Instrumentation of NMR spectroscopy MCQs With Answer

Instrumentation of NMR spectroscopy MCQs With Answer

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Instrumentation of NMR spectroscopy MCQs With Answer helps M. Pharm students master the essential components and working principles of modern NMR spectrometers. This quiz focuses on magnets, shims, probes, RF electronics, digitization, gradients, lock systems, decoupling, and temperature control—areas critical for accurate, high-resolution pharmaceutical analysis. By tackling application-oriented and theory-grounded questions, you will strengthen your understanding of how field homogeneity, sampling parameters, and probe tuning directly affect sensitivity, resolution, and data quality. These MCQs also touch on operational safety, cryogenic maintenance, and advanced features like quadrature detection and MAS, ensuring you are well-prepared for research and practical lab work in Modern Pharmaceutical Analytical Techniques.

Q1. What is the primary function of the superconducting magnet in an NMR spectrometer?

  • To provide a highly homogeneous, strong static magnetic field (B0) for nuclear alignment.
  • To generate RF pulses at the Larmor frequency.
  • To perform Fourier transform of the FID to a spectrum.
  • To control the sample temperature during experiments.

Correct Answer: To provide a highly homogeneous, strong static magnetic field (B0) for nuclear alignment.

Q2. What is the role of shim coils in NMR instrumentation?

  • Compensate spatial inhomogeneities in B0 via fine current adjustments.
  • Increase RF power for 90° pulses.
  • Deliver deuterium lock irradiation.
  • Rotate the sample at the magic angle.

Correct Answer: Compensate spatial inhomogeneities in B0 via fine current adjustments.

Q3. The deuterium lock system primarily serves to:

  • Stabilizes the B0 field by tracking the 2H signal from solvent and applying feedback.
  • Provides gradient pulses for diffusion experiments.
  • Eliminates chemical exchange broadening.

Correct Answer: Stabilizes the B0 field by tracking the 2H signal from solvent and applying feedback.

Q4. Which statement best describes the NMR probe?

  • Contains RF coil(s) and tuning/matching network around the sample for transmit/receive.
  • Houses the superconducting magnet windings.
  • Supplies cryogens to maintain superconductivity.
  • Measures helium levels in the magnet.

Correct Answer: Contains RF coil(s) and tuning/matching network around the sample for transmit/receive.

Q5. The flip angle produced by an RF pulse is determined by which relationship?

  • Flip angle θ = γ B1 tp (in radians) set by RF amplitude and pulse duration.
  • Flip angle depends only on B0 strength.
  • Flip angle equals 90° for any short pulse.
  • Flip angle is independent of the gyromagnetic ratio.

Correct Answer: Flip angle θ = γ B1 tp (in radians) set by RF amplitude and pulse duration.

Q6. In digital acquisition, spectral width (SW) is related to dwell time (dw) by:

  • Spectral width (Hz) = 1 / dwell time (s).
  • Spectral width (Hz) = dwell time (s).
  • Spectral width depends only on B0 and not on sampling.
  • Spectral width equals ADC bit depth.

Correct Answer: Spectral width (Hz) = 1 / dwell time (s).

Q7. The receiver preamplifier in an NMR spectrometer is designed to:

  • Low-noise amplification of the NMR signal prior to digitization to maximize SNR.
  • Deliver shimming currents to the magnet.
  • Produce continuous-wave decoupling fields.
  • Control sample spinning speed.

Correct Answer: Low-noise amplification of the NMR signal prior to digitization to maximize SNR.

Q8. Why is a Fourier transform applied to the FID in FT-NMR?

  • Converts the time-domain FID into a frequency-domain spectrum.
  • Increases B0 homogeneity.
  • Generates RF pulses with defined flip angles.
  • Warms the sample to reduce viscosity.

Correct Answer: Converts the time-domain FID into a frequency-domain spectrum.

Q9. What is the purpose of gradient coils in modern NMR systems?

  • Create controlled spatial variations of B0 used for imaging, shimming, and pulsed-field gradients.
  • Maintain the sample at constant temperature.
  • Provide proton decoupling in 13C experiments.
  • Detect the FID with higher sensitivity.

Correct Answer: Create controlled spatial variations of B0 used for imaging, shimming, and pulsed-field gradients.

Q10. In high-resolution solution NMR, slow sample spinning primarily:

  • Slowly averages residual field inhomogeneities and susceptibility variations to improve line shape.
  • Achieves magic-angle averaging of anisotropic interactions.
  • Increases Larmor frequency to expand spectral dispersion.
  • Prevents RF coil arcing at high power.

