Gas chromatography – introduction, theory and instrumentation MCQs With Answer

Gas chromatography (GC) is an essential analytical technique in pharmaceutical analysis, used for separation, identification, and quantitation of volatile and semi-volatile compounds. This introduction covers GC theory — retention time, partitioning, adsorption, capacity factor (k’), plate theory (HETP, number of theoretical plates) and the Van Deemter relationship — and instrumentation — injector types, capillary and packed columns, stationary phases, carrier gases, oven temperature programming, and detectors like FID, TCD and MS. Key concepts also include sample preparation, split/splitless injection, resolution, selectivity and quantitative methods such as calibration and internal standards. Now let’s test your knowledge with 30 MCQs on this topic.

Q1. What property of analytes primarily determines their separation in gas chromatography?

• Polarity of the detector
• Interaction with the stationary phase
• Size of the sample injector
• Detector response time

Correct Answer: Interaction with the stationary phase

Q2. Which carrier gas is commonly used for high-sensitivity Flame Ionization Detector (FID) applications?

• Carbon dioxide
• Helium
• Nitrogen
• Oxygen

Correct Answer: Helium

Q3. What does retention time (tR) represent in GC?

• The time a detector takes to respond
• The time between injection and detector signal peak
• The time to replace the carrier gas
• The time required for column conditioning

Correct Answer: The time between injection and detector signal peak

Q4. Which detector is most selective for halogenated compounds and used for trace analysis?

• Flame Ionization Detector (FID)
• Thermal Conductivity Detector (TCD)
• Electron Capture Detector (ECD)
• Nitrogen-Phosphorus Detector (NPD)

Correct Answer: Electron Capture Detector (ECD)

Q5. The Van Deemter equation describes the relationship between:

• Retention time and detector sensitivity
• Column efficiency (HETP) and linear velocity
• Carrier gas purity and baseline noise
• Injector temperature and sample loss

Correct Answer: Column efficiency (HETP) and linear velocity

Q6. What is a major advantage of capillary columns over packed columns?

• Lower resolution
• Higher capacity for large sample loads
• Higher separation efficiency and resolution
• Less need for carrier gas

Correct Answer: Higher separation efficiency and resolution

Q7. In split injection, why is a portion of the vaporized sample vented?

• To increase detector noise
• To prevent column overload and improve peak shape
• To cool the inlet rapidly
• To change the stationary phase polarity

Correct Answer: To prevent column overload and improve peak shape

Q8. What does the capacity factor (k’) indicate?

• Detector linear range
• Relative retention of an analyte compared to dead time
• Column temperature stability
• Injector split ratio

Correct Answer: Relative retention of an analyte compared to dead time

Q9. Which detector measures changes in thermal conductivity of the gas stream?

• FID
• TCD
• ECD
• NPD

Correct Answer: TCD

Q10. Which parameter directly improves resolution between two adjacent peaks?

• Decreasing column length
• Increasing selectivity (α) or plate number (N)
• Using a lower purity carrier gas
• Decreasing detector sensitivity

Correct Answer: Increasing selectivity (α) or plate number (N)

Q11. What is the primary role of the GC oven?

• To detect eluted compounds
• To maintain and program column temperature for optimal separation
• To vaporize the sample
• To provide carrier gas flow

Correct Answer: To maintain and program column temperature for optimal separation

Q12. Kovats retention index is used to:

• Quantify analyte concentration
• Standardize retention times across different columns and conditions
• Measure detector baseline noise
• Set injector split ratio

Correct Answer: Standardize retention times across different columns and conditions

Q13. Which stationary phase is most suitable for separating nonpolar hydrocarbons?

• Highly polar polyethylene glycol
• Nonpolar dimethylpolysiloxane
• Polar cyano phase
• Strong anion exchange

Correct Answer: Nonpolar dimethylpolysiloxane

Q14. What is the effect of increasing column temperature on retention times?

• Retention times generally increase
• Retention times generally decrease
• Retention times remain unchanged
• Retention times fluctuate randomly

Correct Answer: Retention times generally decrease

Q15. Which detector is nearly universal but less sensitive than FID and often used for permanent gases?

• ECD
• TCD
• NPD
• SCD (Sulfur Chemiluminescence Detector)

Correct Answer: TCD

Q16. How does a Flame Ionization Detector (FID) detect analytes?

