Test for Gasoline—Even without Residue
ARSON INVESTIGATION
An on-site test for lead in fire debris can provide the arson investigator with a clue as to whether or not a lead-based residue is present and an incentive to continue with more expensive, timeconsuming and exacting lab tests.
One of the major problems facing arson investigators in the field is identifying the agents that may or may not have been used as accelerants. As budgets for the running of laboratory samples are pinched tighter in both the public and private investigation areas, the ability to steer an inquiry away from a dead end in other ways will become more and more important.
Barker W. Davie, Jr., a partner in Barker and Herbert Analytical Labs in New Haven, IN, has found an inexpensive way to look for lead content in fire samples, which may give the investigator an indication that gasoline was a possible addition to the area under investigation.
Davie has been a forensic chemist for over 10 years. He specialized in fire investigations with the Fort Wayne, IN, Police Department for seven years before beginning private full time work.
“Initially, I was asked by another investigator if there was any way of telling whether or not gasoline had been used if all of the hydrocarbon residue had been destroyed,” says Davie. “After thinking about it for awhile, I realized that trace elements such as lead should still be remaining. After all, if it wasn’t for the lead deposits left behind after gasoline combustion in an auto engine, spark plugs and other components would probably last a lot longer.”
Working from the assumption that if there are lead deposits left behind in car engines, they should also be found when wood, paper, cloth, or other combustibles are burned, Davie set out to find a detection method.
“It is just basic chemistry to know that lead forms easily seen precipitates with chromate compounds,” notes Davie. “After that, it was basically tinkering with different percentages of solution (potassium chromate and distilled water) until the one that worked best was found.”
To put things into a technical perspective, the one that worked best was a .5 molar solution of potassium chromate (K2CrO4) added to a sample soaked in 6 molar acetic acid (glacial).
Despite the last paragraph, the test itself is simple to perform. Whatever needs to be tested is put into a standard plastic or glass beaker to which the acetic acid is added until it just covers the sample. After stirring the solution for two minutes, it is drained through filter paper into a second beaker.
It is very important that none of the sediment from the first beaker passes through the filter paper. If it does, let the solution set for about five minutes so that the extra material will sink to the bottom, at which time the solution can be carefully poured into a clean beaker for final processing.
Now, add two or three drops of the potassium chromate to the filtered solution. After allowing it to set for about 10-20 seconds, a yellow cloud should begin to form. As the process continues, the cloud should begin to solidify into the precipitate. If there are problems in getting the solution to solidify, placing the solution in cold water for about 10 minutes should help.
In addition to the chemicals, all that is really needed to perform this test is the filter paper, a supply of beakers, a funnel, and some stirring rods. It is also handy to include a roll of paper towels to clean out the materials for reuse during the same investigation.
Davie noted that the test is still valid if the gasoline used is “unleaded.” “Most gasoline is not totally unleaded. Unleaded just means that there is less than .5 parts per million (ppm) of lead in the formulation. This is still easily detectable by the test.
“There are a number of tests that are readily available for finding out the content of various types of fire residue,” notes Davie. “I wanted a test that was very economical, quick, and yet easy enough to be performed accurately by someone without a chemistry background. I wanted it to go together into a ‘cookbook’ that was very easy to follow. It needed to say that if you did this, this, and this, you will be able to get these results which are easily interpreted.”
It was also a requirement that the chemicals involved be readily available. The acetic acid can be purchased at most well-stocked photography stores, as it is used in the stop-bath portion of the developing process. However, caution should be used in purchasing the acetic acid, as many photographic stores will carry’ a weaker solution which will not work for the lead test. Make sure that the container states “glacial acetic acid” on its label. The glacial acetic is usually available to those who “homebrew” their own chemicals.
The hardest part of the mixture to find may be the potassium chromate. While not readily available in hobby stores, it is used extensively in high school chemistry classes and for blood testing in hospitals. Since both of these areas have a long history of cooperation with the fire service, there should be few problems with finding the proper chemicals.
After finding the needed ingredients, they must be made into a working solution before being taken into the field. According to Davie, 97.1 grams of the potassium chromate is added to 1,000 cc of distilled water to come up with the required solution. The glacial acetic acid is added to the water until one part of the chemical is diluted in two parts of the distilled water.
Essentially, the lead deposits that were left behind during combustion have been saturated into the material that is to be tested. The acetic acid floats this material into suspension. After filtering, the potassium chromate is added and by a process known as complexing, binds with the lead to form a visible precipitate.
The principle is much the same as taking some river water and letting it set in a jar. The sediment will eventually sink to the bottom and form an easily seen residue.
Davie stressed that this is only a presumptive test designed solely to give the investigator a good idea of whether or not gasoline is present. Since the precipitate forms whenever lead is present, false positives may be found from things such as lead-based paints or certain types of manufacturing processes.
“This is not a specific test,” says Davie. “Since things other than gasoline can contain lead, a follow-up at a well-staffed lab is still needed. But if you find a positive sample from something that should not contain lead, you can then work from the assumption that gasoline may have been involved.”
Davie feels that this test should not be used unless the investigator has a reason to think that something is not quite right. “There should be some reason to think the area is suspicious before performing the test. No test in the field will come close to replacing common sense and intuition.”
Questions on the test and interpretation of results may be obtained by writing to Barker W. Davie, Jr., c/o Barker and Herbert Analytical Labs, 207 Main Street, New Haven, IN 46774.
