Forensic chemistry

In short

Forensic chemistry is the application of analytical chemistry to legal questions: what a seized powder is, whether a fire was set with petrol, whether a person fired a gun, whether a paint smear on a victim came from the accused's car. It uses presumptive colour tests, chromatography, spectroscopy and elemental analysis to identify and compare non-biological evidence, and to report it in a form that stands cross-examination in court. It matters because conviction under the NDPS Act, 1985, the Explosive Substances Act, 1908 and most arson and adulteration prosecutions rests on a chemical examiner's report. It is examined in B.Sc/M.Sc Forensic Science papers, CSIR-UGC NET (Forensic Science), and recruitment tests for CFSL/State FSL Scientific Assistant and Scientific Officer (Chemistry) posts, and is used daily in FSL chemistry and narcotics divisions, excise laboratories and private testing labs.

Explained

Scope, principles and legal framework

• Definition: the branch of forensic science dealing with chemical identification and comparison of evidence — drugs, poisons in trade form, explosives, accelerants, gunshot residue, paints, glass, fibres, inks, petroleum products, illicit liquor, adulterated food and metals.
• Identification vs comparison: identification answers "what is this substance?" (drug, explosive); comparison answers "did these two samples share a source?" (paint, glass, ink). Comparison evidence is usually class evidence, not individualising.
• Analytical scheme (always two-tier): presumptive/screening test → confirmatory test by an orthogonal technique. A colour test alone is never a conclusion; confirmation must rest on a structure-specific method (GC-MS, FTIR, NMR).
• Locard's exchange principle underpins all trace evidence: contact leaves transfer; chemistry recovers and identifies it.
• Chain of custody: sealed sample, forwarding note, seal impression, laboratory register, examiner's signed report. Any break is attacked in cross-examination.
• Statutes and provisions commonly asked:
  • NDPS Act, 1985 — narcotic drugs and psychotropic substances; punishment graded by small / commercial quantity (quantities are in the Government notification under the Act and are amended from time to time). Standing Order 1/88 and 1/89 govern seizure, sampling and disposal.
  • Bharatiya Nagarik Suraksha Sanhita, 2023 (BNSS), in force from 1 July 2024, replaced the CrPC: the report of a Government Scientific Expert (Chemical Examiner, Director of FSL, Serologist, Explosives expert and others listed) is admissible in evidence without the examiner being called, under Section 329 BNSS (corresponding to the old Section 293 CrPC). The court may still summon the expert.
  • Bharatiya Sakshya Adhiniyam, 2023 (BSA) replaced the Evidence Act — expert opinion is admissible under its expert-opinion provision (formerly Section 45 of the Evidence Act); electronic instrument records are electronic evidence.
  • Explosive Substances Act, 1908; Explosives Act, 1884 and Explosives Rules, 2008; Arms Act, 1959 for firearms and ammunition; Poisons Act and Drugs and Cosmetics Act, 1940 for pharmaceutical and poison cases; FSS Act, 2006 for food adulteration; Petroleum Act, 1934 and oil-marketing company specifications for fuel adulteration.
  • Arson and mischief by fire are prosecuted under the Bharatiya Nyaya Sanhita, 2023 (BNS) (which replaced the IPC from 1 July 2024) — the mischief-by-fire and arson provisions correspond to the old IPC Sections 435/436.
• Quality system: FSLs seek NABL accreditation to ISO/IEC 17025 for testing and ISO/IEC 17020 for inspection; a forensic report should state method, uncertainty where applicable, and reference standards used.

