I often recommended the “anonymous” submission of comparison samples, as suggested by cda.

I have reprinted below the IAAI Forensic Science Committee’s 1991 recommendations on comparison samples. Since that time, the lab’s ability to detect really small quantities of hydrocarbons has improved by a factor of more than 100, and so I suggest collecting comparison samples in all cases when it is possible.
See [www.firescientist.com]

I just appeared in a trial where the state’s chemist erroneously called a “control” sample negative, even though it contained not only the same heavy petroleum distillate as the suspect sample, it also contained Permethrin, a well-known insecticide (unfortunately, the State’s chemist did not know that, and by the way, reporting the comparison sample to be positive would not help the DA obtain his conviction—So it goes).


FORENSIC SCIENCE COMMITTEE POSITION ON COMPARISON SAMPLES Fire and Arson Investigator, 1991

Over the past year, the IAAI Forensic Science Committee has attempted to formulate a consensus position on comparison samples. This is an issue which has been around for a number of years and has generated a lot of controversy. This article was prepared in an attempt to resolve some of the major outstanding questions surrounding comparison samples, and to give the lAAI membership the Committee's position on how comparison samples should be approached.

First of all, there is universal agreement in the scientific community that the word “control” is inappropriate terminology when applied to samples of debris collected from fire scenes which are believed by the investigator to contain no accelerants. These are samples which are collected in order to help the laboratory analyst better understand the chemical composition of substrate materials at a fire scene. The word “control” has connotations to scientists and lay people alike which are just not appropriate for describing these samples. Fire investigators have used the word “control sample” for may years to describe comparison samples, and it is our position that this usage should stop.

(Liquid samples collected from a scene for possible comparison with flammable or combustible liquid residues separated from debris samples have also been inappropriately called "control" samples. Since these liquids are technically “unknowns” they should be described as “liquid samples for comparison”).

Comparison samples of background materials are useful on occasion to a laboratory analyst who is examining a gas chromatogram in order to tell whether or not flammable or combustible liquid residues are present. This is because occasionally, the background materials (or substrate materials, or matrix materials) such as wood, carpet linoleum, or other building materials can produce volatile hydrocarbons when they are burned. For this reason, it is sometimes necessary for the laboratory analyst to be able to look at a sample of uncontaminated material,i.e., material on which the investigator believes no accelerant has been poured, to distinguish between hydrocarbons foreign to the background. and hydrocarbons which are part of the natural background.

In most cases, common petroleum distillates are easily recognizable, even when the substrate material creates background peaks. No comparison sample will be needed for analysis, and the absence of a comparison sample will not compromise either the integrity or the accuracy of the results.

Some substrate materials may overwhelm some accelerants with high background peaks. In these cases, unless there is a comparison sample, the results will be falsely reported as negative. The presence of comparison samples may help to sort out the various peaks in the chromatogram and allow a correct identification to be made.

Some substrate materials, most of which are well known to laboratory analysts, produce residues on burning which are very similar to some common accelerants. For instance, asphalt containing materials such as roof shingles can produce a series of peaks which can be misinterpreted as fuel on residue. Some newspapers contain trace quantities of mid-range petroleum distillates used as ink solvent or as a cleaning agent. It is possible that, in the absence of a comparison sample, falsely positive results may be reported Comparison samples in these cases help to prevent miscalls.

Whether or not a comparison sample is necessary or useful in a particular analysis is a decision which the laboratory analyst must make based on the analyst’s experience, and the situation at hand. While it is the laboratory analyst’s call to decide whether a comparison sample is necessary, it is obviously the field investigator who will collect the sample. Because the field investigator cannot know whether comparison samples will be required or not, it is necessary to collect them whenever possible, and at the time of the original field inspection. A fire scene is a fleeting thing, and if a comparison sample is not collected at the same time as suspect samples are collected, it may be impossible to go back at a later time and collect an appropriate comparison sample.

It is recognized that there are occasions when a comparison sample is not available for any number of reasons. A comparison sample is defined as a sample which, to the best of the investigator’s knowledge, is identical to the suspect sample in every respect except that no accelerant has been applied to the material being selected for comparison. Obviously, if the suspect sample consists of a piece of cloth which has been soaked with accelerant and used as a trailer, a comparison sample may be unavailable.

A melted plastic jug containing a flammable liquid is another example of a type of suspect sample for which no comparison sample may be available: Any time a sample consists of multiple types of material, it is recognized that selection of a comparison sample will be difficult if not impossible. Another situation where there may be no comparison sample available would be in the case of a total fire loss, or black hole. Suspect samples may be collected from areas where there are no bum patterns, but where one would reasonably expect an arsonist to pour accelerant such as doorways and hallways. Comparison samples could be selected from areas away from doorway, and hallways, but it Is not always possible to tell where those area, and it is always difficult to determine the composition of the original material.

In many cases, the selection of appropriate comparison samples can be as difficult or even more difficult than finding the positive samples In the fire scene. The Committee recognizes these difficulties, and wishes to emphasize that these recommendations concerning collection of comparison samples are just that, recommendations, not absolute requirements.

To sum up, the position of the Forensic Science Committee on comparison samples is as follows:

1. The word control sample is inappropriate terminology.

2. Comparison samples are sometimes necessary for the proper analysis of samples from a fire scene, but will usually not be required for routine identification of common liquid accelerants.

3. The necessity for comparison samples is a judgment that can only be made by the laboratory analyst, but the selection of comparison samples can only be done by the fire scene investigator. Therefore, comparison samples should be collected when the original fire scene inspection is performed.

4. It is recognized that collection of proper comparison samples is sometimes impossible, and always difficult.

The Committee realizes that the publication of this article will not resolve all of the questions regarding comparison samples, but hopes that the above position will make the fire investigator's task a little easier. The Committee Chair and membership welcome any questions or comments from the IAAI membership.

John J. Lentini, Chair
Mary Lou Fultz, Co-Chair
Andrew Armstrong
Barker Davie
John DeHaan·
Richard Henderson
Jerry O'Donnell
Betty Jean Rogers
James Small

John Lentini, CFI, D-ABC
Fire Investigation Consultant
Florida Keys
[www.firescientist.com]