Mass spectrometry imaging of latent fingerprints using titanium oxide development powder as an existing matrix

Applications of MALDI mass spectrometry in forensic science

Forensic chemistry literature has grown exponentially, with many analytical techniques being used to provide valuable information to help solve criminal cases. Among them, matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS), particularly MALDI MS imaging (MALDI MSI), has shown much potential in forensic applications. Due to its high specificity, MALDI MSI can analyze a wide variety of compounds in complex samples without extensive sample preparation, providing chemical profiles and spatial distributions of given analyte(s). This review introduces MALDI MS(I) to forensic scientists with a focus on its basic principles and the applications of MALDI MS(I) to the analysis of fingerprints, drugs of abuse, and their metabolites in hair, medicine samples, animal tissues, and inks in documents.

Matrix- and surface-assisted laser desorption/ionization-mass spectrometry analysis of fingermark components for forensic studies: current trends and future prospects

The chemical analysis of fingermarks (FMs) has attracted considerable attention in the realm of forensic investigations. Techniques based on direct ionization of a sample by laser irradiation, specifically matrix-assisted laser desorption ionization-mass spectrometry (MALDI-MS), have provided excellent figures of merit for analyzing high molecular-weight compounds. However, it can be challenging to analyze low molecular-weight compounds using MALDI-MS owing to potential interference produced by the organic matrices in the low molecular-weight region, which can impede the detection of small molecules (m/z < 700 Da). Alternately, surface-assisted laser desorption/ionization-mass spectrometry (SALDI-MS) has shown great promise for small molecules analysis owing to the unique properties of the nanostructures used, particularly, minimal chemical background in low m/z region improved the production of ions involved in this method. The advancement of MALDI-MS and SALDI-MS has propelled their application in the analysis of FM components, focused on gaining deep insights into individual traits. This review aims to outline the current role of MALDI-MS and SALDI-MS in the chemical analysis of FMs. It also describes the latest achievements in forensic intelligence derived from fingermark analysis using these powerful methods. The accomplishments include the understanding of certain characteristics and lifestyles of donors. The review offers a comprehensive overview of the challenges and demands in this field. It suggests potential enhancements in this rapidly expanding domain to bridge the gap between research and practical police casework.

Desorption electrospray ionization mass spectrometry imaging of latent fingerprints revealed by Oil Red O

Mass spectrometry imaging (MSI) is increasingly used to produce chemical images of latent fingerprints. Yet, the actual benefits of MSI for real case studies have to be assessed for fingerprints previously processed by forensic techniques. Here, we have evaluated the compatibility of desorption electrospray ionization (DESI) with the fingerprint enhancement technique involving Oil Red O (ORO).

METHODS

To optimize the ionization step independently from surface extraction, the ORO reagent and its mixture with model compounds (triolein and linoleic acid) were first studied in solution using high-resolution electrospray ionization tandem mass spectrometry (ESI-MS/MS). Then, DESI-MSI experiments were performed in both polarity modes for ORO-processed fingermarks deposited on pieces of paper used as porous substrates.

RESULTS

ESI-MS of ORO reveals a complex mixture of azo dyes. Two main impurities detected beside the targeted species were characterized using MS/MS and then were usefully employed to produce DESI-MS images of fingermarks, decreasing the scanning rate to get sufficient ion abundance from natural fingerprints. ORO did not prevent chemical profiling, and one major added value of this pink dye was to produce MS images with contrast that cannot be obtained optically for some colored substrates.

CONCLUSIONS

DESI-MS has demonstrated imaging compatibility with the application of ORO used to enhance latent fingerprints on paper and could also enable chemical profiling in natural fingermarks. In addition, MS images of ORO impurities were of higher quality than optical ones for fingerprints revealed on colored paper.

Imaging of Neurotransmitters and Small Molecules in Brain Tissues Using Laser Desorption/Ionization Mass Spectrometry Assisted with Zinc Oxide Nanoparticles

Inorganic nanostructured materials such as silicon, carbon, metals, and metal oxides have been explored as matrices of low-background signals to assist the laser desorption/ionization (LDI) mass spectrometric (MS) analysis of small molecules, but their applications for imaging of small molecules in biological tissues remain limited in the literature. Titanium dioxide is one of the known nanoparticles (NP) that can effectively assist LDI MS imaging of low molecular weight molecules (LMWM). TiO₂ NP is commercially available as dispersions, which can be applied using a chemical solution sprayer. However, aggregation of NP can occur in the dispersions, and the aggregated NP can slowly clog the sprayer nozzle. In this work, the use of zinc oxide (ZnO) NP for LDI MS imaging is investigated as a superior alternative due to its dissolution in acidic pH. ZnO NP was found to deliver similar or better results in the imaging of LMWM in comparison to TiO₂ NP. The regular acid washes were effective in minimizing clogging and maintaining high reproducibility. High-quality images of mouse sagittal and rat coronal tissue sections were obtained. Ions were detected predominately as Na⁺ or K⁺ adducts in the positive ion mode. The number of LMWM detected with ZnO NP was similar to that obtained with TiO₂ NP, and only a small degree of specificity was observed.