Sample Collection, Packaging and Chain of Custody for Chemical Evidence

The packaging of a chemical exhibit is itself a chemical decision. The container and its closure material must not react with, absorb, or release compounds that would alter the composition of the exhibit, and they must not allow external contamination to enter. These requirements follow from the chemistry of the analytes and the mechanical properties of packaging materials.

Polyethylene (PE) bags, widely available and cheap, are a common packaging material in field settings. They are adequate for many solid drug exhibits at ambient temperatures for short periods. But polyethylene is permeable to volatile organic compounds at a molecular level: a bag containing a volatile accelerant such as petrol will lose measurable quantities of lighter fractions through permeation within hours at elevated ambient temperatures. A fire debris sample packed in a PE bag and left in a hot vehicle or a non-climate-controlled evidence room can lose the very compounds that would identify the accelerant. ASTM E1492 (Standard Practice for Receiving, Documenting, Storing, and Retrieving Evidence in a Forensic Science Laboratory) specifically prohibits PE bags for fire debris samples for this reason. The required container is an airtight, solvent-rinsed metal paint can or a heat-sealed nylon bag, both of which have negligible permeability to volatile organics.

For drug exhibits, PE bags are generally acceptable for non-volatile solids, provided they are heat-sealed and the seal integrity is verified. But for liquid drug samples (GHB solutions, liquid MDMA, ketamine solutions), PE can leach plasticisers into the sample matrix, potentially interfering with GC-MS analysis. Glass vials with PTFE-lined caps are preferred for liquid samples destined for GC-MS or LC-MS/MS analysis.

For post-blast debris, the packaging must prevent cross-contamination between different scene zones. A fragment from the seat of explosion and a swab from the perimeter are different exhibits with potentially different residue profiles; mixing them (even by re-using gloves without changing between items) defeats the spatial information the chemist needs to link an explosive type to a specific location. The full zonal sampling strategy is covered in post-blast residue sampling and the IMS, GC-ECD, LC-MS/MS workflow.

At the scene, the investigator's packaging decisions create or destroy the analytical possibilities available to the laboratory. The forensic chemist, in an ideal world, provides written guidance to investigators before scenes are processed. In practice, field officers in Indian states, US jurisdictions, and UK police forces use standard exhibit-packaging kits with pre-printed labelling and written instructions that reflect laboratory requirements, under the guidance of forensic investigators or scenes-of-crime officers (SOCOs) who have received laboratory-provided training.

Solid drug exhibits include powders, compressed tablets, plant material (cannabis herb and resin), crystalline substances (methamphetamine, MDMA, cocaine HCl), and paste forms (heroin No. 4, cannabis oil pressed into slabs). Each physical form requires a tailored collection and packaging approach.

At the point of seizure. The seizing officer photographs the exhibit in situ before touching it. The photograph establishes the context of discovery (location, surrounding packaging, proximity to other items). The exhibit is then handled with gloves to prevent contamination from the officer's skin (which could introduce human DNA or interfere with any biological evidence adjacent to the drug exhibit). For powders and small quantities, the entire exhibit is packaged in a primary container (heat-sealed PE evidence bag or similar). For bulk seizures (multiple kilograms), the officer typically packages each discrete unit (each bag, each brick, each bale) separately, labels each with a unique exhibit number, and then packages all into a bulk outer container.

In India, under the NDPS Act 1985 (Section 52A) and Rules 2 and 3 of the Narcotic Drugs and Psychotropic Substances (Seizure, Storage, Sampling and Disposal) Rules 2014, the seizing officer is required to draw samples in the presence of a Magistrate or a Gazetted Officer not below the rank of Sub-Divisional Officer. For bulk seizures above a specified quantity, the Rules require that duplicate samples are drawn: one for examination and one for retention as the "court sample" that can be produced in evidence at the trial. The containers must be sealed, signed, and labelled with the case details, the exhibit number, and the date of seizure.

In the US, DEA field agents follow the DEA Agents Manual and SWGDRUG guidance. Chain-of-custody documentation begins with the seizing agent's field notes and continues with a DEA-7 (Report of Drug Property Collected, Purchased or Seized) form that records every custodian of the evidence from seizure to destruction. The exhibit is transported to a DEA Diversion field office or directly to a DEA laboratory, depending on the case type.

In the UK, police officers follow the Police and Criminal Evidence Act 1984 (PACE) Code of Practice B (searching and seizing) and the MoJ's Forensic Science Regulator's evidence management provisions. Exhibits are packaged in tamper-evident bags, sealed with evidence tape across the opening, and signed by the seizing officer. A unique exhibit reference (typically the officer's warrant number followed by a sequential number) appears on both the bag and in the Police National Computer record.

