The Core Lab Sciences: Biology, Chemistry, Physics

Forensic biology covers biological material deposited at scenes or on individuals: blood, semen, saliva, sweat, urine, vaginal secretions, hair, fingernails, and tissue. The workflow starts with detection (is biological material present?), moves through characterisation (which fluid is it?), and ends with profiling (whose DNA is in it?). Each step uses different methods, and not every sample makes it to the profiling stage.

Serology provides the presumptive and confirmatory tests for fluid type. Blood is detected at scene with luminol or the Kastle-Meyer test (a phenolphthalein-based colour screen), the traditional presumptive test for blood. Semen is identified by acid phosphatase activity or the presence of prostate-specific antigen (PSA). Saliva is detected by salivary amylase. These tests tell the analyst what biological material is worth submitting for DNA.

STR profiling is the current standard for DNA identification. It measures the number of repeated short sequences at each of a defined set of loci. The CODIS system in the United States uses 20 core STR loci; the UK NDNAD uses 16. The probability of two unrelated individuals sharing the same profile at all loci is vanishingly small, typically stated as one in several billion or more for full profiles. Partial profiles (fewer loci amplified, common with degraded samples) carry a weaker statistical weight and should be reported accordingly.

Forensic chemistry handles the largest volume of casework in most national laboratory systems. Drug seizure analysis alone accounts for the majority of submissions to many labs. The chemist's fundamental task is the same in every case: identify the chemical composition of an unknown substance, and compare samples to determine whether they share a common source or origin.

Drug analysis follows a tiered protocol. A colour presumptive test (Scott for cocaine, Marquis for opiates and amphetamines, Duquenois-Levine for cannabis) gives an initial screen, not a confirmed identification. Confirmation requires an instrumental technique, with GC-MS as the accepted gold standard. The result must identify the specific compound (e.g., MDMA not just 'amphetamine-type substance') and, in some jurisdictions, quantify the purity, because sentencing in many legal systems scales with quantity and concentration.

• Controlled drugs: colour tests screen, GC-MS or HPLC-MS confirms identity and purity.
• Explosives residues: ion chromatography (IC) and GC-MS detect characteristic ions (nitrates, peroxides, TATP, PETN) from post-blast debris or suspected devices.
• Fire debris: headspace extraction followed by GC-MS screens for C6-C12 hydrocarbons indicating petroleum-based accelerants.
• Ink and printing: HPTLC and mass spectrometry compare ink formulations in questioned-document support work.
• Toxicological chemistry: when forensic toxicology is integrated into the chemistry section, immunoassay screens blood or urine; GC-MS or LC-MS-MS confirms the specific compound and concentration.

Explosives work carries additional complexity because the blast destroys most of the original material. Post-blast analysis targets residues on surfaces near the seat of the explosion, swabs from a suspect's hands, and fragments from the device itself. Inorganic components (perchlorates, nitrates) survive heat better than organic compounds, so IC plays a larger role in post-blast work than in intact-device analysis.

Trace examination works with fragments so small they are often invisible at the scene: glass shards, paint flakes, single fibres, soil particles, tool marks, tyre marks, and shoe prints. The physical scientist's task is comparison: does this sample from a suspect share a common origin with this sample from the scene? The question is about the physical and optical properties of the material, not its chemistry at the molecular level (though chemical methods are frequently combined).

Glass comparison is a well-validated area of trace evidence. When a window is broken, fragments scatter backward toward the person who broke it as well as forward. Those fragments can be collected from clothing or hair. The refractive index is measured precisely using the glass refractive index measurement (GRIM3) technique, and the elemental profile is determined by SEM-EDX or LA-ICP-MS. A match between scene glass and suspect clothing is expressed statistically using population databases of glass types.

Fibre evidence requires both discrimination (is this fibre type consistent with the source garment?) and transfer context (how and when did it move?). A fibre from a victim's jumper on a suspect's car seat is meaningful only if its persistence is considered: the same fibre would be meaningless if the two people had any innocent prior contact. This is why trace evidence almost always needs to be assessed alongside a statement from both suspect and victim about their contact history.

Gunshot residue (GSR) is the microscopic particulate cloud ejected from a firearm's barrel, cylinder gap, and breech when a round is fired. It lands on the hands, face, and clothing of the shooter and, to a lesser extent, on bystanders and surfaces nearby. The primary analytical method is scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX), which identifies the characteristic three-element particles of lead, barium, and antimony from conventional primer formulations.

The interpretation of a positive GSR result is more complex than it first appears. The three-element particles are highly characteristic of a firearms discharge, but lead-barium-antimony particles from non-firearms sources (some industrial processes, certain fireworks, brake dust) have been documented. Lead-free 'green' primers produce different elemental signatures, requiring updated analytical criteria. And persistence is short: active hand-washing removes most particles within an hour or two, and normal activity erodes the count over the same timeframe.

Across biology, chemistry, and trace examination, the forensic scientist's job ends not when the instrument produces a number but when that number is correctly communicated to the investigator and, eventually, to the court. The UK Forensic Science Regulator and the European Network of Forensic Science Institutes (ENFSI) have both published guidance requiring scientists to report findings within a likelihood ratio (LR) framework where possible, stating how much more probable the evidence is if the prosecution hypothesis is true than if the defence hypothesis is true.

For DNA, this is routine: a full STR match is reported as a likelihood ratio in the billions. For trace evidence, the calculation is harder because the relevant population databases (how many vehicles have this paint sequence? how many blue polyester jumpers of this dye lot?) are often incomplete. In those cases, scientists express their findings qualitatively, using terms calibrated to their laboratory's standard scale, from 'weak support' to 'very strong support' for a common origin. The scale must be defined in the report, not assumed.

• A match does not equal proof of contact. It raises the probability that two samples share an origin. The investigator and the court assign that probability its evidential weight.
• A non-match does not equal proof of no contact. Transfer may not have occurred, or the evidence may not have persisted to collection.
• Validated methods, accredited labs, and independently audited interpretation criteria are the quality markers a court should expect.