Making Use of What’s Not Visible in Trace Evidence Examinations

Engineered nanoparticles have become ubiquitous in modern society, in everything from sunscreen to beauty products to clothing. Scientists at Microtrace, a research company based in Elgin, Illinois, realized that except for gunshot residue analysis, nanoparticles and subvisible particles are routinely overlooked by forensic investigators. Their NIJ-funded research was intended to establish the “first systematic forensic approach to the characterization of subvisible and nanoscale evidence.”

The scientists, led by microscopist Christopher Palenik, focused on developing a framework that would allow forensic laboratories to use existing equipment to identify and interpret the significance of smaller particles in a practical manner. “While the smallest particles in this range will require higher resolution instrumentation,” the researchers said, “a wide range of particles can be characterized effectively by more effectively applying the suite of microanalytical methods present in most trace evidence laboratories today.”

The scientists first developed a list of nanoparticles and subvisible particles that could be used as forensic evidence, then established isolation methods so the particles could be found and identified by traditional evidence collection techniques, such as tape lifts and swabs. They then identified the particles using instruments and techniques typical in trace evidence laboratories, such as stereomicroscopes and polarized light microscopy.

The scientists noted that there “is virtually no published information on the population of subvisible particles present in real-world forensic samples, nor is there data on the prevalence, transfer, and persistence of nanoparticles within a given substrate or environment.” They worked to correct that by examining samples, environments, and substrates to better understand the “unintended contributions of subvisible particles.”

They looked at an array of such particles in several types of trace evidence, including automotive paint, and created a survey that documents the range of particles encountered. The scientists also looked at four specific types of particles: glass microspheres, printer toner particles, 3D printer dust, and titanium dioxide (white pigment prevalent in background environmental samples).

More than 50 examples of glass microspheres were characterized because they are common and “indicative of certain products or activities” and can be “interpreted in a forensic science context.” The researchers gathered 53 toner samples, some from random locations in their laboratory, to determine if the particles could be specifically identified by size, shape, and color.

The 3D printer dust samples were collected from 36 printers so they could be classified and methods could be developed to identify them. Finally, titanium dioxide, a naturally occurring mineral that is in widespread use in paints, paper, plastics, foodstuffs, and coatings, was examined so that protocols could be developed to identify the particles in dust and soil.

“Subvisible and nanoparticles comprise an entirely new class of forensic evidence, one which requires a new way of thinking about sample collection, analysis, and interpretation,” the researchers said. As they attempted to put together an electronic database of subvisible and nanoparticles, they realized that it would not allow for detailed explanations, particle comparisons, or the challenges and nuances that are distinct to certain particles.

Instead, the database “was reimagined as a collection of scientific research papers and presentations at forensic science conferences,” the scientists said. This research project resulted in 27 research papers and presentations at scientific meetings.

About This Article

The research described in this article was funded by NIJ award 2015-DN-BX-K033, awarded to Microtrace LLC of Elgin, Illinois. This article is based on the grantee report “Nanotrace: Applications of Subvisible to Nanoscale Particles in Trace Evidence,” by Christopher S. Palenik.

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