Techniques: how each one works and what it is used for

• Presumptive colour (spot) tests: a reagent forms a coloured product with a functional group. Fast, cheap, destructive of a little sample, not specific — only exclusionary value when negative.
  • Marquis (formaldehyde + conc. H₂SO₄): purple→violet with opiates (morphine, heroin, codeine); orange→brown with amphetamine/methamphetamine, dark purple-black with MDMA.
  • Mecke (selenious acid/H₂SO₄): blue-green with opiates. Nitric acid test: morphine orange-red, heroin yellow-green.
  • Duquenois–Levine (vanillin + acetaldehyde in ethanol, then HCl, then chloroform): purple transferring to the chloroform layer — cannabis.
  • Cobalt thiocyanate (Scott test): blue with cocaine. Dille–Koppanyi (cobalt acetate): violet with barbiturates. Mandelin (ammonium vanadate/H₂SO₄): broad alkaloid screen.
  • Van Urk / p-DMAB: blue-violet with LSD and other indoles.
• Microcrystal tests: a reagent precipitates the drug as characteristic crystals (e.g. gold chloride with methamphetamine) viewed under a polarising microscope — cheap, fairly selective, operator-dependent.
• Thin-layer chromatography (TLC): separation on silica by differential adsorption; sample compared with a reference standard run on the same plate. Reported as Rf. Screening-level.
• Gas chromatography (GC): volatile analytes partition between carrier gas (N₂, He) and a stationary phase in a capillary column; detected by FID (hydrocarbons, accelerants), TCD, ECD (halogenated), NPD/TSD (N- and P-containing drugs and pesticides). Retention time identifies, peak area quantifies.
• GC-MS: the gold standard — GC separates, the mass spectrometer fragments each eluting compound (electron impact, 70 eV) and gives a mass spectrum that is a molecular fingerprint, searchable against NIST/SWGDRUG libraries. Selected ion monitoring (SIM) raises sensitivity for a target.
• HPLC and LC-MS/MS: for thermolabile and non-volatile analytes — LSD, benzodiazepines, dyes in ink, drug formulations. LC-MS/MS gives very low detection limits and quantification against deuterated internal standards.
• FTIR spectroscopy: mid-IR absorption (4000–400 cm⁻¹) gives functional-group and fingerprint information; ATR-FTIR handles solids and films directly. Routine for polymers, paints, fibres, explosives, drug cutting agents.
• Raman and portable Raman / handheld FTIR: non-destructive, through-container screening of white powders and suspect explosives at scene.
• UV-visible spectrophotometry: quantification via Beer–Lambert law; λmax characterises a chromophore. Used for colorimetric kits, dyes, and quantifying drugs after extraction.
• AAS, ICP-OES and ICP-MS: elemental analysis — heavy metals (As, Pb, Hg, Tl), gunshot residue elements, trace elements in glass; ICP-MS is best for ppb-level multi-element profiles.
• SEM-EDX: images a particle and gives its elemental spectrum — the standard for GSR (characteristic Pb-Sb-Ba spheroids) and for small inorganic particles.
• XRD: identifies crystalline phases; distinguishes polymorphs and inorganic fillers.
• Capillary electrophoresis, ion chromatography: inorganic ion profiles of post-blast residue (NO₃⁻, NO₂⁻, ClO₃⁻, ClO₄⁻, SO₄²⁻, NH₄⁺, K⁺).
• DSC/TGA, pyrolysis-GC-MS: polymer and paint binder characterisation.
• NMR and elemental (CHNS) analysis: structure elucidation of new psychoactive substances.

Drug and narcotics analysis

• Classification: narcotics/opiates (opium, morphine, heroin), cannabinoids (ganja, charas/hashish, hash oil), cocaine and coca derivatives, synthetics/psychotropics (amphetamine, methamphetamine, MDMA, LSD, mephedrone), depressants (barbiturates, benzodiazepines), and new psychoactive substances (NPS) that need NMR/high-resolution MS.
• Workflow: physical description and weighing of whole seizure → homogenisation → representative sampling and sealing → presumptive colour test → TLC → confirmation by GC-MS or FTIR → quantitative purity assay (GC-FID or HPLC against certified reference standard) → report.
• Why purity matters in India: under the NDPS Act, as settled by the Supreme Court in *Hira Singh v. Union of India* (2020), the entire weight of the mixture/preparation (not merely the pure drug content) is reckoned for small/commercial quantity. Purity is still reported because it bears on sentencing, source linkage and drug profiling.
• Sampling under Standing Orders: drawing representative samples from each package, or from bunched packages of the same description within the limits the Standing Order allows, in duplicate — one for analysis, one kept for court.
• Opium and cannabis specifics: opium is assayed for morphine content; charas is the separated resin, ganja the flowering/fruiting tops, and the seeds and leaves alone (when not accompanied by the tops) are outside the Act's definition of ganja — a favourite exam point.
• Pharmaceutical dosage forms: tablet/capsule identification by markings, weight uniformity, dissolution and HPLC assay; spurious-drug cases under the Drugs and Cosmetics Act.
• Limitations: adulterants and diluents (lactose, mannitol, caffeine, paracetamol, chalk, quinine) complicate quantification; isomers (d- vs l-methamphetamine; cathinone analogues) need chiral or derivatised analysis.

