ACS CP Analytical Chemistry & Instrumentation 3 — Questions and Answers
Question 1: In X-ray fluorescence (XRF) spectrometry, the energy of the emitted characteristic X-ray is determined by:
- The intensity of the X-ray source
- The difference in binding energies between the electron shells involved in the transition (Correct answer)
- The mass of the element being analyzed
- The sample preparation method used
Correct answer: The difference in binding energies between the electron shells involved in the transition
Characteristic X-ray energy equals the difference in electron binding energies between the two shells involved (e.g., Kα = EK − EL), making it unique to each element.
Question 2: In capillary electrophoresis (CE), electroosmotic flow (EOF) results from:
- The electric field acting directly on the analyte ions
- The movement of the diffuse double layer of cations near the negatively charged capillary wall (Correct answer)
- Pressure-driven flow applied at the capillary inlet
- The viscosity difference between sample and buffer zones
Correct answer: The movement of the diffuse double layer of cations near the negatively charged capillary wall
EOF arises when the applied electric field drives the excess cations in the diffuse double layer near the negatively charged silica wall, dragging the bulk solution toward the cathode.
Question 3: Which of the following describes the correct definition of the detection limit (LOD) according to IUPAC guidelines?
- The smallest concentration that can be measured with 95% confidence above a blank
- The signal equal to three times the standard deviation of the blank (3σ blank) (Correct answer)
- The concentration at which the calibration curve becomes nonlinear
- The minimum concentration quantifiable with 10% RSD
Correct answer: The signal equal to three times the standard deviation of the blank (3σ blank)
IUPAC defines the LOD as the analyte concentration corresponding to a signal equal to the blank signal plus 3 times the standard deviation of the blank (k=3, 99.86% confidence).
Question 4: In NMR spectroscopy, the chemical shift is referenced against an internal standard. What compound is most commonly used as the reference standard in ¹H NMR?
- Benzene (δ = 7.27 ppm)
- Tetramethylsilane (TMS, δ = 0.00 ppm) (Correct answer)
- Chloroform (δ = 7.27 ppm)
- Water (δ = 4.79 ppm)
Correct answer: Tetramethylsilane (TMS, δ = 0.00 ppm)
Tetramethylsilane (TMS) is the universal ¹H NMR reference standard set at 0.00 ppm because it gives a single sharp peak in a chemically clean region, is inert, and is easily removed.
Question 5: A flame photometer measures lithium content in a serum sample. Which interferent must be most carefully controlled due to its spectral overlap with the lithium emission line at 671 nm?
- Sodium (589 nm) (Correct answer)
- Potassium (766 nm)
- Calcium (622 nm)
- Cesium (852 nm)
Correct answer: Sodium (589 nm)
Sodium, though emitting primarily at 589 nm, can cause background interference; more critically, high sodium concentrations cause matrix effects that alter lithium emission intensity.
Question 6: In cyclic voltammetry (CV), the ratio of the cathodic peak current (ipc) to the anodic peak current (ipa) equal to 1.0 indicates:
- An irreversible electrochemical reaction
- A reversible electrochemical process (Correct answer)
- Chemical decomposition of the product
- Adsorption of the analyte on the electrode
Correct answer: A reversible electrochemical process
For a reversible, diffusion-controlled electrochemical reaction, the Randles-Ševčík conditions predict ipc/ipa = 1.0 regardless of scan rate.
Question 7: Which type of chromatography detector is considered universal (responds to nearly all analytes) but is destructive and incompatible with gradient elution in HPLC?
- UV-Vis diode array detector (DAD)
- Refractive index (RI) detector
- Flame ionization detector (FID) (Correct answer)
- Evaporative light scattering detector (ELSD)
Correct answer: Flame ionization detector (FID)
The FID is a near-universal detector for organic compounds in GC that destroys the sample in a hydrogen flame, making it incompatible with HPLC or gradient methods.
ACS CP Analytical Chemistry & Instrumentation 3
In X-ray fluorescence (XRF) spectrometry, the energy of the emitted characteristic X-ray is determined by: