Chapter 23 — Self-Check Quiz

24 questions: multiple choice and short answer. Try them closed-book. The answer key is in the collapsed block at the bottom.

Multiple choice

1. The defining difference between a presumptive and a confirmatory test is that a confirmatory test: - A. Is always more expensive - B. Is highly specific — it establishes identity in a way other substances do not reproduce, ideally by interrogating molecular structure - C. Is faster - D. Can be done at the roadside

2. Chromatography separates a mixture by exploiting: - A. The color of each component - B. How differently each component distributes between a moving (mobile) phase and a stationary phase - C. The weight of the whole sample - D. The temperature of the room

3. On a chromatogram, a peak's retention time tells you: - A. Exactly which compound it is, with certainty - B. Which compound it might be (a class characteristic), to be confirmed against a standard - C. The mass of the molecule - D. Who handled the sample

4. GC-MS is considered confirmatory largely because it combines: - A. Two copies of the same measurement - B. Two independent dimensions of information — retention time (chromatography) and fragmentation pattern (mass spectrometry) — that fail differently - C. Color and crystal shape - D. Two different analysts' opinions

5. In a mass spectrum, the molecular ion corresponds to: - A. The smallest fragment - B. The intact, charged molecule, giving its molecular weight - C. The instrument's background noise - D. The base peak, always

6. The base peak in a mass spectrum is: - A. The heaviest fragment - B. The most abundant fragment (scaled to 100%) - C. The molecular ion, always - D. The retention time

7. FTIR identifies a substance by measuring how it: - A. Scatters X-rays - B. Absorbs infrared light at frequencies characteristic of its chemical bonds - C. Conducts electricity - D. Dissolves in water

8. A key practical advantage of FTIR over GC-MS is that FTIR is: - A. Better at separating complex mixtures - B. Non-destructive and excellent for identifying bulk solid materials taken whole - C. Able to identify who touched the sample - D. Always more specific for every sample

9. Raman spectroscopy is especially useful (versus FTIR) because it can often: - A. Identify the suspect - B. Work through transparent containers and handle aqueous samples - C. Replace the chain of custody - D. Vaporize heavy molecules

10. UV-Vis spectroscopy is best regarded as: - A. A confirmatory identification method on its own - B. A screening and quantification tool, not a confident stand-alone identifier (its spectra are broad and shared by many substances) - C. The gold standard for drug identification - D. A type of electron microscopy

11. The SEM uses a focused beam of __ to image a specimen, achieving resolution far beyond a light microscope: - A. Infrared light - B. Electrons - C. X-rays only - D. Visible light

12. The EDX detector on a SEM reads: - A. The molecular weight of organic compounds - B. The elemental composition of the spot the beam strikes (via characteristic X-rays) - C. The retention time - D. The DNA of the particle

13. SEM-EDX evidence about a particle's elemental composition is, in forensic terms: - A. An individualization to one unique source - B. Class evidence — it tells you what a particle is made of, not which unique gun/car/person it came from - C. Proof of who fired a weapon - D. Equivalent in strength to a single-source DNA match

14. The classic gunshot-residue (GSR) signature confirmed by SEM-EDX is a particle combining: - A. Carbon, hydrogen, and oxygen - B. Lead, barium, and antimony, with characteristic morphology - C. Sodium and chlorine - D. Iron and nickel only

15. Compared to a bite-mark comparison (Chapter 16), GC-MS identification of a compound sits on the validity spectrum as: - A. Equally unvalidated - B. Far higher — grounded in analytical chemistry, quantified, reproducible, and genuinely confirmatory of a compound's identity - C. Lower, because instruments make mistakes - D. Identical, since both involve "matching"

16. "Garbage in, garbage out," in this chapter, means: - A. Instruments produce random output - B. A flawless instrument run on a contaminated or mislabeled sample yields a confident, wrong answer — validity lives in the method and the practice - C. Old samples should be discarded - D. GC-MS cannot be trusted

17. In the cold case, the GC-MS result on the fire debris most defensibly establishes: - A. Who poured the gasoline - B. When the fire was set - C. That gasoline is present in the fire debris, identified instrumentally - D. That the defendant is guilty

18. A "97% library match" reported by a spectrometer should be treated by the analyst as: - A. A final verdict - B. A starting point for the analyst's judgment, to be interrogated (whole spectrum explained? clean blank? sensible in context?), not read aloud as a conclusion - C. Legally binding - D. More reliable than a same-day standard

Short answer

19. In two sentences, explain why a retention-time match alone is weak identification but a retention-time match plus a full mass-spectral match is confirmatory.

20. Name the three controls/comparisons that make an instrumental identification defensible (§23.1), and state in one phrase what each one rules out or establishes.

21. Explain why elemental composition from SEM-EDX is class evidence, using one sentence and the class-vs-individual idea from Chapter 1.

22. A detective tells the analyst which drug the sample "should" be before analysis. State the bias risk in interpreting an ambiguous spectrum and the safeguard (preview of Chapter 31).

23. In the cold case, explain in one or two sentences what changed scientifically between Chapter 21's presumptive gasoline finding and this chapter's confirmed gasoline finding — and what did not change about who the evidence implicates.

24. Write one sentence an honest expert could say on the stand about the Mill Creek SEM-EDX sleeve particle, and one sentence that would overstate it.

Answer key (click to expand) **Multiple choice:** 1-B · 2-B · 3-B · 4-B · 5-B · 6-B · 7-B · 8-B · 9-B · 10-B · 11-B · 12-B · 13-B · 14-B · 15-B · 16-B · 17-C · 18-B **Short answer (model points):** **19.** Retention time is a *class* characteristic — several different compounds can coincidentally share one, so a match on retention time alone could be a coincidence. A full mass-spectral match adds an independent, structure-specific dimension (the fragmentation fingerprint); agreement on *both* retention time and the whole mass spectrum, against a same-day standard, is so unlikely to occur by chance for two different substances that it counts as confirmatory. **20.** Any reasonable phrasing of: (a) a **reference standard** of the suspected substance, run the same day under identical conditions — establishes that the unknown behaves identically to the known; (b) a **blank** — rules out contamination of the instrument/glassware as the source of the signal; (c) the **chain of custody** (Chapter 2) — establishes that the sample analyzed is the sample seized. (Calibration of the instrument is a defensible fourth.) **21.** Elemental composition tells you which *class* of particles a speck belongs to (e.g., particles made of lead/barium/antimony, produced by a class of events), not which unique source produced it — many guns, cartridges, or persons could yield the same elements, just as many shoes share a class tread pattern. **22.** **Risk:** being told the "expected" drug *anchors* the analyst, so an ambiguous spectrum or a marginal library match gets read toward the wanted answer (contextual/confirmation bias). **Safeguard:** keep domain-irrelevant information away from the analyst until the identification is fixed, then compare (context management / sequential unmasking, Chapter 31); a blind interpretation is worth more than one made knowing the wanted answer, even when they agree. **23.** What changed is the *certainty of the accelerant's identity*: a presumptive test suggested gasoline chemistry; GC-MS, against a standard and a clean blank, *confirmed* gasoline's identity by retention time plus matching mass spectra — a move from hypothesis to conclusion. What did *not* change is who or what is implicated: the chemistry still says only that gasoline is present in the debris, not who introduced it, when, or why. **24.** **Honest:** "The particle recovered from the sleeve has a morphology and elemental composition consistent with [the relevant class of particles]; it cannot be excluded and may corroborate, but elemental analysis identifies a *class* of particle, not a unique source, and cannot establish how the particle reached the sleeve." **Overstated:** "The SEM-EDX particle proves this defendant was present and is the source of the residue."