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Department of Forensics and Investigative Sciences Tatiana Trejos Associate Professor
Eberly College of Arts and Science

Research

Gunshot Residue (GSR), Polymers/Physical Fits, and Glass/Paint Evidence Interpretation

Projects:

Gunshot Residue (GSR):

The GSR group focuses on developing novel methods of analysis and interpretation of GSR evidence using LIBS, LA-ICP-MS, LC/MS/MS, GC-MS, SEM-EDS, particle counters, laser scattering videography, and alternative light sources methods. Our research has evolved through the following phases: (a) development of emerging technologies; (b) development of databases for the interpretation of GSR evidence; (c) development of micro-particle GSR reference standards for experiment to visualize gunshot residue after firing. Pictures a firearm, on the left side of the image, held by a person and a green laser with a dust pattern on the right side of the image.fundamental studies; (d) testing of technology transfer for laboratory and crime scene use. 

With over three hundred thousand shooting incidents a year, gun violence affects the U.S. society like no other part of the world. Thus, research advances in this field directly impact how safety and justice are maintained in our communities. Assessing relationships between an individual of interest and the activities and circumstances surrounding the case is of utmost importance to these firearm-related cases. 

Scientist collecting a sample from a hand by pressing a metal stub with adhesive to the surface of the volunteer's hands.

The identification of gunshot residues can provide valuable leads to this matter. However, despite existing reliable and standardized methods  for GSR analysis, the field still faces several challenge s. For instance, the turnaround time required to produce a forensic report is lengthy (~2 months). Unlike most forensic disciplines, no screening methods are routinely used for rapid field and  laboratory GSR testing. On top of that, the increased occurrence of environmentally friendly ammunition requires the modernization of current methodologies, and more overarching interpretation approaches are needed t o contend with the complex mechanisms of transfer and persistence of GSR. 

Our research team has been focusing on addressing these needs by:

  1. Developing innovative, reliable, fast screening methods to detect organic and inorganic GSR.
  2. Comparing the performance and cost-efficiency of portable and bench-top LIBS and electrochemical (E.C.) systems (in collaboration with Arroyo's group and industry partners).
  3. Building the largest database of gunshot residues using multi-techniques (SEM-EDS, LC-MS/MS, E.C., and LIBS) on populations of legitimate shooters and background populations, including those individuals that may represent a high risk for occupational/environmental false positives.
  4. Developing statistical-based objective methods for interpreting GSR data for shooting distance estimation and evaluating traces from the hands of individuals of interest.
  5. Designing one-of-a-kind organic and inorganic GSR reference standards representative of modern ammunition that simulate the size, morphology, and composition of gunshot residue micro-particles.
  6. Developing breakthrough models using human skin, synthetic skin membranes, and laser scattering videography to reveal the complex deposition, transfer, and persistence mechanisms of microscopic inorganic particles and vaporous organic GSR compounds.

Our research has focused on developing SMARTER (Simpler, Modern, Affordable, Rapid, Transformative, Effective, and Reliable) solutions for GSR examinations. We have achieved that using universal collection methods compatible with current practice, providing for the first-time reliable screening tools (LIBS, EC) that can generate results in under 5 minutes per sample, detecting both IGSR and OGSR, while leaving the sample intact for further testing. 

LIBS spectra, highlighting the 3 characteristic elements of gunshot residue, lead barium and antimony. In the middle there is an image of a gunshot residue particle that is circular, black and 1 micrometer in diameter.

Our technology can analyze more significant numbers of GSR samples, lower the costs of GSR examinations, and assist with better decisions at the laboratory and the scene, allowing a more efficient process. LIBS and EC performance rates have been evaluated through large datasets (>3,600 authentic samples and >100,000 data files) with accuracy better than 95%, which is outstanding for a screening test (vs. ~70%).

