Comprehensive Study Notes on Forensic Chemistry and Presumptive Testing

Overview of Advanced Presumptive Testing

• Conceptual Framework: Presumptive tests are preliminary screening tools used in forensic science to determine the general class of a substance. They focus on rapid, visible results rather than definitive chemical identification.

• Chemistry-Based Approach: The pedagogy focuses on understanding the underlying chemical mechanisms (e.g., redox, organic dye formation) rather than just memorizing color changes.

• Legacy Material: Lecture 1 materials concluded with non-drug presumptive tests (dogs, breathalyzers) before transitioning into Lecture 2 materials regarding non-metal based organic tests.

Non-Traditional Presumptive Testing Methods

• Hemostix: Swabs designed to detect blood in serological fluids. These strips typically undergo a color change (blue or green) to indicate a presumptive positive result. They are used in both medical and forensic contexts.

• K9 Units (Detection Dogs): Dogs serve as biological presumptive "instruments" due to their highly sensitive olfactory systems.

  • Search Targets: Cadavers (still or deceased), accelerants/hydrocarbons (arson investigations), explosives, and illicit drugs.

  • Quarantine Measures: In Australia, dogs are frequently used to detect fruit and biological hazards at airports.

  • Limitation: A major drawback is that a dog cannot testify in court; their detection must be followed by scientific confirmation.

• Breathalyzers: Used for roadside screening of alcohol consumption. While accurate, they are considered presumptive because potential calibration drift over time prevents them from being absolute confirmatory evidence without follow-up lab tests.

• Saliva/Lick Tests: Similar in function to COVID-19 rapid antigen tests (RATs). These utilize biological components (antigens) to identify specific molecules like Cannabis, Cocaine, or MDMA.

  • Mechanism: These are biological tests where molecules are trained (often using the immune systems of animals like rabbits) to recognize illicit substances.

Electrochemical Foundations of Breathalyzers

• Mechanism: Breathalyzers operate on the principles of electrochemistry using a redox reaction involving ethanol (alcohol).

• The Redox Process: Oxidation is the loss of electrons, while reduction is the gain. The instrument measures the movement of electrons as an electric current/potential.

• Oxidation Half-Reaction: Alcohol in the breath is oxidized by catalysts inside the device.

  • Products: $CO_2$, carboxylic acids, and aldehydes.

  • Electron Action: As alcohol is oxidized, it loses electrons.

• Reduction Half-Reaction: Oxygen ($O_2$) from the breath is reduced to form water ($H_2O$).

• Specificity: $CO_2$ cannot be further oxidized by the catalysts, so it does not trigger a current. Only oxidizable substances like alcohol allow the current to flow, which the instrument converts into a Breath Alcohol Concentration (BrAC) reading.

Classification and Structural Diversity of Drugs

• The "White Powder Problem": Presumptive tests do not identify specific drugs but rather classify them.

• Functional vs. Arbitrary Classes:

  • Structurally Useful Classes (Good): Opiates, Cannabinoids, Barbiturates, Steroids. These names imply a specific chemical backbone.

  • Effect-Based Classes (Bad for Forensic Chemistry): Stimulants, Depressants, Hallucinogens, Narcotics, "Party Drugs," "Psychoactive Drugs." These terms describe biological effects but include chemically unrelated structures.

• Case Study: Hallucinogens: PCP and Psilocybin both cause hallucinations but have vastly different chemical structures. Designing a single presumptive test for such a diverse class is difficult.

• Opiates vs. Opioids:

  • Opiates: Substances directly extracted from the opium poppy (e.g., Morphine, Morphine-derivatives). They share a core structural motif.

  • Opioids: Any substance that interacts with the opioid receptors in the brain, including synthetic substances like Fentanyl, which is structurally unrelated to morphine.

Principles of Organic Dye Formation and Color

• Transition Metal vs. Organic Color: While transition metals use $d$-orbital splitting to absorb light, organic dyes use conjugated $\pi$-bonds.

• Conjugation: This occurs when double bonds are separated by a single bond ("one jump"), allowing electrons to share density across a large system.

• Conditions for Color:

  • Chain Length: A minimum of approximately $6-7$ conjugated double bonds is required to absorb visible light.

  • UV Range: Systems with fewer double bonds absorb in the Ultraviolet range (invisible to the eye).

  • IR Range: Systems with too many double bonds (e.g., $13-15$) absorb in the Infrared range (invisible).

  • Charges: The presence of a positive (carbocation) or negative charge in the system can act as an additional "double bond" enhancer, deepening the color.

