Derivatization techniques and temperature programming in GC MCQs With Answer

Derivatization techniques and temperature programming in GC MCQs With Answer

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Derivatization techniques and temperature programming in GC are essential topics for B. Pharm students learning gas chromatography (GC) and GC‑MS. Derivatization (silylation, acylation, alkylation) improves volatility, thermal stability, and detectability of polar drugs and metabolites; common reagents include BSTFA, MSTFA, TMCS and BSA. Temperature programming (initial temp, ramp rate, hold time, final temp) optimizes separation across wide boiling‑point ranges, influencing retention time, resolution and peak shape. Key keywords: derivatization techniques, temperature programming, GC, GC‑MS, silylation, acylation, retention time, ramp rate, splitless injection, capillary column. Now let’s test your knowledge with 30 MCQs on this topic.

Q1. Which primary purpose of derivatization in GC analyses of pharmaceutical compounds?

  • To increase analyte polarity
  • To decrease analyte volatility
  • To increase analyte volatility and thermal stability
  • To remove the analyte from solution

Correct Answer: To increase analyte volatility and thermal stability

Q2. Which reagent is most commonly used for silylation of hydroxyl and amino groups in GC‑MS?

  • Acetic anhydride
  • BSTFA (N,O‑bis(trimethylsilyl)trifluoroacetamide)
  • Methanol
  • Sodium borohydride

Correct Answer: BSTFA (N,O‑bis(trimethylsilyl)trifluoroacetamide)

Q3. Silylation of an alcohol typically replaces the active hydrogen with which group?

  • Acetyl group (–COCH3)
  • Methyl group (–CH3)
  • Trimethylsilyl group (–Si(CH3)3)
  • Nitro group (–NO2)

Correct Answer: Trimethylsilyl group (–Si(CH3)3)

Q4. Why is pyridine often used in derivatization reactions?

  • As a strong oxidizing agent
  • As a solvent and base catalyst that scavenges acids
  • To increase moisture in the reaction
  • To reduce analyte volatility

Correct Answer: As a solvent and base catalyst that scavenges acids

Q5. Which functional group is NOT typically targeted for derivatization prior to GC analysis?

  • Carboxylic acids
  • Alcohols
  • Alkanes without heteroatoms
  • Amines

Correct Answer: Alkanes without heteroatoms

Q6. Which side effect can moisture cause during silylation with BSTFA?

  • Enhanced derivatization efficiency
  • Formation of silanols and reduced derivatization yield
  • Complete conversion of all analytes
  • No effect at all

Correct Answer: Formation of silanols and reduced derivatization yield

Q7. In GC temperature programming, what is the main advantage of using a temperature ramp instead of isothermal analysis?

  • Shorter column lifetime
  • Improved separation across a wide boiling range and reduced analysis time
  • Increased baseline noise always
  • Elimination of need for carrier gas

Correct Answer: Improved separation across a wide boiling range and reduced analysis time

Q8. What effect does a fast oven ramp rate typically have on closely eluting early-eluting compounds?

  • Improves resolution between them
  • Reduces resolution and may cause coelution
  • Has no effect on resolution
  • Converts them into derivatized products

Correct Answer: Reduces resolution and may cause coelution

Q9. Which detector is most influenced by derivatization to enhance response for polar analytes in GC‑MS?

  • Thermal conductivity detector (TCD)
  • Electron capture detector (ECD)
  • Flame ionization detector (FID) and MS ionization efficiency
  • Refractive index detector (RID)

Correct Answer: Flame ionization detector (FID) and MS ionization efficiency

Q10. For splitless injection of trace-level derivatized analytes, which practice is recommended?

  • Use a very high split ratio
  • Use cold trapping or low initial oven temperature to focus analytes at the head of the column
  • Keep the injector temperature below ambient
  • Always use on-column injection instead

Correct Answer: Use cold trapping or low initial oven temperature to focus analytes at the head of the column

Q11. Which of the following is a common acylation reagent used for derivatizing amines or alcohols?

  • BSTFA
  • Pyridine alone
  • Acetic anhydride
  • Helium gas

Correct Answer: Acetic anhydride

Q12. What is a primary reason to choose MSTFA over BSTFA for some applications?

  • MSTFA gives bulkier derivatives
  • MSTFA is less volatile than BSTFA
  • MSTFA often reacts faster and produces fewer byproducts for sterically hindered sites
  • MSTFA is not moisture sensitive

Correct Answer: MSTFA often reacts faster and produces fewer byproducts for sterically hindered sites

Q13. In GC temperature programming, what is the significance of the initial oven temperature being too high?

  • It improves focusing of volatile analytes
  • It may cause poor peak shapes and loss of early-eluting analytes due to lack of focusing
  • It increases column polarity
  • It has no influence on retention time

Correct Answer: It may cause poor peak shapes and loss of early-eluting analytes due to lack of focusing

Q14. Which derivatization approach increases GC detectability by adding electronegative groups for ECD detection?

  • Silylation
  • Fluoroacylation or introduction of halogens
  • Methylation with methanol
  • Hydrogenation

Correct Answer: Fluoroacylation or introduction of halogens

Q15. Which factor does NOT directly influence the choice of temperature program for a GC separation?

  • Boiling point range of analytes
  • Stationary phase polarity
  • Carrier gas purity only
  • Column length and film thickness

Correct Answer: Carrier gas purity only

Q16. What is an advantage of on‑column injection for thermally labile derivatized samples?

