GC detectors and quantification MCQs With Answer

Introduction: This quiz set on GC detectors and quantification is tailored for M.Pharm students preparing for Advanced Instrumental Analysis (MPA 201T). It emphasizes detector principles, strengths and limitations, and practical considerations for accurate quantification. Questions cover common gas chromatography detectors — FID, TCD, ECD, NPD, PID, MS — and delve into sensitivity, selectivity, linear dynamic range, noise sources, and calibration strategies such as external calibration, internal standardization, and standard addition. Students will also be tested on response factors, chromatographic peak integration, and method validation concepts required for reliable assay and impurity determination in pharmaceutical analysis.

Q1. Which GC detector is based on ionization of organic compounds in a hydrogen-air flame and produces a signal proportional to the number of carbon atoms that form ions?

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

Correct Answer: Flame Ionization Detector (FID)

Q2. Which detector is most suitable for detecting trace halogenated pesticides at very low concentrations due to its high selectivity for electronegative species?

• Flame Ionization Detector (FID)
• Electron Capture Detector (ECD)
• Photoionization Detector (PID)
• Thermal Conductivity Detector (TCD)

Correct Answer: Electron Capture Detector (ECD)

Q3. Which detector type measures changes in thermal conductivity of the carrier gas caused by eluting analytes and is considered universal but relatively low in sensitivity?

• Mass Spectrometer (MS)
• Flame Photometric Detector (FPD)
• Thermal Conductivity Detector (TCD)
• Electron Capture Detector (ECD)

Correct Answer: Thermal Conductivity Detector (TCD)

Q4. For quantitative GC using an internal standard, which statement about the internal standard is correct?

• The internal standard must be chemically identical to the analyte
• The internal standard should co-elute with the analyte
• The internal standard should not be present in the sample and should have similar response behavior
• The internal standard must be detected by a different detector

Correct Answer: The internal standard should not be present in the sample and should have similar response behavior

Q5. Which detector provides structural information and is commonly used for confirmation after GC separation, but requires vacuum and more complex instrumentation?

• Flame Ionization Detector (FID)
• Mass Spectrometer (MS)
• Photoionization Detector (PID)
• Nitrogen-Phosphorus Detector (NPD)

Correct Answer: Mass Spectrometer (MS)

Q6. The linear dynamic range of a detector refers to:

• The range of temperatures over which the detector operates
• The concentration range over which detector response is directly proportional to analyte amount
• The retention time window for which the detector produces peaks
• The range of carrier gas flow rates compatible with the detector

Correct Answer: The concentration range over which detector response is directly proportional to analyte amount

Q7. Which detector is highly selective for nitrogen- and phosphorus-containing compounds and often used in pesticide and drug residue analysis?

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

Correct Answer: Nitrogen-Phosphorus Detector (NPD)

Q8. In GC quantification using external calibration, which practice is essential to obtain accurate results?

• Use an internal standard only in standards, not samples
• Match the matrix of standards to the sample matrix when possible
• Dilute samples to arbitrary concentration ranges after measurement
• Ignore detector linearity because software corrects automatically

Correct Answer: Match the matrix of standards to the sample matrix when possible

Q9. Which factor most directly affects the limit of detection (LOD) for a GC detector?

• Column length only
• Signal-to-noise ratio of the detector response
• Injector temperature alone
• Carrier gas type only

Correct Answer: Signal-to-noise ratio of the detector response

Q10. For peak area quantification, why is area preferred over height when peaks are asymmetric or partially resolved?

• Area is independent of peak width and better represents total analyte mass
• Height is easier to integrate with noisy baselines
• Area requires less accurate integration settings
• Height increases linearly with retention time

Correct Answer: Area is independent of peak width and better represents total analyte mass

Q11. Which detector is most appropriate for detecting aromatic hydrocarbons and VOCs at low ppb levels using ultraviolet photons for ionization?

• Photoionization Detector (PID)
• Electron Capture Detector (ECD)
• Thermal Conductivity Detector (TCD)
• Flame Ionization Detector (FID)

Correct Answer: Photoionization Detector (PID)

Q12. Relative Response Factor (RRF) in GC quantification using an internal standard is defined as:

• Ratio of analyte concentration to internal standard concentration in the sample
• Ratio of detector response per unit concentration of analyte to that of the internal standard
• Difference between peak areas of analyte and internal standard
• Sum of calibration curve slopes for analyte and internal standard

Correct Answer: Ratio of detector response per unit concentration of analyte to that of the internal standard

Q13. Which characteristic makes FID unsuitable for detecting inorganic gases like H2O, CO2, or permanent gases?

• FID responds primarily to ions formed from carbon-containing species
• FID requires halogenated compounds to produce a signal
• FID operates only at cryogenic temperatures
• FID is selective for nitrogen and phosphorus

Correct Answer: FID responds primarily to ions formed from carbon-containing species

Q14. Standard addition is most beneficial in GC quantification when:

• Instrument response is perfectly linear across all concentrations
• Matrix effects cause suppression or enhancement of analyte response
• Analyte is the major component of the sample matrix
• No internal standard is available and matrix matching is unnecessary

Correct Answer: Matrix effects cause suppression or enhancement of analyte response

Q15. Which detector uses a beta-emitting radioisotope source and is particularly sensitive to halogens and nitrated species?

• Electron Capture Detector (ECD)
• Mass Spectrometer (MS)
• Flame Ionization Detector (FID)
• Thermal Conductivity Detector (TCD)

Correct Answer: Electron Capture Detector (ECD)

Q16. Which statement about splitless injection for trace analysis is correct?

• Splitless mode is used to introduce only a small fraction of the injector vapor to the column
• Splitless injection maximizes sample transfer of low-concentration analytes onto the column
• Splitless injection is preferred for highly concentrated samples to prevent column overload
• Splitless mode eliminates the need for an inlet liner

Correct Answer: Splitless injection maximizes sample transfer of low-concentration analytes onto the column

Q17. Which error source predominantly affects quantitative GC results when integration baseline placement is inconsistent?

• Systematic error in retention time measurement
• Random detector electronic noise only
• Bias in calculated peak areas leading to inaccurate concentrations
• Change in carrier gas purity

Correct Answer: Bias in calculated peak areas leading to inaccurate concentrations

Q18. In creating calibration curves, the best practice to verify linearity of the detector response is to:

• Use only two calibration points at extremes
• Plot response vs concentration across the working range and evaluate residuals and R²
• Assume linearity because most detectors are linear by design
• Fit a polynomial curve without assessing residuals

Correct Answer: Plot response vs concentration across the working range and evaluate residuals and R²

Q19. Which carrier gas is commonly preferred for mass spectrometer coupling due to good vacuum pump compatibility and efficient ionization?

• Nitrogen
• Helium
• Hydrogen
• Argon

Correct Answer: Helium

Q20. In quantification, what is the primary advantage of using an internal standard over external calibration?

• Internal standard eliminates the need to prepare calibration standards
• Internal standard corrects for injection and instrumental variability between runs
• Internal standard doubles detector sensitivity
• Internal standard guarantees complete separation of analyte peaks

Correct Answer: Internal standard corrects for injection and instrumental variability between runs

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