Headspace sampling in GC MCQs With Answer

Introduction

This MCQ collection focuses on headspace sampling for gas chromatography and is tailored for M.Pharm students studying Advanced Instrumental Analysis. It condenses key theoretical and practical aspects — static and dynamic headspace, thermodynamic partitioning, vial selection, septa compatibility, incubation parameters, salting-out, matrix effects, and automation with headspace autosamplers. Questions explore method development, calibration strategies, quality control, and troubleshooting common problems like carryover and leakage. Also included are comparisons with purge-and-trap and solid-phase microextraction to clarify applicability. Use these questions to test and deepen your understanding, prepare for exams, and reinforce best practices for reliable, quantitative headspace-GC analyses. Answers with explanations are included to aid conceptual clarity and practical application.

Q1. What is the primary analytical advantage of static headspace sampling in GC?

• It allows direct injection of particulate matter into the GC column
• It selectively analyzes volatile and semi‑volatile compounds in the sample vapor without injecting the bulk matrix
• It chemically derivatizes non‑volatile analytes to improve GC response
• It increases detector linear range by dilution of analytes

Correct Answer: It selectively analyzes volatile and semi‑volatile compounds in the sample vapor without injecting the bulk matrix

Q2. In static headspace, the partition coefficient K (concentration in liquid / concentration in gas) is 100. Which statement is true for quantitation at equilibrium?

• A small sample mass will favor a larger fraction of analyte in the gas phase
• A K of 100 means most analyte is in the gas phase at equilibrium
• A high K indicates the analyte strongly favors the liquid phase, reducing headspace concentration
• K has no effect on sensitivity in static headspace

Correct Answer: A high K indicates the analyte strongly favors the liquid phase, reducing headspace concentration

Q3. Which operational change typically increases analyte concentration in the headspace for static HS‑GC?

• Decreasing incubation temperature
• Adding salt (salting‑out) to the sample matrix
• Increasing vial volume while keeping sample amount constant
• Using a glass vial instead of a plastic vial without changing other parameters

Correct Answer: Adding salt (salting‑out) to the sample matrix

Q4. Which of the following best describes dynamic headspace (purge-and-trap) compared with static headspace?

• Dynamic headspace collects analytes by continuous purging and trapping, generally achieving higher sensitivity for trace volatiles
• Dynamic headspace always yields better quantitative precision than static headspace
• Dynamic headspace does not require a trap or concentrating step
• Dynamic headspace equilibrates the sample overnight before sampling

Correct Answer: Dynamic headspace collects analytes by continuous purging and trapping, generally achieving higher sensitivity for trace volatiles

Q5. Which vial septum material is commonly recommended to minimize bleed and contamination in automated HS‑GC?

• Butyl rubber septa with a Teflon (PTFE) liner
• PVC septa with silicone coating
• Plain natural rubber without liner
• Aluminum foil lids without septa

Correct Answer: Butyl rubber septa with a Teflon (PTFE) liner

Q6. For a volatile analyte, increasing incubation temperature usually results in which effect on headspace concentration and partition coefficient?

• Headspace concentration decreases and partition coefficient increases
• Headspace concentration increases and partition coefficient decreases
• Both headspace concentration and partition coefficient increase
• Neither headspace concentration nor partition coefficient change

Correct Answer: Headspace concentration increases and partition coefficient decreases

Q7. Which calibration approach is most appropriate when matrix effects significantly alter headspace partitioning?

• External calibration in solvent only
• Matrix‑matched calibration or standard addition
• Single point calibration using pure gas standards
• Calibration using detector response factor for a different matrix

Correct Answer: Matrix‑matched calibration or standard addition

Q8. What is the main cause of carryover in headspace autosamplers?

• Insufficient detector inlet temperature
• Adsorption of analytes on syringe needle, transfer lines, or septa leading to release in subsequent runs
• Using too small a sample vial
• Excessive salting of samples

Correct Answer: Adsorption of analytes on syringe needle, transfer lines, or septa leading to release in subsequent runs

Q9. Which parameter is critical to determine experimentally when developing a static HS‑GC method for a new analyte?

• Equilibrium time at chosen incubation temperature
• Detector type (only FID can be used with headspace)
• Minimum sample vial color
• Manufacturer of GC column

Correct Answer: Equilibrium time at chosen incubation temperature

Q10. How does agitation during incubation affect headspace sampling?

