Forensic Chemistry: GC-MS, GC-ECD and LC-MS for Post-Blast Analysis

UGC-NET Forensic Science Unit VI advanced drill on the instrumental methods used for detecting and confirming organic explosive residues in post-blast debris. Coverage spans GC-MS column selection (DB-5, DB-XLB), electron ionisation characteristic ions for TNT (m/z 210 base peak, [M-OH]+) and RDX (m/z 128, 81, 42), chemical ionisation for thermally labile nitrate esters, and the distinction between EI and CI ionisation modes in forensic explosive analysis.

GC-ECD and GC-TEA questions focus on the electron capture mechanism (Ni-63 source, nitrogen makeup gas, sub-picogram LODs for PETN and NG), TEA pyrolysis at 400 to 500 degrees C and NO-ozone chemiluminescence, and the complementary selectivity relationship between the two detectors for nitro versus halogen interferences. LC-MS and LC-MS-MS questions address why peroxide explosives HMTD and TATP require electrospray rather than GC methods, negative-mode MRM transitions for TNT (226 to 46) and RDX (chloride adduct m/z 257), APCI versus ESI source selection, and ammonium adduct formation for TATP detection. Ion mobility spectrometry coverage includes the Smiths IONSCAN 600 drift-tube principle, the K0 reduced-mobility calculation, DNT cross-reactivity causing false positives, and the confirmatory role of GC-MS after a presumptive IMS positive.

The final section covers portable Raman and FTIR for bulk explosive identification (RDX fingerprint at 800 to 1000 cm-1, TNT asymmetric NO2 stretch at 1540 cm-1), advantages of Raman over FTIR for wet samples at scenes, and the identification criteria and reporting obligations from ASTM E2998-16 and the ENFSI Best Practice Manual for the Forensic Analysis of Explosions.

This mock is calibrated for UGC-NET Paper II Unit VI top-decile candidates, NFSU MSc Forensic Chemistry applicants, and practising forensic chemists reviewing post-blast analytical protocols for accreditation.

Topics covered:

• GC-MS: column choice DB-5 vs DB-XLB, EI at 70 eV, CI ionisation
• Characteristic m/z for TNT (210) and RDX (128, 81, 42)
• GC-ECD: Ni-63 source, nitrogen makeup gas, sub-picogram LODs
• GC-TEA: pyrolysis at 400 to 500 degrees C, NO-ozone chemiluminescence
• LC-ESI-MS-MS: peroxide explosives HMTD and TATP, MRM for TNT and RDX
• APCI vs ESI source selection for moderately non-polar explosives
• IMS: IONSCAN 600 drift-tube, K0 formula, DNT false-positive risk
• Raman and FTIR: portable instruments, fingerprint regions, scene use
• ASTM E2998-16 and ENFSI guidelines: identification criteria and reporting

Allow 30 minutes.