An arson investigation typically requires collecting one or two liters of ashes and debris from various locations within a fire scene in metal cans similar to those used for paint, and sending the material to a lab. The testing methods typically include gas and liquid chromatography or various versions of spectroscopy, with gas chromatography being the most widely used in fire debris analysis. When the fire debris is received at the testing facility, samples are taken for testing. Sometimes this will involve suspending a strip with activated charcoal in the air or “headspace” directly above the sample in the metal can for a period of time that can vary, depending on the judgment of the analyst, for 2-3 hours or up to 16 hours.
Other testing methods include dynamic purge and trap of the headspace. And still another sampling method involves a newer solid-phase microextraction method (SPME) that does not destroy the sample. This later method, however, has a high displacement rate of heavier over lighter ignitable liquid components, is difficult to automate, makes preserving and archiving samples difficult, and has not shown a consistent ability to obtain repeatable and quantitative results. Also, the SPME sampling method requires expensive equipment, and the fibers are easily damaged. Still other methods are less sensitive and produce large amounts of chemical waste.
The vapor collection method developed in this work involves the dynamic adsorption of headspace vapors on short porous layer open tubular (PLOT) columns maintained at low temperature (as low as −40 °C). The underlying PLOT-cryoadsorption, or PLOT-cryo, is sensitive, quantitative, and more broadly useful than many competing techniques. It can identify compounds that don’t readily evaporate, and is not limited to samples dissolved in water, for example. The method recovers vapors by suction or by sweeping a gas across the air above a sample of interest. The benefits of this method are many. The collection sensitivity is high — below 1 part per billion (ppb). The low temperature is achieved using a vortex tube connected to compressed air; it has no moving parts, and is attractive for use in environments with explosive or flammable materials.
Vapors are collected in sturdy, inexpensive tubes embedded in an epoxy wafer. The wafer can be used inside either an insulated handpiece for manual sampling, or a longer probe for remote sampling of soil and spaces under buildings, in luggage, or in other containers. With either the handpiece or the probe, the wafer can be chilled to collect vapors and then heated to help remove them.
After vapor collection, the PLOT capillaries can be heated (up to 160 °C, again with the vortex tube), releasing the vapor. The capillaries used are robust and inexpensive, and this process is especially effective with relatively nonvolatile substances because of its wide operating temperature range. It also is not limited to water-borne samples, as is the case with most commercial sampling instruments.
The PLOT-cryo method can be used to simultaneously test for up to eight different ignitable liquids from a single sample. This allows investigators to take multiple samples from each of several locations in a fire scene (such as a grid approach) in a short amount of time. This method also enables high repeatability and quality assurance of the testing process, and is available in a portable unit that can perform the sampling in remote locations.
The briefcase-sized kit could enable detectives, field inspectors, and others to carry with them a convenient version of the system. The kit is powered by compressed air, which enables operation without electrical power, and ensures safety in potentially flammable and explosive environments. Compressed air is available on many emergency response vehicles.