Chemistry 439

Arson samples by GC

Objectives:

    1. Learn about the chromatograms of common fuels.
    2. Learn about weathering of mixtures
    3. Learn how to trace a fuel to the place where it was sold.
    4. Learn how to collect and analyze arson samples.
Background:
Most fuels are delivered to the retail outlet by different routes. They may have come from different refineries, through different pipelines and in different trucks. Each of these "containers" probably contained different materials and some trace (ppm or ppb) of each material is frequently left in each container. Therefore, most fuel products, say gasoline, will be unique, at the trace chemical level, for the store that sold the fuel. Sometimes even lots or truckloads are unique. The challenge is to find the unique chemicals in the many more chemicals that are alike.

Procedure:

    1. Obtain a sample of a material that has been contaminated with a fuel from the TA. The material may be soil, cloth or any other material commonly found around the home. As described below in more detail, extract the material with methylene chloride, dry the extract with anhydrous sodium sulfate and carefully evaporate the methylene chloride. Determine which fuel was used to contaminate the material by comparing the gas chromatogram of the residue with that of standards provided by the TA.
    2. Obtain several samples of fuels that may include lighter fluid, petroleum ether, gasoline, terpentine, paint thinners, kerosene, jet fuel and diesel fuels #1 & #2 from your TA. Dilute them as appropriate in methylene chloride to get good chromatograms and record a good chromatogram of each mixture on a capillary boiling point column using a flame ionization detector operating close to the detection limit of the detector (you want to see the trace compounds). You may need to use different GC columns. Do not inject more than one ul of the sample extract or of any fuel.
    3. After you have obtained good chromatograms of each candidate fuel, (You should not have to run all fuels because you should be able to make an educated guess of what is in your unknown), extract the contaminated material from step 1 above 3x with methylene chloride, dry the methylene chloride with anhydrous sodium sulfate and concentrate by evaporating most of the solvent. Caution: Do not destroy your sample by evaporating all of the fuel.
    4. Inject an aliquot of the extract residue into the GC and compare the chromatogram to the chromatograms of the pure fuels. Explain any differences.
    5. Could you, on the basis of your chromatograms, prove which fuel was used to prepare the extract? If not, how could you do so?