Issues in Explosives Residue Analysis: Interpretation of data in explosive scene investigation

Issues in Explosives Residue Analysis A Primer for the Bar Frederic Whitehurst, Ph.D.[1]

[Editor’s Note: This is a multi-part series deigned to educate the defense bar on important issues concerning explosive and explosive residue investigations]

Part 1: Introduction

Part 2: Back to the Basics: Was it the result of an explosive device in the first place? How do we know that?

Part 3: Daubert provides guidance and a means to expose limitations and evaluate explosive investigations, methods, and interpretation

Part 4: The Explosion Crime Scene: Sampling and Homogeneity Issues

Part 5: Disposition Homogeneity in explosive scene investigation

Part 6: Contamination and Cross Contamination in explosive scene investigation

Part 7: Contamination by “Render-Safe” acts of explosives

Part 8: Transportation and storage of evidence in explosive scene investigation

Part 9: Chemical analysis in explosive scene investigation

Part 10: Identifying Techniques in explosive scene investigation

Part 11: Interpretation of data in explosive scene investigation

Part 12: Experience: What makes for a proper expert in explosive scene investigation?

Part 13: Conclusion

At some point in the analytical scheme, the residue analyst will conclude that she can be reasonably sure that she has detected various materials which might be signatures of initiated explosives. Such materials include uninitiated/unreacted explosives, such as TNT and nitroglycerine and also materials such as nitrates, chlorides, sulfates, chlorates, and perchlorates. At this point the analyst must begin the attempt at interpretation of the data. That interpretation is the most critical test of the “expert” training. Imwinkelried [102] points out an important factor which I shall refer to as the “aberrant” character of the materials found in the matrix analyzed. He uses as his example the neutron activation analysis (NAA) detection of antimony and barium, the components of gunshot residue, which are found on the hands of shooters of guns. He asks “If NAA reveals the presence of antimony and barium,…must we conclude that the subject recently fired a weapon?…In the past some NAA experts have attempted to express such conclusive opinions to lay jurors. But, upon reflection, it is clear that in framing those opinions, the analysts were guilty of classic non sequiturs. We cannot draw those inferences until we have developed a comparative database. It may be true that there are antimony and barium on the subject’s hand and that they are the components of primer. However… first we must ask: In what quantities do antimony and barium naturally occur in the environment?”

The explosives residue analyst must also ask this question? Do the materials that are being detected normally occur in the matrix from which they were collected or are they aberrations in the background of environmental material. Dr. Magee [103] describes the situation well in the investigation of the bombing of United Airlines Flight 629 on November 1, 1955. After the plane exploded for no apparent reason eleven minutes into its flight, Magee collected evidence at the crime scene for analysis. He found blackish-gray-colored metal particles which when analyzed were found to have sodium carbonate along with small amounts of nitrate and sulfur adhering to them. He states, “We had no idea what the heck they were doing there. Sodium carbonate is normally found in scouring powder. There was no logical reason for it to be on those parts of the airplane.” Magee is expressing the fact that what he was seeing was an aberration in the background, material which normally would not be found in that type of matrix. In contrast to Magee’s findings are those described by Maloney and Thornton. [104] These authors note that a particular test for nitrates found on the hands of shooting suspects was discredited, because “nitrates are ubiquitous in the environment and because there is no differentiation between nitrates from gunpowder and those from other sources…. These interfering factors, which may cause false positives, are tobacco ash, pharmaceuticals, fertilizers, leguminous plants, urine, and colored fingernail polishes.”

The explosives residue analyst must also ask the more complex question, “Are the materials we are detecting a result of the breakdown of nonexplosive substances present at the scene of the explosion which were effected by the extreme heat and pressure from the explosion?” For instance, it is known without citation by most laymen, that various parts of the United States and the world suffer from high concentrations of acid rain formed from nitric acid and sulfuric acid discharges into the atmosphere. The deposition of those materials on the surface of objects that later are involved in bombing crime scenes may negate the value of finding those materials at the crime scene. Of what use is information about the presence of nitrate and sulfate ions in explosives residue if we can only conjecture about their origin. And objects in the world are more and more often made of plastics which might produce materials that we would see in the residues from an explosive but which did not originate from the explosive. Sulfates, nitrates, ammonium ions, chloride ions and other materials are produced by the burning of plastic and other nonexplosive materials.

We have already also explored the possibilities of contamination so that even materials such as TNT and nitroglycerine may be detected on evidence where those materials were not in the bomb’s explosive at all. Counsel should closely look at the interpretations of data in explosives residue expert opinions to determine if materials found in the residues could have likely other sources than the bomb itself. Consider, for instance, a bombing in a parking garage. The presence of nitrates and sulfates in the residue from the crime scene might not necessarily be probative if the garage exists in an acid rain belt of the United States. Also the presence of urea, possibly coming from a urea nitrate-based explosive, could have originated from urea being used to melt ice on the streets. Urea found on evidence can also have been transferred to the evidence from the urea naturally occurring in the sweat on the hands of evidence collectors. The detection of urea in that instance is exactly like the detection of barium and antimony mentioned by Imwinkelried.

Of what use is the finding of ammonium ions normally originating from the detonation of ammonium nitrate-based explosives when many fires are extinguished at a bombing crime scene with fire extinguishers containing ammonium sulfate and ammonium phosphate. Unless the analyst can do more than conjecture about whether the ammonium ions originated from fire extinguishers or bomb components of ammonium nitrate based explosives, he would have to admit that an explanation of the ammonium residues as having originated from the bomb was not the only reasonable explanation. Therefore under Stokes [105] the court would not admit that testimony. Counsel must critically review the matrix from which residue was extracted. For instance, where explosives would not normally be considered a component of matrices such as residential housing, a home presently or formerly occupied by a munitions worker or a military soldier, or a home in the same area as a military base, or a home of an individual using nitroglycerine or PETN heart medicine may very well have explosives residues adhering to surfaces. If a law enforcement bomb technician were to go from a bombing crime scene directly to an arrest situation, that technician may transfer explosives residues picked up at the crime scene to the suspect arrested shortly thereafter. If the suspect was arrested by law enforcement personnel who then placed him in a vehicle used in the past to hold explosives evidence the suspect may pick up explosives residues on his clothing from contact with surfaces in the vehicle. Some components of residue such as sodium, potassium and chloride ions occur naturally. They also occur in residues from exploding bombs.

Counsel that remains confused may refer to law school professional responsibility classes’ teachings concerning the commingling of client and attorney funds. Once the dollars are mixed together, one cannot say with certainty whose pocket they came from. Once the components of bombs’ residues are mixed together with the same kinds of materials from the matrix in which the bombs exploded they are no longer separable nor can any analyst say that those materials originated from the bomb. In the interpretation of explosives residue analysis data, though, the most important aspects to understand are the significance of what is not seen. Explosives are built to efficiently react and to totally decompose. Many decomposition products are gases that do not stay around to be analyzed but simply mix with the atmosphere.

For instance, explosive peroxides, used by a number of terrorist groups [106] leave no solid residue at all upon initiation. A bomb could have this material present in it along with other types of explosives and the analyst never realize it from analysis of the residues. The blast damage would not categorically reflect the presence of these high explosives and the analyst could and most likely would miss the material’s signature entirely. Some homemade explosives are mixtures of materials some of the components of which could be missed in an analysis due to the lack of deposition homogeneity mentioned before. And environmental impact could erode some energetic components to the point that they were not detectable even with the highly sensitive equipment that science uses today. Rain or water used to extinguish fires can and does dissolve inorganic oxidizing salts used in explosives as well as the salts found in the residues from explosives. There can be any number of very feasible reasons why an explosive could be misidentified because of component materials not being detected during analysis of the explosives residue. Without knowing whether these processes took place and without being able to rule them out, the explosives residue analyst cannot present a “scientific” opinion to the trier of fact concerning the type of explosive used in an initiated device. The analyst can only present conjecture concerning the total makeup of the explosive used. Imagine that the analyst does hypothesize from the observation of materials that a particular type of explosives mixture was present. Then he must validate that hypothesis by building a bomb filled with that type of explosive. Even if the residue signature is the same for the test bomb as that from the crime scene, then he must also test other mixtures, knowing that there is a very real possibility that other explosives could have been present in the bomb and that his crime scene crew may have missed a piece of material with relevant residues on it or that the residues may have decomposed or not been detectable by the protocol he used. There are many feasible and reasonable explanations which cannot be discounted for not finding particular explosives residues.