Correct Answer: Slowly averages residual field inhomogeneities and susceptibility variations to improve line shape.

Q11. In a 13C{1H} experiment, the decoupler channel is used to:

  • Irradiates 1H to remove 1H–13C scalar couplings and enhance 13C sensitivity via NOE.
  • Irradiates 13C to remove 13C–13C couplings only.
  • Applies gradients to suppress convection.
  • Tunes the magnet to a new field strength.

Correct Answer: Irradiates 1H to remove 1H–13C scalar couplings and enhance 13C sensitivity via NOE.

Q12. Automatic tuning/matching (ATM) systems are implemented to:

  • Minimizes reflected RF power by adjusting probe tuning and impedance matching to the sample.
  • Locks the spectrometer to an external frequency standard.
  • Cools the probe to reduce thermal noise.
  • Increases the ADC dynamic range.

Correct Answer: Minimizes reflected RF power by adjusting probe tuning and impedance matching to the sample.

Q13. What is the role of cryogens (liquid He/N2) in a superconducting NMR magnet?

  • Keep the magnet coils at superconducting temperature to sustain persistent current and stable B0.
  • Cool the probe for cryoprobe operation only.
  • Provide pressure for pneumatic sample eject.
  • Cool the ADC to reduce Johnson noise.

Correct Answer: Keep the magnet coils at superconducting temperature to sustain persistent current and stable B0.

Q14. Which statement about a magnet quench is correct?

  • A quench causes rapid helium boil-off and field collapse, creating asphyxiation risk if venting fails.
  • A quench improves field homogeneity by removing trapped flux.
  • A quench is a routine step before each experiment.
  • A quench only affects RF electronics, not the magnet.

Correct Answer: A quench causes rapid helium boil-off and field collapse, creating asphyxiation risk if venting fails.

Q15. Digital resolution in the frequency domain is given by:

  • Digital resolution (Hz/point) = Spectral width (Hz) / number of points in the processed spectrum.
  • Digital resolution equals the ADC bit depth.
  • Digital resolution is independent of acquisition time and points.
  • Digital resolution (Hz/point) = 1 / repetition time (TR).

Correct Answer: Digital resolution (Hz/point) = Spectral width (Hz) / number of points in the processed spectrum.

Q16. The transmitter offset (carrier) in NMR is used to:

  • Sets the center frequency of the observed spectral window for the nucleus of interest.
  • Determines the gradient strength during diffusion experiments.
  • Controls the temperature of the sample.
  • Sets the bandwidth of the ADC anti-aliasing filter only.

Correct Answer: Sets the center frequency of the observed spectral window for the nucleus of interest.

Q17. Why is quadrature detection used in FT-NMR receivers?

  • Simultaneous acquisition of sine and cosine components enables phase-sensitive spectra without mirror images and improves SNR by √2.
  • It doubles the B0 field strength.
  • It eliminates the need for shimming.
  • It removes all noise from the FID.

Correct Answer: Simultaneous acquisition of sine and cosine components enables phase-sensitive spectra without mirror images and improves SNR by √2.

Q18. In solid-state NMR, magic-angle spinning (MAS) sets the rotor axis at approximately:

  • Approximately 54.74° relative to B0 to average anisotropic interactions in solids.
  • 0° relative to B0 to maximize alignment.
  • 90° relative to B0 to minimize dipolar couplings.
  • 35.26° relative to B0 to match quadrupolar splitting.

Correct Answer: Approximately 54.74° relative to B0 to average anisotropic interactions in solids.

Q19. The variable temperature (VT) unit in an NMR spectrometer primarily:

  • Regulates sample temperature via controlled gas flow through the probe for precise thermal control.
  • Measures T1 and T2 directly from FID.
  • Sets the deuterium lock frequency.
  • Provides magnetic shielding against external fields.

Correct Answer: Regulates sample temperature via controlled gas flow through the probe for precise thermal control.

Q20. What does “actively shielded” mean for a superconducting NMR magnet?

  • Uses additional superconducting windings to confine the fringe field and reduce the 5-gauss line footprint.
  • Contains a mu-metal room to remove external fields.
  • Does not require cryogens due to permanent magnets.
  • Eliminates the need for shimming in high fields.

Correct Answer: Uses additional superconducting windings to confine the fringe field and reduce the 5-gauss line footprint.

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