Tropylium Cation (M/z 91)
Definition
The C7H7+ fragment ion produced by rearrangement of alkylbenzenes in the EI ion source of a mass spectrometer. The diagnostic ion for the aromatic fraction in fire debris analysis; its presence and distribution characterise gasoline and aromatic-class ignitable liquid residues.
- Formula
- C7H7+
- m/z value
- 91
- Origin
- Rearrangement of alkylbenzenes in the EI ion source
- Diagnostic use
- Marker for the aromatic fraction in fire debris analysis
- Associated liquid class
- Gasoline and other aromatic ignitable liquids
Common questions
Why is m/z 91 specifically useful in fire debris analysis?+
Alkylbenzenes such as toluene, xylenes, and trimethylbenzenes are major gasoline components, and they nearly all fragment to give a strong tropylium ion. Extracting the m/z 91 chromatogram highlights this whole aromatic family even when the total ion chromatogram is dominated by pyrolysis products from the burned matrix.
Does detecting m/z 91 alone prove gasoline was present?+
No. The ion confirms alkylbenzene content generally, and ASTM E1618 classification requires evaluating the pattern and relative abundance of several diagnostic ions and target compounds together, not a single mass fragment in isolation.
Related terms
- ASTM E1618
- Standard Test Method for Ignitable Liquid Residues in Extracts from Fire Debris Samples by Gas Chromatography-Mass Spectrometry. The active analytical standard defining...
- Carbon Disulfide (CS2)
- The standard elution solvent for activated charcoal strips in ASTM E1412 passive headspace analysis. Selected for its low UV absorption, minimal GC-FID...
- Evaporative Weathering
- The preferential loss of volatile components (benzene, toluene, C5-C7 alkanes) from an ignitable liquid residue during and after a fire, altering the...
- Extracted Ion Profile (EIP)
- A GC-MS chromatographic display plotting only ions of a specified m/z value against retention time, effectively filtering out all compounds that do...
- Ignitable Liquid Reference Collection (ILRC)
- A database of GC-MS chromatographic data for over 1,100 ignitable liquid products maintained by the National Centre for Forensic Science at the...
- Light Petroleum Distillate (LPD)
- The ASTM E1618 ignitable liquid class covering C4-C9 aliphatic products (charcoal lighter fluid, camping fuel, some paint thinners). The class most frequently...
- Passive Headspace Concentration
- The ASTM E1412 method in which an activated charcoal strip is suspended in a sealed container above heated fire debris (60-80°C, 16...
- Total Ion Chromatogram (TIC)
- A GC-MS chromatographic output plotting the summed intensity of all detected ions at each retention time. Provides the full-pattern overview of a...
- Unresolved Complex Mixture (UCM)
- The broad hump of hundreds of co-eluting cycloalkane and isoalkane compounds in the TIC of heavy petroleum products (diesel, heavy fuel oil,...