Issues in Explosives Residue Analysis: Chemical analysis 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

In the mind of the trier of fact, it is this area which generally comes forward when the expert analytical chemist is testifying. “In the minds of most lay jurors, these techniques epitomize scientific evidence.” [69] And it is this area which has been most highly addressed in the field of explosives residue analysis over the past twenty-five to thirty years. [70] The papers of Beveridge and Yinon and Zitrin mentioned above give good overall histories and descriptions of the development of the field, especially paying close attention to the analytical techniques that have been utilized in this field. These techniques may come with very complicated names but in fact are used in such a manner by the explosives examiner that the average juror and judge could understand the data if properly instructed and counsel could raise valid questions during the teaching phase of testimony.

Of immediate importance in this discussion of instrumental analysis tools that are used in explosives residue analysis is an understanding of the “identity hypothesis.” When a scientist “identifies” a material, the scientist is declaring that he has made observations about the material, has hypothesized that the material has a particular identity and has tested his hypothesis and not found it to be false. The scientist then is making an inference or assertion which is “derived by the scientific method” and is in keeping with Daubert. [71] The scientific method requires observation, hypothesis based on observation and then testing of hypotheses. This is very important where explosives residues are concerned. Many explosives residues can normally not be seen by the naked eye or by even a scanning electron microscope. The analyst does not normally know ahead of time what residues will be found in bomb debris. Even when a suspect has confessed to the use of a particular type of explosive or intelligence information has indicated that terrorists are using a particular type of explosive, the job of the analyst is to independently corroborate that information through scientific analysis. The analyst may, for example, receive intelligence information that C-4 plastic explosive containing the explosive RDX was used in a bomb. If the analyst simply looks for RDX during her analysis, she may find RDX but miss other materials which are found in combination with RDX in other types of explosives. RDX can be and is combined with other types of explosives such as TNT and PETN, many of which combinations are used by criminal bombers. Simply stopping the analysis with a finding of the presence of RDX in residue may very well lead to an interpretation of the data that misleads the trier of fact. The trier of fact would have no way of knowing that other types of explosives are used in combination with RDX and would therefore be prejudiced to think that the detection of RDX was indicative of the use of C-4 plastic explosive. The wrong weight would be given to the evidence. If the explosive residue material in debris can not be seen by the naked eye, by microscope, or by scanning electron microscope, and cannot be physically tested for such characteristics as color or melting point due to its limited quantity, then the first observation the analyst will make will be with an instrumental analysis. The analyst that rushes forward with the hypothesis presented by that first observation from whatever instrument is used will not be presenting scientific evidence to the court. No one instrument can be utilized to “identify” possible components of an explosive found in an initiated bomb. The burden of proof requirement will not have been satisfied and the evidence will not be admissible.

The explosive residue scientific community has spent twenty-five years fine tuning analytical protocols which are presented in the Proceedings of the International Symposia on Explosives Analysis. [72] The protocols are well documented and validated for many types of explosives and explosive crime scenes. However, there are many, many types of explosives not considered by the protocols because of the impracticability of including all of the different types of explosives into the protocols. [73]

The identity hypothesis which purports to totally identify within any degree of certainty all of the energetic components of an explosive mixture which has been initiated should be very closely scrutinized. The article on instrumental techniques by Robert and Joan Watkins [74], though dated at this time, offers fundamental principals which are necessary and helpful for an understanding of explosives residue analysis as it is practiced in the forensic community today. In order to understand the science that the analytical chemist practices, one must realize that the task of analysis is divided into four areas: 1.) Physical analyses, such as optical microscopy, boiling and melting point determination, color comparison, and crystal morphology comparison, etc., 2.) Elemental analysis which is accomplished with atomic absorption spectrometry, x-ray fluorescence or energy dispersive x-ray analysis, 3.) functional group and atomic spatial arrangement analyses which are conducted with x-ray powder diffractometry (XRD), high performance liquid chromatography (HPLC), gas chromatography (GC), mass spectrometry (MS), nuclear magnetic resonance spectrometry (NMR), infrared spectrometry (IR), capillary zone electrophoresis (CZE), thin-layer chromatography (TLC) and a host of other methods of analysis, and 4.) interpretation of the analytical data.

The explosives analyst relies initially on microscopic analyses. However because components of the original explosive may be present after the explosion in submicroscopic amounts, the analyst must proceed with instrumental analysis. The analytical chemist’s tools can be divided into separation techniques and “identification” techniques each of which have their strengths and weaknesses. Separation Techniques Chromatographic techniques are essentially separation techniques and cannot be used alone as “identifying” techniques.

Daubert can be seen as dictating this requirement. “Proposed testimony must be supported by appropriate validation…” [75] Anderson [76] teaches that a court should “disregard expert opinion if it is mere speculation, not supported by facts.” These separation technique tools are needed by the forensic examiner to separate materials before a detector or analyzer tries to make sense of the material. The trier of fact must know that if these tools are utilized, the scientist can never really identify a material but must present his conclusions with the word “consistency.”

To be fair to the trier of fact, the unbiased expert must present alternative explanations for the data, of which there are many, with these separation techniques. The reason for this is that one can never be sure that material eluted from one of these devices equipped with a nonspecific detector is not one of a number of materials that would elute in the same manner. For example, high performance liquid chromatography functions by flowing a liquid through a tube, called a column, which is filled with a substrate or surface to which many materials dissolved in the liquid flowing through the column will adhere to different degrees. The materials go into the column together at one end and leave the column at the other end at different times referred to as “retention times” depending upon their attraction to or interaction with the material in the column. The Walters’ description of gas chromatography closely follows and further fills out this description. [77] As they also point out, without a specific detector, confirmation with another analytical technique [78], and good chemical standards also being injected into the column, one cannot be sure of the identity of materials emerging from the column.

