Analytical Toxicology Methods

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Sample Handling & Basics - Toxin Toolkit Intro

  • Sample Collection & Preservation:
    • Blood: While 10 mL is common, 10-20 mL or more is recommended for comprehensive analysis (Sodium Fluoride for alcohol; EDTA/Heparin for drugs - specific anticoagulant varies by protocol).
    • Urine: 50-100 mL or entire voided sample preferred for screening and confirmatory tests including novel psychoactive substances.
    • Vitreous Humor: 2-5 mL (resists putrefaction; primarily for postmortem alcohol, glucose, specific drugs - not universal).
    • Tissues (Liver, Kidney, Brain): 50-100g (varies by suspected substance and analytical method).
    • Critical: Proper labeling, appropriate container, correct preservative.
  • Storage & Transport:
    • Refrigerate at 2-8°C; Freeze at -20°C or -70°C for unstable compounds; avoid freeze-thaw cycles.
    • Maintain strict Chain of Custody (CoC) documentation per BNSS procedures.
  • Analytical Principles:
    • Screening: Modern techniques include HRMS and targeted LC-MS/MS beyond traditional Immunoassays, TLC.
    • Confirmation: GC-MS, LC-MS/MS, HRMS with ISO/IEC 17025 validation standards.

Forensic toxicology sample preparation tubes

⭐ In postmortem toxicology, peripheral blood (e.g., femoral) is preferred over central (heart) blood to minimize postmortem redistribution (PMR) from tissue diffusion and pH changes. Urine, vitreous humor aid PMR interpretation.

Screening Techniques - Quick Toxin Scan

  • Initial broad-spectrum analysis to detect presence or class of potential toxins.
  • Immunoassays (ELISA, EMIT, RIA, FPIA):
    • Principle: Antigen-antibody binding.
    • Pros: Rapid, sensitive, automatable.
    • Cons: Cross-reactivity (potential false positives); often qualitative/semi-quantitative.
  • Thin Layer Chromatography (TLC):
    • Principle: Differential adsorption on a stationary phase.
    • Identifies substances by $R_f$ value (Retention factor) & color reactions with spraying reagents.
    • Pros: Simple, inexpensive, versatile for many drug classes.
  • Colorimetric/Spot Tests:
    • Rapid, presumptive tests based on color change.
    • Examples: Trinder’s test (salicylates), Reinsch test (heavy metals like As, Sb, Bi, Hg).
    • Pros: Quick, field-usable.
    • Cons: Non-specific, prone to interferences.
  • Gas Chromatography (GC - basic screening):
    • Primarily for volatile substances (e.g., alcohols via headspace GC).
    • Separates compounds based on volatility. ELISA principle diagram

⭐ Immunoassays are highly sensitive for detecting classes of drugs but require confirmatory tests (e.g., GC-MS) due to potential cross-reactivity and to achieve definitive identification and quantification.

Confirmatory Analysis - Definitive ID

  • Confirms presence & quantifies specific drugs/poisons post-presumptive tests; legally defensible.
  • Chromatography-Mass Spectrometry (MS) based: Gold standard for definitive ID.
    • Separation by chromatography; ID & quantification by MS.
    • Gas Chromatography-Mass Spectrometry (GC-MS):
      • For volatile & thermally stable compounds.
      • Analyte vaporized, GC separation, MS ionization, fragmentation & detection.
      • Provides unique mass spectrum ('molecular fingerprint').
    • Liquid Chromatography-Tandem Mass Spectrometry (LC-MS/MS):
      • For non-volatile, polar, or thermally labile compounds.
      • Tandem MS (MS/MS) enhances specificity & sensitivity.
  • High-Performance Liquid Chromatography (HPLC):
    • Used with specific detectors (e.g., Diode Array Detector - DAD, UV-Vis).
    • Primarily for quantification; MS coupling for confirmation.
  • Elemental Analysis (Metals):
    • Atomic Absorption Spectroscopy (AAS): Detects specific metals (e.g., Pb, Hg, As).
    • Inductively Coupled Plasma-MS (ICP-MS): Ultra-sensitive for trace/toxic metals.

⭐ GC-MS is widely regarded as the gold standard for the confirmation of most organic drugs of abuse and their metabolites in forensic toxicology.

LC-MS/MS principle diagram

Results Interpretation - Case Conclusions

  • Correlate analytical results with autopsy, clinical history, & circumstances.
  • Critical factors:
    • Pharmacokinetics (ADME, $t_{1/2}$, metabolites).
    • Post-Mortem Redistribution (PMR): drug levels change post-death.
    • Drug interactions, tolerance.
    • Contamination, chain of custody integrity.
  • Reporting: Qualitative (drug presence) & Quantitative (concentration).
  • Interpret levels: therapeutic, toxic, lethal. Link to cause of death.
  • Final opinion: based on all available evidence.

⭐ Significant PMR occurs with lipophilic drugs (e.g., TCAs, digoxin, amphetamines). Prefer femoral/iliac blood over heart blood for analysis to minimize PMR effects, as heart blood concentrations can be falsely elevated due to diffusion from nearby organs like the liver or lungs after death for certain drugs.

High‑Yield Points - ⚡ Biggest Takeaways

  • Screening tests (e.g., immunoassays) are presumptive; confirmatory tests (e.g., GC-MS) are definitive.
  • GC-MS is the gold standard for identifying most organic poisons.
  • HPLC is preferred for non-volatile or thermolabile substances.
  • Atomic Absorption Spectroscopy (AAS) is key for metallic poison detection (e.g., Arsenic, Lead).
  • Immunoassays (ELISA, EMIT) offer rapid screening, especially in urine.
  • Strict adherence to Chain of Custody is mandatory for legal evidence.

Practice Questions: Analytical Toxicology Methods

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Which of the following tests is commonly used for the detection of heavy metals?

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Flashcards: Analytical Toxicology Methods

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Color of post mortem hypostasis in case of opium poisoning is _____.

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Color of post mortem hypostasis in case of opium poisoning is _____.

black/grayish

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