Ion Mobility Spectrometry (IMS)
Definition
A field-deployable analytical technique that separates ions by their mobility in an electric field at atmospheric pressure. Used for trace explosive detection at airports and post-blast scenes. Standard IMS instruments detect nitro-explosive markers; dedicated TATP detection modes use different ionisation chemistry.
- Primary Use
- Field screening for explosive residue at airports, blast scenes, and security checkpoints
- Detection Speed
- Seconds for presumptive identification
- Core Principle
- Separates ions by drift velocity in a carrier gas under electric field (atmospheric pressure)
- Forensic Requirement
- IMS results require confirmation by GC-MS or LC-MS/MS before use in court
Common questions
What does ion mobility spectrometry do at a crime scene?+
IMS separates ions by their drift velocity through a buffer gas under an electric field, providing presumptive identification of explosives in seconds. It's used for on-scene screening of swabs and residue to detect explosive compounds quickly, with results later confirmed by lab techniques like GC-MS or LC-MS/MS for court.
Why is IMS deployed at airports if results need confirmation?+
IMS is field-portable and gives results in seconds, making it ideal for fast security screening of passengers and cargo. The confirmation methods (GC-MS, LC-MS/MS) require labs and more time, so IMS serves as the rapid initial filter at checkpoints and post-blast scenes before evidence moves to the laboratory.
Can IMS detect all explosives the same way?+
Most IMS instruments detect nitro-explosive markers using standard ionisation chemistry. However, explosives like TATP require dedicated detection modes with different ionisation chemistry because their chemical structure behaves differently in the instrument.
Related terms
- ASTM E2998
- Standard Guide for Sampling Improvised Explosive Devices and Post-Blast Debris for Explosive Residue Analysis. Specifies cotton swabs, isopropanol or methanol pre-wetting, swabbing...
- Attenuated Total Reflectance (ATR) FTIR
- An infrared spectroscopy configuration where the IR beam passes through an ATR crystal and generates an evanescent wave that samples a thin...
- CFSL Hyderabad Explosives Wing
- The explosives examination section of India's Central Forensic Science Laboratory in Hyderabad, the designated court-certifying facility for explosives analysis in major Indian...
- Chain of Custody
- The documented chronological record of who collected, handled, transferred, and examined a piece of evidence. For digital evidence, chain of custody includes...
- Control Swab (Background)
- A swab collected from an uncontaminated surface outside the outer cordon of a blast scene, processed identically to scene swabs, to establish...
- Crater Glass
- Glass fragments recovered from within or immediately adjacent to the blast crater, originating from the device container or adjacent glazing; distinguished by...
- Critical Diameter
- The minimum charge diameter below which a detonation wave cannot sustain itself, because energy losses to the sides of the charge exceed...
- Detonating Cord
- A cord with a PETN explosive core (5-40 g/m) that detonates at approximately 6,400 m/s when initiated. Used to connect multiple charges...
- Diphenylamine (DPA) Test
- A broad-spectrum presumptive colour test for nitrate and nitrite compounds, producing a blue-green coloration. Reactive with multiple oxidising agents, so environmental context...
- DNT Isomers (2,4-DNT and 2,6-DNT)
- Dinitrotoluene intermediates and impurities present in manufactured TNT. Their molar ratio in post-blast debris or intact samples provides a manufacturing signature that...
- Expanding Concentric Search Grid
- The systematic spatial framework for post-blast scene searches, consisting of concentric rings at measured radii from the seat of blast, divided into...
- Explosive-Detection Canine (EDC)
- A trained dog used to detect vapour-phase explosive signatures in open areas, vehicles, or baggage. Certified under national programmes (TSA CTCP, UK...
Explained in these topics
- Field Explosives Detection: IMS, ETD, Canines and RamanA gas-phase separation technique that measures the drift velocity of ions through a buffer gas under an electric field. Used in ETD devices at airports worldwi...
- Lab Equipment, Evidence Handling and Chain of CustodyAn atmospheric-pressure analytical technique that separates ionised explosive residue molecules by their drift time in a carrier gas under a uniform electric f...
- Post-Blast Residue Sampling and the IMS, GC-ECD, LC-MS/MS WorkflowA field-portable analytical technique that separates ions by their drift velocity through a buffer gas under an electric field. Provides presumptive identifica...
- Post-Blast Scene: Search Grid, Fragment Collection, Seat of BlastA field-portable technique that separates gas-phase ions by their drift velocity in an electric field; used for on-scene screening of swabs for explosive resid...
- Specific Explosives: TNT, RDX, PETN, HMX, ANFO, TATP, Urea NitrateA field-deployable analytical technique that separates ions by their mobility in an electric field at atmospheric pressure. Used for trace explosive detection...