Chapter 23 — Exercises

Work these without looking back at the chapter first; then check yourself. Items marked † have full worked solutions in the answers appendix. There are no answers in this file. Mix of recall, applied reasoning, evidence interpretation, "spot the overstatement," ethics, and a cold-case extension.

A. Recall and definitions

  1. Define chromatography in one sentence, and name the two "phases" whose interaction does the separating.

  2. † State the three properties that make a result confirmatory rather than presumptive (§23.1). For each, give a one-phrase reason it matters.

  3. Define mass spectrum, and explain what the horizontal axis (m/z) and the bar heights each represent.

  4. What does the abbreviation GC-MS stand for, and what distinct job does each half of the instrument do?

  5. Define FTIR and state, in one phrase, the physical property of molecules it exploits.

  6. Define SEM-EDX, and state separately what the SEM part contributes and what the EDX part contributes.

  7. What is analytical microscopy, and roughly what magnification ceiling limits an ordinary light microscope (and why)?

  8. Distinguish GC from HPLC by the one thing that differs between them, and name the kind of sample that forces a lab to choose HPLC.

B. Applied reasoning

  1. † A compound matches a reference standard on its retention time but no mass spectrum is run. Your colleague reports the identification as confirmed. Using the class-vs-individual idea from Chapter 1, explain why retention time alone is not a confirmation, and what is missing.

  2. Explain, step by step, how a single peak in Figure 23.1 becomes a mass spectrum in Figure 23.2. What happens to the molecule inside the mass spectrometer, and what is recorded?

  3. A toxicology lab must confirm a large, heat-sensitive opioid metabolite that decomposes when vaporized. Which separation method should it use, and why is GC the wrong tool here?

  4. † Gasoline is "a soup of hundreds of hydrocarbons." Explain why a fire-debris analyst confirms an accelerant from the whole chromatographic pattern compared to a standard, rather than from any single peak. What does the pattern-based approach guard against?

  5. You have a tiny unknown powder and only enough of it for one test before it is consumed. You want to keep the sample intact for possible re-testing. Which instrument in this chapter is non-destructive and well suited to identifying a bulk solid, and what is its main limit on mixtures?

  6. A field officer screens a sealed transparent vial of suspected drugs with a handheld instrument without opening it, and gets a "match." Which spectroscopy makes through-the-glass screening possible, and what is the honest status of that field result (presumptive or confirmatory)? Why?

  7. Sequence these from "weakest identification on its own" to "strongest," and justify the ranking: a UV-Vis spectrum; a GC retention time; a full GC-MS run (retention time + matching mass spectrum) against a same-day standard.

C. Evidence interpretation

  1. † Re-read Figure 23.1. (a) What does the flat baseline mean? (b) What does a peak's position tell you, and what does its height/area tell you? (c) Why is the "signature ridge" of aromatic peaks, not any single peak, what the analyst recognizes as gasoline?

  2. Re-read Figure 23.2. Identify and explain the difference between the base peak and the molecular ion, and state what each one contributes to identifying the compound.

  3. An analyst's report says: "The substance was identified as cocaine by GC-MS: the retention time matched the reference standard, the mass spectrum matched the standard and the NIST library across the full fragmentation pattern, and the instrument blank was clean." Identify three separate things this sentence does well (where it is appropriately rigorous).

  4. Figure 23.3 (the cold case) concludes the accelerant is "confirmed instrumentally." Write the single sentence an honest expert could say on the stand about the GC-MS result, and the single sentence that would overstate it by reaching from the chemistry to a person.

D. Spot the overstatement / junk-science alert

  1. † An instrument prints a "97% library match" for an unknown spectrum, and the report states the substance is identified on that basis alone. Using the §23.4 Junk-Science Alert, name two ways a high match score can be confidently wrong, and state what the analyst must do instead of reading the percentage aloud.