In addition to its high accuracy, LIBS is a versatile technique with spectral and 3D chemical imaging capabilities that we  have utilized to quickly identify GSR residues on carbon stubs collected from the hands of a person or around a bullet orifice on clothing and non-movable objects such as doors, windows, and walls. The 3D chemical patterns can be used to predict the muzzle-to-target shooting distance to aid in reconstructing firearm-related events. Most importantly, unlike current colorimetric methods that change color in the presence of elements like lead, LIBS expands the monitored elements, does not suffer from interferences from blood or dark-color substrates, has superior selectivity and sensitivity, does not destroy the evidence, does not depend only on human judgment. This type of research is anticipated to transform current practice by providing more accurate and objective approaches. Also, such technology would make it easier for practitioners to find and process evidence while minimizing evidence contamination.

Moreover, we have developed and characterized unique organic and inorganic GSR reference standard materials that can be used in research and crime laboratories. The tailor-made microparticle is created in a controlled environment by firing a cartridge with the primer only (no bullet or propellant), collecting the IGSR microparticles in an organic solvent, and then adding a composite of OGSR compounds. Those suspensions can prepare up to 5000 individual standards, facilitating future interlaboratory testing. Furthermore, the standards can be used in suspension or dry form, allowing for various uses that otherwise would be unavailable to the field. We are expanding these standards' applicability by incorporating fluorescent markers and using synthetic skin models that can allow fundamental transfer and persistence studies and lead to a leap of knowledge in this field. The standards are fully characterized by a multi-technique approach (SEM-EDS, LIBS, ICP-MS, LA-ICP-MS, LC/MS, GC/MS, and electrochemical sensors EC) and through inter-laboratory studies.

Finally, we are applying ground-breaking statistical methods to interpret GSR evidence considering probabilistic approaches and artificial intelligence, using the data from our extensive population database and persistence studies. This research's overall framework and interconnected objectives address several needs in gunshot residues and are gaining interest with established collaborations at crime laboratories, federal institutions, and industry.

Polymers and Physical Fit Research:

The Polymer and Physical Fit groups focus on providing experimental scientific assessments of the value of fracture fits of trace materials (tapes, fabrics, documents, automotive plastics). Also, the team develops and validates (via interlaboratory studies) analytical methods for the characterization and interpretation of tapes and polymers by micro-XRF, SEM-EDS, FTIR, LA-ICP-MS and LIBS. We have created partnerships with the WVU's Department of Physics and Astronomy and practitioners from over 20 crime laboratories and research institutions.

Duct tape edges next to a ruler, being compared for a match by a hand with a blue glove.

In the area of physical fits, we received NIJ funding to develop quantifiable metrics to assess the quality of a physical fit. Fracture matches are considered the highest degree of association between two trace materials. Still, an objective statistical assessment of the weight of the evidence is not yet feasible in current practice. This research is generating, for the first time, a large dataset of fractured duct tapes, textiles, and plastics aimed to provide:

  1. Systematic methods of analysis and quantifiable methods for the evaluation of the quality of a fracture match,
  2. Decision criteria thresholds for human-based and computational-based approaches,
  3. Assessment of the accuracy and reliability of the fracture fit comparisons and conclusions,
  4. Formal assessment of intra and inter-examiner error rates that can serve as a basis for optimal content on proficiency testing and
  5. A model for an effective, transparent, and traceable peer-review process.

Our study is designed to address the research needs identified by the NIST-OSAC Trace subcommittee and six of the top ten operational requirements specified by the NIJ-TWG on pattern and trace evidence (e.g., scientific foundations, standardization, validation, interpretation, casework review, and proficiency assessment). Moreover, the strategic multi-disciplinary team of practitioners and researchers (statistics, physics, computer science, forensics) is critical for transformative and adoptable end-products in this subdiscipline.

We have developed a systematic and reproducible metric named the "edge similarity score, ESS," and various statistical tools used to estimate error rates and the probabilistic interpretation of the probative value of the evidence. To date, we have analyzed over 2500 duct tapes, 100 textiles, 600 automotive plastics, and 100 paper stamps to meet the objectives listed above. Interlaboratory studies conducted among 205 crime laboratories showed promising results for future implementation. Also, population datasets are used to develop user-friendly automated interfaces to estimate the significance of a given fracture match and substantiate expert conclusions. These computational tools will be available to forensic practitioners and the legal community. The methods developed in this research can further serve as models that can be generalized to other disciplines, expanding impact.