In order for the trier of fact to determine if the analyst has given reliable data, a determination has to be made concerning the analytical method used in the technique. Specifically, the trier of fact should be presented with clear, well documented data which will include analytical conditions and pedigree of any standards used. In Murphy [79] the court disallowed the portion of an expert witness’ testimony concerning thin-layer chromatography because the standard used in conducting the test had never itself been tested against any known standard. Daubert requires reliability, the primary focus of which obligation is Rule 702.80 The trier of fact should look for documentation associated with the analytical data and require that the offerer of expert opinion satisfy her burden of preponderance of proof. The charts must be presented correctly with all the analytical conditions of the method in order for the court to determine if the method utilized is reliable and can be repeated. If, as Daubert points out [81], “Proposed testimony must be supported by appropriate validation, ‘good grounds’ based on what is known”, one must start with what is known from the data itself.

Without clear presentation of analytical conditions, determining what is truly known about the data is impossible. Partial and incomplete presentation of the data and analytical conditions results in data that is not scientific data in that it is not falsifiable. Daubert tells us that “a key question to be answered in deciding whether something is scientific knowledge will be whether it can be tested.” [82]

Despite Justice Rehnquist’s confusion concerning falsifiability expressed in his dissent in Daubert, the ability to falsify takes us out of the realm of “black magic” into modern science. This ability is particularly important in the complex field of explosives residue analysis. The trier of fact can easily understand techniques that are so simple as determining how long it takes for an analyte to emerge from a column and be detected. The trier of fact can also determine if standards are traceable to some valid source. (Until about 1994, acquiring explosives standards with documented pedigree was virtually impossible due to the inherent cost and danger with manufacture and shipping of explosives standards. Standards were derived in some laboratories by those laboratories extracting the materials from commercial explosives and comparing data from the analyses of the extracts to data in the literature. [83])

And the trier of fact can easily see, if presented the correctly documented data, whether evidence, which is described by these techniques as having a particular material, does in fact have that material. The process of determination simply involves looking at the data to see if 1.) the analytical conditions are the same for analyses of unknowns and standards; 2.) if the retention times for the standards and the unknowns are the same or within the limits of the known variability of the times; and 3.) determining if the standards have well documented pedigree. These simple determinations can be addressed by the court and are applicable to any of the methods that can be described as “chromatography.”

Though Justice Rehnquist in his dissenting opinion, [84] describes scientific knowledge, methods and validity as “far afield from the expertise of judges,” in reality there is no mystery, nor should there be, to understanding the data presented by these techniques. The fact that the “criterion of the scientific status of a theory is its falsifiability, or refutability, or testability,” impinges itself on the data that is presented from these methods. Without presentation of the analytical conditions and standards’ pedigrees, the court is not allowed to test the evidence nor can appropriate cross-examination take place and therefore it is not presentable to the trier of fact as “scientific evidence.”

When the court is faced with explosives analysis data which has been gleaned from chromatographic analyses followed by nonspecific detectors, the court should not accept opinions purporting to “identify” materials without questioning the underlying scientific validity of the evidence. Though chromatographic retention times may be consistent with those of known explosives, there may very well be other materials with the same retention times which would be as likely found in the matrices in which explosives would be found. As a chromatographer gains expertise and experience with chromatographic methods, more and more often that experience includes knowledge of specific situations where different materials responded in the same way to a particular chromatographic instrument. The analyst must make this known to the trier of fact if indeed she is to present a scientific opinion. Chromatographic methods are merely separation methods. As in Anderson [85], the expert must admit that her explanation of the data is not the only reasonable one and therefore her testimony will be mere speculation and the court may disregard it.


[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.

[69] Edward Imwinkelried, A New Era in the Evolution of Scientific Evidence-A Primer on Evaluating the Weight of Scientific Evidence, 23 WM.& MARY L. REV. 261, at 274 (1981).

[70] Yinon & Zitrin, supra note 12, at 163.

[71] Daubert, at 2795.

[72] Conference Proceedings, supra note 15.

[73] B.T. Fedoroff & O.E. Sheffield, Picatinny Arsenal, Encyclopedia of Explosives And Related Items (1960) lists thousands of types of explosives and explosives mixtures in ten different volumes.

[74] Robert Watkins & Joan Watkins, 22 American Jurisprudence, PROOF OF FACTS, Identification of Substances by Instrumental Analysis, 385(1969).

[75] Daubert, 113 S.Ct. 2786 2795 (1993).

[76] Anderson v. National R.R. Passenger Corp., 866 F.Supp.937 943 (1994) cites Stokes v. L. Geisman, S.A., 815 F.Supp. 904,909 (E.D.Va. 1993) where the expert himself admitted that his explaination of the cause was not the only reasonable one.

[77] WATKINS, supra note 74, at 401.

[78] The reader should note that instrumental confirmation of hypotheses is extremely important in areas of investigation where the human eye, even aided by microscope, can not detect analytes. Such a situation exists with explosives residues.

[79] State v. Murphy, 234 N.W.2d 54 58 (1975).

[80] Daubert, supra note 17, at 2795.

[81] Id. at 2795.

[82] Id. at 2796.

[83] This method of creating standards was followed, for instance, in the FBI Laboratory from 1986 to 1994 when commercial sources of standards became available

[84] Daubert at 2799.

[85] Anderson, supra note 76, at 943. 86 Imwinkelried, supra note 69, at 276-277. 47

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