• By measuring change in thermal conductivity
• By ionizing organic compounds in a flame and measuring current
• By capturing electrons from electronegative compounds
• By measuring mass-to-charge ratios

Correct Answer: By ionizing organic compounds in a flame and measuring current

Q17. What is splitless injection primarily used for?

• Analyzing very volatile gases only
• Trace analysis when sample concentration is low
• To decrease detector sensitivity
• To improve column lifetime

Correct Answer: Trace analysis when sample concentration is low

Q18. Which factor does NOT influence the number of theoretical plates (N)?

• Column length
• Mobile phase linear velocity
• Detector type
• Stationary phase efficiency

Correct Answer: Detector type

Q19. GC-MS coupling provides which major advantage?

• Reduced separation efficiency
• Direct mass spectral identification of eluted compounds
• Elimination of carrier gas
• Higher sample capacity than LC-MS

Correct Answer: Direct mass spectral identification of eluted compounds

Q20. Which sample property makes a compound unsuitable for GC analysis?

• High volatility
• Very high molecular weight and thermal instability
• Low polarity
• Moderate boiling point

Correct Answer: Very high molecular weight and thermal instability

Q21. Which procedure reduces contamination and prolongs column life?

• Overloading the column
• Using guard columns or retention gap
• Using impure carrier gas
• Running very high split ratios always

Correct Answer: Using guard columns or retention gap

Q22. What does resolution (Rs) between two peaks depend on?

• Only retention times
• Retention factor (k’), selectivity (α), and efficiency (N)
• Detector manufacturer
• Injector color

Correct Answer: Retention factor (k’), selectivity (α), and efficiency (N)

Q23. Which carrier gas offers highest optimal linear velocity for best efficiency according to Van Deemter?

• Nitrogen
• Helium
• Carbon dioxide
• Argon

Correct Answer: Helium

Q24. In pharmaceutical GC quantitation, why is an internal standard used?

• To change stationary phase polarity
• To correct for injection variability and matrix effects
• To reduce detector sensitivity
• To speed up oven programming

Correct Answer: To correct for injection variability and matrix effects

Q25. Temperature programming in GC is beneficial because it:

• Makes all peaks elute at the same time
• Improves analysis of mixtures with wide boiling point ranges
• Prevents any interaction with the stationary phase
• Eliminates need for carrier gas

Correct Answer: Improves analysis of mixtures with wide boiling point ranges

Q26. What is the main principle of an Electron Capture Detector (ECD)?

• Measuring mass spectra
• Detecting compounds that capture electrons, reducing current
• Measuring flame ionization current
• Detecting thermal conductivity changes

Correct Answer: Detecting compounds that capture electrons, reducing current

Q27. Which maintenance issue most likely causes peak tailing in GC?

• Column perfectly deactivated
• Active sites in the inlet or column due to contamination
• Using helium as carrier gas
• Too narrow a split ratio

Correct Answer: Active sites in the inlet or column due to contamination

Q28. Headspace GC is especially suitable for analyzing:

• Non-volatile polymers
• Volatile organic compounds from solids or liquids without direct injection
• High-boiling triglycerides
• Metals

Correct Answer: Volatile organic compounds from solids or liquids without direct injection

Q29. The nitrogen-phosphorus detector (NPD) is selective for:

• Hydrocarbons only
• Nitrogen- and phosphorus-containing compounds
• Inorganic gases like CO2
• All electronegative compounds

Correct Answer: Nitrogen- and phosphorus-containing compounds

Q30. Which practice improves quantitative accuracy in GC calibration?

• Using single-point external calibration without replication
• Using multi-point calibration and quality control standards
• Not using any standards and relying on retention time only
• Changing detector type between calibrations

Correct Answer: Using multi-point calibration and quality control standards

G S Sachin

I am a Registered Pharmacist under the Pharmacy Act, 1948, and the founder of PharmacyFreak.com. I hold a Bachelor of Pharmacy degree from Rungta College of Pharmaceutical Science and Research. With a strong academic foundation and practical knowledge, I am committed to providing accurate, easy-to-understand content to support pharmacy students and professionals. My aim is to make complex pharmaceutical concepts accessible and useful for real-world application.

Mail- Sachin@pharmacyfreak.com