Arson accelerants, explosives and gunshot residue

• Arson: debris (burnt carpet, soil, wood, cloth) and a comparison/substrate control are sealed in nylon bags or clean metal cans — never polythene, which leaks hydrocarbons and can contribute them.
  • Recovery: passive headspace concentration on activated charcoal strip (ASTM E1412), dynamic headspace, steam distillation or solvent extraction; eluted with CS₂ or pentane.
  • Analysis: GC-FID / GC-MS and pattern recognition of the chromatogram against ASTM E1618 classes — gasoline (strong aromatics C₆–C₁₀, toluene/xylene/trimethylbenzene cluster), kerosene and diesel (normal-alkane "picket fence", C₉–C₂₀), light petroleum distillates, alcohols.
  • Pyrolysis interference: burnt carpet backing and plastics generate hydrocarbons; hence the substrate control.
  • Fire chemistry terms: flash point, fire point, autoignition temperature, flammability (explosive) limits LEL/UEL, heat of combustion, flashover, backdraught.
• Explosives:
  • Low explosives deflagrate (<1,000 m/s): black powder (KNO₃ + S + charcoal), smokeless powder, chlorate/perchlorate mixtures, potassium nitrate–sugar and fireworks compositions. High explosives detonate (several thousand m/s): primary — lead azide, mercury fulminate, PETN in detonators; secondary — TNT, RDX, HMX, PETN, tetryl, nitroglycerine, dynamite, ANFO, PEK/PETN-based sheet explosives, and commercial slurry/emulsion explosives widely used in Indian mining.
  • Terms: VOD (velocity of detonation), brisance, oxygen balance, sensitivity, detonator, booster, initiating train.
  • Post-blast residue: swabs from crater and fragments; inorganic ions by ion chromatography/CE/spot tests (Griess test for nitrite, diphenylamine for nitrate/chlorate); organic explosives by HPLC-UV, LC-MS/MS, GC-ECD/TEA and confirmation by FTIR/Raman. Trace detection at airports uses IMS (ion mobility spectrometry) swab machines.
  • Indian context: PESO (Petroleum and Explosives Safety Organisation) licenses explosives under the Explosives Rules, 2008; NSG/BDDS and FSL explosives divisions handle post-blast work.
• Gunshot residue (GSR):
  • Comes from the primer (lead styphnate, barium nitrate, antimony sulphide) and propellant. Confirmatory: SEM-EDX detection of Pb–Sb–Ba containing spheroidal particles on adhesive-stub lifts from hands, face and clothing; heavy-metal-free primers give Sr/Ti/Zn-based signatures instead.
  • Chemical tests: dermal nitrate (diphenylamine) test — now considered unreliable; sodium rhodizonate for lead (pink→blue with HCl); Walker test on cloth for nitrite distribution around a bullet hole; Griess test for nitrites; Harrison–Gilroy test for Sb and Ba (less specific). Modified Griess and dermal nitrate are presumptive only.
  • Uses: firing range estimation from residue pattern, entry vs exit hole, distinguishing suicide from homicide, bullet-hole direction.
  • Limitation: GSR transfers and is lost rapidly — sampling within a few hours is advised; occupational exposure (mechanics, battery workers, firework handlers) gives false Pb/Sb/Ba.

