MS MS in Chemistry 2 — Questions and Answers
Question 1: In gas chromatography (GC), the stationary phase separates compounds primarily based on:
- Molecular weight
- Polarity and volatility interactions with the column (Correct answer)
- Ionic charge
- Refractive index
Correct answer: Polarity and volatility interactions with the column
GC separation depends on differential partitioning of analytes between the mobile gas phase and the stationary liquid or solid phase, governed by volatility and polarity interactions.
Question 2: The Henderson-Hasselbalch equation, pH = pKa + log([A−]/[HA]), is most useful for:
- Strong acid titrations near the equivalence point
- Calculating buffer pH when the ratio of conjugate base to weak acid is known (Correct answer)
- Predicting solubility of sparingly soluble salts
- Determining electrode potentials in electrochemical cells
Correct answer: Calculating buffer pH when the ratio of conjugate base to weak acid is known
The Henderson-Hasselbalch equation directly calculates buffer pH from the pKa and the ratio of deprotonated to protonated species, provided neither form is negligible.
Question 3: Which electrochemical technique measures the current as a function of applied potential while analyte concentration is varied to generate calibration data?
- Potentiometry
- Coulometry
- Voltammetry (Correct answer)
- Conductimetry
Correct answer: Voltammetry
Voltammetry (e.g., cyclic or differential pulse voltammetry) sweeps the applied potential and records resulting current, allowing quantitative determination of analyte concentrations.
Question 4: In HPLC with a reverse-phase column, which solvent system would elute a polar compound first?
- 100% acetonitrile
- 90% water / 10% acetonitrile (Correct answer)
- 50% water / 50% methanol
- 100% methanol
Correct answer: 90% water / 10% acetonitrile
In reverse-phase HPLC, polar compounds have less affinity for the nonpolar C18 stationary phase and elute earlier in high-aqueous (polar) mobile phases.
Question 5: Beer-Lambert Law (A = εbc) relates absorbance to concentration. If you double the path length (b) while halving the concentration (c), absorbance will:
- Double
- Halve
- Remain the same (Correct answer)
- Quadruple
Correct answer: Remain the same
Since A = ε·b·c and both b and c change by reciprocal factors (×2 and ×0.5), their product b·c is unchanged, so absorbance remains constant.
Question 6: In inductively coupled plasma–mass spectrometry (ICP-MS), the plasma temperature (~6,000–10,000 K) primarily serves to:
- Ionize analyte atoms for mass analysis (Correct answer)
- Separate isotopes based on charge
- Vaporize the solvent only
- Reduce polyatomic interferences
Correct answer: Ionize analyte atoms for mass analysis
The extremely high plasma temperature atomizes and ionizes nearly all elements with >99% efficiency, producing M⁺ ions that are then separated and detected by the mass spectrometer.
Question 7: Which statistical parameter describes the precision of an analytical method by expressing the standard deviation as a percentage of the mean?
- Limit of detection (LOD)
- Coefficient of variation (CV%) (Correct answer)
- Bias
- Recovery percentage
Correct answer: Coefficient of variation (CV%)
The coefficient of variation (CV% = SD/mean × 100) normalizes standard deviation to the mean, allowing precision comparison across different concentration ranges.
MS MS in Chemistry 2
In gas chromatography (GC), the stationary phase separates compounds primarily based on: