Unresolved Complex Mixture (UCM)
Definition
The broad hump of hundreds of co-eluting cycloalkane and isoalkane compounds in the TIC of heavy petroleum products (diesel, heavy fuel oil, lubricating oil base). The UCM hump is characteristic of biodegraded or highly refined oils and is an important pattern element in oil-spill source attribution for heavy fuel oil types.
- Composition
- Hundreds of co-eluting cycloalkane and isoalkane isomers
- Typical sources
- Diesel, heavy fuel oil, lubricating oils, biodegraded petroleum products
- Also produced by
- Pyrolysis of asphalt and roofing materials
- Key use
- Pattern element in fire debris and oil-spill source attribution
Common questions
What does the UCM hump look like in a GC-MS chromatogram?+
It appears as a broad, dome-shaped baseline elevation in the total ion current (TIC). Rather than distinct peaks, you see a gradual rise and fall across a range of retention times. This happens because hundreds of similar hydrocarbon compounds co-elute together and the column cannot separate them individually.
Why is UCM so common in diesel and heavy fuel oils?+
These products contain hundreds of cycloalkane and isoalkane isomers of similar boiling points. Standard GC columns are not fine enough to resolve them one by one, so they bunch together into a single broad hump. It is a signature of the way heavy petroleum products are chemically structured.
Does UCM mean an accelerant was used?+
Not by itself. While UCM is characteristic of diesel and heavy fuel oil accelerants, it also appears as a pyrolysis product from materials like asphalt and roofing. Identifying accelerant requires the whole chromatographic pattern, not just the presence of a UCM hump.
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...
- 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...
- 4-Vinylcyclohexene (4-VCH)
- A pyrolysis product of styrene-butadiene rubber (SBR) carpet backing, appearing at m/z 108 in the GC-MS. Elevated m/z 108 in a fire...
- Alkene Triplet Pattern
- The GC-MS signature of polypropylene pyrolysis: a repeating triplet of n-alkane, 1-alkene (terminal double bond), and diene peaks at each carbon number...
- Aroclor
- A series of polychlorinated biphenyl (PCB) commercial mixtures manufactured by Monsanto (US) and marketed as electrical insulating fluids. Aroclors are designated by...
- Background Subtraction
- The analytical practice of comparing fire debris EIPs against comparison sample EIPs from unburned substrate of the same type. Features present in...
- C30 17Alpha-Hopane
- The C30 pentacyclic triterpane used as the primary internal normalisation standard in petroleum biomarker fingerprinting. It is resistant to evaporation, biodegradation, and...
- 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...
- Comparison (Control) Sample
- A sample of unburned substrate collected from the same room or proximate unburned area as the fire debris sample, of the same...
- DB-5 Column
- A gas chromatography capillary column with a 5% phenyl-95% dimethylpolysiloxane stationary phase, the standard column for fire debris GC-MS analysis. Provides adequate...
- Dissolved Gas Analysis (DGA)
- The analysis of gases dissolved in transformer oil (H2, CH4, C2H4, C2H2, CO, CO2) by ASTM D3612 or IEC 60599 to diagnose...
- 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...
Explained in these topics
- Accelerant Detection: ASTM E1387 / E1618 and Passive Headspace ConcentrationThe broad hump of co-eluting compounds visible in the TIC of diesel and heavy petroleum distillate extracts, produced by the hundreds of cycloalkane and isoalk...
- Fire Debris Analysis: GC-MS and ASTM E1618 Pattern RecognitionA broad, dome-shaped baseline elevation in the GC-MS TIC chromatogram, visible in diesel and heavy fuel oil extracts. Arises from hundreds of overlapping isome...
- Petroleum Products, Lubricants and Transformer OilThe broad hump of hundreds of co-eluting cycloalkane and isoalkane compounds in the TIC of heavy petroleum products (diesel, heavy fuel oil, lubricating oil ba...
- Pyrolysis Products, Substrate Interferents and Background SubtractionA broad, dome-shaped baseline hump in the GC-MS TIC arising from hundreds of co-eluting high-boiling isomers. Characteristic of asphalt, heavy fuel oil, and bi...