Trace evidence: paint, glass, fibre, soil, ink and petroleum

• Paint: layer structure examined under a stereomicroscope (a multi-layer match is strong); pigments and binder by FTIR, pyrolysis-GC-MS, SEM-EDX, XRD; automotive paints compared against the PDQ (Paint Data Query) type of database for make/model. Chips with physical fit (jigsaw match) are individualising.
• Glass: refractive index by the hot-stage oil immersion (Emmons/GRIM) method; density by flotation; elemental profile by LA-ICP-MS; fracture analysis — radial and concentric cracks, 4R rule (Radial cracks form Right angles on the Reverse side of the force), direction of impact and sequence of shots from cone (Hertzian) fractures.
• Fibres: natural (cotton, wool, silk, jute) vs regenerated (viscose, rayon) vs synthetic (nylon, polyester, acrylic); identified by microscopy, melting behaviour, FTIR and dye comparison by microspectrophotometry/TLC.
• Soil and dust: colour (Munsell), density gradient column, mineral content by polarising microscopy and XRD, pH and organic matter.
• Inks and questioned documents (chemistry part): TLC/HPTLC of extracted dyes, video spectral comparator for ink discrimination, ink dating by solvent-loss ratios (indicative only); obliterated/charred documents restored with reagents and IR imaging; counterfeit currency examined for security features, optically variable ink and chemical bleaching.
• Petroleum product adulteration: kerosene in petrol/diesel detected by GC distribution, density and distillation (ASTM D86 / IS 1448 series), and historically by the marker/filter-paper tests; BIS specifications IS 2796 (motor gasoline) and IS 1460 (automotive diesel) give the quality limits (verify the current edition before quoting values). Lubricating oil and solvent cases also come to FSL.
• Illicit liquor: the classic Indian hooch tragedy case — distinguish ethanol from methanol by GC-FID (headspace), by specific chemical tests (chromotropic acid test for methanol after oxidation to formaldehyde) and refractometry; the toxic agent is methanol's metabolites (formic acid). Analysis under state Excise Acts; the medico-legal/toxicology side belongs to toxicology.
• Miscellaneous: tear gas and chemical agents, acid-attack residues, chemical tampering of seals, metallurgical failure and restoration of erased engine/chassis numbers (etching with acidic reagents such as Fry's or dilute HCl-based reagents exploiting the strained metal below the stamped number).

Reporting, interpretation and the chemist in court

• A report states: particulars of sealed parcels and seal condition, physical description and weight, methods used, results, and an opinion limited to what the data support.
• Statistical caution: chemistry rarely individualises. Correct phrasing is "consistent with having a common origin" / "cannot be distinguished", not "is from".
• Blanks and controls: reagent blank, substrate control, negative control, positive control with a certified reference material; carry-over blanks between GC runs.
• Validation parameters examined in exams: specificity/selectivity, LOD (commonly 3×signal-to-noise or 3.3σ/S) and LOQ (10×S/N or 10σ/S), linearity (r² typically ≥0.99), accuracy, precision (repeatability, intermediate), robustness, range, and measurement uncertainty.
• Destructive vs non-destructive order: always non-destructive first (visual, microscopy, XRF, Raman, ATR-FTIR), then micro-destructive, then destructive; preserve part of the sample for defence re-analysis.
• Court exposure: admissibility under Section 329 BNSS; cross-examination attacks sampling adequacy, chain of custody, instrument calibration records and library-match quality.

Key points

• Presumptive test - rapid colour/crystal test; indicative only, never a conclusion on its own.
• Confirmatory test - structure-specific method (GC-MS, FTIR, NMR) that identifies the compound.
• Marquis reagent - formaldehyde in conc. H₂SO₄; purple-violet for opiates, orange-brown for amphetamine (dark purple-black for MDMA).
• Duquenois–Levine - purple colour passing into the chloroform layer indicates cannabis.
• Scott (cobalt thiocyanate) test - blue colour for cocaine.
• GC-MS - separation plus mass fingerprint; the accepted gold standard for drug and accelerant identification.
• ASTM E1412 / E1618 - charcoal-strip passive headspace recovery and ignitable-liquid classification in arson work.
• Picket-fence chromatogram - evenly spaced n-alkanes, typical of kerosene/diesel; aromatic cluster indicates petrol.
• Low vs high explosive - deflagration (<1,000 m/s) versus detonation (thousands of m/s); brisance and VOD describe the latter.
• GSR confirmation - SEM-EDX spheroids containing Pb, Sb and Ba; chemical colour tests are presumptive.
• Sodium rhodizonate - lead-specific test for bullet holes and residue (pink, turning blue with HCl).
• 4R rule - radial cracks form right angles on the reverse of the side where force was applied.
• Refractive index - key glass comparison property, measured by hot-stage oil immersion.
• NDPS Act, 1985 - small and commercial quantities decide punishment; whole mixture weight counts (*Hira Singh*, 2020).
• Section 329 BNSS - Government Scientific Expert's report is admissible without the expert appearing (old Section 293 CrPC); BNSS replaced the CrPC from 1 July 2024.
• Chain of custody - unbroken, documented, sealed custody; the commonest ground for demolishing chemical evidence.
• Beer–Lambert law - basis of all spectrophotometric quantification: A = εcl.
• LOD/LOQ - 3 and 10 times signal-to-noise (or 3.3σ/S and 10σ/S) — quote with the method.