  2. A witness testifies that SEM-EDX found a particle of lead, barium, and antimony on the defendant's sleeve and that this "proves he fired the gun." Name the two distinct overstatements (one about what elemental analysis can establish, one about how a particle reaches a sleeve), and give the defensible version.

  3. A prosecutor's slide reads: "GC-MS proves the defendant poured gasoline in the cabin." Separate the part of this claim the chemistry does support from the part it does not, and rewrite it honestly.

  4. A report gives a confident identification but never mentions a blank, a reference standard, or the chain of custody. Using §23.1 and §23.3, explain why the absence of these should make a reader distrust an otherwise impressive-looking instrumental result.

E. Ethics and reasoning

  1. † "Garbage in, garbage out." Explain, with a concrete example, how a flawless GC-MS run can still produce a confident, wrong answer. Whose responsibility is the input, and why does this mean the instrument's validity is not the same as the result's reliability?

  2. An analyst is told by the lead detective, before running the sample, exactly which drug it "should" be. Previewing Chapter 31, explain the bias risk in interpreting an ambiguous spectrum, and the safeguard. Is a result interpreted blind worth more than one interpreted knowing the wanted answer, even if they agree?

  3. A lab is asked to report a confirmatory drug identification but, due to backlog, performed only a presumptive color test and a retention-time check. Recalling the Massachusetts lab scandals (Chapter 4), explain why reporting this as "confirmed" is both a scientific and an ethical failure — even if the substance really was the drug.

  4. Two competent analysts agree that a mass spectrum matches a reference, but disagree about whether a minor unexplained peak signals a second component. Is this disagreement evidence that GC-MS is unreliable? Explain the difference between disputing an interpretation at the margins and disputing the core method.

F. Synthesis and the validity spectrum

  1. † Place these four on the NAS 2009 / PCAST 2016 validity spectrum (strong → discredited), justifying each: GC-MS identification of a compound (this chapter); SEM-EDX elemental identification of a particle (this chapter); microscopic hair comparison (Chapter 19); bite-mark comparison (Chapter 16). Be explicit about which claim you are ranking for each.

  2. Explain how this chapter's GC-MS confirmation of gasoline and Chapter 22's fire-science finding of an incendiary fire relate to each other: which establishes what, and why neither alone establishes a homicide (recall Chapter 11).

  3. In one paragraph, explain how this chapter advances at least two of the book's four themes (exclusion over proof; the validity spectrum; cognitive bias; the CSI effect cutting both ways). Name which themes and how.

  4. The chapter calls instrumental analysis "the high-validity end" of the spectrum and insists "the instrument is the easy part." Explain why both statements are true at once, and why holding them together is the chapter's central discipline.

G. Cold-case extension

  1. † Cold Case. Using only what the GC-MS result on the fire debris establishes, write the entry you would add to the Mill Creek evidence log (Appendix I). State (a) the defensible conclusion at its true strength, (b) the honest verb, (c) at least three things this evidence specifically does not establish, and (d) what role this confirmation plays relative to the Chapter 21 presumptive finding and the Chapter 22 arson finding.

  2. Cold Case. The SEM-EDX particle work on a sleeve is reported at "consistent-with" strength only, with no name attached. Explain precisely why the chapter refuses to tie the particle to a person — name the two reasons (one about class vs. individual evidence, one about how particles transfer) — and state what additional kinds of evidence would be required before any person could be associated with that sleeve.

  3. Cold Case, integrative. Suppose a defense expert argues the gasoline in the debris could have been present innocently (e.g., a contractor's stored fuel). Using the chapter (what GC-MS does and does not establish) and Chapter 22 (origin and cause), explain what other findings in the file would have to do the work of distinguishing "gasoline was present" from "gasoline was used as an accelerant" — and why the chemistry alone cannot settle that question.

H. Short writing

  1. † In 150–200 words, explain to a juror what "confirmed by GC-MS" actually means and why it is one of the strongest statements a forensic chemist can make — and what such a confirmation still cannot tell them about who did what.