answer: Lower the electron impact energy below 70 eV

Q28. EI is most appropriate for analysis of which class of pharmaceutical molecules?

  • Large non-volatile peptides (>2000 Da)
  • Low-to-moderate molecular weight volatile and thermally stable compounds
  • Highly polar, water-soluble polymers without derivatization
  • Intact proteins in biological matrices

Correct Answer: Low-to-moderate molecular weight volatile and thermally stable compounds

Q29. Why is derivatization often performed before GC-EI-MS analysis of some drugs?

  • To make compounds less volatile and more polar
  • To increase volatility, thermal stability, and produce better EI spectra
  • To convert molecules into salts for LC-MS
  • To decrease molecular weight for easier detection

Correct Answer: To increase volatility, thermal stability, and produce better EI spectra

Q30. Why are EI spectra particularly useful for library searching and compound identification?

  • EI spectra are unique, highly reproducible fragmentation patterns enabling database matching
  • EI always produces only the molecular ion with no fragments
  • EI spectra vary randomly with each run, preventing reliable matching
  • EI cannot be used with standardized libraries

Correct Answer: EI spectra are unique, highly reproducible fragmentation patterns enabling database matching

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