Development of high-sensitivity atmospheric pressure (ap) matrix-assisted laser desorption/ionization (maldi) and open air ionization techniques for the analysis of biomolecules by mass spectrometry
Navare, Arti T.
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Matrix-assisted laser desorption/ionization (MALDI) has been celebrated as a soft ionization method for analyzing very diverse biological species including large proteins, peptides, carbohydrates, lipids and metabolites. The fact that MALDI is tolerant to salts and buffers and that it mostly produces singly charged ions from intact biomolecules is considered highly advantageous over electrospray ionization (ESI). Almost two decades after the introduction of vacuum MALDI, the technique was successfully implemented under atmospheric pressure (AP) conditions by Laiko and co-workers. Some of the most salient advantages of AP-MALDI over vacuum MALDI are its ability to generate intact ions from labile species with minimal fragmentation due to collisional cooling under AP, the ability of performing MSn experiments, and its exchangeability with other ion sources. However, AP-MALDI suffers from limited sensitivity due to low ion transmission efficiency under AP conditions. Because sensitivity is a function of the sample pretreatment method of choice, both preconcentration and selective sample fractionation can be used during the initial stages of the analytical pipeline to improve detectability. To that end, the first part of the work presented in this thesis is aimed at investigating various approaches to improve the sensitivity of AP-MALDI for mass spectrometric analysis of biomolecules. Chapter 1 reviews the history of laser desorption ionization (LDI), presenting salient features of vacuum MALDI and AP-MALDI, and concludes with a brief overview of leading ambient ionization techniques, such as Direct Analysis in Real Time (DART) ionization. Chapter 2 presents an investigation of an on chip sample preconcentration approach coupled to AP-MALDI for high-sensitivity analysis of neuropeptides extracted from Aedes aegypti mosquito heads. The theme of exploring efficient and reproducible purification methods for complex biosamples is continued in Chapter 3, where an evaluation of new on-tip solid-phase extraction (SPE) micro columns with various functional groups is presented. A second approach for enhancing AP-MALDI sensitivity by constructing a new pneumatically-assisted (PA) AP-MALDI ion source is presented in Chapter 4, where various factors affecting the performance of this device are investigated. Chapter 5 describes work involving the evaluation of DART ionization as a high-throughput method for the detection and identification of small terpene molecules central to the Aedes aegypti mosquito lifecycle.