Avoiding False-Positive Results in Fire Investigations

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As fire investigators, we are all aware of the need to follow protocols and procedures to prevent contamination of samples collected for ignitable liquid testing. Ignitable liquid testing is often used to identify if there were accelerants used to ignite or intensify an intentionally set fire. Laboratory verification of ignitable liquids can lead to denial of claims or arrest and potential incarceration. As investigators, it is imperative that we reduce or eliminate the possibility of false-positive results through proper recovery and handling of samples collected for laboratory analysis.

Testing for Ignitable Liquids

Ignitable liquid testing involves heating a sealed metal can to cause volatile components within the materials to off-gas. The resulting headspace fumes are extracted and passed through a Gas Chromatograph /Mass Spectrometer (GC/MS) to identify the physical spectra of the components. These spectra are then compared to known spectra for ignitable liquids.

As such, evidence samples are collected and placed in sealed cans at the fire scene before being forwarded to the lab for analysis. The laboratory testing will identify ignitable liquids that are present in the can at the time it is sealed at the fire scene. This may include contaminants that are introduced into the can prior to sealing. Any hydrocarbon contaminants would cause the sample to test positive, even though the sample itself may not contain any ignitable liquid residue. This is defined as a “false-positive” result.

NFPA 921 Requirements

Because of the potential for contamination and false-positive results, NFPA 921, “Guide for Fire and Explosion Investigations” contains a section regarding contamination. Section 17.4 - Contamination of Physical Evidence, outlines procedures for minimizing contamination. These procedures include:

  • Sealing containers as soon as they are received from the supplier
  • Wearing clean disposable gloves for each sample recovered
  • Properly cleaning all recovery tools between samples
  • Not utilizing gas-fired equipment in the area of sampling

Use of Exemplar Cans for Control Sample

Another procedure for ensuring the accuracy of your lab results is to send an empty exemplar can to the laboratory as a control sample. This ensures that no residue is present in the cans prior to testing. The use of a control sample is a recognized method for providing an additional layer of verification to your test results. We, at J. S. Held, adhere to these procedures and protocols for minimizing contamination and false-positive results.

Case Study

During a recent investigation, we were surprised to get a false-positive result on an exemplar can during a routine lab analysis of debris samples. After immediately confirming that proper procedures had been followed by the investigator, sample evidence cans were pulled from multiple locations within the company, both from investigators as well as new shipments from the supplier, for testing. A total of seven exemplar cans were sent to two different independent laboratories, which were different from the original lab that identified the false-positive result. All eight cans, from all three labs, tested positive for oxidized hydrocarbons. This verified that the cans were being received from the supplier in a contaminated state. It was hypothesized by the labs that the cans had been treated with a hydrocarbon as a rust inhibitor during the manufacturing process.

Since this incident, we have added an additional internal control process of testing cans from all new shipments received from our suppliers. Additionally, we learned that various governmental agencies have also encountered contaminated cans. In its June 10, 2020 Notice of DFS Policy Change, the Commonwealth of Virginia’s Department of Forensic Science advised that they have experienced false positives due to contaminated cans and are “effective immediately” requiring the submission of a control can.

Through proper adherence to procedures and protocols outlined in NFPA 921, including comparison samples, we were able to identify a contamination issue from our supplier. This allowed us to react quickly, remove the contaminated evidence containers from service, identify new equipment suppliers, and ultimately prevent future contamination issues and false-positive results.


We would like to thank Stuart Morrison, PE, IAAI-CFI for providing insight and expertise that greatly assisted in this research.

Stuart Morrison is a Senior Engineer and expert in Fire Origin & Cause Investigations. Stu has over 20 years of experience in fire investigation and the analysis of mechanical and electrical equipment related to the cause of fires. He also has nearly 25 years’ experience as a volunteer firefighter. He was a fire investigator with Montgomery County and is a certified fire investigator with designations including IAAI-CFI and NAFI-CFEI. Prior to founding Morrison Engineering in 1996, Stu was the Technical Director in Failure Analysis at Encotech in Schenectady, NY. He is a licensed engineer in both New York and Vermont.

Stu can be reached at [email protected] or +1 518 504 6317.

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This publication is for educational and general information purposes only. It may contain errors and is provided as is. It is not intended as specific advice, legal, or otherwise. Opinions and views are not necessarily those of J.S. Held or its affiliates and it should not be presumed that J.S. Held subscribes to any particular method, interpretation, or analysis merely because it appears in this publication. We disclaim any representation and/or warranty regarding the accuracy, timeliness, quality, or applicability of any of the contents. You should not act, or fail to act, in reliance on this publication and we disclaim all liability in respect to such actions or failure to act. We assume no responsibility for information contained in this publication and disclaim all liability and damages in respect to such information. This publication is not a substitute for competent legal advice. The content herein may be updated or otherwise modified without notice.

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