Mobilized Electrospray Device for On-Tissue Chemical Derivatization in MALDI-MS Imaging.

Applying solutions of a matrix or derivatization agent via microdroplets is a common sample preparation technique for matrix-assisted laser desorption/ionization-mass spectrometry imaging (MALDI-MSI) experiments. Mobilized nebulizer sprayers are commonly used to create a homogeneous matrix or reagent layer across large surfaces. Electrospray devices have also been used to produce microdroplets for the same purpose but are rarely used for large tissues due to their immobility. Herein, we present a movable electrospray device that can be used for large tissue sample preparation through a simple modification to an automatic commercial nebulizer device. As demonstrated for on-tissue chemical derivatization (OTCD) with Girard's reagent T using a mimetic tissue model, the sprayer has the additional benefit of being able to investigate reaction acceleration in OTCD when comparing electrostatically charged spray to electrostatically neutral spray. Finally, MALDI-MSI of fatty aldehydes is successfully demonstrated in rat brain tissues using this device for both OTCD and matrix application.

Predicting Fingerprint Age Based on Ozonolysis Kinetics of Unsaturated Triacylglycerol Degradation.

Answering the question, "How old is a fingerprint?", is a highly sought-after aim in forensic science. Despite several decades of studies to find an empirical correlation in fingerprint aging, there has been no reliable method so far. In this study, we attempt to determine the time since deposition (TSD) of aged fingerprints from the chemical profile captured within a matrix-assisted laser desorption/ionization mass spectrometry data set. Our approach is based on the chemical kinetics associated with the ambient ozonolysis of unsaturated triacylglycerols (TGs), a major component in fingerprint lipids. First, ozone concentration and ambient temperature were determined to be the major factors in the degradation of unsaturated TGs. A simple kinetics model is then developed to describe the decay of unsaturated TGs, dictated only by the temperature and ozone concentration. This model is then applied to the degradation of TGs in a mixture of TG standards and multiple individuals' fingerprints. The overall decay of unsaturated TGs follows the pseudo-first-order reaction kinetics, validating our hypothesis; however, there are significant person-to-person variations in the initial abundance of unsaturated TGs and the decay rate, hampering the accurate prediction of TSD unless they are corrected for each individual. Nevertheless, the model's applicability for ambient fingerprint aging data was successfully demonstrated.

On-tissue chemical derivatization of volatile metabolites for matrix-assisted laser desorption/ionization mass spectrometry imaging.

Mass spectrometry imaging (MSI) of volatile metabolites is challenging, especially in matrix-assisted laser desorption/ionization (MALDI). Most MALDI ion sources operate in vacuum, which leads to the vaporization of volatile metabolites during analysis. In addition, tissue samples are often dried during sample preparation, leading to the loss of volatile metabolites even for other MSI techniques. On-tissue chemical derivatization can dramatically reduce the volatility of analytes. Herein, a derivatization method is proposed utilizing N,N,N-trimethyl-2-(piperazin-1-yl)ethan-1-aminium iodide to chemically modify short-chain fatty acids in chicken cecum, ileum, and jejunum tissue sections before sample preparation for MSI visualization.

Use of Nanoparticle Decorated Surface-Enhanced Raman Scattering Active Sol-Gel Substrates for SALDI-MS Analysis.

Spectroscopy and mass spectrometry techniques are sometimes combined into the same analytical workflow to leverage each technique's analytical benefits. This combined workflow is especially useful in forensic and medical contexts where samples are often precious in nature. Here, we adopt metal nanoparticle (NP) doped sol-gel substrates, initially developed for surface-enhanced Raman scattering (SERS) analysis, as surface-assisted laser desorption/ionization-mass spectrometry (SALDI-MS) substrates. Using dried blood and sample protocols previously developed for SERS analysis, we observe heme-related spectral features on both silver and gold NP substrates by SALDI-MS, demonstrating dual functionality for these orthogonal techniques. Modifying the dried blood extraction procedures also allows for the observation of blood triacylglycerols by SALDI-MS. This is the first demonstration of a SERS/SALDI-MS substrate based on a sol-gel scaffold and the first demonstration of a gold NP sol-gel substrate for SALDI-MS which features lower substrate-related SALDI-MS background compared to the silver substrate.

Novel Ambient Oxidation Trends in Fingerprint Aging Discovered by Kendrick Mass Defect Analysis.

A Kendrick mass defect (KMD) plot is an efficient way to disperse complex high-resolution mass spectral data in a visually informative two-dimensional format which allows for the rapid assignment of compound classes that differ by heteroatom content and/or unsaturation. Fingerprint lipid oxidation has the potential to be used to estimate the time since deposition of a fingerprint, but the mass spectra become extremely complex as the lipids degrade. We apply KMD plot analysis for the first time to sebaceous fingerprints aged for 0-7 days to characterize lipid degradation processes analyzed by MALDI-MS. In addition to the ambient ozonolysis of fingerprint lipids previously reported, we observed unique spectral features associated with epoxides and medium chain fatty acid degradation products that are correlated with fingerprint age. We propose an ambient epoxidation mechanism via a peroxyl radical intermediate and the prevalence of omega-10 fatty acyl chains in fingerprint lipids to explain the features observed by the KMD plot analysis. Our hypotheses are supported by an aging experiment performed in a sparse ozone condition and on-surface Paternò-Büchi reaction. A comprehensive understanding of fingerprint degradation processes, afforded by the KMD plots, provides crucial insights for considering which ions to monitor and which to avoid, when creating a robust model for time since deposition of fingerprints.

Three-Dimensional Profiling of OLED by Laser Desorption Ionization-Mass Spectrometry Imaging.

Organic light emitting devices (OLEDs), especially in a screen display format, present unique and interesting substrates for laser desorption/ionization-mass spectrometry imaging (LDI-MSI) analysis. These devices contain many compounds that inherently absorb light energy and do not require an additional matrix to induce desorption and ionization. OLED screens have lateral features with dimensions that are tens of microns in magnitude and depth features that are tens to hundreds of nanometers thick. Monitoring the chemical composition of these features is essential, as contamination and degradation can impact device lifetime. This work demonstrates the capability of LDI-MSI to obtain lateral and partial depth resolved information on multicolored OLED displays and suggests the application to other mixed organic electronics with minimal sample preparation. This was realized when analyzing two different manufactured OLEDs, in an active-matrix display format, without the need to remove the cathode. By utilizing low laser energy and high lateral spatial resolution imaging (10 µm), depth profiling can be observed while maintaining laterally resolved information, resulting in a three-dimensional MSI approach that would complement existing OLED characterization methods.