Selected Ion Monitoring (SIM)
Definition
A GC-MS operating mode in which the mass spectrometer monitors only a set of pre-selected diagnostic m/z values, rather than scanning the full mass range. SIM provides higher sensitivity for target compounds but does not produce full mass spectra for library matching; used for confirmation and quantitation of specific ILR markers.
- Instrument
- GC-MS
- Mode
- Monitors pre-selected diagnostic m/z values only
- Tradeoff
- Higher sensitivity, no full mass spectrum
- Use
- Confirmation and quantitation of specific markers
Common questions
Why does SIM improve sensitivity compared to full-scan mode?+
By dwelling on only a few chosen ions instead of sweeping the entire mass range, the instrument spends more time detecting the ions that matter, which raises signal relative to background noise.
What is the practical drawback of relying on SIM for identification?+
Because it does not capture a full mass spectrum, SIM cannot be matched against a spectral library to confirm compound identity from scratch, so it is normally paired with an earlier full-scan or reference standard to establish what is being targeted.
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...
- 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...
- Friction Lid
- The press-on metal lid used to seal evidence cans for fire debris. Seated by pressing firmly around the rim; allows field packaging...
- Ion Mobility Spectrometry (IMS)
- A field-deployable analytical technique that separates ions by their mobility in an electric field at atmospheric pressure. Used for trace explosive detection...
- LC-MS/MS
- Liquid chromatography coupled to tandem mass spectrometry. The preferred confirmatory method for non-volatile organic explosives (PETN, RDX, TNT, NG) because it avoids...
- Mahazar
- The formal seizure list used in Indian criminal procedure (BNSS 2023) to document the collection of physical evidence at a scene. Prepared...
- Passive Headspace Concentration (PHC)
- ASTM E1412 extraction method. The sealed evidence container is heated at 60-80°C; volatilised hydrocarbons adsorb onto an activated charcoal strip suspended in...
- Tamper-Evident Sticker
- A self-adhesive label applied across the lid-rim or closure interface of an evidence container. Any disturbance of the container after sealing leaves...
- Unlined Metal Can
- The standard evidence container for fire debris analysis. 'Unlined' refers to the absence of internal plastic or epoxy coating, which would contribute...