Visualisation of Amphetamine Contamination in Fingerprints Using TOF-SIMS Technique

Markedly Enhanced Analysis of Mass Spectrometry Images Using Weakly Supervised Machine Learning

Supervised and unsupervised machine learning algorithms are routinely applied to time-of-flight secondary ion mass spectrometry (ToF-SIMS) imaging data and, more broadly, to mass spectrometry imaging (MSI). These algorithms have accelerated large-scale, single-pixel analysis, classification, and regression. However, there is relatively little research on methods suited for so-called weakly supervised problems, where ground-truth class labels exist at the image level, but not at the individual pixel level. Unsupervised learning methods are usually applied to these problems. However, these methods cannot make use of available labels. Here a novel method specifically designed for weakly supervised MSI data is presented. A dual-stream multiple instance learning (MIL) approach is adapted from computational pathology that reveals the spatial-spectral characteristics distinguishing different classes of MSI images. The method uses an information entropy-regularized attention mechanism to identify characteristic class pixels that are then used to extract characteristic mass spectra. This work provides a proof-of-concept exemplification using printed ink samples imaged by ToF-SIMS. A second application-oriented study is also presented, focusing on the analysis of a mixed powder sample type. Results demonstrate the potential of the MIL method for broader application in MSI, with implications for understanding subtle spatial-spectral characteristics in various applications and contexts.

Molecular detection of per- and polyfluoroalkyl substances in water using time-of-flight secondary ion mass spectrometry

Detection of per- and polyfluoroalkyl substances (PFASs) is crucial in environmental mitigation and remediation of these persistent pollutants. We demonstrate that time-of-flight secondary ion mass spectrometry (ToF-SIMS) is a viable technique to analyze and identify these substances at parts per trillion (ppt) level in real field samples without complicated sample preparation due to its superior surface sensitivity. Several representative PFAS compounds, such as perfluorooctanesulfonic acid (PFOS), perfluorobutanoic acid (PFBA), perfluoropentanoic acid (PFPeA), perfluoheptanoic acid (PFHpA), and perfluorononanoic acid (PFNA), and real-world groundwater samples collected from monitoring wells installed around at a municipal wastewater treatment plant located in Southern California were analyzed in this work. ToF-SIMS spectral comparison depicts sensitive identification of pseudo-molecular ions, characteristic of reference PFASs. Additionally, principal component analysis (PCA) shows clear discrimination among real samples and reference compounds. Our results show that characteristic molecular ion and fragments peaks can be used to identify PFASs. Furthermore, SIMS two-dimensional (2D) images directly exhibit the distribution of perfluorocarboxylic acid (PFCA) and PFOS in simulated mixtures and real wastewater samples. Such findings indicate that ToF-SIMS is useable to determine PFAS compounds in complex environmental water samples. In conclusion, ToF-SIMS provides simple sample preparation and high sensitivity in mass spectral imaging, offering an alternative solution for environmental forensic analysis of PFASs in wastewater in the future.

Advancements in ToF-SIMS imaging for life sciences

In the last 2 decades, Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) has gained significant prominence as a powerful imaging technique in the field of life sciences. This comprehensive review provides an in-depth overview of recent advancements in ToF-SIMS instrument technology and its applications in metabolomics, lipidomics, and single-cell analysis. We highlight the use of ToF-SIMS imaging for studying lipid distribution, composition, and interactions in cells and tissues, and discuss its application in metabolomics, including the analysis of metabolic pathways. Furthermore, we review recent progress in single-cell analysis using ToF-SIMS, focusing on sample preparation techniques, in situ investigation for subcellular distribution of drugs, and interactions between drug molecules and biological targets. The high spatial resolution and potential for multimodal analysis of ToF-SIMS make it a promising tool for unraveling the complex molecular landscape of biological systems. We also discuss future prospects and potential advancements of ToF-SIMS in the research of life sciences, with the expectation of a significant impact in the field.

Fingerprints: A New Specimen for Innovative Applications for the Detection of Xenobiotics

Fingerprints are invisible traces that result from a deposition of sweat and sebum present on the papillary ridges. As sweat and sebum contain drugs, fingerprints are promising since collection is rapid, non-invasive and difficult to falsify. Very limited data are available in the literature, and therefore, it seems opportune to study the transfer of xenobiotics onto the items taken in hand via the fingerprints. Two studies were implemented using the ballpoint pen as a model. The objective of the first study was to compare the nicotine concentrations found on the pens of three smokers and three non-smokers. Five pens, belonging to each subject and used regularly, were rubbed with a cotton swab dipped in methanol and analyzed by liquid chromatography coupled with tandem mass spectrometry (LC-MS-MS). The second study was to analyze the transfer via fingerprints of four volunteers, after administration of 30 mg of codeine. The objective was to determine the feasibility of this study and the time corresponding to the highest concentration of codeine. Over a 24-h period, new pens were handled for 5 min by the four volunteers, rubbed with a cotton swab dipped in methanol, and then analyzed by LC-MS-MS. The nicotine study showed a major difference between the nicotine concentrations obtained from smokers (between 6 and 276 ng/pen) and non-smokers (between 2 and 4 ng/pen). After administration of 30 mg of codeine, the analysis of the pens of the four volunteers allowed to demonstrate the presence of codeine up to 24 h between 9 and 544 pg/pen. Normal hygiene practices did not influence the final result. The highest concentration was observed after 2 h. Morphine was also detected (between 19 and 33 pg/pen). These preliminary results should be considered a demonstration of the interest of fingerprints testing to document drug exposure.