Introduction to Trace Evidence

Contributed by Patrick Buzzini

Department of Forensic Science, Sam Houston State University

Trace evidence is that subdiscipline of criminalistics that is concerned with the recognition, detection, collection, characterization, comparison, and the interpretation of a large array of clue materials. These materials consist of different types of mass-produced or naturally occurring substances that transfer, often, but not necessarily, in small amounts and sizes during a given activity. Typical examples of trace evidence are textile fibers, ignitable liquid residue potentially used as accelerants in arson cases, gunshot residue, surface coating (or paint), glass, cosmetics, human and animal hair, soil and minerals, tapes, lamp filaments, explosives, wood chips, botanical substances such as pollen, and much more.

As a corollary it can be stated that “anything can be trace evidence.” Indeed, a well-known pragmatic definition describes trace evidence as any type of evidence that does not fall into a particular unit or department of a forensic laboratory. Therefore, trace evidence examiners are the most versatile scientists within a forensic laboratory setting. As mentioned above, they are confronted with different types of materials. This clearly evokes the notion of variety. This notion does not only limit itself to the type of evidence, but also to the variety of sizes of specimens that can be recovered, the variety of their forms, and, as a consequence, to the variety of methods and procedures that are utilized for their collection and examination.

For example, glass fragments can be recovered in large numbers and sizes at accident scenes, or in millimetric sizes on the garments of an individual suspected of having smashed a window; glass particles can also occur in powder form on a bullet having impacted through a window. Paint can be recovered in the form of multilayered fragments, in the form of abrasions, or in the form of droplets if produced by a spray can. Textiles can be recovered as individual fibers, but also in the form of yarns (e.g. cordage), or in the form of torn pieces of cloths. Our environment is extremely rich in the macroscopic and microscopic forms of a large variety of dust particles. Morphology is one of the main features that a trace evidence examiner exploits particularly using microscopical methods, whether it be light microscopy or electron microscopy. Gunshot residue, for example, exhibits distinctive spherical shapes, and fibers display different morphological properties depending on their natural or man-made origins. Ignitable liquid residues are volatile compounds. The detection of drink adulterants involves the study of liquids. Physical matches are carried out on fragmented solid debris.

Trace evidence, as vestiges left behind or taken with during activities between individuals or objects follow the same process as any other types of physical evidence. While the focus of forensic laboratories seems to be confined to testing operations and the delivery of reliable outcomes, the utility of trace evidence to a particular case depends on the entire process that trace evidence undergoes, and on the uncertainties that arise along that process. This process starts with the generation of traces in their various forms, sizes, and quantities following a particular activity within a particular context. Depending on their quality and quantity, these remnants produced involuntarily under no controlled conditions and during a unique event constitute what De Forest refers to as an imperfect record. These traces need to be recognized and their potential relatedness to the case at hand has to be assessed. This requires a judgment of their relevancy to the case. In many instances, trace evidence is not immediately recognized at the scene, especially when it occurs in microscopic forms. This requires a microscopical approachfrom the part of individuals investigating in the field. Questions such as “if a suspected activity occurred in a given location and in a given mode, which traces are expected to be transferred and having persisted?” become of the upmost importance to detect trace evidence.

Various methods of collection, that ensure proper preservation, are utilized contingent upon, again, the variety of forms and sizes of the recovered clue materials. Methods such as taping, scratching, vacuuming, nail clippers, combing, manual picking are applied. Various containers like paper bags, petri dishes, or metallic paint cans are used.

Depending on the case, trace evidence may be collected along with other types of physical evidence. Prior to conducting laboratory examinations, it is then advisable to conduct a case assessment to evaluate what the contribution of trace evidence would be in the context of the case at hand. This is a helpful endeavor to prioritize pertinent examinations and to pre-evaluate the impact of potential analytical outcomes.