Tamper-evident sealing. The sealing mechanism must make tampering evident. Simple knot-tied PE bags are inadequate; they can be untied, material added or removed, and re-tied without obvious trace. Tamper-evident packaging uses heat sealing (the seal cannot be opened without visible destruction), adhesive tape with a pattern that reveals lifting (security evidence tape often contains a latent "OPENED" or void pattern that transfers to the bag when the tape is lifted), or purpose-designed adhesive evidence bags in which the flap adhesive bonds permanently and leaves a void message if removed.

At the laboratory, the forensic chemist records the condition of the tamper-evident seal before opening the exhibit. A seal that shows signs of tampering must be documented, photographed, and reported; the chain of custody must be able to account for any such breach.

Sub-sampling for analysis. When a portion of the exhibit is removed for analysis, the sampling event is a chain-of-custody event: it must be documented, the sub-sample assigned a sub-exhibit number or analytical portion identifier, and the remainder of the exhibit re-sealed (or retained in an open but controlled analytical container within the secure laboratory area) until the case is concluded. The subsequent analytical workflow, from this sub-sample to a signed court report, is described in Reference Standards, CRMs and the Quantification Baseline.

Liquid samples and fire debris require container choices that are driven by the chemistry of the target analytes, not by whatever is conveniently available at the scene.

Liquid drug samples. Liquid seizures (GHB solutions, ketamine injection vials, clandestine drug-precursor solutions, counterfeit pharmaceutical syrups) must be collected in containers compatible with their intended analytical use. Glass vials with PTFE-lined caps are the preferred primary container for any liquid destined for GC-MS or LC-MS/MS analysis: glass does not absorb or leach organic compounds, and PTFE closures are chemically inert to all forensic solvents. Capped with aluminium crimp seals, these containers are effectively tamper-evident because recrimping without specialist equipment leaves tooling marks.

Volume is a chain-of-custody datum for liquid exhibits: the container volume and the observed volume in the container at receipt, transfer, and analysis must be recorded. For a GHB solution in a prosecution for drug-facilitated sexual assault, the available volume may be very small (residues in a glass, dregs in a bottle), and every microliter must be accounted for across the custody chain.

Biological fluids (blood, urine) submitted as drug evidence are not in the forensic chemistry chain-of-custody scope at the point of biological collection; they enter it when they are submitted for drug identification or alcohol quantification, typically from a medical professional who has collected them using a standard collection kit (grey-top tube with sodium fluoride/potassium oxalate for blood ethanol, red-top for drug analysis). The collection kit itself constitutes part of the exhibit and must travel with the sample.

Fire debris. Fire debris is the exhibit class with the highest packaging-failure rate in forensic chemistry. The analytes of interest (C8 to C18 petroleum hydrocarbons from gasoline, kerosene, diesel, or white spirit) are volatile enough to permeate or evaporate through inadequate containers within hours.

The required containers are: (1) solvent-rinsed, unlined metal paint cans with friction-fit lids (the ASTM E1492 standard), providing excellent retention of volatile organics; or (2) heat-sealed nylon evidence bags (such as Kapak or equivalent nylon/polyethylene laminates with a nylon outer layer), which have been validated by multiple studies to retain C8-C12 hydrocarbons for periods of at least 90 days without significant loss. Standard PE bags are prohibited for fire debris by ASTM E1492; they cannot retain the volatile compounds that constitute the accelerant signature.

The correct procedure for fire debris collection is documented in ASTM E1388 (Standard Practice for Sampling of Headspace Vapors from Fire Debris Samples). Samples are collected from areas showing burn patterns, char, pour patterns, or concentrated burning. The sampler (the fire investigator or a scenes-of-crime officer under laboratory guidance) uses a clean, unused collection tool (trowel, spoon) for each sample, changing tools between collection areas. Each sample is immediately placed in the can or nylon bag, which is then sealed. The sealed container is labelled and a control sample (unburned substrate from an area of the scene not exposed to the fire) is also collected; without a control, distinguishing accelerant residues from pyrolysis products of the substrate is impossible.

In India, fire debris is typically collected by police officers or forensic investigation team members and submitted to the CFSL or state FSL chemistry section. The Bureau of Indian Standards (BIS) has not published a dedicated standard for fire debris container requirements, so in practice, CFSL SOPs reference ASTM E1492 and E1388. In the US, the National Fire Protection Association's NFPA 921 Guide for Fire and Explosion Investigations (2021 edition) provides the collection framework, with ASTM methods governing the analytical side. In the UK, the Forensic Science Regulator's codes reference the ISO 17025 general framework; specific fire debris guidance is maintained by the CAST and the ENFSI Fire Debris Working Group.