  • Higher inlet temperatures are always used
  • No advantages; it always degrades analytes
  • Minimal thermal degradation because analytes enter the column without passing through a hot inlet liner
  • It increases sample discrimination against high boilers

Correct Answer: Minimal thermal degradation because analytes enter the column without passing through a hot inlet liner

Q17. Which byproduct is commonly produced when silylation reagents react with moisture?

  • Trimethylsilanol
  • Methane
  • Water only
  • Carbon dioxide

Correct Answer: Trimethylsilanol

Q18. When optimizing a temperature program, which parameter is typically adjusted first to improve resolution between late-eluting peaks?

  • Initial oven temperature
  • Carrier gas type
  • Ramp rate in the high-temperature region or final hold time
  • Injector liner diameter

Correct Answer: Ramp rate in the high-temperature region or final hold time

Q19. Which derivatization technique is most appropriate to prevent hydrogen bonding that causes broadened peaks for polar analytes?

  • Hydrogenation
  • Silylation to mask OH and NH groups
  • Increasing sample concentration
  • Lowering column temperature

Correct Answer: Silylation to mask OH and NH groups

Q20. In temperature programming, what does “hold time” at a given temperature accomplish?

  • It increases column bleed permanently
  • Allows late‑eluting high‑boiling analytes to elute and stabilizes baseline
  • Always decreases resolution
  • Removes derivatization reagents

Correct Answer: Allows late‑eluting high‑boiling analytes to elute and stabilizes baseline

Q21. Which statement about derivatization for GC‑MS fragmentation is correct?

  • Derivatization never affects mass spectral fragmentation patterns
  • Silyl derivatives often give characteristic fragmentation helpful for identifying functional groups
  • Derivatization always eliminates molecular ions in MS
  • Derivatization converts analytes to inorganic salts

Correct Answer: Silyl derivatives often give characteristic fragmentation helpful for identifying functional groups

Q22. What is column bleed and how is it influenced by temperature programming?

  • Column bleed is baseline noise from stationary phase degradation and increases at high final temperatures
  • Column bleed is caused by carrier gas impurities only and is unaffected by temperature
  • Column bleed improves analyte response
  • Column bleed is the sample leaking out of the injector

Correct Answer: Column bleed is baseline noise from stationary phase degradation and increases at high final temperatures

Q23. For methylation derivatization (e.g., using diazomethane), what is a major safety consideration?

  • Diazomethane is non‑toxic and safe to handle in air
  • Diazomethane is highly toxic and explosive; requires strict safety controls
  • Methylation does not produce volatile byproducts
  • Methylation is ineffective for acids

Correct Answer: Diazomethane is highly toxic and explosive; requires strict safety controls

Q24. How does film thickness of the GC capillary column influence temperature-programmed separations?

  • Thicker films increase capacity and retention, often requiring higher final temperatures or slower ramps
  • Film thickness has no influence on retention
  • Thicker films always decrease analysis time
  • Thicker films make the column nonpolar

Correct Answer: Thicker films increase capacity and retention, often requiring higher final temperatures or slower ramps

Q25. Which technique can be used to remove excess derivatization reagent before GC analysis?

  • Direct injection without cleanup
  • Quenching and liquid–liquid extraction or solid‑phase extraction (SPE) cleanup
  • Reducing column oven temperature to ambient
  • Heating the sample to 300 °C

Correct Answer: Quenching and liquid–liquid extraction or solid‑phase extraction (SPE) cleanup

Q26. What is the effect of too low an initial oven temperature on high‑volatility analytes during splitless injection?

  • They may be strongly retained and give broad peaks due to cold trapping
  • They will always degrade
  • They will not be injected into the column
  • They will convert to less volatile derivatives

Correct Answer: They may be strongly retained and give broad peaks due to cold trapping

Q27. Which derivatization reagent is commonly used to introduce electron‑capturing groups for ECD analysis?

  • Trimethylsilyl chloride
  • Fluoroacyl chloride or pentafluorobenzoyl chloride
  • DMSO
  • Hydrochloric acid

Correct Answer: Fluoroacyl chloride or pentafluorobenzoyl chloride

Q28. Why is it important to match the final oven temperature to the boiling point of the highest-boiling analyte?

  • To ensure all analytes, including high boilers, elute within a reasonable time without degrading the column
  • Final temperature should always be below 100 °C
  • To prevent early-eluting volatiles from eluting
  • Final temperature only affects carrier gas flow

Correct Answer: To ensure all analytes, including high boilers, elute within a reasonable time without degrading the column

Q29. Which parameter best describes the rate of temperature increase in a temperature program?

  • Initial temperature
  • Ramp rate (°C/min)
  • Final hold time
  • Injector split ratio

Correct Answer: Ramp rate (°C/min)

Q30. When derivatizing a pharmaceutical compound containing both a carboxylic acid and an alcohol, what strategy ensures complete derivatization?

  • Derivatize only one functional group and ignore the other
  • Use a reagent or sequential reagents that can derivatize both groups (e.g., silylation after converting acid to its ester or using reagents that derivatize both OH and COOH)
  • Reduce the sample concentration drastically
  • Freeze the sample before injection

Correct Answer: Use a reagent or sequential reagents that can derivatize both groups (e.g., silylation after converting acid to its ester or using reagents that derivatize both OH and COOH)

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