Principles of FT-NMR MCQs With Answer

Introduction:

Principles of FT-NMR MCQs With Answer is a concise question set designed for M.Pharm students to strengthen their understanding of Fourier Transform Nuclear Magnetic Resonance (FT‑NMR). This collection focuses on core principles—pulse excitation, free induction decay (FID), Fourier transformation, relaxation times (T1 and T2), spectral resolution, sensitivity enhancement, and common pulse techniques used in pharmaceutical analysis. Each multiple-choice question challenges conceptual and practical aspects encountered in advanced instrumental analysis, reinforcing theory and its application to drug characterization, structure elucidation, and quantitative NMR. Answers are provided to aid rapid self-assessment and targeted study for exams and laboratory practice.

Q1. Which phenomenon directly produces the time-domain signal called the free induction decay (FID) in an FT‑NMR experiment?

• Continuous-wave absorption of RF energy by nuclei
• Precessing transverse magnetization induced after a radiofrequency pulse
• Relaxation of longitudinal magnetization back to equilibrium
• Gradient echo formation due to magnetic field gradients

Correct Answer: Precessing transverse magnetization induced after a radiofrequency pulse

Q2. What is the principal mathematical operation used to convert the FID into a frequency‑domain NMR spectrum?

• Laplace transform
• Z-transform
• Fourier transform
• Wavelet transform

Correct Answer: Fourier transform

Q3. In FT‑NMR, increasing the spectral resolution (smaller Hz per point) is achieved primarily by which action during acquisition?

• Using stronger magnetic field gradients
• Extending the acquisition time (acquiring more points in time domain)
• Increasing pulse flip angle from 90° to 180°
• Reducing receiver gain

Correct Answer: Extending the acquisition time (acquiring more points in time domain)

Q4. Which sampling requirement ensures faithful digitization of the NMR FID without aliasing?

• Sampling at least twice the highest frequency component (Nyquist criterion)
• Sampling once per T1 relaxation period
• Sampling at a rate equal to the Larmor frequency
• Sampling only after applying apodization

Correct Answer: Sampling at least twice the highest frequency component (Nyquist criterion)

Q5. Apodization (window functions) applied to FID before Fourier transform is used mainly to:

• Increase chemical shift dispersion
• Smooth noise at the cost of altering relaxation times
• Improve signal-to-noise ratio and modify line shapes
• Eliminate the need for phasing the spectrum

Correct Answer: Improve signal-to-noise ratio and modify line shapes

Q6. Which relaxation time governs the recovery of longitudinal magnetization and affects repetition time selection for quantitative FT‑NMR?

• T2 (transverse relaxation time)
• T1ρ (spin‑lock relaxation time)
• T1 (longitudinal relaxation time)
• Td (diffusion relaxation time)

Correct Answer: T1 (longitudinal relaxation time)

Q7. What is the principal effect of magnetic field inhomogeneity on an NMR signal if not corrected by shimming?

• Increase in chemical shift values
• Broadening of resonance lines and reduced resolution
• Change in the sign of J‑coupling constants
• Conversion of heteronuclear peaks into homonuclear ones

Correct Answer: Broadening of resonance lines and reduced resolution

Q8. Which pulse sequence technique is commonly used in FT‑NMR to suppress solvent signals during acquisition?

• NOESY mixing
• Spin‑echo T2 filter
• Presaturation or water suppression pulses
• Inverse gated decoupling

Correct Answer: Presaturation or water suppression pulses

Q9. In heteronuclear 1H–13C HSQC experiments, what is the main advantage of using INEPT transfer?

• Direct observation of carbonyl carbons only
• Maximized sensitivity by polarization transfer from 1H to 13C
• Suppression of 1H–1H scalar coupling
• Removal of dipolar coupling effects

Correct Answer: Maximized sensitivity by polarization transfer from 1H to 13C

Q10. Which of the following describes the effect of strong decoupling during 13C acquisition?

• Splits 13C resonances into multiplets due to 1H coupling
• Collapses 13C–1H multiplets into singlets improving sensitivity
• Decreases relaxation times of 13C dramatically
• Causes chemical shift anisotropy to appear

Correct Answer: Collapses 13C–1H multiplets into singlets improving sensitivity

Q11. The Bloch equations describe which fundamental behavior in NMR?

• Quantum mechanical energy levels of nuclei
• Time evolution of nuclear magnetization under RF and relaxation
• Chemical exchange kinetics only
• Polarization transfer rates in NOESY

Correct Answer: Time evolution of nuclear magnetization under RF and relaxation

Q12. Pulsed field gradients are used in FT‑NMR primarily to:

• Increase longitudinal relaxation time T1
• Select coherence pathways and suppress artifacts such as radiation damping
• Double the chemical shift dispersion
• Convert scalar couplings into dipolar couplings

Correct Answer: Select coherence pathways and suppress artifacts such as radiation damping

Q13. Which parameter directly limits the smallest chemical shift difference that can be resolved between two peaks in Hz?

• The number of scans (NS)
• Digital resolution (Hz per point) determined by spectral width and number of points
• Pulse flip angle accuracy
• Type of probe used (room temperature vs cryoprobe)

Correct Answer: Digital resolution (Hz per point) determined by spectral width and number of points

Q14. Which statement about signal averaging in FT‑NMR is correct?

• Signal-to-noise ratio improves linearly with the number of scans
• Signal-to-noise ratio improves as the square root of the number of scans
• Signal-to-noise ratio is independent of number of scans
• Increasing scans reduces spectral linewidths directly

Correct Answer: Signal-to-noise ratio improves as the square root of the number of scans

Q15. Chemical shift referencing in proton NMR is commonly performed relative to which internal standard?

• Deuterium oxide (D2O)
• Tetramethylsilane (TMS)
• Chloroform (CHCl3)
• Formic acid

Correct Answer: Tetramethylsilane (TMS)

Q16. Which of these is a reason for using cryogenic probes (cryoprobes) in FT‑NMR?

• To increase the chemical shift range
• To reduce thermal noise and improve sensitivity
• To lengthen T2 relaxation times
• To avoid the need for shimming

Correct Answer: To reduce thermal noise and improve sensitivity

Q17. In a 2D COSY (correlation spectroscopy) experiment, cross peaks indicate:

• Through-space dipolar interactions only
• Scalar (J) coupling between protons indicating spin connectivity
• Absolute configuration of chiral centers
• Direct heteronuclear correlations

Correct Answer: Scalar (J) coupling between protons indicating spin connectivity

Q18. The effective spectral width in Hz must be set considering which characteristic of the nucleus being observed?

• T1 relaxation time only
• Range of chemical shifts (in ppm) multiplied by the spectrometer frequency (MHz)
• Number of coupled spins in the molecule
• The solvent boiling point

Correct Answer: Range of chemical shifts (in ppm) multiplied by the spectrometer frequency (MHz)

Q19. What is the principal advantage of phase cycling in FT‑NMR experiments?

• To increase the spectrometer magnetic field strength
• To filter unwanted coherence pathways and suppress artifacts
• To replace the need for Fourier transformation
• To directly measure T1 without inversion recovery

Correct Answer: To filter unwanted coherence pathways and suppress artifacts

Q20. Which factor most directly increases quantitative accuracy in an FT‑NMR assay for drug content?

• Using the highest possible receiver gain
• Ensuring fully relaxed conditions by using repetition time ≥ 5×T1 and an internal standard
• Applying heavy apodization to sharpen peaks
• Collecting a single scan at maximum spectral width

Correct Answer: Ensuring fully relaxed conditions by using repetition time ≥ 5×T1 and an internal standard

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