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GC-MS Peak-Match Confirmation

Run the GC-MS Peak-Match MicroSim Fullscreen

About This MicroSim

A field color test is fast and cheap, but it can only ever be presumptive — a color suggests a class of substances, never a specific molecule. To name the molecule, a crime lab turns to the confirmatory gold standard: gas chromatography–mass spectrometry (GC-MS).

This MicroSim shows the two pieces of evidence GC-MS produces:

  • The top panel is a gas chromatogram — detector abundance plotted against retention time. Every compound travels through the column at its own speed, so it exits at a characteristic time. The unknown appears as one dominant peak.
  • The bottom panel is a mass spectrum — relative intensity plotted against m/z (mass-to-charge ratio). The mass spectrometer shatters the molecule into fragments, and the pattern of labeled bars is as unique as a fingerprint.

You pick a candidate from a four-substance reference library — Caffeine, Aspirin, Pseudoephedrine, or Compound X — and overlay it on the unknown. A candidate only earns a CONFIRMED MATCH when both the retention time and the fragmentation pattern line up.

How to Use It

  1. Study the unknown's chromatogram peak and its labeled mass-spectrum fragments.
  2. Pick a Candidate from the dropdown that you think matches.
  3. Press Overlay & Score. The candidate's retention peak and spectrum appear in orange over the unknown's, and the verdict panel reports the retention-time match, the fragment-pattern match, and an overall percentage. Shared fragments turn their m/z labels green.
  4. Press New Unknown to test yourself on a different substance, and Reveal Match to check the true identity.

Learning Objective

Apply (L3): Confirm the identity of an unknown by matching its retention time and mass-spectrum fragmentation pattern to a reference library. Investigators practice why a presumptive color test must be followed by GC-MS confirmation before any substance can be named in court — matching two independent features is far stronger than matching either one alone.

You can embed this MicroSim on your own web page with this iframe:

<iframe src="https://dmccreary.github.io/forensic-science/sims/gcms-peak-match/main.html"
        width="100%" height="580" scrolling="no"></iframe>

Lesson Plan

Audience: High-school forensic science (grades 9–12) Time: 15–20 minutes Bloom level: Apply (L3) — match retention time and fragmentation pattern to confirm identity.

Warm-up. Ask investigators: "A field color test just turned purple. A prosecutor asks, 'So it's definitely this drug?' Why can't you say yes yet, and what instrument would you reach for?"

Guided questions:

  • Overlay a candidate that matches the retention time but not the fragmentation pattern. Why is that not a confirmed match, and what does each of the two features tell you?
  • Why is matching two independent features (retention time and mass spectrum) far stronger evidence than matching either one alone?
  • GC-MS is called a confirmatory "gold standard." What makes a mass-spectrum fragmentation pattern almost as unique as a fingerprint?

Extension. Look up how real labs build and search mass-spectral reference libraries (such as the NIST library), and discuss why a good library match still requires an analyst's judgment.

References

Specification

This MicroSim was generated from a specification in Chapter 9: Forensic Toxicology and Chemical Analysis.

Design note: the chromatogram peaks and mass spectra are stylized for teaching — retention times and fragment bars are chosen to make the two-feature confirmation logic visible, not to reproduce instrument-grade data.