• Agitation prevents equilibrium and should be avoided
• Agitation speeds equilibration and can improve precision and sensitivity for many matrices
• Agitation reduces analyte volatility by cooling the sample
• Agitation has no effect on partitioning between phases

Correct Answer: Agitation speeds equilibration and can improve precision and sensitivity for many matrices

Q11. Which detection issue is most likely if the headspace sample contains non‑volatile particulates accidentally injected into the GC?

• Column bleed will be eliminated
• System backpressure increase and potential column contamination/damage
• Improved resolution due to matrix filtering
• Decreased baseline noise

Correct Answer: System backpressure increase and potential column contamination/damage

Q12. For USP <467> residual solvent analysis in tablets by HS‑GC, what is a common practice to improve recovery of low volatility residual solvents?

• Decrease vial temperature to prevent losses
• Add an organic solvent to dilute the sample matrix
• Use an internal standard in the headspace vial and consider heating or adding water to aid release
• Freeze the sample before analysis

Correct Answer: Use an internal standard in the headspace vial and consider heating or adding water to aid release

Q13. Which is a limitation of static headspace compared with SPME (solid‑phase microextraction)?

• Static headspace cannot be automated
• Static headspace generally offers lower sensitivity for trace analytes than SPME or dynamic trapping
• Static headspace always requires derivatization
• Static headspace is incompatible with polar analytes

Correct Answer: Static headspace generally offers lower sensitivity for trace analytes than SPME or dynamic trapping

Q14. Which factor least affects headspace equilibrium for a semi‑volatile analyte?

• Incubation temperature
• Sample matrix composition
• Barometric pressure during sampling
• Color of the vial cap

Correct Answer: Color of the vial cap

Q15. When using HS autosamplers with syringe transfer, what practice reduces sample loss and variance?

• Using a very small needle bore to avoid contamination
• Pre‑conditioning the syringe and performing multiple pre‑draw/discard cycles to eliminate dead volume effects
• Skipping syringe cleaning between runs to maintain temperature
• Always drawing the largest possible aliquot regardless of instrument limits

Correct Answer: Pre‑conditioning the syringe and performing multiple pre‑draw/discard cycles to eliminate dead volume effects

Q16. Which statement about salting‑out in HS‑GC is correct?

• Adding salt always reduces headspace analyte concentration
• Salting‑out increases aqueous analyte activity and shifts partitioning toward the gas phase for many polar analytes
• Salting‑out is only useful for non‑polar analytes
• Salting‑out chemically reacts with analytes to form volatile complexes

Correct Answer: Salting‑out increases aqueous analyte activity and shifts partitioning toward the gas phase for many polar analytes

Q17. Which performance characteristic is most challenging to validate for quantitative static headspace GC?

• Linearity over multiple orders of magnitude for gases used to flush the system
• Matrix independence of the response for analytes with high partition coefficients
• Detector selectivity for FID versus MS
• Retention time reproducibility on isothermal runs

Correct Answer: Matrix independence of the response for analytes with high partition coefficients

Q18. In headspace sampling, leakage of the vial seal during incubation will most likely cause:

• Increased headspace concentration due to air ingress
• Loss of volatile analytes resulting in low response and poor reproducibility
• No change because partitioning compensates
• Improved equilibration speed and higher sensitivity

Correct Answer: Loss of volatile analytes resulting in low response and poor reproducibility

Q19. When comparing purge‑and‑trap to headspace for trace environmental volatiles, the main reason to choose purge‑and‑trap is:

• Purge‑and‑trap is simpler and requires no sample preparation
• Purge‑and‑trap provides superior enrichment for very low concentration analytes
• Purge‑and‑trap eliminates the need for a GC column
• Purge‑and‑trap always gives better compound identification than MS

Correct Answer: Purge‑and‑trap provides superior enrichment for very low concentration analytes

Q20. For method troubleshooting: if two headspace injections of the same vial give systematically different peak areas, the most likely cause is:

• Insufficient chromatographic resolution
• Non‑equilibrium during sampling or progressive depletion of analyte due to repeated sampling
• Detector saturation unrelated to sampling
• Wrong carrier gas type used in the GC

Correct Answer: Non‑equilibrium during sampling or progressive depletion of analyte due to repeated sampling

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