Isolation and Concentration of Accelerant Residues
A number of methods are commonly used for isolating and concentrating fire scene residues. The one(s) chosen will depend upon personal preference, available equip ment, and the nature of the exhibits being processed. Below is a list of the typical types of exhibits that are encountered:
• Neat ignitable liquid—Occasionally, residues will contain some intact, unburned accelerant mixed with debris from the fire. Even if the exhibit is wet, hydrocarbons are not miscible with water so they would form a separate layer. It may be possible to pour off the liquid from the residue, filter it to remove solid particulates, separate the hydrocarbon from the water, and make a direct injection into the gas chromatograph.
• Partially burned accelerants—A much more common occurrence in fire scene residues is accelerants that have been partially burned. The major change that these substances undergo is evaporation of the most volatile components, leaving the higher boiling components behind. Usually, such exhibits must be extracted from the matrix in order to be concentrated.
• Nearly completely burned accelerants—If an accelerant has been subjected to extreme heat for a significant period, nearly all of the substances present will evaporate or burn. The best that can be hoped for, in such cases, is that there will be some nonvolatile residue left that can be extracted. Identification of these residues can be difficult owing to a lack of characteristic chromatographic information.
There are four methods and some variants that are used for isolation of accelerant residues. They are described below.
Headspace methods Consider a fire residue containing some small amount of liquid accelerant. This residue is put in a sealed metal container at the scene by the fire scene investigator. Some of the accelerant will vaporize, while the rest will remain a liquid. The amount of liquid that becomes a vapor depends upon the vapor pressure of the substance and the temperature. The higher the temperature, the greater percentage of vapor there will be. Eventually, equilibrium will be established between the liquid accelerant in the residue and the vapor in the headspace above. Henry’s Law describes this equilibrium. Once the equilibrium is established, then some of the vapor above the matrix, the headspace, can be sampled with a gastight syringe and injected into a gas chromatograph. The amount of heating that the container can be subjected to is limited. If there is too much heat applied then the ensuing increase in pressure in the container can cause the top to come off of a can or the glass to break in a jar. Typically, a container will be heated to no more than about 60 °C. A diagram of a can showing the headspace can be seen in Figure 1.
FIGURE 1 The headspace in a can. The fire debris is at the bottom. The headspace is the air layer on top of this that contains accelerant vapors.
The headspace in a can. The fire debris is at the bottom. The headspace is the air layer on top of this that contains accelerant vapors.
overnight at room temperature or for a shorter time while being heated. Sometimes the fire scene investigator will put a charcoal strip into the container with the evidence at the crime scene. This means that the container won’t have to be opened to insert the strip at the laboratory, thus minimizing the loss of accelerant vapors. In active adsorption, sometimes called adsorption-elution, two tubes containing charcoal or Tenax are inserted partway into the container through holes in the top. Then air is pumped through one of the tubes into the container. This causes air to flow from inside the tube out through the other tube. As the container is heated, more accelerant evaporates into the headspace. It is swept through the outlet tube along with the air and is trapped or adsorbed onto the charcoal or Tenax. This upsets the vapor–liquid equilibrium in the container and the consequence is for more of the liquid accelerant residue to evaporate. This continues until there is no more accelerant in the container—it has all been trapped in the outlet tube. In some laboratories, a vacuum is applied to one of the tubes, drawing in air from the outside through the other one. The accelerant vapors are trapped in the tube where the vacuum is applied. The result is the same. See Figure 2 for a diagram showing absorption-elution using the vacuum method.
FIGURE 2 Absorption-elution. The paint can has two tubes that can trap accelerants. The vacuum pulls air through one tube into the can. Any contaminants in the air that might interfere with the analysis are trapped in the first tube. The headspace vapors are pulled out through the other tube and trapped in the charcoal in the tube.
Once the accelerant has been adsorbed onto the charcoal or Tenax, then it is eluted off using a suitable solvent. Carbon disulfide (CS2) has been used for many years for this purpose, but it is toxic and highly flammable. Other solvents have been tried, including butane and pentane, but they are less satisfactory because they are also constituents of many accelerants. Another variant of the adsorption methods is solid phase microextraction. In this method, a fiber made from fused silica is coated with an adsorbent such as charcoal or Tenax. This is inserted into the heated fire residue container. After adsorption is complete, the fiber can be inserted directly into the inlet of a gas chromatograph, where the high heat of the injector zone rapidly elutes the accelerant into the mobile phase stream for analysis. The advantages of this technique are extreme sensitivity and removing the necessity of a separate elution step.
Solvent extraction Solvent extraction is a very simple and sensitive technique, useable with a wide range of accelerants. The evidence container is opened and a small quantity (depending on the amount of debris in the container) of a suitable solvent is added. Carbon disulfide is the most popular solvent for this process. The solvent is then poured off and filtered and then evaporated to a small volume leaving behind the accelerant residue. This solution can then be introduced into a gas chromatograph. Disadvantages of solvent extraction are, first, that the solvent will also dissolve unwanted pyrolysis products, matrix materials, and other substances, some of which may interfere with the subsequent analysis and second, that evaporation of the solvent may also cause evaporation of some of the volatile components of the accelerant residues. Steam distillation This is the oldest technique for isolation of accelerant residues. Some of the accelerant residue is put in a distillation apparatus with some water, which is then boiled and distilled. The steam will heat and carry over accelerant residues. Those that are immiscible with water will form a layer on top of the distilled water. If water soluble residues suspected to be present, then the first aliquot of water must be collected and analyzed. Steam distillation is not very sensitive and relatively large quantities of matrix are needed. It is not as subject to contamination interferences as is solvent extraction, but it does favor high boiling fractions. It is also the most complicated to run.