Formulas and examples

Example 1 — Purity of a heroin seizure by HPLC (external standard). A 50.0 mg portion of seized powder is dissolved and made up to 50.0 mL; 10.0 µL is injected and gives a diacetylmorphine peak area of 182,400. A 0.500 mg/mL reference standard injected identically gives 240,000.

• C_sample = (182,400 / 240,000) × 0.500 = 0.380 mg/mL.
• Mass of heroin in the 50.0 mL solution = 0.380 × 50.0 = 19.0 mg.
• Purity = (19.0 / 50.0) × 100 = 38.0% w/w.
• If the total seizure was 300 g, pure heroin = 300 × 0.380 = 114 g. (Note: for small/commercial quantity under the NDPS Act the whole 300 g is reckoned, per *Hira Singh*.)

Example 2 — Beer–Lambert quantification of a dye extracted from ink. A 1.00 cm cell gives A = 0.620 at λmax; ε = 12,400 L mol⁻¹ cm⁻¹.

• c = A/(εl) = 0.620/(12,400 × 1.00) = 5.00 × 10⁻⁵ mol L⁻¹.
• If the molar mass is 320 g/mol, concentration = 5.00 × 10⁻⁵ × 320 = 0.0160 g/L = 16.0 mg/L.
• Transmittance: T = 10⁻⁰·⁶²⁰ = 0.240, i.e. 24.0 %T.

Example 3 — TLC Rf comparison in a cannabis case. Solvent front moved 8.0 cm. The questioned extract spot centre is at 4.8 cm; the THC reference standard spot at 4.9 cm; a cutting-agent spot at 2.4 cm.

• Rf (questioned) = 4.8/8.0 = 0.60; Rf (standard) = 4.9/8.0 = 0.61; Rf (other spot) = 2.4/8.0 = 0.30.
• Conclusion: the questioned spot is not distinguishable from the standard on this system — screening-level only; confirm by GC-MS. (Rf agreement on one plate is not identification.)

Example 4 — LOD and LOQ for a GC-MS explosive method. Blank response SD σ = 180 counts; calibration slope S = 5,400 counts per ng/µL.

• LOD = 3.3 × 180 / 5,400 = 0.11 ng/µL.
• LOQ = 10 × 180 / 5,400 = 0.33 ng/µL.
• A residue extract reading 0.25 ng/µL is detectable but not reliably quantifiable — report as "detected, below quantification limit".

Example 5 — Oxygen balance of TNT (C₇H₅N₃O₆, M = 227.13 g/mol). Using OB = −1600 (2x + y/2 − z)/M with x = 7, y = 5, z = 6:

• 2x = 14; y/2 = 2.5; sum = 16.5; 16.5 − 6 = 10.5.
• OB = −1600 × 10.5 / 227.13 = −73.96 %, i.e. strongly oxygen-deficient — hence TNT's sooty detonation and its use in oxygen-rich mixtures (amatol, ammonal and other ammonium-nitrate blends).

Where it is used

• CFSL (DFSS, MHA) at Delhi, Chandigarh, Hyderabad, Kolkata, Bhopal, Guwahati and Pune, CBI's CFSL, and every State FSL and Regional FSL — chemistry, narcotics, explosives, ballistics-support and document divisions; posts of Scientific Assistant, Scientific Officer, Assistant Director and Director.
• Enforcement agencies: Narcotics Control Bureau, Central Bureau of Narcotics, DRI and Customs laboratories, state excise laboratories, NIA/NSG BDDS support, and Revenue Laboratories for petroleum and liquor cases.
• Regulatory and industrial testing: FSSAI-notified food laboratories, state Drugs Control laboratories and CDSCO testing labs, oil-marketing company quality labs checking fuel adulteration, PESO-licensed explosives manufacturers (e.g. mining-explosive producers) running QC on emulsions and detonators.
• Private and corporate: NABL-accredited private testing laboratories, insurance-linked fire investigation consultancies, and anti-counterfeiting/brand-protection labs using ink and packaging chemistry.
• Academic and R&D roles: teaching posts in forensic science departments, DRDO and CSIR laboratories working on explosive trace detection, and development of portable Raman/IMS detectors for Indian airports and metro security.
• Courtroom work: appearing as a Government Scientific Expert, preparing opinions for prosecution and defence, and training police investigation officers on sealing and sampling.

