Forensic analysis of cosmetic smudges using surface-assisted laser desorption/ionization mass spectrometry: recovery and ageing study

Abstract

Owing to its tendency to persist over prolonged periods, cosmetic evidence recovered from crime scenes can assist in linking a suspect to a certain crime or victim, or exonerating a suspect. This work presents a preliminary study in which surface-assisted laser desorption/ionization-mass spectrometry (SALDI-MS) was employed to analyze five cosmetic products recovered from different surfaces and collected at different time points. Topical over-the-counter (OTC) preparations, including Bepanthen cream, Reparil gel, Eucerin cream, Soskin serum, and Clofen creamagel containing dexpanthenol, diethylamine salicylate, urea, salicylic acid (SA), and diclofenac sodium (DCS) as active pharmaceutical ingredients (APIs), respectively, were used in this study. Due to their intricate nature and texture, five cosmetic product smudges were freshly deposited in small amounts on the target plate and directly subjected to SALDI-MS analysis without performing an extraction. The five APIs were successfully detected, along with other ingredients present in the intricate cosmetic matrices; the presence of waxes and polymers in the preparations did not hinder ionization of the target analytes. Since only trace amounts of cosmetics are often encountered at crime scenes; therefore, limits of detection (LOD) were determined for each API using optimum solvents. The LOD were determined as 300, 30, 6, 1.20, and 600 ng/mL for dexpanthenol, salicylate, urea, SA, and DCS, respectively. Furthermore, the relative standard deviations of the amounts of each API detected were calculated and estimated to be between 0.40 and 17.95%, indicating good reproducibility of the results. Subsequently, models of forensic samples were prepared wherein cosmetic smudges were deposited on three different surfaces (plastic, glass, and stainless steel) and recovered via extraction and swabbing. The model indicated that direct swabbing of the sample onto the target plate was the best recovery approach which, owing to its simplicity, can be implemented at a crime scene. Finally, an ageing study was performed, which showed successful detection of all the APIs on their optimum surfaces, even after 38 days post-deposition. The imposed lockdown during the global COVID-19 pandemic has increased the necessity for more advanced recovery and detection techniques that can coincide with massive delays and sample backlogs in forensic laboratories. Therefore, SALDI-MS provides better discrimination power, faster analysis duration, and reduces the matrix effect when employed for small sample volumes, such as cosmetic smudges.