Development and performance improvement of novel portable mass spectrometer

Tsung-Chi Chen, Purdue University

Abstract

With significant improvement during the last few decades, mass spectrometry (MS) features high sensitivity and specificity as well as fast speed in analysis of chemical and biological compounds.[1] Great efforts have also been put on the development of miniature MS for increasing needs to in-situ chemical analysis.[2-4] Equipped with many powerful techniques, such as the tandem mass spectrometry (MSn) [5, 6] and discontinuous atmospheric pressure interface (DAPI) [7-9], the MS instrument can now achieved high performance in the man-portable scale. In the more recent, it is desirable to perform complicated mass analysis in harsh environment, which relies on advanced analytical techniques and puts more challenges on miniature MS instrumentation researches. The overall goal of my researches is to improve the analytic performance of miniature mass spectrometry systems [2, 3, 10, 11] for analysis of chemicals of great interests, such as explosives detection in battle field or the searching of sign of life in planetary explorations. [12, 13] . To achieve the goal, three problems have been identified and the corresponding solutions were explored. The specific objectives were to develop novel ionization techniques, to investigate flexible ion transmission paths for API and to improve the performance of the MS/MS processes. The brief of thesis objectives are descripted as follows. Thesis objective 1: The first part of this study (Ch2) discusses a novel ionization method which increases the mass analysis sensitivity and lowers the power consumption for a portable MS. The sensitivity of the mass analysis highly depends on the analyte introduction and ionization. For miniature mass spectrometers (MSs) with compromised pumping systems, it is challenging to introduce the analytes or ions at comparable amount levels to those for lab-scale instruments. In this study, a discharge ionization source inherently synchronized with discontinuous atmospheric pressure interface (DAPI)[7-10] operation was developed to analyze volatile organic compounds (VOCs) in ambient air using a miniature mass spectrometer. Intact molecular ions of analytes were generated and subsequently trapped in the rectilinear ion trap (RIT) near the source. Analysis of a variety of VOCs in ambient air has also been performed, and the limit of detection (LOD) in part-per-trillion levels of naphthalene with good linearity in quantitation was achieved. In comparison with other conventional ionization methods such as an electron impact (EI) or chemical ionization (CI), SDI has advantages of softness in ionization, ease for implementation, low power for operation, and no need for vulnerable components like a filament. Thesis objective 2: Ion introduction using bent capillaries was explored in the second part of this study (Ch3). Discontinuous atmospheric pressure interfaces (DAPI) with bent capillaries represents a highly simplified and flexible means for introducing ions into vacuum manifold for mass analysis or gas phase ion reactions. In this work, a series of capillaries of different radians and curvatures were used with DAPI for studying the impact of the capillary bending on the ion transfer. The variation of transfer efficiency was systematically characterized for dry and solvated ions. The efficiency loss for dry ions was less than one order of magnitude, even with a three-turn bent capillary. The transfer of solvated ions generated by electrospray was found to be minimally impacted by the bending of the transfer capillary. For multiply protonated ions, the transfer efficiency for ions at lower charge states could be relatively well retained, presumably due to the lower reactivity associated with proton transfer reaction and the compensation in intensity by conversion of ions at higher charge states. Thesis objective 3: For the mass analyzer, the ion trap with the additional buffer gas dramatically improves the resolution and sensitivity of the mass spectra.[14, 15] Accordingly, this part of this study focuses on a method of introducing helium as a second buffer gas other than air for a miniature mass spectrometer using a DAPI configuration. The effects of the buffer gas on the performance of a linear ion trap (LIT) with hyperbolic electrodes were characterized for ion isolation, fragmentation and a mass selective instability scan. Significant improvement in spectral resolution and ion isolation was obtained with helium, while moderate advantage was gained with air for collision induced dissociation. The buffer gas can also be switched between air and helium for different steps within a tandem MS analysis process prior to instability scan, which allows further optimization of the instrument performance for tandem mass spectrometry.

Degree

Ph.D.

Advisors

Chiu, Purdue University.

Subject Area

Chemistry|Biomedical engineering|Mechanical engineering

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