Interview questions

• Q: Why can a colour test never be the basis of a drug report? A: Colour tests respond to a functional group shared by many compounds, so they give false positives; they only screen, and a structure-specific confirmation such as GC-MS is required.
• Q: What reagent and colour would you expect for heroin, cocaine and cannabis? A: Marquis gives purple-violet with heroin, cobalt thiocyanate (Scott) gives blue with cocaine, and Duquenois–Levine gives a purple colour that transfers into the chloroform layer with cannabis.
• Q: How is an accelerant recovered from fire debris and why is polythene avoided? A: By passive headspace adsorption on an activated charcoal strip (ASTM E1412) and solvent elution; polythene bags are permeable to hydrocarbons and can themselves contribute hydrocarbon peaks.
• Q: How do you distinguish petrol from kerosene on a chromatogram? A: Petrol shows a dense cluster of C₆–C₁₀ aromatics (toluene, xylenes, trimethylbenzenes); kerosene shows an evenly spaced n-alkane "picket fence" from about C₉ to C₁₆.
• Q: What makes SEM-EDX the confirmatory test for gunshot residue? A: It images the characteristic spheroidal morphology of fused primer particles and simultaneously confirms the Pb-Sb-Ba elemental combination in a single particle, which colour tests cannot do.
• Q: Why is the dermal nitrate test no longer relied upon? A: Diphenylamine reacts with nitrates and nitrites from fertilisers, tobacco, cosmetics and urine, so a positive result does not establish firing.
• Q: Does purity determine the offence category under the NDPS Act? A: No — following *Hira Singh v. Union of India* (2020) the entire weight of the mixture or preparation is reckoned for small and commercial quantity; purity is reported for sentencing, profiling and source linkage.
• Q: What is the difference between a low and a high explosive? A: A low explosive deflagrates with a subsonic reaction front (below about 1,000 m/s) and works by pressure build-up, while a high explosive detonates with a supersonic shock wave at several thousand metres per second and shows brisance.
• Q: Which part of the cannabis plant is outside the NDPS definition of ganja? A: The seeds and leaves when not accompanied by the flowering or fruiting tops; charas (the separated resin) and the tops are covered.
• Q: Under which provision is a chemical examiner's report admissible without his appearing in court? A: Section 329 of the Bharatiya Nagarik Suraksha Sanhita, 2023 (which replaced Section 293 CrPC when the BNSS came into force on 1 July 2024); the court may still summon the expert.
• Q: A hooch tragedy sample comes in — what do you look for and how? A: Methanol against ethanol by headspace GC-FID, supported by the chromotropic acid test for methanol after oxidation to formaldehyde; methanol is quantified because its metabolites formic acid and formaldehyde cause the blindness and deaths.
• Q: Why is the order of examination non-destructive first? A: To preserve the sample's integrity and leave enough material for re-analysis by the defence or a second laboratory, which the court may direct.
• Q: What is the role of a substrate control in arson analysis? A: It is unburnt material of the same type from an unaffected area, analysed in parallel to show that any hydrocarbons found are not pyrolysis products of the substrate itself.

Common mistakes

• Treating a positive presumptive colour test, or a matching TLC Rf, as identification and omitting the confirmatory method.
• Quoting NDPS small and commercial quantities from memory — they are set by Government notification and amended; and wrongly applying pure-drug weight instead of the whole mixture weight.
• Citing the CrPC Section 293, the IPC and the Indian Evidence Act in answers — these have been replaced by the BNSS (Section 329), the BNS and the BSA from 1 July 2024.
• Packing arson debris in polythene, or sending debris with no substrate control, then reporting pyrolysis hydrocarbons as an accelerant.
• Reporting GSR as proof of firing when the person could have been near a discharge, handled a fired weapon, or been exposed occupationally to Pb/Sb/Ba.
• Overstating comparison evidence: writing "the paint is from the accused's vehicle" instead of "cannot be distinguished from / is consistent with".
• Unit and dilution slips in purity calculations — forgetting the dilution factor, mixing mg/mL with µg/mL, or ignoring the injection volume.
• Neglecting blanks, calibration records and reference standards, which is exactly where cross-examination begins; and failing to document an unbroken chain of custody.

Related topics

Forensic toxicology · Forensic biology and serology · DNA profiling · Forensic medicine basics · Questioned documents and fingerprints · Instrumental methods of analysis