NMR instrumentation MCQs With Answer

NMR Instrumentation MCQs With Answer

This quiz set on NMR instrumentation is designed specifically for M.Pharm students preparing for Advanced Instrumental Analysis (MPA 201T). It covers core and advanced aspects of nuclear magnetic resonance instrumentation including magnet types, radiofrequency (RF) coils and probes, pulse sequences, relaxation phenomena, shimming, Fourier transform processing, sensitivity and resolution factors, gradient systems, decoupling, cryoprobes and practical considerations for sample handling and solvent suppression. Each multiple-choice question is crafted to deepen understanding of how hardware and pulse-programming choices influence spectral quality and interpretation, helping bridge theoretical principles with practical laboratory troubleshooting and optimization.

Q1. Which of the following properties of a superconducting NMR magnet most directly determines the Larmor frequency of nuclei in the sample?

• Magnet bore diameter
• Field homogeneity across the sample
• Main static magnetic field strength (B0)
• Cryogen temperature around the coils

Correct Answer: Main static magnetic field strength (B0)

Q2. In FT-NMR, the time-domain signal detected after a pulse is called:

• Sweep signal
• Free induction decay (FID)
• Spin echo
• Transient resonance

Correct Answer: Free induction decay (FID)

Q3. Which pulse angle maximizes transverse magnetization for a single 90° pulse experiment?

• 30 degrees
• 90 degrees
• 180 degrees
• 45 degrees

Correct Answer: 90 degrees

Q4. Chemical shift is measured in parts per million (ppm) because:

• It depends on absolute frequency only
• It normalizes frequency differences relative to spectrometer frequency
• It corrects for magnetic susceptibility variations
• It is defined by the gyromagnetic ratio

Correct Answer: It normalizes frequency differences relative to spectrometer frequency

Q5. Which probe feature most strongly improves sensitivity in modern high-resolution liquid-state NMR?

• Use of larger sample tubes
• Higher RF power amplifiers
• Cryogenically cooled preamplifiers and coil (cryoprobe)
• Longer acquisition times

Correct Answer: Cryogenically cooled preamplifiers and coil (cryoprobe)

Q6. The main purpose of shimming in NMR instrumentation is to:

• Adjust RF power for pulses
• Improve static magnetic field homogeneity across the sample
• Calibrate the chemical shift scale
• Reduce sample heating during experiments

Correct Answer: Improve static magnetic field homogeneity across the sample

Q7. In an NMR spectrometer, magnetic field gradients are primarily used for:

• Cooling the superconducting magnet
• Generating spatial encoding for diffusion and imaging and for pulse field gradient experiments
• Enhancing magnet homogeneity
• Measuring gyromagnetic ratios directly

Correct Answer: Generating spatial encoding for diffusion and imaging and for pulse field gradient experiments

Q8. The J-coupling constant between two spin-1/2 nuclei is expressed in:

• Hertz (Hz)
• Parts per million (ppm)
• Tesla (T)
• Seconds (s)

Correct Answer: Hertz (Hz)

Q9. In a typical proton NMR experiment, tetramethylsilane (TMS) is used as:

• A solvent to dissolve polar samples
• A chemical shift reference at 0 ppm
• A relaxation agent to shorten T1
• A decoupling nucleus for 13C experiments

Correct Answer: A chemical shift reference at 0 ppm

Q10. The parameter T1 in NMR refers to:

• Spin–spin relaxation time (transverse)
• Recovery time of longitudinal magnetization (spin–lattice relaxation)
• Time required for shimming
• Pulse repetition delay only

Correct Answer: Recovery time of longitudinal magnetization (spin–lattice relaxation)

Q11. Proton decoupling during 13C acquisition primarily results in:

• Line splitting due to heteronuclear coupling
• Signal multiplication from multiple coherence pathways
• Collapse of carbon multiplicities to singlets and increased 13C sensitivity
• Broadening of 13C lines due to power deposition

Correct Answer: Collapse of carbon multiplicities to singlets and increased 13C sensitivity

Q12. Which factor most directly increases spectral resolution in a 1D NMR experiment?

• Increasing acquisition time (longer FID sampled)
• Using a smaller sample volume
• Reducing spectrometer field strength
• Applying stronger decoupling during acquisition

Correct Answer: Increasing acquisition time (longer FID sampled)

Q13. Fourier transform in NMR converts:

• Frequency domain to time domain
• Time-domain FID to frequency domain spectrum
• Magnetization vectors into scalar values
• Chemical shift into coupling constants

Correct Answer: Time-domain FID to frequency domain spectrum

Q14. Which technique is routinely used to suppress the large solvent signal in proton NMR?

• Increase receiver gain
• Presaturation or selective solvent suppression using shaped pulses
• Lowering static magnetic field strength
• Using longer pulse widths for 180° pulses

Correct Answer: Presaturation or selective solvent suppression using shaped pulses

Q15. In a 2D COSY experiment the cross-peaks indicate:

• Long-range heteronuclear correlations
• Scalar (J) coupling correlations between protons, typically through bonds
• Spatial proximity within 5 Å only
• Direct chemical shift referencing errors

Correct Answer: Scalar (J) coupling correlations between protons, typically through bonds

Q16. The primary advantage of pulsed NMR over continuous-wave (CW) NMR is:

• Ability to operate without superconducting magnets
• Faster data collection with Fourier transform processing and higher sensitivity (multiplex advantage)
• Improved chemical shift referencing without standards
• Elimination of relaxation effects

Correct Answer: Faster data collection with Fourier transform processing and higher sensitivity (multiplex advantage)

Q17. Which of the following best describes the effect of poor shimming on an NMR spectrum?

• Sharper, higher amplitude peaks
• Increased line broadening and distorted multiplet shapes, reducing resolution
• No effect on spectral quality but changes chemical shifts uniformly
• Only affects heteronuclear experiments, not 1H spectra

Correct Answer: Increased line broadening and distorted multiplet shapes, reducing resolution

Q18. The Nyquist theorem in digital NMR acquisition dictates that:

• Receiver bandwidth must be zero for accurate spectra
• Sampling frequency must be at least twice the highest frequency present to avoid aliasing
• Number of scans must equal number of points in FID
• Acquisition time should be shorter than T2

Correct Answer: Sampling frequency must be at least twice the highest frequency present to avoid aliasing

Q19. Nuclear Overhauser Effect (NOE) observed in NMR primarily provides information about:

• Scalar coupling constants between bonded nuclei
• Through-space dipolar proximity between nuclei, useful for distance constraints
• Absolute chemical shifts in ppm
• Magnet homogeneity across the sample

Correct Answer: Through-space dipolar proximity between nuclei, useful for distance constraints

Q20. In solid-state NMR, magic-angle spinning (MAS) is used to:

• Reduce radiofrequency power needed for pulses
• Average out anisotropic interactions such as chemical shift anisotropy and dipolar couplings to narrow lines
• Increase T1 relaxation times uniformly
• Change the gyromagnetic ratio of nuclei

Correct Answer: Average out anisotropic interactions such as chemical shift anisotropy and dipolar couplings to narrow lines

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