Derivatization in GC MCQs With Answer is a focused quiz resource tailored for M.Pharm students studying Advanced Instrumental Analysis (MPA 201T). This collection emphasizes practical and conceptual aspects of chemical derivatization used to make non-volatile or thermally labile pharmaceutical analytes amenable to gas chromatography. Questions cover reagents, mechanisms, reaction conditions, analytical benefits (volatility, stability, detectability), common pitfalls, on-line vs off-line strategies, safety concerns and interpretation in GC and GC–MS. Designed to reinforce classroom learning and exam preparation, the set balances fundamental theory with application-based scenarios that reflect real laboratory considerations in pharmaceutical analysis.
Q1. What is the primary analytical purpose of chemical derivatization in gas chromatography?
- To increase analyte volatility and thermal stability for GC analysis
- To decrease analyte molecular weight so it elutes faster
- To permanently bind analytes to the stationary phase
- To convert all analytes into charged species for GC separation
Correct Answer: To increase analyte volatility and thermal stability for GC analysis
Q2. Which general derivatization strategy is most commonly used to modify hydroxyls, carboxyls and amines for GC?
- Silylation (formation of trimethylsilyl or other silyl derivatives)
- Oxidation to carbonyls
- Hydrogenation to reduce polarity
- Complexation with crown ethers
Correct Answer: Silylation (formation of trimethylsilyl or other silyl derivatives)
Q3. N,O-bis(trimethylsilyl)trifluoroacetamide is the chemical name for which commonly used silylation reagent?
- BSTFA
- MSTFA
- MTBSTFA
- PFPA
Correct Answer: BSTFA
Q4. Which reagent is specifically used to introduce the tert-butyldimethylsilyl (TBDMS) protecting group for increased derivative stability?
- MTBSTFA (N-tert-butyldimethylsilyl-N-methyltrifluoroacetamide)
- BSTFA
- Diazomethane
- PFBBr (pentafluorobenzyl bromide)
Correct Answer: MTBSTFA (N-tert-butyldimethylsilyl-N-methyltrifluoroacetamide)
Q5. Which reagent is most commonly used to methylate carboxylic acids to form methyl esters for GC analysis, but requires extreme safety precautions?
- Diazomethane
- BSTFA
- Pyridine
- PFPA
Correct Answer: Diazomethane
Q6. Mechanistically, silylation of an alcohol involves which key step?
- Nucleophilic attack of the oxygen on silicon, displacing a leaving group to form a Si–O bond
- Free radical abstraction of hydrogen to form a radical intermediate
- Hydrogen transfer to the reagent followed by elimination of water
- Formation of a coordination complex without covalent bond formation
Correct Answer: Nucleophilic attack of the oxygen on silicon, displacing a leaving group to form a Si–O bond
Q7. How does derivatization commonly improve GC–MS detection of polar analytes?
- By producing more volatile and thermally stable derivatives with characteristic fragmentation patterns in EI-MS
- By converting analytes into non-ionizable neutral species to avoid MS suppression
- By significantly increasing molecular weight to prevent fragmentation
- By binding analytes irreversibly to ionization sources
Correct Answer: By producing more volatile and thermally stable derivatives with characteristic fragmentation patterns in EI-MS
Q8. Which statement best describes silylation reagents like BSTFA or MSTFA regarding moisture?
- They are highly moisture-sensitive and are hydrolyzed by water, reducing derivatization efficiency
- They are inert to water and perform equally well in aqueous solutions
- They require water to activate the silyl group for reaction
- They form stable hydrates that improve GC peak shape
Correct Answer: They are highly moisture-sensitive and are hydrolyzed by water, reducing derivatization efficiency
Q9. Which solvent often acts both as reaction medium and acid–base catalyst for silylation reactions?
- Pyridine
- Hexane
- Water
- Methanol
Correct Answer: Pyridine
Q10. Which derivatization reagent is commonly used to introduce an electron-capturing pentafluorobenzyl (PFB) group to enhance detection by electron capture or negative-ion MS?
