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Report No: DOT/FAA/AM-02/15

Title and Subtitle: An Accurate Method for the Determination of Carbon Monoxide in Postmortem Blood Using GC/TCD

Report Date: August 2002

Authors: Lewis, R.L., Johnson, R.D. and Canfield, D.V.

Abstract: During the investigation of aviation accidents, postmortem samples from accident victims are submitted to the FAA’s Civil Aerospace Medical Institute for toxicological analysis. To determine if the accident victim was exposed to an in-flight/post crash fire or faulty heating/exhaust system, the analysis of carbon monoxide (CO) is conducted. While our laboratory predominantly uses a spectrophotometric method for the determination of carboxyhemoglobin (COHb), we consider it essential to confirm with a second technique based on a different analytical principle.

Our laboratory encountered difficulties with many of our postmortem samples while employing a commonly used GC method. We believed these problems were due to elevated methemoglobin (MetHb) concentration in our specimens. MetHb does not bind CO, thus elevated MetHb levels will result in a loss of CO binding capacity. Since most commonly employed GC methods determine %COHb from a ratio of unsaturated blood to CO-saturated blood, a loss of CO binding capacity will result in an erroneously high %COHb value.

Our laboratory has developed a new GC method for the determination of %COHb that incorporates sodium dithionite, which will reduce any MetHb present to Hb. Using numerous fresh human blood controls ranging from 1% to 67% COHb, we found no statistically significant differences between %COHb results from our new GC method and our spectrophotometric method. We then applied our new GC method to putrefied and non-putrefied postmortem samples. To validate the new GC method, postmortem samples were analyzed with our existing spectrophotometric method, a GC method commonly used without reducing agent, and our new GC method with the addition of sodium dithionite.

As expected, we saw errors up to and exceeding 50% when comparing the unreduced GC results with our spectrophotometric method. With our new GC procedure, which incorporates a reducing agent, the error was virtually eliminated.

Key Words: Combustion Toxicology, Carbon Monoxide, Carboxyhemoglobin, Methemoglobin, Aircraft Accident Investigation

No. of Pages: 9