Forensic Analysis of Explosives and Post-Blast Residues
UGC-NET Paper 2 Unit VI notes on explosives analysis: Griess, Janowski, diphenylamine, GC-ECD, GC-TEA, LC-MS, IC, IMS, Raman, FTIR and SEM-EDX with the NSG / NIA / CFSL workflow.
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Post-blast residue analysis is the analytical half of UGC-NET Forensic Science Unit VI's explosives bullet. The sibling order-3 topic covers definition and types; this topic covers how the lab finds and identifies the explosive after it has gone off. NTA tests it through color-test chemistry (which reagent gives which colour for which functional group), instrumental selection (GC-ECD vs GC-TEA vs LC-MS vs IC), and the Indian institutional frame (NSG on site, CFSL Hyderabad in the lab, NIA in court).
Treat this as a screen-then-confirm workflow topic. Field swabs and IMS screening narrow the search, color tests bin the residue by functional group, TLC separates closely related compounds, and chromatography-mass-spectrometry gives the courtroom-grade identification. Memorise the reagent-to-colour pairings and the instrument-to-analyte pairings. The Indian casework anchors (Mumbai 1993, 7/11 train blasts, Pulwama 2019) recur in short-answer questions.
- Post-blast residue (PBR)
- Unconsumed explosive plus reaction by-products left at the seat of an explosion, on debris, on fragments and on the hands or clothing of anyone who handled the device. The forensic analyte of every blast case.
- Modified Griess test
- Color test for nitrites (NO2-). Sulphanilic acid is diazotised by the nitrite, then coupled with alpha-naphthylamine to give a pink azo dye. Indicates smokeless powder, NG and partially degraded nitramines / nitrate esters.
- Janowski test
- Color test for nitroaromatics. Sample plus DMSO plus alcoholic KOH gives a violet Meisenheimer complex. Positive for TNT, picric acid, tetryl and dinitrotoluenes.
- Diphenylamine test
- Color test for nitrates and nitro groups. Diphenylamine in conc. H2SO4 turns deep blue with oxidising NO3-/NO2-. Also a primer-residue test, which is why it overlaps with the GSR bullet.
- GC-TEA
- Gas chromatography with a Thermal Energy Analyser. Pyrolytic cleavage of the nitro group to NO followed by chemiluminescent reaction with ozone. Nitro-specific, sub-nanogram sensitivity.
- Ion chromatography (IC)
- Anion / cation separation with suppressed conductivity detection. The only practical instrument for ANFO (NH4+, NO3-), urea nitrate and chlorate / perchlorate mixes.
- IMS
- Ion Mobility Spectrometry. Drift-tube separation of gas-phase ions at ambient pressure. Smiths IONSCAN 500DT and Sabre 5000 give 5-second field screening on swabs.
- VBIED
- Vehicle-Borne Improvised Explosive Device. Pulwama 2019 is the recent Indian reference case (RDX plus ammonium nitrate).
What ends up at the scene
Unconsumed explosive plus reaction by-products, scattered over the blast seat.
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 show up in MCQs. 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 tracks.
Color tests: NTA's favourite quick screens
One reagent, one colour, one functional-group family.
Color tests are the cheapest analytical step after IMS, and they map cleanly to one-line MCQ answers.
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 test appears in the GSR bullet, which is the classic NET trap: same reagent, different analyte.
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.
| Reagent | Target functional group | Positive colour | Explosive class indicated |
|---|---|---|---|
| Modified Griess | Nitrite (NO2-) | Pink to orange |
TLC and instrumental confirmation
Pick the right detector for the analyte.
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 is a decision tree. Memorise the pairings.
GC with 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.
Suspect specimens and the Indian workflow
Hand swabs, clothing, then NSG to CFSL to NIA.
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 trial-court records identify RDX plus ammonium-nitrate-based slurry charges in pressure-cooker IEDs.
- Pulwama 14 February 2019 CRPF convoy attack. NIA chargesheet identifies an RDX plus ammonium-nitrate VBIED, with initial IMS confirmation at the scene and laboratory confirmation following.