Maximizing chemical information obtained during surface analysis with laser desorption/ionization mass spectrometry

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Paulson, Andrew Elliot
Major Professor
Lee, Young Jin
Anand, Robbyn
Gundlach-Graham, Alexander
Rossini, Aaron
Smith, Emily
Committee Member
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This dissertation presents four projects that aim to maximize the chemical information obtained during surface analysis by laser desorption/ionization mass spectrometry (LDI-MS) techniques. The first chapter serves as a brief introduction to mass spectrometry techniques utilized in this work to establish a foundation for further discussion. The content is not exhaustive and is focused on information relevant to the use of soft ionization techniques paired with high-resolution mass spectrometers, specifically for the application of surface analysis and mass spectrometry imaging. This chapter introduces the important components of a mass spectrometer; ionization sources, mass analyzers, and detectors. Specific details are provided about electrospray ionization, laser desorption/ionization, and Orbitrap mass analyzers, as these are routinely used in the following chapters. Mass spectrometry imaging (MSI) is then described, followed by an explanation of why Orbitrap technology is well suited for MSI. The four middle chapters, brief descriptions below, feature work published in manuscripts within peer-reviewed journals (chapters two, three, and four) or work in preparation to be submitted for publication (chapter five). Chapter six gives a general conclusion and future outlook for further development in regard to each project presented. The second chapter describes the use of LDI-MSI for probing the layered and pixilated architecture of screen display formats based on organic light-emitting diodes (OLED). This method was developed to complement existing OLED characterization techniques used for quality assurance and research and development of these devices. Importantly, the LDI-MSI method allows for assessing chemicals with both lateral and depth relevance, and the method was demonstrated on two OLED displays. Ultimately, with appropriate tuning of the laser fluence, number of laser shots, and the number of sequential mass spectrometry imaging passes, lateral and depth-relevant information can be gleaned from the OLED's architecture. Given the added spatial information, this technique is anticipated to streamline investigations of degradation processes, leading to informed interventions concerning the design of the devices. The third chapter introduces a substrate with surface characteristics that allow for surface-enhanced Raman scattering (SERS) and surface-assisted LDI-MS (SALDI-MS) analysis. This work is discussed in the context of the forensic analysis of blood, as this is an application where two forms of confirmatory analysis using a minimal amount of sample is desirable. Despite the shared material reliance on nanostructured surfaces, few dual-functional substrates for SERS and SALDI-MS approaches have been developed. Thus, we adopt previously developed SERS substrates, along with sample preparation protocols, and demonstrate SALDI-MS analysis from the same sample substrate. Also, preliminary investigations into modified extraction procedures for dried blood and the assessment of SALDI-MS analyte coverage outside of blood-related compounds are discussed. The fourth chapter describes the use of matrix-assisted LDI-MS (MALDI-MS) and Kendrick mass defect (KMD) plots for the analysis of lipid species and their oxidation products in sebaceous fingerprints that have been deposited and aged on a surface. Understanding these lipid oxidation trends will assist in developing time since deposition models of fingerprint evidence. Our previous work largely focused on using triacylglycerol ozonolysis to model the time since deposition of a fingerprint. However, these models have had limited success. Using KMD plots, it was observed that our MALDI-MS methods allow us to monitor other sebaceous fingerprint compounds, including squalene, diacylglycerols, wax esters, fatty acids, and associated oxidation products. The improved understanding of the observed molecular complexity will help guide our selection of compounds used in the development of more robust fingerprint-aging models. Specifically, including the other observed sebaceous compounds in a model may allow us to account for person-to-person variations in the rates of fingerprint aging that are observed when monitoring only triacylglycerols. Also, decanoic acid was identified as a promising feature that may help improve our accuracy in late-stage fingerprint age predictions. In chapter five, a mobilized electrospray device for sample preparation is developed. Specifically, we investigate the use of the device for the derivatization of analytes to improve ionization efficiency during surface analysis by MALDI-MSI. Reaction acceleration of a commonly used derivatization reaction is observed when comparing the electrospray to an electrostatically neutral spray. The acceleration is hypothesized to be due to characteristics specific to charged microdroplets. However, for the reaction investigated, there are limited practical implications for improved sample derivatization when typical reagent concentrations are used. Regardless, the mobilized electrospray device allows for more reproducible sample preparations compared to a stationary electrospray device and opens up the opportunity for future investigations of electrospray in MALDI-MSI sample preparation workflows.
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