All of this leaves the explosives residue expert having to seriously weigh the evidence that he is to present and having to be very careful in rendering a categorical description of the explosive.

And finally Starrs [107] brings up the possible problems associated with bias. His description of a police drug investigation meeting the forensic laboratory is instructive. “In a substance abuse investigation the police will transport the confiscated substance to the crime laboratory noting that a field test was conducted with positive results. The police, uninitiated into the line drawing between screening and confirmatory tests, are satisfied their drug bust was well made in true Kojak-style. They further muddy the waters of a dispassionate scientific evaluation by mentioning, with heightened gusto, that the drug cost them $10,000 of law enforcement money which will be lost if the laboratory analysis does not concur with their conclusions. The analyst, mindful of these police importunities, conducts further spot tests, microscopic analyses and a number of thin layer chromatographic systems. These tests all correlate perfectly with the tableau of the events recited by the police. But the scientific analysis could be taken to a more assured, indisputably confirmatory level using gas chromatography coupled with mass spectrometry.

But will it?

The laboratory is drowning in a tidal wave of substance abuse cases. Most such cases are plea-bargained out short of trial anyway. These incentives to short circuit the more confirmatory procedures are exacerbated by the fact that the police will be plenty ireful with a laboratory result contrary to their preordained conclusions.

Where does this leave the forensic scientist who strives to reflect an open mind in testing physical evidence?” Starrs also quotes significantly from a paper by Evan Hodge, formerly Firearms-Toolmarks Unit Chief at the FBI laboratory. [108] According to Starrs, Hodge notes that no forensic scientist is immune from police pressure tactics. Starrs notes that Hodge goes so far as to say that “We all do this (give in to investigative pressure) to one extent or another.” One may ask if the “giving in to investigative pressure” has manifested itself in the analysis of explosives residues. How could a field investigator, without conducting a chemical analysis, have any idea about what was in an explosive charge and therefore pressure a forensic chemist into a position. One piece of data that the crime scene investigator might try to use is the admission of the perpetrator himself or the statements of witnesses to the explosion. One must remember that the bomber is very often one of the least trustworthy sources of information and may give false information.

Witnesses to explosions often come forward with statements identifying the type of explosive used based on the sounds of the blast, or the color of the smoke from the blast, none of which is very good data from which to judge what type of explosive was used. Another kind of data that the contributor of evidence uses to “identify” the explosive is from the blast damage itself. Blast damage depends upon the shattering ability of the explosive, referred to as the “brisance,” as well as the temperature of the explosion, the location of the explosive, the types of materials effected by the explosion, the confinement of the explosive and a number of other factors. Even if all factors were held constant except the type of explosive used, one could only postulate about what specific explosive was used. Many explosives share the same brisance values. Therefore the hypothesis presentable by field investigators at best can only narrow down the type of explosive to a relatively broad range.

Indeed there is a body of science which involves itself with this question. Dr. Frank Tatom, a researcher in this field, has stated that “I cannot conceive of a means by which an observer, no matter how experienced, could inspect the damage pattern to a structure, and from such inspection identify the explosive used with any degree of confidence.” [109] Scientists have the nasty habit of measuring things. They measure such things as distance, velocity, temperature, mass, and time. When they put forth hypotheses, these hypotheses are based upon observations which in the physical sciences are based upon measurements. For instance, one way of measuring the performance of an explosive is the sand test. [110] The test is described as follows: “A known amount of the explosive is exploded in sand consisting of a single grain size (sieve) fraction; the magnitude determined is the amount of the sand which passes a finer-meshed sieve following the fragmentation.” The measurement is one of weight of the sand passing through the sieve.