Post-blast debris. Post-blast samples share many packaging requirements with fire debris but introduce additional complexity: different exhibit types (swabs, fragments, soil) require different containers, and the potential for energetic material residues adds a safety dimension. Metal containers, which are appropriate for fire debris, should not be used for unexploded ordnance remnants or live energetic material samples, which are handled by the Explosive Ordnance Disposal (EOD) teams separately.

For swabs collected from blast scene surfaces, tamper-evident plastic tubes (analogous to buccal swab containers) or polypropylene vials with secure caps are used. The solvent choice and analytical workflow for these swabs follows from the explosives classification framework, different explosive classes leave different ionic and organic residue profiles that dictate extraction chemistry. Metal fragment samples are placed in separate plastic evidence bags. Soil samples from around the seat of explosion are collected in clean glass jars or PE bottles (no volatile accelerant is at issue here; the targets are low-volatility explosives residues). Each sample area is uniquely numbered on a scene diagram so the laboratory can reconstruct the spatial distribution of residues.

Chain of custody (CoC) is the documented sequence of individuals who had possession of, access to, or control over a physical exhibit from the moment of its collection to its production in court. A complete chain of custody does not merely list names; it records dates, times, the purpose of each transfer, the condition of the exhibit at each transfer, and the mechanism by which each custodian confirmed receipt.

India: BSA 2023 and the NDPS framework. The Bharatiya Sakshya Adhiniyam 2023 (which replaced the Indian Evidence Act 1872 with effect from 1 July 2024) governs the admissibility of evidence in Indian courts. Section 57 of the BSA 2023 addresses the admissibility of documents produced as evidence; the forensic chemistry report, as an expert opinion, is governed by Sections 39 to 43. But the evidentiary chain that makes the report meaningful is established by the prosecution's obligation to prove continuity of possession of the material exhibit, which is a principle developed through decades of Supreme Court and High Court case law under the old IEA and continuing under the BSA framework.

The NDPS Act 1985 adds specific requirements. Section 52 governs the procedure for arrests and seizures. Section 55 requires that seized property be produced before a Magistrate at the earliest. Rule 3 of the NDPS (Seizure, Storage, Sampling) Rules 2014 specifies that a sample shall be drawn and sealed in a manner as directed by the State Government, in the presence of the person from whom the seizure is made or two independent witnesses. The sealed sample must be signed by the seizing officer and the witnesses; one sample goes to the FSL, one is retained as the "court sample."

In landmark drug prosecution appeals (State of Punjab v. Baldev Singh, Supreme Court, 1999), the Supreme Court has emphasised that mandatory procedural requirements under the NDPS Act (including witness requirements for search and seizure) are not mere formalities but conditions for admissibility. A failure to observe these requirements, absent a reasonable explanation, can result in acquittal even where the forensic chemistry identification is not in doubt.

United States: Federal Rules of Evidence and the DEA protocol. Under the Federal Rules of Evidence (FRE), authentication of evidence (FRE Rule 901) requires that the proponent produce evidence sufficient to support a finding that the item is what the proponent claims it is. For physical drug evidence, this means establishing a chain of custody sufficient to show that the analysed sample is the same as the sample seized at the scene and has not been altered. Courts have held that an unbroken chain is not required (gaps can be explained and go to the weight rather than the admissibility of evidence, United States v. Howard-Arias, 679 F.2d 363, 366 (4th Cir. 1982)), but intentional gaps or significant unexplained gaps may lead to exclusion.

DEA Exhibit handling follows Form DEA-7a (Report of Drug Property Collected, Purchased or Seized), which records every transfer of custody with date, time, custodian signature, and seal condition. Laboratory receipt is logged in the DEA LIMS (STARLab). Each analyst who works on the exhibit signs into the LIMS, creating an electronic chain-of-custody record. When the analyst opens the exhibit packaging, the condition of the tamper-evident seal is noted, photographed, and recorded. Any discrepancy between the labelled weight and the received weight is immediately flagged as a critical chain-of-custody event.

State-level drug exhibits in the US follow analogous protocols under state evidence rules and the relevant state crime laboratory's accreditation requirements. Many states require chain-of-custody forms that track the exhibit through each transfer, a laboratory receipt log, an analyst sign-in log, and storage log entries for any period in which the exhibit is in a laboratory freezer or vault.

