Browsing by Author "Wu, Tianyang"

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    Analysis of Ion Motion and Diffusion Confinement in Inverted Drift Tubes and Trapped Ion Mobility Spectrometry Devices
    (ACS, 2019) Larriba-Andaluz, Carlos; Chen, Xi; Nahin, Minal; Wu, Tianyang; Fukushima, Nobuhiko; Mechanical and Energy Engineering, School of Engineering and Technology
    Ion motion in trapped ion mobility spectrometers (TIMS) and inverted drift tubes (IDT) has been investigated. The two-dimensional (2D) axisymmetric analytical solution to the Nernst–Planck equation for constant gas flows and opposed linearly increasing fields is presented for the first time and is used to study the dynamics of ion distributions in the ramp region. It is shown that axial diffusion confinement is possible and that broad packets of ions injected initially into the system can be contracted. This comes at the expense of the generation of a residual radial field that pushes the ions outward. This residual electric field is of significant importance as it hampers sensitivity and resolution when parabolic velocity profiles form. When radio frequency (RF) is employed at low pressures, this radial field affects the stability of ions inside the mobility cell. Trajectories and frequencies for stable motion are determined through the study of Mathieu’s equation. Finally, effective resolutions for the ramp and plateau regions of the TIMS instrument are provided. While resolution depends on the inverse of the square root of mobility, when proper parameters are used, resolutions in the thousands can be achieved theoretically for modest distances and large mobilities.
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    Ion Mobility Spectrometry : Optimization of Parameters in Collision Cross Sections and Trace Detection of Explosives
    (2018-12) Wu, Tianyang; Larriba Andaluz, Carlos; Zhu, Likun; Tovar, Andres
    Ion mobility spectrometry is a powerful technique for the study related to molecule. The work of tow major applications are introduced in this paper. The first application is the optimization of parameters in CCS. The accurate calculation of the collision cross section for multiple molecules is a long-time interested topic in the research for substances detection in micro scale. No reliable analytical approach to calculate the collision cross section has been established to date. Different approaches rely on different mechanism will provide different results in significant extent. This work introduce a method for the determination of parameters in the Lennard Jones potential. Experimental data combined with numerical computation was the fundamental strategy during the optimization of the parameters. In the experiment, electrospray is used as the ion source of IMS while a nebulizer was utilized to electrify the aromatic compounds. New parameters show no less accuracy and equal efficiency while can explain the physical meaning of the collision more clearly. The second application is the trace detection of explosives with very low concentration. The detection of explosives is an important topic in security, while the detection will be difficult due to the low vapor pressure of explosives. In this work, two types of devices are designed for the trace detection of explosives at an extremely low concentration. TNT is selected as the explosives in the experiment. The experiment succeed to reach a sensitivity of 1 part per quintillion, and even find out a linear relationship between the logarithm of TNT concentration and TNT vapor pressure.
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    Optimization of long range potential interaction parameters in ion mobility spectrometry
    (AIP, 2018) Wu, Tianyang; Derrick, Joseph; Nahin, Minal; Chen, Xi; Larriba-Andaluz, Carlos; Mechanical Engineering, School of Engineering and Technology
    The problem of optimizing Lennard-Jones (L-J) potential parameters to perform collision cross section (CCS) calculations in ion mobility spectrometry has been undertaken. The experimental CCS of 16 small organic molecules containing carbon, hydrogen, oxygen, nitrogen, and fluoride in N2 was compared to numerical calculations using Density Functional Theory (DFT). CCS calculations were performed using the momentum transfer algorithm IMoS and a 4-6-12 potential without incorporating the ion-quadrupole potential. A ceteris paribus optimization method was used to optimize the intercept σ and potential well-depth ϵ for the given atoms. This method yields important information that otherwise would remain concealed. Results show that the optimized L-J parameters are not necessarily unique with intercept and well-depth following an exponential relation at an existing line of minimums. Similarly, the method shows that some molecules containing atoms of interest may be ill-conditioned candidates to perform optimizations of the L-J parameters. The final calculated CCSs for the chosen parameters differ 1% on average from their experimental counterparts. This result conveys the notion that DFT calculations can indeed be used as potential candidates for CCS calculations and that effects, such as the ion-quadrupole potential or diffuse scattering, can be embedded into the L-J parameters without loss of accuracy but with a large increase in computational efficiency.