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