From a laboratory standpoint, the characterization of the various materials is carried out my means of microscopical examinations and chemical analyses, often instrumental. The proper use of light microscopy is a building block for trace evidence examinations. Using instruments such as the stereomicroscope, the compound microscope or the comparison microscope, the trace evidence examiner shall master different microscopical applications to characterize unknown specimens and to compare them to reference samples or other unknown specimens. Typical examples of these applications are polarized light microscopy (i.e. double polarization techniques or pleochroïsm), fluorescence microscopy, phase contrast and thermal microscopy, or cross sectioning. Traditional methods of chemical analysis include Fourier transform infrared (FTIR) spectroscopy, gas chromatography coupled to mass spectrometry (GC-MS), and elemental analysis by scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDS) or X-ray fluorescence (XRF). Other methods like visible or UV-visible microspectrophotometry (MSP) or the glass refractive index measurement system are also used. It is important to state that more methods are available to the trace evidence examiner.

Trace evidence examinations are conducted to answer different types of questions. The interpretation of the forensic findings depends on these questions as well as on the context of the case. However, the starting point is the characterization of the recovered specimens or identification of unknowns. For example, an examiner may state that particles of lead, barium and antimony were identified on a given surface, which is a characteristic of gunshot residue. Questions pertaining to false positives may arise at this level.

A traditional question of interest is about the source of the recovered clue materials. If a suspected source is not available it is possible to develop investigative leads by means of searching reference collections, computerized databases, or conducting manufacturer inquiries. An example among these possibilities would be the use of the PDQ (Paint Data Query) database to develop a list of potential vehicles from paint chips recovered at a scene of a hit-and-run accident. Another formidable use of trace evidence would be the comparison between clue materials of a given type recovered at a given scene, and clue materials of the same type recovered at other scenes. A comparative approach such as this has the potential to help investigators conducting case linkages. For example, fibers collected from dead bodies recovered at different places and times have been used, in concert with other investigative information, to attribute the homicides to the same individual. If a putative source is available a comparative examination is typically conducted between recovered specimens and reference samples: the goal would be to explore the possibility to reach a decision of a common source between the compared sets (individualization). Given the impossibility to reach a decision of source attribution in the most part of cases, an approach would be to evaluate the evidential value based on the evaluation of the rarity of the clue materials in a relevant population or environment. The more peculiar the observed attributes are, the higher their evidential value.

Other questions such as the occurrence of a particular contact between two surfaces or objects during a well-defined alleged activity may be advanced. In that case, the presence of the evidence must be explained, rather than their source only. The trace evidence examiner shall then typically evaluate if the location and the quantity of the recovered clue materials are in agreement with the supposed alleged activity. Important notions such as transfer, persistence, and evidence recovery efficiency need to be considered. For example, what would be the expectations to recover a single hair (microscopically indistinguishable from the victim’s hair) in a car trunk for reasons other than transporting a dead body, and given the particular case circumstances? Finally, trace evidence examinations can play a key role in the reconstructions of incidents (i.e. crimes, suicides or accidents). They can help the criminalist to understand the relative positions of individuals and objects at the scene, to establish a sequence of events, and to corroborate or infirm claims from victims, suspects, and witnesses.

Additional References:

  1. Locard E. Dust and Its Analysis – An Aid to Criminal Investigation. Police Journal 1928; 177-192.
  2. Palenik S, Palenik C. Microscopy and Microchemistry of Physical Evidence (Ch. 5). In: Saferstein R (editor). Forensic Science Handbook – Volume II (2nd edition). Pearson Prentice Hall, Upper Saddle River, NJ (2005): 175-230.
  3. De Forest P. R. What is Trace Evidence? (Ch. 1). In: Caddy B (ed.). Forensic Examination of Glass and Paint – Analysis and Interpretation. Taylor & Francis, London, Philadelphia (2001).
  4. Cook R, Evett IW, Jackson G, Jones PJ, Lambert JA. A Hierarchy of Propositions: Deciding Which Level to Address in Casework. Science & Justice 1998: 38(4): 231-239.