Two torn textile pieces that have been realigned at their edges. The fabric is primarily white with red, blue and yellow stripes horizontally.

Also, our group has evaluated the utility of using elemental profiles of electrical tape to associate samples originating from the same roll and differentiate samples originating from different sources. The performance of SEM-EDS, LA-ICP-MS, and micro-XRF has been compared on a dataset of over 150 tape sources. LA-ICP-MS and micro-XRF shown to be well suited for quick screening with accuracy and discrimination greater than 94%-97% and superior performance than the combined conventional methods used for tape examinations (SEM-EDS, FTIR, PyGC-MS). Further, applying more objective comparison methods using spectral contrast angle and probabilistic outputs enhanced performance in interpreting tape evidence. Likewise, supervised discriminant analysis is proposed as a tool for intelligence and investigative leads when used with existing tapes with known origin databases.

Glass and Paint Evidence Interpretation Research:

The Glass and Paint group studies transfer and persistence mechanisms of glass and paint evidence prior, during, and after breaking events and the occurrence of trace particulates in the population. NIJ has funded the research to survey glass and paint background occurrence, partnering with the University of Auckland, N.Z., and Sam Houston State University. Another ongoing research is characterizing modern nail polish formulations.

A microscopic image of a blue glittery paint on the right, a triangular-shaped fragment of glass on the top middle, and a few paint shavings in the bottom middle on a white background

Also, our group is currently funded through NIJ to collaborate with different glass examiners at the state, federal and private sectors to conduct interlaboratory studies. The collaborative studies evaluate the effect of technological advances on micro-XRF and LIBS instruments on assumptions about microheterogeneity and modern glass and tape discrimination.

Finally, another area of interest of our group is improving interpretation approaches to assess the significance of trace evidence and communication of the findings. As part of this effort, we have published a review manuscript, participated in the development of a standard guide for the interpretation and reporting of trace evidence (NIST-OSAC, to be submitted to ASTM), and conducted an interlaboratory study among 117 paint examiners to evaluate the effectiveness of the guide in helping to arrive at consensus criteria and reporting language. We collected 1247 responses from practitioners who evaluated 30 paint case scenarios. The findings demonstrate a high agreement among practitioners regarding the significance of results in comparative examinations when using the proposed guide. In these studies, we collaborate with practitioners, statisticians, and psychologists with expertise in human factors to minimize bias in the interpretative process. These studies are laying the foundations for transformative interpretation approaches to modernize the use of trace evidence in our field.

In collaboration with statisticians, we are using these datasets to narrow down the data needed for using comprehensive Bayesian frameworks that evaluate trace evidence at the source and activity levels.

List of Techniques Used:

  • Gas Chromatography-Mass Spectrometry (GC-MS, GC-MS-FID, GC-MS-ECD)
  • Liquid Chromatography-Tandem Mass Spectrometry (LC-MS/MS)
  • Laser-Induced Breakdown Spectroscopy (LIBS)
  • Inductively-Coupled Plasma-Mass Spectrometry (ICP-MS)
  • Laser-Induced Inductively-Coupled Mass Spectroscopy (LA-ICP-MS)
  • Stereomicroscopy & Polarizing-Light Microscopy (PLM)
  • Glass Refractive Index Measurement (GRIM3)
  • Scanning Electron Microscopy-Energy Dispersive X-Ray Spectrometry (SEM-EDS)
  • Fourier Transform Infrared Spectroscopy (FTIR)
  • Video Spectral Comparison (VSC)
  • Electrochemical detectors (ECD)
  • Stereomicroscopy & Polarizing-Light Microscopy (PLM)

sample vial being placed into an instrument named Liquid Chromatography-Tandem Mass Spectrometry
A professor helping a study study a sample under a microscope, both smiling.
A student pointing to a sample image on an instrument named Scanning electron microscopy energy dispersive spectrometry. The sample appears as a large white sphere on the monitor.