Fire/Arson Investigations : Fire/Arson Investigations
A place to ask questions and add to probative and informative discussions associated with the various aspects of the field of fire investigation. -- FORUM RULES---BE CIVIL AND NO NAME CALLING, NO BELITTLING, NO BERATING, NO DENIGRATING others. Postings in violation of these rules can be removed or editted to remove the offending remarks at the discretion of the moderators and/or site administrator.
Re: Definition of "Weathered" Ignitable Liquid Residue (ILR)
Posted by: John J. Lentini, CFEI (IP Logged)
Date: July 11, 2007 10:44PM
Weathering is a term used to describe ignitable liquids that are not "fresh." There is currently no way to distinguish the effects of evaporation at ambient temperature versus the effects of evaporation due to fire exposure. Many chemists have tried. At one time, it was common to refer to "burned gasoline," but the term "weathered gasoline" is more accurate.
The term is usually applied to gasoline, because with the other distillates, at least with the medium and heavy ones, one really doesn't know how it started out, unless there is a sample of known "fresh" liquid. One batch of mineral spirits or kerosene or diesel fuel may have a higher average molecular weight than another, so the one with the higher average molecular weight will appear more weathered.
What we can state with some certainty is that liquids do not change in the direction of lighter average molecular weight upon evaporation, so it is possible to address some issues of identity of source based on average molecular weight.
With gasoline, one knows generally what the starting material is, so it is more easily characterized as to weathering. Unless it is in the long cylinder that Dr. Hurst describes, the lighter compounds will evaporate predictably, leaving behind a mixture relatively richer in the heavier compounds. It is difficult to estimate the degree of evaporation more closely than plus or minus 10%. I use standards of fresh, 25% evaporated, 50, 75, 90 and 98% evaporated for comparison purposes. There is a section in Chapter 5 of my book dealing with estimating the degree of evaporation.
If a sample of residue gives the appearance of not being weathered, then it is certainly appropriate to address contamination as a possible source of the residue. Some matrix effects, however can lead to confusion. Concrete, for some reason, tends to hold on to the lighter compounds, so extracts from fire exposed concrete may give the appearance of not having been exposed to a fire.
John Lentini, CFI, D-ABC
Fire Investigation Consultant
Florida Keys
[www.firescientist.com]
The term is usually applied to gasoline, because with the other distillates, at least with the medium and heavy ones, one really doesn't know how it started out, unless there is a sample of known "fresh" liquid. One batch of mineral spirits or kerosene or diesel fuel may have a higher average molecular weight than another, so the one with the higher average molecular weight will appear more weathered.
What we can state with some certainty is that liquids do not change in the direction of lighter average molecular weight upon evaporation, so it is possible to address some issues of identity of source based on average molecular weight.
With gasoline, one knows generally what the starting material is, so it is more easily characterized as to weathering. Unless it is in the long cylinder that Dr. Hurst describes, the lighter compounds will evaporate predictably, leaving behind a mixture relatively richer in the heavier compounds. It is difficult to estimate the degree of evaporation more closely than plus or minus 10%. I use standards of fresh, 25% evaporated, 50, 75, 90 and 98% evaporated for comparison purposes. There is a section in Chapter 5 of my book dealing with estimating the degree of evaporation.
If a sample of residue gives the appearance of not being weathered, then it is certainly appropriate to address contamination as a possible source of the residue. Some matrix effects, however can lead to confusion. Concrete, for some reason, tends to hold on to the lighter compounds, so extracts from fire exposed concrete may give the appearance of not having been exposed to a fire.
John Lentini, CFI, D-ABC
Fire Investigation Consultant
Florida Keys
[www.firescientist.com]
Sorry, only registered users may post in this forum.