Chapter 6 — Case Study 1: Forensic Chemistry and the Identification of Unknown Substances

How IR and MS let a forensic chemist identify seized materials.

Every year, crime labs across the world receive tens of thousands of unknown substances for identification: powders, tablets, leaves, residues, liquids. Each sample might be an illegal drug, a prescription medication, a poison, a cutting agent, or a completely benign substance. The forensic chemist's job is to say, with high confidence, what it is.

The workflow is essentially the one you learned in this chapter:

  1. Visual inspection and presumptive testing. A drug chemist first looks at the substance and performs simple color tests (Marquis reagent, Mandelin, Scott tests, etc.) that give preliminary clues. A powder turning purple with Marquis reagent might be an MDA/MDMA; a powder turning orange might be an amphetamine. These are screening tests only — positive results require confirmation.

  2. Mass spectrometry. The sample is dissolved and analyzed by GC-MS (gas chromatography-mass spectrometry): the GC separates components of a mixture, and each separated component's MS spectrum is recorded. The molecular ion reveals molecular weight. The fragmentation pattern is compared to a reference library of over 200,000 compounds. Usually, one compound matches with > 99% confidence.

  3. Infrared spectroscopy. Especially for drugs in pure or semi-pure form, IR of the solid sample (using an attenuated-total-reflectance, ATR, attachment) provides an additional fingerprint. The combination of MS and IR is essentially unique — two independent techniques, both confirming the same compound.

For a compound with unusual isotope pattern (containing chlorine, bromine), the halogen identity is obvious from the MS alone. For a compound with a characteristic C=O, that region of the IR distinguishes it from non-carbonyl compounds. For nitrogen content, the nitrogen rule on the molecular ion.

The stakes

A forensic misidentification can cost someone years of freedom. A forensic chemist does not guess. She runs the MS, runs the IR, compares to library standards, confirms with a second instrument, and only then reports a result. The chain of evidence — physical sample, reagent logs, instrument runs — is documented at every step.

This is an example where Chapter 6's techniques are not hobbies but the tools of a professional judgment.

A representative workflow: identifying MDMA from a pill

Suppose a pill is seized and submitted to the lab. The analyst:

  1. Visually inspects: A 10 mg tablet with a butterfly imprint, off-white. Consistent with a "club drug" appearance.

  2. Dissolves a small portion in methanol for GC-MS. Runs the sample.

  3. Examines the MS: The GC shows two peaks — a major one at retention time 7.2 min, a minor one at 4.1 min. The major peak's MS shows $M^+ = 193$ (weak), base peak at $m/z = 135$ (loss of 58 = $CH_2N(CH_3)H$, characteristic of ring-opened amphetamines).

  4. Compares to library: Matches MDMA (3,4-methylenedioxymethamphetamine) with 98% confidence. MW = 193. Fragment at 135 = the [M-CH2NHCH3]⁺.

  5. Confirms by IR: The solid tablet is pressed and scanned. IR shows characteristic peaks for the methylenedioxy ring (unusual doublet around 930/1045 cm⁻¹), aromatic C=C, N-H (secondary amine around 3330).

  6. Writes the report: "The major component was identified as MDMA. Concentration determined to be 85 mg per tablet."

Every one of these steps uses Chapter 6's framework. The analyst reads MS and IR fluently, knows the expected fragmentation of amphetamines, and knows what structural features give rise to which peaks.

What this tells the chemistry student

Forensic chemistry is one of many applied fields where the basic spectroscopic vocabulary of this chapter is the working language. Pharmaceutical analysis, environmental monitoring, food-safety testing, metabolomics research, clinical chemistry, archaeological artifact analysis — all rely on the same combination of IR, MS, and (for harder cases) NMR.

The underlying theory is the same. The specific compounds vary. The pragmatic skill — looking at a spectrum and asking "what functional groups, what molecular weight, what fragments?" — is the same.

Chapter 6 has given you the basics. Chapter 9 gives you NMR. From there, the tools to identify almost any organic molecule are in your hands.