Forensic Analysis of Explosives and Post-Blast Residues

No explosion is 100 percent efficient. Even a properly detonated military charge leaves microgram quantities of unconsumed explosive on the casing, on nearby debris, in blast-seat soil and on anyone within arm's length of the device. The recovered explosive type is the single biggest investigative lead: RDX points to military or trained-terrorist origin, ANFO to mining-grade bulk supply, AN plus fuel oil to improvised fertiliser-based VBIEDs.

Collection is layered. The state Bomb Disposal Squad or NSG clears any secondary device first. Samplers then move in with sterile gloves and pre-cleaned glass vials. Adhesive cotton swabs (or sterile gauze) are wetted with acetone for organic nitroaromatics and with water for inorganic ions, and rubbed over fragments and surfaces. Tweezers pick up casing fragments, packaging remnants, blast-seat soil and any intact detonator wire. Suspect hand swabs and clothing samples are taken under BNSS Section 176(3) once a suspect is in custody. Fine particulate goes to a portable vacuum collector with an inline filter. Field IMS units (Smiths IONSCAN, Sabre 5000) screen each sample so the lab knows which vials to prioritise.

Two SOP rules govern sample integrity. First, never use plastic containers for organic residues: plasticisers leach into the swab and confound the GC-MS run. Second, keep the acetone-wet swab and the water-wet swab separate; mixing them dilutes both analytical tracks.

Color tests are the cheapest analytical step after IMS, each targeting a specific functional-group family.

Modified Griess. Sulphanilic acid plus alpha-naphthylamine in acetic acid. Nitrite diazotises the sulphanilic acid; the diazonium then couples with alpha-naphthylamine to a pink azo dye. Positive for smokeless powder, NG, and partially decomposed nitramine / nitrate-ester residue.

Janowski. A few drops of DMSO (or acetone) plus alcoholic KOH on a suspected nitroaromatic sample. Deep violet to purple confirms TNT, picric acid, tetryl or DNT. The colour is a Meisenheimer sigma-adduct between hydroxide and the electron-poor aromatic ring.

Diphenylamine in conc. H2SO4.Deep blue with oxidising nitrate or nitro groups. Strong test for inorganic nitrates (AN, KNO3), nitrate esters (NG, PETN) and nitramines (RDX, HMX). The same reagent is used in gunshot-residue analysis, where it targets primer-derived nitrates rather than main-charge residues.

Bratton-Marshall. Sulphanilamide plus N-(1-naphthyl)ethylenediamine. Pink for NO2-. Backup for Griess.

Tetramethylbenzidine variants. Lower-toxicity replacements for benzidine and o-tolidine; adapted with vanadate or peroxide for chlorate / perchlorate screening.

Color tests are sensitive but not specific (same lesson as the presumptive blood tests). A positive Griess means "treat as nitrite-bearing, continue to TLC and GC-MS". It never proves the substance was explosive.

TLC bridges the color test and full instrumental confirmation. A drop of extract is spotted on silica gel alongside TNT, RDX, PETN, NG and tetryl standards, developed in chloroform-methanol-acetone, then visualised under shortwave UV (nitroaromatics quench fluorescence, dark on green) and sprayed with Griess (pink for nitrite-bearers) and Janowski (violet for nitroaromatics).

The instrumental confirmation step follows a decision tree keyed to analyte class and thermal stability.

GCwith electron capture detector (GC-ECD). Picogram-level detection for nitroaromatics and nitramines that survive the GC injector. The workhorse explosives screen at most state SFSLs.

GC-TEA. Pyrolyses the analyte, captures the NO radical, reacts it with ozone for chemiluminescent NO2*. Nitro-specific, sub-nanogram sensitivity. Classical gold standard; still used at CFSL Hyderabad for difficult traces.

GC-MS.Unambiguous ID of TNT, DNT and any thermally stable nitroaromatic. Limited for RDX, HMX, PETN, which decompose in the GC injector.

LC-MS and LC-MS-MS. Room-temperature separation, so RDX, HMX, PETN, urea nitrate and other thermally labile species survive. ESI / APCI in negative-ion mode gives clean adduct ions. The courtroom-grade confirmation at CFSL Hyderabad and Chandigarh.

Ion chromatography (IC). Suppressed conductivity detection of NO3-, NO2-, ClO3-, ClO4-, NH4+, K+ and Cl- at low ppm. The only practical instrument for ANFO, urea nitrate and chlorate / perchlorate mixes. The water-wet swab goes straight to IC.

IMS. Smiths IONSCAN 500DT and Sabre 5000. Five-second result on a swab. Deployed at airports, Parliament-security checkpoints and every post-blast scene. Lower specificity than LC-MS but unmatched speed.

Raman spectroscopy.Bulk ID of intact explosive crystals without sample prep. A 1064 nm laser (not 785 nm) avoids the fluorescence background that swamps the Raman signal from many real samples. Rigaku Progeny ResQ and similar handhelds are standard NSG and CISF kit.

FTIR with ATR.Solid-state ID. Asymmetric NO2 stretch around 1550 cm-1 and symmetric stretch around 1340 cm-1 are diagnostic for nitro compounds; the exact pattern is compound-specific.

SEM-EDX.Identifies metallic composition of fragments (steel, aluminium, copper of casing, brass of detonator caps) and inorganic primer residues on detonator fragments.

Once a suspect is detained, the team takes hand swabs (back of the hand, web of the thumb, palm), clothing samples (cuffs, pockets, footwear) and, where the case warrants, hair and nail clippings. The same acetone-wet / water-wet split applies and the same GC / LC / IC instruments confirm. A suspect who handled a pipe bomb with a primer-based initiator carries Pb / Ba / Sb primer particles (the GSR triad) plus nitroamine or nitroester residues from the main charge, so the report has to separate primer signatures from main-charge signatures and from environmental contamination.

The Indian institutional chain runs: state BDS clears any secondary device; for terrorism-linked or high-casualty incidents the NSG Post-Blast Investigation Team moves in; forensic sampling under BNSS Section 176(3) is led by the nearest CFSL or state SFSL; sealed samples go to CFSL Hyderabad (the designated explosives lab) or CFSL Chandigarh; the analytical report is filed under BSA 2023 Section 39. For terrorism cases the NIA takes the chargesheet under UAPA 1967 plus the Explosive Substances Act 1908.

Indian casework anchors (factually documented in court records):

• Mumbai 1993 serial blasts. TADA Court and Bombay High Court records confirm recovery and identification of RDX traces from multiple sites.
• Mumbai 11 July 2006 train blasts (7/11). Maharashtra ATS chargesheet and forensic laboratory analysis identify a mixture of RDX and ammonium nitrate packed into pressure-cooker IEDs, enhanced with ball bearings.
• Pulwama 14 February 2019 CRPF convoy attack. NIA chargesheet identifies a VBIED comprising RDX, calcium-ammonium nitrate, gelatin sticks and aluminium powder, with initial IMS confirmation at the scene and laboratory confirmation following.