United Kingdom: FSR Codes and PACE. The Forensic Science Regulator's Codes of Practice and Conduct (2023 edition, published under Section 2 of the Forensic Science Regulator Act 2021) mandate specific requirements for exhibit management in accredited forensic laboratories. Exhibit packaging must be examined on receipt; the condition of any tamper-evident seals must be recorded; any signs of damage, contamination, or previously opened packaging must be noted and reported to the submitting authority. The exhibit must be stored in conditions appropriate to the exhibit type, with access restricted to laboratory personnel with a documented need to access it. Each access event must be logged.

Under PACE 1984 Code B, police officers in England and Wales have specific obligations regarding the seizure, handling, and recording of items taken from premises. A police officer who seizes a quantity of drugs at a search under a warrant must complete an exhibit label, place the item in a tamper-evident bag, seal it with evidence tape, sign the seal, and record the item in a property register. The continuity of custody from the seizing officer through to the forensic laboratory is maintained through the property register and the laboratory submission form (typically a police form specifying the case number, the exhibit reference, the date and time of submission, and the name of the officer making the submission).

In Scotland, Scots criminal procedure (governed by the Criminal Procedure (Scotland) Act 1995) has equivalent requirements; SPAFS operates under UKAS accreditation and FSR-equivalent protocols.

1. Seizure documentation
   Seizing officer photographs the exhibit in situ, completes the field seizure form (DEA-7 in the US, NDPS seizure register in India, PACE exhibit label in the UK), applies tamper-evident seal and signs across the seal.
2. Witness and sample duplication
   In India under NDPS Rules: draw two samples in the presence of a Gazetted Officer or Magistrate. One for FSL analysis, one as the court sample. Both sealed, signed by the seizing officer and witnesses, labelled identically.
3. Transfer to secure storage
   Exhibit transferred to evidence room or field office. Transfer signed and dated by both the outgoing and incoming custodians. Condition of seals verified at each handoff.
4. Laboratory submission
   Sealed exhibit submitted to forensic laboratory with a submission form listing the case number, exhibit reference, seizing officer's details, alleged substance, and request for analysis. Laboratory issues a receipt.
5. Laboratory receipt and registration
   Laboratory officer verifies the seal condition, photographs the sealed exhibit, records the intact weight (exhibit in packaging), assigns a LIMS case number, and logs the receiving analyst's name and date-time.
6. Analytical access
   Each time an analyst accesses the exhibit for analysis or sub-sampling, this is logged in the LIMS with analyst name, date, time, and purpose. The exhibit is re-sealed or maintained under controlled access conditions between access events.
7. Court production
   The exhibit is produced in court in its sealed, labelled condition. The prosecution calls the chain-of-custody witnesses (seizing officer, transferring officer, laboratory receiving officer, reporting analyst) to establish continuity.

The label on a forensic chemistry exhibit must carry enough information that any qualified person, examining the exhibit at any point in the chain of custody, can uniquely identify it, trace its origin, and verify that it has not been tampered with. The following fields are non-negotiable.

Unique exhibit reference. A number or alphanumeric code that is unique to this exhibit within this investigation. In India, CFSL uses a reference format that combines the state abbreviation, the year, and a sequential number. DEA uses the DEA-7 exhibit number. UK police use the officer's warrant number plus a sequential suffix (e.g. DT/47/2024/001).

Case details. The investigating agency's case file number (FIR number in India, case number in the US and UK). This links the exhibit to the specific investigation.

Description of contents. What the officer believes the exhibit to be, based on field observation: "white crystalline powder, approximately 500 g, in clear PE bag" or "brown plant material, loosely packed, in cardboard box." The description at seizure is the baseline against which the laboratory's examination will be compared; if the laboratory receives "brown resinous substance" when the label says "white crystalline powder," that discrepancy must be explained.

Date, time, and location of seizure. The precise date and time of seizure, and the location (address or GPS coordinates for field seizures). In India, this must match the entries in the seizing officer's field diary and the Mahazar/Panchnama drawn at the time of seizure.

Seizing officer's identification. Name, rank, and badge/warrant number of the seizing officer, plus a signature across the tamper-evident seal.

Witness information. In India (under NDPS Rules), the names and signatures of the two independent witnesses (Panchas) who were present at seizure. In the US, any co-seizing agent. In the UK, any other officer present at the seizure.

Storage conditions required. If the exhibit requires particular storage (refrigeration for biological co-exhibits, darkness for light-sensitive compounds, ambient temperature for most drug powders), this must be indicated on the label so that storage facility staff can comply.