Counsel, when faced with proffered testimony by experts purporting to identify the type of explosive used from the blast damage, should ask of the expert just what measurements have been made to support the testimony. Physical scientists don’t just “look” at a pile of rubble and wave a wand. This situation was addressed very clearly in O’Conner. [111] In this case the expert testified to the court that he could simply observe cataracts and determine if they have characteristics specific to radiation-induced cataracts. The scientific studies that he offered did not support this testimony. The articles did note that “although radiation-induced cataracts are of the posterior subcapsular type, all posterior subcapsular cataracts are not radiation-induced.”

Taking this theme back to blast damage, counsel must recognize that many, many types of explosives can cause the same type of damage in any particular matrix. Without the scientific measurements which would be extremely time-consuming, and the known data base of information to compare to [112], the court may properly exclude this type of evidence because it is not well-grounded in the scientific method. The explosives residue analyst also may exclude this type of evidence when attempting to determine the type of explosive used in a device. Field investigators could present hypotheses which they expect to have verified, not tested.

But has it happened in the past?

Once asked, this question is quickly answered by the Royal Courts of Justice in the Court of Appeal, Criminal Division. Before Lord Justices Glidewell, Nolan and Steyn on June 4, 1992, in the matter Regina v. Judith Theresa Ward, the court record reads “An incident of a defendant’s right to a fair trial is a right to timely disclosure by the prosecution of all material matters which affect the scientific case relied on by the prosecution, that is, whether such matters strengthen or weaken the prosecution case or assist the defence case.” The record reads further “In Miss Ward’s case the disclosure of scientific evidence was woefully deficient. Three senior RARDE scientists took the law into their own hands, and concealed from the prosecution, the defence and the court, matters which might have changed the course of the trial. The catalogue of lamentable omissions included failures to reveal actual test results, the failure to reveal discrepant Rf values, the suppression of the boot polish experimental data, the misrepresentation of the first firing cell test results, the concealment of subsequent positive firing cell test results, economical witness statements calculated to obstruct enquiry by the defence, and …” The record continues, describing why these individuals failed. “For the future is it important to consider why scientists acted as they did. For lawyers, jurors and judges a forensic scientist conjures up the image of a man in a white coat working in a laboratory, approaching his task with cold neutrality, and dedicated only to the pursuit of scientific truth. It is a sombre thought that the reality is sometimes different. Forensic scientists may be partisan.”

The “failure” here involved the analysis of explosives residues. If indeed Starrs and Hodge are correct and if indeed the British scientists bent to pressures, then where would counsel see manifestations of this pressure on the forensic explosives residue examiner?

In order to combat this type of failure of science, counsel should look directly to the question that is answered by the examiner. If the question that is answered following the explosion of a bomb is “Was the explosive of type X?” and the examiner narrowly interprets his data, saying that “Yes, it very possibly originated from explosive X,” then one might infer that the examiner has narrowed his interpretation of the data down to the point of being useless or has insufficient expertise to recognize the many other types of explosives that could have given the same residue signature following explosion, or ignored the fact that the matrix in which the bomb exploded may have had the same materials in it as the explosives residue. If the contributor of the evidence presents an hypothesis established from field investigation that the explosive was, for instance, C-4 plastic explosive, then the explosive examiner’s job is not to prove that C-4 was present but to try to determine what explosive was present, to test the hypothesis of the field investigator. If the examiner simply looks for nothing but the component RDX which is in C-4 plastic explosive then she may miss other materials which RDX also is combined with in other explosives. She may also miss a signature from many other types of explosives which are unrelated to C-4 plastic explosive. The author has personally analyzed many unexploded homemade explosives containing mixtures of explosives which represent the combination of up to six different types of explosives. If the laboratory report should read “The residues found could have originated from C-4 plastic explosive.”, counsel should look very closely at the way the analysis was conducted. That opinion fails to inform the trier of fact that many, many types of explosives contain RDX, and there is presently no way of determining with any great degree of certainty how many of the other candidates were also part of the explosive that caused the explosion. It may very well be true that the residue signature matches that of a particular type of explosive and therefore passes the Rule 401 relevancy test. [113]

