Advanced fabrication, simulation, and characterization of single and multichannel miniature ion trap chemical sensors
With miniature mass spectrometry high performance chemical analysis becomes available to the broader community. In areas of defense, law enforcement, pharmaceutical science, and research industries such portable instrumentation begins to fill technology gaps, increase safety and efficiency, decrease sample preparation time, and decrease lead time waiting for results. A key technology facilitating miniature mass spectrometry is the ion trap mass analyzer. In this thesis miniature ion traps have demonstrated lower RF voltage, lower power, increased mass range, smaller form factor, ion trap and circuit integration, increased trapping capacity through arrays, the use of wideband power amplifiers, and amenability to planar and other alternative fabrication strategies all of which can significantly reduce device size and cost. ^ Several obstacles associated with the miniaturization process have been investigated through simulation and experiment in order to mitigate their effects. First, limitations of precision machining at very small dimensions are overcome by advanced fabrication through stereolithography. Second, the large space requirement of bulky interconnects and assembly hardware is reduced through assembly-free integration of circuit board and stereolithography. Third, ion trap arrays are used to compensate for the loss of ion trapping capacity per element, thereby increasing dynamic range. Fourth, the array effect, caused by dimensional variance between array elements, is characterized and overcome by tuning at each element. Fifth, the effects of perturbations are analyzed through simulation and characterization of a ceramic ion trap. Sixth, the increased effects of nonlinear resonances caused by higher order multipoles in miniature traps are analyzed through corroboration of simulation and experiment. Seventh, limitations on scaling are described on account of collisional cooling and decreased spectral resolution. ^ New methods are introduced for ion trap fabrication, based on the fabrication method stereolithography. One rapid methodology combines 2D and 3D parallel processing techniques to create miniature ion traps and arrays as single contiguous units while requiring no hand assembly. This process allows the array to be designed with freedom for tuning at each element. A second extends the use of stereolithography to the fabrication of a ceramic ion trap. ^ The development of an ion trajectory simulator with a collision model has been invaluable for the characterization of events observed in miniature traps. The trajectory simulator illustrates the minimum ion cloud size under typical operating parameters, and accurately predicts the resonance ejection patterns of miniature nonlinear ion traps. It has been shown that mass range, buffer gas temperature and type, and ion trap dimensions have an increased impact on performance with the decreased scale of the ion trap. ^ The reduced scale ion trap and circuit board integration have led to the design of a small integrated ion trap vacuum chamber. With this configuration low voltage waveforms are passed through circuit board into a small polymer vacuum chamber form fit to the miniature ion trap. A power operational amplifier permits operation at 1MHz for a mass range of over 1000 m/z in a miniature trap. Finally, the future of miniature mass spectrometry is proposed by combining the ion trap vacuum chamber with the power amplifier on a single substrate to decrease the size and cost of mass spectrometers while maintaining high quality performance expected of lab based equipment.^
William J. Chappell, Purdue University.
Chemistry, Analytical|Engineering, Electronics and Electrical