Most chemists do not compare ignitable liquid residues. (ILR). Comparisons are beyond the scope of the standard test method, ASTM E-1618, but the science is not complicated.

The source of ILRs may be of interest to fire investigators. This is particularly true if two samples contain the same classification of ILR.

One way to compare any type of ILR to a suspected source or to see if two ILRs within the same class came from the same source is to compare the average molecular weights. When exposed to a fire, ignitable liquid evaporates, and its average molecular weight necessarily increases. No predictable experience causes the average molecular weight of the fire-exposed liquid to decrease. If the suspected source of an ignitable liquid, which was not exposed to the fire, exhibits a higher average molecular weight than the residue extracted from samples collected at the scene, the suspected source can be conclusively eliminated.

This was a crucial finding in the case of California versus George Souliotes, who had MPD on his shoes, and there was MPD found in the fire scene. The prosecution claimed that this "match" proved that Souliotes was guilty. In fact, the MPD found in the fire scene in an area exposed to the fire had a lower average molecular weight than the MPD on his shoes. Thus it was possible to eliminate the proposition that the MPD inside the scene was accidentally spilled onto the defendant's shoes. All the chemists who examined the data agreed that the two MPDs had a different source.

With respect to any painted surface, a finding of a trace amount of MPD in the absence of a comparison sample is meaningless. In the Fegley fire, this was especially meaningless because it was found on an exterior surface of the basement window. A photo of the window is attached.

You will only find a report of a comparison on a lab report if you ask for the comparison to be done.

A copy of the affidavit that I filed questioning the significance of the MPD finding can be found here [app.box.com]

John J. Lentini, CFI
Scientific Fire Analysis
Islamorada, FL
www.firescientist.com
scientific.fire@yahoo.com