Continuity endorsements. Each transfer of custody adds a dated, signed endorsement to the label (or to a secondary label affixed alongside the primary one). A label that has space for only one endorsement is inadequate for a case that will spend months in a laboratory before going to trial. Pre-printed forms with multiple endorsement rows are standard in professional forensic evidence management.

Handling the label in the laboratory. When the forensic chemist opens the exhibit to take a sub-sample, the opening event is itself a chain-of-custody event that must be documented. The chemist records the condition of the original label, notes whether the tamper-evident seal is intact, and photographs the exhibit and label before opening. The original outer packaging, with its original label, is retained as part of the exhibit (not discarded), because it may be required in court. A common error is to discard the original packaging after removing the exhibit contents for analysis; this destroys the identity document for the exhibit.

Trace residues span a wide range of chemical evidence categories: explosive residues on a suspect's hands or clothing, gunshot residue (elemental and organic components), accelerant residues on fabric from an arson scene, drug residues on packaging material, and chemical warfare agent residues on environmental surfaces. All share the characteristic that the amount of material is very small (micrograms to nanograms), the analyte may be labile or volatile, and the collection window is often limited.

Explosive residue swabs. At a post-blast scene, the investigator collects swabs from surfaces where explosive residue is likely to have deposited: the inside surfaces of a blast chamber (walls, floor immediately adjacent to the seat of explosion), vehicle interiors if an IED was detonated in a vehicle, and the hands and clothing of suspects if they are detained promptly. Each swab uses a cotton or polyester swab moistened with a validated collection solvent (typically acetonitrile or methanol/acetonitrile mixture, per ENFSI Explosives Working Group guidance and TWGFEX/SWGMAT guidance in the US). A reference swab (blank solvent swab of the same batch, swabbed on an unexposed clean surface) is collected alongside to identify any contribution from the collection materials themselves.

Swabs are placed immediately in solvent-rinsed glass or polypropylene vials. They are not allowed to air-dry in the open (which would cause evaporative loss of volatile explosive compounds). Transport time to the laboratory is minimised; refrigerated transport is recommended for peroxide-based explosive residue samples (TATP, HMTD), which are thermally labile and decompose at elevated temperatures.

In India, post-blast residue collection is typically performed by the state police's bomb disposal unit or a forensic investigation team, following protocols issued by the Bureau of Police Research and Development (BPR&D) and the CFSL. In the US, the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) and the FBI's Explosives Unit follow the TWGFEX (Technical Working Group for Fire and Explosion Investigation) guidelines. In the UK, the Metropolitan Police Counter Terrorism Command (SO15) uses protocols developed by CAST and aligned with ENFSI Explosives Working Group best practice.

GSR collection. Gunshot residue collection for chemical analysis (as distinct from SEM-EDX particle analysis) uses adhesive lifts or solvent swabs depending on the analytical target. For SEM-EDX (identifying Pb-Sb-Ba particles), adhesive aluminium stubs (the standard ASTM E1588 collection method) are used. The chemistry of those three-component Pb-Sb-Ba particles and the sampling persistence window are detailed in the GSR module; the GSR sampling protocols and secondary transfer from a ballistics perspective covers the persistence and transfer dynamics that govern collection timing. For organic GSR analysis (nitrite compounds, ethyl centralite, diphenylamine oxidation products), cotton swabs moistened with 1 per cent acetic acid or methanol are used. GSR is a rapidly depleting trace: hand washing, rubbing, or even wind exposure can remove a significant portion of deposited particles within 2 hours. Collection must occur as soon as possible after the suspected discharge event, ideally before the suspect is allowed to use toilet facilities.

Drug residue contact samples. Packaging material associated with a drug seizure (bags, wrapping, tape) can itself be a useful exhibit even after the primary drug exhibit has been consumed in analysis. Residual drug on packaging surfaces can be detected by LC-MS/MS at nanogram levels, linking the packaging to the drug and potentially (through profiling) to a manufacturing source. These samples are collected by wiping the inner surfaces of packaging material with a solvent-moistened swab and submitting the swab for LC-MS/MS analysis. The chain of custody for these secondary samples follows the same documentation principles as for the primary exhibit.

The contamination window. Every trace residue sample has a contamination window: the period and pathways during which exogenous material can be introduced. For GSR, the contamination window begins at the moment the officer approaches the suspect (secondary transfer of GSR from the officer's clothing) and ends when the sample is sealed. For explosive residue swabs, the contamination window includes any shared equipment, any shared vehicle, and any handling by personnel who have been in contact with explosives training materials. A single anti-contamination measure used universally: change gloves between each swab collection, and use new tools for each sample area. This is not optional. A single glove failure or shared tool can introduce contamination that renders an entire suite of scene samples uninterpretable.