However the probative value of that match may be very limited if any number of other explosives mixtures could have given the same residue signature. Or even more reasonably, the probative value of that match may be nonexistent due to the present inability of crime-scene evidence collectors to always know what evidence to collect that might have representative residues on it.

As Berger [114] reminds us, “The Daubert Court reminded judges that they have the power to exclude prejudicial or confusing evidence pursuant to Rule 403: ‘Expert evidence can be both powerful and quite misleading because of the difficulty in evaluating it. Because of this risk, the judge, in weighing possible prejudice against probative force under Rule 403 of the present rules exercises more control over experts than over lay witnesses.'”

Therefore a witness that reports a narrow interpretation of the data from residue analysis can very well lend a prejudicial aspect to the significance of the data which can result in exclusion of the testimony despite its probative value. Narrow interpretations of residue analysis data may also inform the counsel that the explosives residue analysis was conducted to support the evidence contributor’s hypothesis, not to test it. Again, one can imagine that the pressures on the forensic examiner may be insurmountable in high profile bombing matters.

Counsel can detect manifestations of the analyst’s succumbing to such pressures and bring out the true meaning of the analytical data through appropriate probing.


[1] Executive Director, Forensic Justice Project, Washington, D.C., B.S. Chemistry, 1974, East Carolina University, Ph.D. in Chemistry, 1980, Duke University, J.D., 1996, Georgetown University School of Law. (202)342-6980.

[102] Imwinkelried, supra note 69, at 277.

[103] Fisher, supra note 8, at 29-30.

[104] R. Maloney & J. Thornton, Color Tests for Diphenylamine Stabilizer and Related Compounds in Smokeless Gunpowder, 27 J. Forensic Sciences 318 (1982).

[105] Anderson, supra note 76, at 943 cites Stokes v. L. Geismar, 815 F.Supp. 904,909 (E.D.Va 1993) holding that an expert’s testimony as to causation was “mere speculation where the expert himself admitted that his explanation of the cause was not the only reasonable one.”

[106] J. Chladek, The Identification of Organic Peroxides, Advances In Analysis And Detection of Explosives, Proceedings of The 4th International Symposium on Analysis and Detection of Explosives 73, September 7-10, 1992, Jerusalem, Israel notes that these peroxides are a “group of substances which draw an attention of forensic scientists as these substances are used in criminal activity and by terrorists.”

[107] James E. Starrs, The Forensic Scientist and the Open Mind, 31 Journal Of Forensic Science Society 111, 137-138 (1991).

[108] Starrs, at 138 cites HODGE E. Guarding Against Error, 20 AFTE J 290-293, July 1988.

[109] Private communication from Frank B. Tatom, Ph.D., P.E., President/Chief Engineer of Engineering Analysis, Inc., Huntsville, Ala. to the author.

[110] Rudolf Meyer, Explosives 296 (1987).

[111] O’Connor v. Commonwealth Edison Co., 13 F.3d 1090 1106(7th Cir. 1994).

[112] Porter v. Whitehall Laboratories, Inc., 9 F.3d 607 614(1993).

[113] Federal Rule of Evidence 401 defines relevant evidence as: “Relevant evidence means evidence having any tendency to make the existence of any fact that is of consequence to the determination of the action more probable or less probable than it would be without the evidence.” FED. R. EVID. 401.

[114] Berger, supra note 5, at 1358.


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