Analysis of Residues

Laboratory Analysis of Debris and Other Samples 1. Preparation of Liquid Samples: Liquid samples are simply drawn into a special syringe and injected into a gas chromatograph (GC) 2. Preparation of Fire Debris samples: There are four commonly used methods to separate or concentrate any ignitable liquids from fire debris samples: heated headspace, steam distillation, carbon strip or tube absorption, and solvent wash

Laboratory Analysis of Debris and Other Samples i. Heated Headspace: The paint can containing the fire debris is punctured with a nail, the hole covered with tape, and the can incubated for 1+ hours at °C After removing the can from the oven, the headspace vapor is quickly sampled with a gas-tight syringe and immediately injected into a GC Advantages: simple with minimal handling Disadvantages: not for trace amounts of ignitable liquid residues

Laboratory Analysis of Debris and Other Samples ii. Steam Distillation: Debris is placed into a flask and either water or ethylene glycol added The flask is attached to a distillation apparatus and brought to a boil The condensate is collected, and if any ignitable fluids are present, it will separate into a second discernible liquid phase Disadvantages: time-consuming and risk of contamination

Laboratory Analysis of Debris and Other Samples iii. Carbon Strip or Tube Absorption: Uses activated carbon to absorb hydrocarbon vapors A strip is suspended above the debris & the can is heated to vaporize the residues Hydrocarbons are removed from the strip by a solvent wash, which can be analyzed directly by GC A very sensitive technique

Laboratory Analysis of Debris and Other Samples iv. Solvent Wash: Fire debris is placed into a flask, solvent added, mixed and allowed to sit for a period of time The solvent is separated from the debris, then analyzed as with the other approaches Works best for high boiling point ignitable liquids that cannot be easily vaporized

Laboratory Analysis of Debris and Other Samples 3. Laboratory Examination of Prepared Samples: Gas chromatography separates mixtures of compounds by a partitioning process between a mobile and a stationary phase Ignitable fluids such as gasoline produce a very complex pattern of peaks

Laboratory Analysis of Debris and Other Samples Different ignitable liquids will produce very different GC patterns A library of patterns is obtained from known standards of all possible accelerants With fire debris samples, many of the most volatile components tend to be lost The GC chromatograms can be compared to give an indication of the type of accelerant used, but the GC should not be considered an identification technique

Laboratory Analysis of Debris and Other Samples Individual peaks from the GC can be identified using a mass spectrometer (MS) As compounds reach the end of the GC column, they pass into the MS The MS bombards the compounds with high energy electrons, fragmenting the compounds and yielding a mass spectrum that can be used to identify the compound

Classification of Ignitable Liquids

Laboratory Analysis of Debris and Other Samples Comparison specimens are samples of the surface or substratum on which ignitable liquid residues might be present Natural and synthetic materials may contain volatile compounds that produce GC peaks when heated or extracted Specimens should be collected from unburnt areas of the fire scene