ACS CP Analytical Chemistry & Instrumentation 2 — Questions and Answers
Question 1: In atomic absorption spectroscopy (AAS), what is the primary purpose of the hollow cathode lamp?
- To ionize the sample atoms
- To emit a narrow line spectrum characteristic of the analyte element (Correct answer)
- To provide a continuous background radiation source
- To atomize the sample in the flame
Correct answer: To emit a narrow line spectrum characteristic of the analyte element
The hollow cathode lamp emits a narrow-line spectrum specific to the cathode element, providing radiation at exactly the wavelength absorbed by the analyte atoms.
Question 2: Which ionization technique in mass spectrometry is considered a 'soft' ionization method that preserves the molecular ion?
- Electron ionization (EI)
- Chemical ionization (CI)
- Electrospray ionization (ESI) (Correct answer)
- Fast atom bombardment (FAB)
Correct answer: Electrospray ionization (ESI)
Electrospray ionization (ESI) is a soft ionization technique that produces multiply charged ions with minimal fragmentation, preserving the intact molecular ion.
Question 3: In gas chromatography, the van Deemter equation describes the relationship between plate height (H) and mobile phase velocity (u). Which term accounts for longitudinal diffusion?
- A term (Eddy diffusion)
- B/u term (Correct answer)
- Cu term
- D term
Correct answer: B/u term
The B/u term in the van Deemter equation represents longitudinal (axial) diffusion of solute along the column, which decreases as mobile phase velocity increases.
Question 4: A potentiometric sensor uses a glass electrode to measure pH. The Nernst equation predicts a slope of approximately how many mV per pH unit at 25°C?
- 25.7 mV/pH unit
- 59.2 mV/pH unit (Correct answer)
- 96.5 mV/pH unit
- 118.4 mV/pH unit
Correct answer: 59.2 mV/pH unit
At 25°C, the Nernst slope for a monovalent ion (H⁺, n=1) is RT/F × ln(10) ≈ 59.16 mV per decade of activity change.
Question 5: In HPLC, what does a high tailing factor (T > 1.5) for a chromatographic peak typically indicate?
- Column overloading with too much sample
- Strong secondary interactions between analyte and stationary phase (Correct answer)
- Insufficient mobile phase flow rate
- Co-elution of two closely related compounds
Correct answer: Strong secondary interactions between analyte and stationary phase
Peak tailing (T > 1.5) is usually caused by secondary interactions such as ionic or adsorptive interactions between the analyte and active sites on the stationary phase.
Question 6: Which spectroscopic method relies on the Raman scattering effect to provide complementary vibrational information to IR spectroscopy?
- UV-Vis absorption spectroscopy
- Nuclear magnetic resonance spectroscopy
- Raman spectroscopy (Correct answer)
- X-ray fluorescence spectroscopy
Correct answer: Raman spectroscopy
Raman spectroscopy detects inelastic scattering of light where the shift in photon frequency corresponds to vibrational modes of the molecule, and it is governed by polarizability changes rather than dipole moment changes.
Question 7: In inductively coupled plasma mass spectrometry (ICP-MS), the primary function of the collision/reaction cell is to:
- Increase the ionization efficiency of the plasma
- Reduce polyatomic interferences by reacting them with a cell gas (Correct answer)
- Separate ions based on their mass-to-charge ratio
- Enhance sensitivity for high-mass analytes
Correct answer: Reduce polyatomic interferences by reacting them with a cell gas
The collision/reaction cell uses gases like He or H₂ to collisionally dissociate or chemically react with polyatomic ions (e.g., ArCl⁺, ArO⁺) that interfere with analyte masses.
ACS CP Analytical Chemistry & Instrumentation 2
In atomic absorption spectroscopy (AAS), what is the primary purpose of the hollow cathode lamp?