FORENSIC INSTRUMENTATION (NMR)
Q1. Which nuclei are most commonly used in forensic
NMR analysis due to their natural abundance and sensitivity?
(a)
¹³C and ¹⁵N
(b)
¹H and ¹³C
(c)
³¹P and ¹⁹F
(d)
²H and ¹⁵N
Answer:
(b)
Explanation: ¹H (high sensitivity, 99.9% abundance) and ¹³C (structural backbone) dominate forensic NMR applications.
Q2.
Which statements about routine broadband proton-decoupled 13C NMR
are correct?
(a) Most
carbon signals generally appear as singlets.
(b) Proton
decoupling removes 13C – 1Hspin-spin splitting.
(c) Signal
intensity directly provides an accurate quantitative count of carbon atoms.
(d) 13C
NMR generally has a wider chemical-shift
range than 1H NMR.
Answer:
(a), (b), (d)
Explanation:
Natural abundance 13C NMR is extremely useful for structural
characterization.
In conventional broadband proton-decoupled 13C NMR, irradiation of protons removes observable 13C – 1H coupling. Therefore, carbon resonances generally appear as singlets.
Q3.
Which of the following best describes the role of chemical shift in forensic
NMR?
(a)
Indicates the number of protons in a molecule
(b)
Reflects the electronic environment of nuclei
(c)
Measures relaxation times directly
(d)
Determines the magnetic field strength
Answer:
(b)
Explanation: Chemical shift (δ, ppm) reveals shielding/deshielding, helping differentiate compounds in forensic samples.
Q4.
Solid‑state NMR is especially important in forensic science for:
(a)
Identifying volatile solvents
(b)
Characterizing polymers and explosives
(c)
Detecting trace metals
(d)
Measuring isotopic ratios
Answer:
(b)
Explanation: MAS solid‑state NMR is used for polymers, fibers, and explosives like RDX/PETN.
Q5.
A forensic laboratory receives an unknown organic substance suspected to be a
controlled drug or pharmaceutical-related compound. NMR is used along with
other analytical techniques.
Which
combination of statements is most scientifically appropriate?
(a) NMR
can provide information about proton environments, integration, and spin-spin
coupling.
(b) NMR
provides information about chemically distinct carbon environments.
(c) NMR
alone always establishes the elemental composition of an unknown with absolute
certainty.
(d) NMR
can help distinguish structural isomers.
(e) Two
compounds with identical molecular formulas must necessarily have identical NMR
spectra.
Answer:
(a), (b), (d)
Explanation:
(a) — Correct
¹H
NMR provides information about proton environments, chemical shifts,
integration, splitting patterns, and coupling constants. For example, CH₃CH₂X
generally shows a triplet for CH₃ and a quartet for CH₂.
(b)
— Correct
¹³C
NMR identifies different carbon environments, including carbonyl, aromatic,
aliphatic, and oxygenated carbons. It is also useful for studying molecular
symmetry.
(d)
- correct
NMR can distinguish structural isomers because compounds with the same molecular formula can have different numbers of signals, chemical shifts, integration, and coupling patterns.
Q6.
Relaxation times (T₁ and T₂) in NMR are important because they:
(a)
Provide information about molecular motion and environment
(b)
Indicate the number of protons in a molecule
(c)
Measure isotopic abundance
(d)
Determine magnetic field strength
Answer:
(a)
Explanation: Relaxation reveals dynamics; forensic metabolomics uses T₁/T₂ to study biological fluids.
Q7.
Assertion (A): FT‑NMR is faster and more sensitive than continuous‑wave NMR.
Reason
(R): FT‑NMR collects all frequencies
simultaneously using a single RF pulse.
Answer:
Both A and R are true, and R explains A.
Explanation: Fourier Transform converts time‑domain FID into frequency‑domain spectra, enabling rapid, high‑resolution analysis.
Q8.
In forensic counterfeit detection, NMR is used to:
(a)
Measure isotopic ratios of metals
(b)
Confirm authenticity of pharmaceuticals
(c)
Detect DNA sequences
(d)
Identify fingerprints
Answer:
(b)
Explanation: NMR verifies active ingredients and detects substitutions in counterfeit drugs.
Q9.
Which advanced NMR technique is most useful for resolving overlapping signals
in forensic samples?
(a)
1D Proton NMR
(b)
2D COSY/HSQC
(c)
IR Spectroscopy
(d)
UV‑Vis Spectroscopy
Answer:
(b)
Explanation: 2D NMR separates overlapping peaks, crucial for complex forensic mixtures.
Q10.
Why is NMR often combined with Mass
Spectrometry (MS) in forensic investigations?
(a)
MS provides molecular formula, NMR provides connectivity
(b)
Both techniques destroy the sample
(c)
MS measures relaxation times, NMR measures isotopic ratios
(d)
Both techniques rely on combustion
Answer:
(a)
Explanation:
MS gives molecular weight/formula; NMR confirms structure. Together they
provide complete forensic evidence.
0 Comments