Principles of FT-NMR MCQs With Answer

Principles of FT-NMR MCQs With Answer

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Principles of FT-NMR MCQs With Answer

Fourier Transform Nuclear Magnetic Resonance (FT-NMR) is central to structural elucidation and quantitative analysis in modern pharmaceutical sciences. For M. Pharm students, a solid grasp of FT-NMR principles—pulsed excitation, time-domain signal (FID) acquisition, Fourier transformation, digital parameters, and spectral processing—is essential for reliable interpretation. This MCQ set emphasizes conceptual and practical aspects: spectral width, dwell time, Nyquist sampling, digital resolution, apodization, zero-filling, signal averaging, phase and baseline correction, shimming, locking, decoupling, and relaxation-informed acquisition strategies. Each question is designed to test both theoretical understanding and instrument-aware decision-making to help you connect pulse sequence physics with real-world data quality and quantitative integrity.

Q1. What is the primary advantage of FT-NMR over continuous-wave (CW) NMR?

  • Higher magnetic field homogeneity without shimming
  • Simultaneous detection of all frequencies (multiplex/Fellgett advantage)
  • No need for deuterated solvents
  • Complete elimination of magnetic field drift

Correct Answer: Simultaneous detection of all frequencies (multiplex/Fellgett advantage)

Q2. What is the role of the Fourier Transform in FT-NMR?

  • To generate the RF pulse that excites nuclear spins
  • To convert the time-domain FID into a frequency-domain spectrum
  • To shim the magnet in real time
  • To remove phase errors introduced by the receiver

Correct Answer: To convert the time-domain FID into a frequency-domain spectrum

Q3. In FT-NMR, what is the Free Induction Decay (FID)?

  • A steady-state signal obtained under continuous RF irradiation
  • A decaying time-domain signal induced in the receiver coil after an RF pulse
  • An artifact arising from imperfect baseline correction
  • A computational filter applied before Fourier transformation

Correct Answer: A decaying time-domain signal induced in the receiver coil after an RF pulse

Q4. Which parameter directly determines the spectral width (SW) in FT-NMR?

  • Magnetic field strength alone
  • 90° pulse length
  • Sampling rate (dwell time), via SW = 1/dwell time
  • Shim current values

Correct Answer: Sampling rate (dwell time), via SW = 1/dwell time

Q5. According to the Nyquist criterion, how should sampling be set to avoid aliasing in FT-NMR?

  • The sampling rate must be lower than the lowest frequency present
  • The sampling rate must match the Larmor frequency exactly
  • The sampling rate must be at least twice the highest frequency component present
  • The spectral width must be half the transmitter offset

Correct Answer: The sampling rate must be at least twice the highest frequency component present

Q6. Digital resolution (in Hz/point) in a processed FT-NMR spectrum is best described as:

  • Number of points divided by spectral width
  • Spectral width divided by the number of points in the spectrum
  • Linewidth divided by T1
  • Receiver gain divided by ADC bit depth

Correct Answer: Spectral width divided by the number of points in the spectrum

Q7. What is the principal effect of zero-filling in FT-NMR data processing?

  • True narrowing of peak linewidths (higher intrinsic resolution)
  • Increase in S/N by a factor proportional to the zero-fill factor
  • Improved digital resolution (denser frequency grid) without improving true spectral resolution
  • Suppression of J-couplings

Correct Answer: Improved digital resolution (denser frequency grid) without improving true spectral resolution

Q8. Applying an exponential apodization (line broadening) to the FID generally:

  • Decreases S/N while narrowing peaks
  • Improves S/N at the expense of increased linewidths
  • Eliminates baseline distortions completely
  • Converts dispersive signals directly to absorptive lines

Correct Answer: Improves S/N at the expense of increased linewidths

Q9. When averaging N transients (scans) in FT-NMR, the signal-to-noise ratio (S/N) improves by:

  • N
  • √N
  • N²
  • 1/N

Correct Answer: √N

Q10. What is quadrature detection in FT-NMR?