- PFBBr (pentafluorobenzyl bromide)
- BSTFA
- Diazomethane
- Imidazole
Correct Answer: PFBBr (pentafluorobenzyl bromide)
Q11. What is a primary advantage of on-line (injection port) derivatization compared with off-line derivatization?
- Greater automation with reduced manual handling and lower contamination risk
- Ability to use larger reaction volumes for higher yield
- Long reaction times without effect on throughput
- Complete elimination of reagent side-products
Correct Answer: Greater automation with reduced manual handling and lower contamination risk
Q12. Which classes of pharmaceutical analytes typically require derivatization before GC analysis?
- Polar and thermally labile compounds such as alcohols, carboxylic acids, amines, sugars and phenols
- All small non-polar hydrocarbons
- Only inorganic salts and metal complexes
- Proteins and large polypeptides without prior hydrolysis
Correct Answer: Polar and thermally labile compounds such as alcohols, carboxylic acids, amines, sugars and phenols
Q13. In quantitative GC, incomplete derivatization most commonly results in what analytical problem?
- Underestimation of analyte concentration due to mixture of derivatized and underivatized species
- False increase in analyte concentration due to reagent contamination
- Complete loss of chromatographic resolution of all peaks
- Transformation of analytes into the same single peak
Correct Answer: Underestimation of analyte concentration due to mixture of derivatized and underivatized species
Q14. Which nucleophilic catalyst is commonly used to accelerate silylation reactions?
- Imidazole
- Sulfuric acid
- Hydrochloric acid
- Sodium chloride
Correct Answer: Imidazole
Q15. For sterically hindered or more stable silyl derivatives that resist hydrolysis, which reagent is preferred?
- MTBSTFA (gives tert-butyldimethylsilyl derivatives)
- BSTFA (gives trimethylsilyl derivatives)
- PFBBr
- Acetic anhydride
Correct Answer: MTBSTFA (gives tert-butyldimethylsilyl derivatives)
Q16. Which detector is particularly enhanced by perfluoroacylation or pentafluorobenzylation of analytes?
- Electron capture detector (ECD) / negative-ion MS (ECNI)
- Flame ionization detector (FID) only
- Thermal conductivity detector (TCD) exclusively
- Refractive index detector
Correct Answer: Electron capture detector (ECD) / negative-ion MS (ECNI)
Q17. What common chromatographic artifact may indicate incomplete or competing derivatization reactions?
- Multiple peaks for a single analyte corresponding to underivatized, partially-derivatized and fully-derivatized species
- Single, sharp peak with unusually high intensity
- Total suppression of all analyte signals only
- Complete co-elution of solvent with all analytes
Correct Answer: Multiple peaks for a single analyte corresponding to underivatized, partially-derivatized and fully-derivatized species
Q18. Which reagent is commonly used to quench residual silylation reagent after the reaction is complete?
- Methanol (to hydrolyze excess silylating agent)
- Sulfuric acid (to protonate the derivatives)
- Hexane (to precipitate reagents)
- Pyridine (to further catalyze the reaction)
Correct Answer: Methanol (to hydrolyze excess silylating agent)
Q19. Typical reaction conditions to achieve complete silylation with reagents like MSTFA or BSTFA are often:
- Moderate heating (e.g., 60–80 °C) for tens of minutes to an hour
- Cryogenic temperatures to stabilize reactive intermediates
- Very high temperatures above 200 °C for rapid conversion
- Strictly anaerobic conditions at room temperature for days
Correct Answer: Moderate heating (e.g., 60–80 °C) for tens of minutes to an hour
Q20. Which of the following safety statements about diazomethane is correct?
- Diazomethane is toxic, potentially explosive and must be generated and handled with strict precautions in a fume hood
- Diazomethane is non-toxic and safe to use on open bench with no precautions
- Diazomethane decomposes into harmless gases at room temperature and does not require special disposal
- Diazomethane is a mild acid used to catalyze silylation reactions
Correct Answer: Diazomethane is toxic, potentially explosive and must be generated and handled with strict precautions in a fume hood
