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).
ACS CP ACS CP Spectroscopy & Chemical Analysis 1
In nuclear magnetic resonance (NMR) spectroscopy, chemical shift (δ) is reported in ppm relative to which standard reference compound for 1H NMR?