Diffusion Constriction of Ions Using Varying Fields for Enhanced Separation, Transmission and Size Range in Ion Mobility Systems

Xi Chen, Purdue University

Abstract

Drift tubes (DT) are prominent tools used in Ion Mobility Spectrometry (IMS) to separate ions in the gas phase due to their difference in mobility. While prominently used for small ions (< 10nm), their use for larger particles (up to 100nm) is limited and can only be attempted at atmospheric pressure due to diffusion. A system that specializes in high sensitivity larger particles (up to 1000nm) is the Differential Mobility Analyzer (DMA), but lacks in resolution (< 10 for particles 30-1000nm). The idea behind this work is to be able to design a new IMS system based on similar principles to the DT but that allows high resolution and sensitivity for a large range of sizes if possible. The primary idea revolves around the principle of nonconstant linear fields to try and control the width of the ion packet as it travels through the system. The first attempt was an Inverted Drift Tube (IDT) which lacked sufficient sensitivity. This was followed by the development of the Varying Field Drift Tube (VFDT) which was the first of such systems to perform better than a regular DT, but only marginally. Finally, the last version of the system included a secondary pulse and labeled High Voltage Pulse - Varying Field Drift Tube (HVP-VFDT), which solved some of the issues of the VFDT and was able to achieve resolving powers of 250, 3-5 times higher than regular DT. In the IDT system, a gas flow is used to drive the packet of ions through the drift region while a linearly increasing electric field which is in the opposite direction of the flow is used to slow down the ions and separate them. In this regime, it is the largest ions that arrive at the detector first, hence the name Inverted Drift Tube. This technique would allow larger ions and particles to be detected. At the same time, the linear field can be shown to have diffusion constriction (auto-correction) properties, where the broad distributions may be narrowed in the axial direction. However, the gas flow is difficult to control well and the parabolic velocity profile of the gas flow in the tube is a unfavorable factor for the system. To avoid the issue of the parabolic velocity but still take into account the VFDT takes the advantage of the diffusion auto-correction, the gas flow is suppressed and a linearly decreasing field is used to drive the ions. By solving the Nernst-Planck equation, we show that the VFDT has a spatial resolving power that is much higher than that of the regular DT. A DT was built and tested using a mixture of tetraalkylammonium salts. The transformation from the raw variable arrival time distribution to collision cross section or mobility diameter is derived and the linear relationship makes it simple for calibration and transformation. A resolving power of over 90 is achieved experimentally although higher resolving powers were expected theory. It turns out that the difference between theory and experiments had to do with the fact that in the VFDT, the spatial and time resolving powers are different.This is due to the low drift velocity at the end of the drift tube. To increase this velocity, a high voltage pulse is applied at a certain time depending on the ions of interest with a new system, HVPVFDT.

Degree

Ph.D.

Advisors

Larriba-Andaluz, Purdue University.

Subject Area

Analytical chemistry|Atmospheric sciences|Atomic physics|Chemistry|Electrical engineering|Electromagnetics|Physics|Polymer chemistry

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