Author ORCID Identifier

https://orcid.org/0009-0007-0549-8468

Semester

Summer

Date of Graduation

2025

Document Type

Thesis

Degree Type

MS

College

Eberly College of Arts and Sciences

Department

Forensic and Investigative Science

Committee Chair

Tatiana Trejos

Committee Member

Glen Jackson

Committee Member

Claudia Martinez Lopez

Abstract

Nail polish can serve as valuable evidence in forensic investigations as it is often found on the hands and feet of individuals. Nail polish can break off and transfer easily during self-defense actions or forceful contact, such as in cases of kidnapping, sexual assault, or homicide. These residues can provide clues about who was at the scene, as well as where, when, and how the event evolved. However, compared to other types of paints commonly analyzed in forensic laboratories, limited forensic research has been conducted on the chemical composition and discrimination capabilities of popular nail products. The lack of information makes it challenging to interpret nail polish evidence because the forensic scientist cannot accurately evaluate how common or rare the evidence’s features are within the relevant population context.

This project expanded the knowledge base on nail polishes from various angles, enabling the forensic community to use nail polish evidence to its full potential. First, the study compiled a collection of over one hundred sources of nail polish chemistry, formulations, and industry regulations, through a comprehensive literature review on cosmetics textbooks, patents, webpages, and scientific articles, with a forensic perspective in mind. The findings of this repository have been organized into detailed documents and supporting figures to provide an overview of the complexity of nail polish chemistry for nitrocellulose-based (NC), UV-curable gels (gel), and dip powder (DP) products.

Following an investigation of supporting literature, the project produced a dataset of 164 nail polishes, comprising NC, Gel, and DP polishes from five brands and eleven colors, for examination. When possible, the set included matching brands and colors across all three types of polish. Characterization and classification of these main polish types were performed using physical and chemical properties, as determined by macroscopical and microscopical examination, micro-solubility testing, Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR), and Scanning Electron Microscopy with Energy Dispersive X-Ray Spectrometry (SEM-EDS). Visually, DP samples were distinguishable from NC and Gel samples by microscopical examination alone, due to the distinct presence of micro-polymeric beads. The NC and Gel polishes were distinguished from one another using micro-solubility tests, where NC is soluble in acetone and produces a deep blue color reaction in the presence of diphenylamine in sulfuric acid, while Gels do not react to these solvents. The microscopic examination and micro-solubility steps, in conjunction with macroscopical examination (color comparison), discriminated all comparison pairs except 33 of them, all of which were samples of the same nail polish type and color. Rapid confirmation of the nail polish type (NG, Gel, or DP) can be achieved using ATR-FTIR, as all three classes contain binders’ functional groups with distinctive IR peaks. Physical and chemical features were documented for each nail polish layer, which may include clear top coats, color coat(s), and, in some products, base coats. In addition to the multi-layered coat system, nail polish traces can also contain residues of the nail substrate, adding significance to the evidence. Therefore, this study also evaluates a subset of natural and artificial nails. The analysis by ATR-FTIR reveals that the characterization of the unknown substrate is possible, providing key insights into the potential preservation and use of biological and chemical evidence on the same item of interest.

Finally, the thesis reports the informative power of each technique, both alone and when combined. Across 13,366 pairwise comparisons, the complete analytical workflow achieved a 99.99% discrimination power, resulting in only one pair that was indistinguishable (IN). The results support the proposed workflow as an adequate sequence for the forensic comparison of nail polish products. Across all techniques, the most discriminating method was microscopical examination, providing a 99.7% discrimination (38 IN pairs), followed by elemental analysis using SEM-EDS (99.5% discrimination, 62 IN pairs). Through the examination of extenders and ratios of main binder constituents, ATR-FTIR alone resulted in a 97.9% discrimination power, with 285 indistinguishable pairs falling in within-group comparisons (i.e., Gel to Gel, DP to DP).

Overall, this project exposes the relevant physical features and chemical information of three contemporary nail polish products and provides recommendations for the examination, characterization, and comparison of these traces. The findings are anticipated to provide critical context and background for forensic examiners to expand the use of modern nail products in investigations and assess their significance to a courtroom audience.

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