ACS CP ACS CP Spectroscopy & Chemical Analysis 1 — Questions and Answers
Question 1: In nuclear magnetic resonance (NMR) spectroscopy, chemical shift (δ) is reported in ppm relative to which standard reference compound for 1H NMR?
- Benzene (C6H6) at δ 7.27 ppm
- Tetramethylsilane (TMS) at δ 0.00 ppm (Correct answer)
- Chloroform (CDCl3) at δ 7.26 ppm
- Water at δ 4.79 ppm
Correct answer: Tetramethylsilane (TMS) at δ 0.00 ppm
Tetramethylsilane (TMS) is the universal reference standard for NMR spectroscopy, assigned a chemical shift of 0.00 ppm because its 12 equivalent protons resonate at a unique, upfield position.
Question 2: Beer-Lambert Law (A = εlc) states that absorbance is directly proportional to concentration only when:
- The sample is measured in the UV range only
- The solution is dilute and the radiation is monochromatic (Correct answer)
- The cuvette path length is exactly 1.00 cm
- The detector operates in photon-counting mode
Correct answer: The solution is dilute and the radiation is monochromatic
Beer-Lambert Law holds for dilute solutions (no solute-solute interactions) and monochromatic light; concentrated solutions, stray light, or polychromatic radiation cause deviations from linearity.
Question 3: In mass spectrometry, the base peak is defined as:
- The molecular ion peak (M+) representing intact molecule
- The most abundant (tallest) ion peak in the mass spectrum, assigned 100% relative abundance (Correct answer)
- The heaviest fragment ion detected in the spectrum
- The peak produced by the isotope with highest natural abundance
Correct answer: The most abundant (tallest) ion peak in the mass spectrum, assigned 100% relative abundance
The base peak is the most abundant ion in the mass spectrum, assigned 100% relative intensity; all other peaks are reported as a percentage of the base peak intensity.
Question 4: Fourier transform infrared spectroscopy (FTIR) measures molecular vibrations. Which bond vibration type appears at approximately 1715 cm⁻¹ in the IR spectrum?
- O–H stretch of alcohols
- N–H bend of primary amines
- C=O stretch of ketones/aldehydes (Correct answer)
- C≡N stretch of nitriles
Correct answer: C=O stretch of ketones/aldehydes
The carbonyl C=O stretching vibration of ketones and aldehydes appears as a strong, sharp absorption around 1715 cm⁻¹ (ketones) to 1725 cm⁻¹ (aldehydes) in the IR spectrum.
Question 5: High-performance liquid chromatography (HPLC) in reverse-phase mode uses which type of stationary and mobile phase?
- Polar stationary phase (silica) with nonpolar mobile phase
- Nonpolar stationary phase (C18) with polar mobile phase (water/acetonitrile) (Correct answer)
- Ion exchange resin with buffered aqueous mobile phase
- Chiral stationary phase with isopropanol modifier
Correct answer: Nonpolar stationary phase (C18) with polar mobile phase (water/acetonitrile)
Reverse-phase HPLC uses a nonpolar C18 (octadecylsilane) bonded silica stationary phase with a polar aqueous/organic mobile phase, retaining nonpolar compounds longer.
Question 6: The precision of an analytical method is best described by:
- How close a measured value is to the true value
- The closeness of agreement among repeated measurements under the same conditions (Correct answer)
- The lowest concentration that can be detected above background noise (LOD)
- The linear dynamic range over which calibration applies
Correct answer: The closeness of agreement among repeated measurements under the same conditions
Precision measures the reproducibility or repeatability of measurements (agreement among replicates), distinct from accuracy (agreement with the true value).