• Resonance: The technical description of electrons moving between different structural forms. It is often the underlying reason for stability and color in chemical species.

• Phenolphthalein Example: A pH indicator that shifts between colorless and colored forms (pink/purple) based on whether the central quaternary carbon is "broken" to allow three rings to conjugate together.

Specific Organic Presumptive Tests

The Marquis Test

• Primary Targets: Opiates (Purple) and Amphetamines (Orange/Brown).

• Reagent Composition: $50\text{ mL}$ of $40\%$ formaldehyde ($HCHO$) in $100\text{ mL}$ of concentrated ($99\%$) sulfuric acid ($H_2SO_4$).

• Role of Components:

  • Sulfuric Acid: A high-strength driver for rapid chemical reactions.

  • Formaldehyde: Acts as "chemical glue," fusing two organic molecules together via an aldehyde bridge.

• Morphine Reaction: Two morphine molecules ($3$ double bonds each) are fused by formaldehyde to create a dimer with $7$ conjugated double bonds and a positive charge, resulting in a purple color.

• Amphetamine Reaction: Similar to the Friedel-Crafts reaction; formaldehyde glues two aromatic rings together to reach the threshold of $6$ double bonds required for an orange hue.

The Duquenoy-Levine Test

• Primary Target: Cannabis (Plant material, resin, or oil).

• Reagents: Vanillin, acetaldehyde, hydrochloric acid ($HCl$), and chloroform ($CHCl_3$).

• Mechanism: A two-step method where vanillin and acetaldehyde fuse onto the phenol ring of Tetrahydrocannabinol (THC).

• Conjugation Shift: Extends the double bond system from $3$ to $8$ conjugated bonds.

• Outcome: A purple/blue color extracted into the bottom chloroform layer.

• False Positives: Coffee, nutmeg, mace, and thyme contain similar phenols that may react. Confirmation is often done via microscopy for the identification of cystolithic trichomes (small spiky microscopic structures unique to Cannabis).

Ehrlich’s Reagent

• Primary Targets: Indoles, specifically LSD and Psilocybin (mushrooms).

• Mechanism: Uses an aldehyde in acid to attach to the indole ring.

• Outcome: Increases double bonds from $4$ to $8$, creating a visible color change.

Presumptive Testing for Blood via Peroxidase Activity

• Blood Composition: Erythrocytes (Red Blood Cells) contain Hemoglobin. Hemoglobin features a Heme group: an Iron ($Fe^{2+}$) porphyrin complex. This iron acts as a catalyst.

• Peroxidase Activity: Hemoglobin catalyzes the breakdown of hydrogen peroxide ($H_2O_2$) into water and oxygen.

• General Mechanism: A redox reaction where the indicator (KM, TMB, or Luminol) loses electrons (Oxidation) and the peroxide gains electrons (Reduction). The blood (iron) facilitates this transfer.

• Thermodynamics vs. Kinetics:

  • The reaction is spontaneous (Thermodynamically favored) but extremely slow without a catalyst.

  • Blood acts as the catalyst, increasing the rate (Kinetics) so that the reaction completes in seconds rather than hours.

Forensic Blood Reagents

• Kastle-Meyer (KM): Uses Phenolphthalein. Changes from colorless to hot pink.

• Tetramethylbenzidine (TMB): Changes from colorless to bright green. It is an accepted method but known to be carcinogenic.

• Luminol: Produces Chemiluminescence (a faint blue glow). It is useful for finding hidden blood or spatter patterns but can interfere with subsequent DNA testing.

• Sensitivity: These tests are highly sensitive, detecting blood even when diluted to $1:10,000,000$.

• Limitations and False Positives:

  • Vegetable Peroxidases: Horseradish, broccoli, and cauliflower can trigger the reaction.

  • Species Identification: These tests do not distinguish between human and animal blood.

Quality Control in Testing

• Positive Control: Testing the reagents against a known sample of the illicit drug to ensure the chemicals are still functional and have not expired.

• Negative Control: Testing the reagents alone (without the drug) to ensure there is no contamination or "false positive" from the testing equipment/background.

Questions & Discussion

• Participant Question: How do you tell human blood from animal blood if the presumptive tests are the same?

• Response: Presumptive tests cannot distinguish the species. DNA analysis or blood typing is necessary for that confirmation.

• Participant Question: Can other metals like copper cause a reaction?

• Response: Copper is a redox-active metal and could potentially catalyze a reaction, though it might result in different colorations (e.g., green instead of red).