Development of a field-portable miniature mass spectrometer designed for in-situ analysis and ion trap miniaturization
Chemical analysis by mass spectrometry has become the gold standard due to the sensitivity, selectivity, and short analysis times that are afforded by the method. Advances in miniaturized vacuum systems, mass analyzers, electronics, and ionization sources have allowed for the development of field-portable mass spectrometers. The performance of portable instruments has received much attention for their utility in the detection of illicit drugs, explosives, environmental contaminants, therapeutic drugs, and more recently, biological tissues. Furthermore, detection of the aforementioned compounds can be completed at atmospheric pressures and in the presence of complex matrices and sample surfaces when used in conjunction with ambient ionization sources. The work contained in this thesis is a summary of investigations that touch on different themes which further the advancement of miniaturized mass spectrometer instrumentation. Chapter 1 details the development and performance characterization of a wearable, backpack-portable, miniature mass spectrometer—the Mini-S. Based on earlier versions, the Mini-S instrument has miniaturized electronics, vacuum system, mass analyzer, and ion detection system. Additionally, a key to achieving chemical analysis on time scales relevant for in-situ detection and real-time analysis is the coupling of ambient ionization to miniature instruments. Also included in chapter 1 is the development of a co-axial low temperature plasma (LTP) probe as an ambient desorption / ionization (ADI) source. Chapter 2 extends the work in chapter 1 with the introduction of the Mini 12 mass spectrometer and application of touch spray for the characterization of tissues based upon detected lipid profiles. Rectilinear ion traps (RITs) are a subset of linear ion traps (LITs) that utilize planar instead of hyperbolic electrodes to generate a quadrupolar field to confine gas phase ions. The simplified geometry of planar electrodes has the advantage of being easier to fabricate which reduces fabrication cost and they are easier to scale to miniaturized dimensions. Chapter 3 details the performance of a 1/3rd scaled RIT (1.66 x 1.33 mm) fabricated with stainless steel and introduces the fabrication of RITs with the printed circuit board (PCB) material and tooling. The performance of a full size (5 x 4 mm) PCB-RIT is also detailed. A method used to offset the reduced ion capacity associated with dimensional miniaturization of the mass analyzer is parallel operation of multiple ion traps, i.e. ion trap array. Chapter 4 details the fabrication and operation of an eight-channel array of 1/3rd scale rectilinear ion traps (RITs) constructed monolithically with stereolithography apparatus (SLA). Each ion trap channel inside the array is independently isolated and can be controlled with a unique set of RF and DC voltages. Bias tee networks and dedicated operational amplifiers were used to improve the overall spectral quality of the device as a compensation for dimensional variation of individual ion trap channels. In this work it was also demonstrated that any given ion trap channel could be scanned at any time point.
Cooks, Purdue University.
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