  • Detection using two magnets for better homogeneity
  • Acquisition of orthogonal sine and cosine components (complex FID) for phase-sensitive spectra
  • Using four phase cycles to cancel all artifacts
  • Simultaneous 1H and 13C detection in one experiment

Correct Answer: Acquisition of orthogonal sine and cosine components (complex FID) for phase-sensitive spectra

Q11. What is the primary purpose of the lock system in FT-NMR?

  • To tune the probe to the correct nucleus
  • To compensate for magnetic field drift using a deuterium signal from the solvent
  • To adjust receiver gain automatically
  • To set the 90° pulse length precisely

Correct Answer: To compensate for magnetic field drift using a deuterium signal from the solvent

Q12. Shimming in FT-NMR is performed mainly to:

  • Increase the spectral width
  • Improve magnetic field homogeneity and narrow peak linewidths
  • Increase the flip angle
  • Reduce T1 relaxation times

Correct Answer: Improve magnetic field homogeneity and narrow peak linewidths

Q13. The flip angle (θ) produced by an RF pulse in FT-NMR is given by which relationship?

  • θ = γB0tp
  • θ = γB1tp
  • θ = 1/(γB1tp)
  • θ = π·(B0/B1)

Correct Answer: θ = γB1tp

Q14. The Ernst angle is the flip angle that maximizes steady-state signal for a given TR and T1. Which expression defines it?

  • α = arctan(T2/T1)
  • α = arccos(e−TR/T1)
  • α = π/2 for all TR
  • α = arcsin(e−TR/T2)

Correct Answer: α = arccos(e−TR/T1)

Q15. Increasing the ADC bit depth in FT-NMR primarily improves:

  • Chemical shift dispersion in ppm
  • Dynamic range and reduces quantization noise
  • Homogeneity of the static magnetic field
  • Intrinsic relaxation times of nuclei

Correct Answer: Dynamic range and reduces quantization noise

Q16. What is a likely consequence of setting the receiver gain too high during acquisition?

  • Under-sampling of the FID
  • FID clipping and spectral distortions (spurious lines, distorted baseline)
  • Excessively narrow peak shapes
  • Loss of lock signal

Correct Answer: FID clipping and spectral distortions (spurious lines, distorted baseline)

Q17. What is the main purpose of phase correction in FT-NMR spectra?

  • To convert dispersive components into pure absorption-mode peaks across the spectrum
  • To eliminate all noise from the spectrum
  • To remove J-coupling splittings
  • To adjust chemical shift referencing to TMS

Correct Answer: To convert dispersive components into pure absorption-mode peaks across the spectrum

Q18. In broadband 1H decoupled 13C FT-NMR, the principal effect of decoupling is to:

  • Suppress NOE enhancements
  • Remove 13C–13C couplings only
  • Collapse 13C–1H multiplets to singlets and enhance signal via NOE
  • Reduce T2* to sharpen lines

Correct Answer: Collapse 13C–1H multiplets to singlets and enhance signal via NOE

Q19. A 90°–τ–180°–τ spin-echo sequence in FT-NMR is primarily used to:

  • Measure T1 by inversion recovery
  • Refocus dephasing from static field inhomogeneity to assess T2
  • Increase chemical shift dispersion
  • Perform proton decoupling

Correct Answer: Refocus dephasing from static field inhomogeneity to assess T2

Q20. For quantitative 1H FT-NMR, the repetition time (TR) relative to T1 should typically be:

  • TR ≈ 1×T1 to maximize throughput
  • TR ≈ 2×T1 for accurate integrals
  • TR ≥ 5×T1 to ensure full relaxation and accurate integrals
  • TR unrelated to T1 if using apodization

Correct Answer: TR ≥ 5×T1 to ensure full relaxation and accurate integrals

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