MChem Master of Chemistry Master of Chemistry: Analytical Chemistry 5 — Questions and Answers
Question 1: Which chromatographic detector responds to virtually all organic compounds but is non-selective and destructive, making it unsuitable for preparative work?
- Electron capture detector (ECD)
- Flame ionization detector (FID) (Correct answer)
- Thermal conductivity detector (TCD)
- Nitrogen-phosphorus detector (NPD)
Correct answer: Flame ionization detector (FID)
FID burns analytes in a hydrogen-air flame, producing ions detected as current; it has near-universal response to organic compounds but destroys the sample.
Question 2: In size-exclusion chromatography (SEC), larger molecules elute:
- Last, because they interact more strongly with the stationary phase
- First, because they are excluded from the pores of the stationary phase (Correct answer)
- First, because they have higher diffusion coefficients
- Last, because they experience greater hydrodynamic drag
Correct answer: First, because they are excluded from the pores of the stationary phase
Large molecules cannot enter the pores of the SEC packing, so they traverse only the interstitial volume and elute earliest.
Question 3: The Debye-Hückel limiting law describes how ionic strength affects:
- The viscosity of electrolyte solutions
- The mean ionic activity coefficient in dilute electrolyte solutions (Correct answer)
- The conductance of strong electrolytes at high concentration
- The diffusion potential at a liquid junction
Correct answer: The mean ionic activity coefficient in dilute electrolyte solutions
The Debye-Hückel limiting law predicts that log γ± = -A|z+z-|√I for very dilute solutions, showing how ionic atmosphere reduces activity coefficients.
Question 4: In tandem mass spectrometry (MS/MS), what occurs during the 'collision-induced dissociation' (CID) step?
- The precursor ion is re-ionized by a secondary electron beam
- The selected precursor ion fragments after collision with neutral gas molecules (Correct answer)
- The detector amplifies the signal from low-abundance ions
- The quadrupole scans all m/z values simultaneously
Correct answer: The selected precursor ion fragments after collision with neutral gas molecules
CID activates a mass-selected precursor ion by collisions with an inert gas (e.g., argon), converting kinetic energy into internal energy that drives fragmentation.
Question 5: Which electroanalytical technique applies a linearly varying potential and measures the resulting current to determine analyte concentration from peak current?
- Chronoamperometry
- Linear sweep voltammetry (LSV) (Correct answer)
- Impedance spectroscopy
- Coulometry at controlled potential
Correct answer: Linear sweep voltammetry (LSV)
LSV scans potential linearly with time and records faradaic current peaks whose height is proportional to analyte concentration via the Randles-Ševčík equation.
Question 6: Which method of background correction in atomic absorption spectroscopy uses a pulsed deuterium (D2) lamp to measure broadband background absorption?
- Zeeman background correction
- Deuterium lamp background correction (Correct answer)
- Smith-Hieftje background correction
- Continuum source background correction
Correct answer: Deuterium lamp background correction
Deuterium lamp correction alternates between the HCL (analyte + background) and a D2 lamp (broadband background only), subtracting the latter to yield net analyte absorbance.
Question 7: In quantitative NMR (qNMR), which feature of NMR spectra makes it inherently suitable for absolute quantification without analyte-specific calibration standards?
- Chemical shifts are matrix-independent
- Signal integrals are directly proportional to the number of contributing nuclei (Correct answer)
- Relaxation times are identical for all compounds
- Coupling constants are invariant with concentration
Correct answer: Signal integrals are directly proportional to the number of contributing nuclei
In qNMR, signal area is directly proportional to the molar amount of nuclei resonating at that frequency, enabling absolute quantification using a single internal reference standard.
MChem Master of Chemistry Master of Chemistry: Analytical Chemistry 5
Which chromatographic detector responds to virtually all organic compounds but is non-selective and destructive, making it unsuitable for preparative work?