Development of Plasma-Based Ambient Mass Spectrometry for Characterization of Polymers and Other Analytes with Limited Desorption/Ionization

Date

2023-08

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Abstract

Ambient mass spectrometry (AMS) allows direct desorption and ionization of analytes with minimal-to-no sample preparation. Compared with spray-based AMS techniques, plasma-based AMS sources offer some unique advantages, including no-solvent requirements, low-power consumption, easy-to-build, and the ability to ionize analytes with a wider range of polarities. Nevertheless, multiple challenges have slowed down the broader application of AMS towards routine analysis. Firstly, the plasma-based AMS may exhibit inadequate sensitivity for analytes with low desorption or ionization efficiencies. Secondly, it has limited capability for isomer differentiation since all analytes are desorbed, ionized, and enter the MS simultaneously without separation. Thirdly, the inherent desorption temperature of plasma-based sources is not high enough to desorb or pyrolyze polymers, which makes it challenging to analyze polymer materials. This dissertation includes several projects aiming to improve the analysis performance of plasma-based AMS for polymers and analytes with limited desorption/ionization. A novel reactive-FAPA ionization source was developed for on-line derivatization of analytes in real-time. In this reactive-FAPA source design, the vapor of derivatization reagent can be incorporated into the plasma effluent and react with the analytes directly. Imine formation and acylation reactions with high derivatization efficiency were shown to increase the sensitivity by up to ten times using methylamine as the derivatization reagent. Two functional isomers, 4-ene valproic acid (4-ene VPA) and γ-valprolactone, can also be differentiated by reactive-FAPA MS. Thin layer chromatography (TLC), which is a well-established separation technique, was coupled with LTP MS to study the analytical figure-of-merit of pharmaceuticals, functional isomers, and 20 different amino acids for both direct desorption and laser assisted desorption (LD) modes. A novel laser assisted micro-pyrolysis (LAMP) FAPA was developed and coupled with high-resolution mass spectrometry (HRMS) for polymer analysis. The diode laser was used to pyrolyze the polymer into monomer and oligomers to be analyzed by the FAPA MS. The pyrolysis temperature can be controlled by modulating the laser duty cycle, thus allows thermal separation of additives and polymer pyrolysis products for better additive identification. Mass spectrometry imaging (MSI) and depth profiling of synthetic polymers were also shown for the first time using plasma-based AMS. Biopolymer such as lignocellulosic biomass analysis via LAMP FAPA HRMS was also demonstrated. The pyrolysis products of lignin, cellulose, and hemicellulose can be identified. Quantitation of lignin, cellulose, hemicellulose and the sugar contents of glucan, xylan, galactan, arabinan, as well as the lignin monomeric unit guaiacyl/syringyl (G/S) with very good accuracy was also shown. In addition, microplastic analysis via LAMP FAPA MS was also demonstrated using CO2 laser to improve the pyrolysis efficiency of transparent materials. Single particle analysis was performed using polyethylene microspheres with different sizes. In addition, a data processing tool was developed to find and integrate pyrolysis peaks automatically. Furthermore, MSI of microplastic single particles was demonstrated.


Embargo status: Restricted until 09/2025. To request the author grant access, click on the PDF link to the left.

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Keywords

Microplastics mass spectrometry imaging directly on filter, Laser assisted sampling, Flowing atmospheric pressure afterglow, Ambient mass spectrometry, Low-temperature plasma probe, Principal components analysis, Complex polymers, Pressure sensitive adhesive tape differentiation, Polymer mass spectrometry imaging, On-line derivertization, Functional isomers differentiation, Thin-layer chromatography plates

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