Department of Mechanical Engineering , Indiana University-Purdue University Indianapolis (IUPUI) , 723 W. Michigan St. , Indianapolis , Indiana 46202 , United States.
Purdue University , West Lafayette , Indiana 47907 , United States.
Anal Chem. 2019 Jan 2;91(1):919-927. doi: 10.1021/acs.analchem.8b03930. Epub 2018 Dec 18.
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.
我们研究了被困离子迁移谱仪（TIMS）和倒置漂移管（IDT）中的离子运动。首次提出了用于恒定气流和相反线性增加场的 Nernst-Planck 方程的二维（2D）轴对称解析解，并用于研究斜坡区域中离子分布的动力学。结果表明，轴向扩散限制是可能的，并且最初注入系统的宽离子包可以被收缩。这是以产生向外推离子的残余径向场为代价的。当形成抛物线速度分布时，这种残余电场非常重要，因为它会降低灵敏度和分辨率。当在低压力下使用射频（RF）时，这种径向场会影响迁移率单元内离子的稳定性。通过研究 Mathieu 方程确定稳定运动的轨迹和频率。最后，提供了 TIMS 仪器斜坡和平台区域的有效分辨率。虽然分辨率取决于迁移率的平方根的倒数，但当使用适当的参数时，对于中等距离和大迁移率，理论上可以实现数千的分辨率。
