Mobility Analysis of 2 nm to 11 nm Aerosol Particles with an Aspirating Drift Tube Ion Mobility Spectrometer

We describe the performance of a drift tube-ion mobility spectrometry (DT-IMS) instrument for the measurement of aerosol particles. In DT-IMS, the electrical mobility of a measured particle is inferred directly from the time required for the particle to traverse a drift region, with motion driven by an electrostatic field. Electrical mobility distributions are hence linked to arrival time distributions (ATDs) for particles reaching a detector downstream of the drift region. The developed instrument addresses two obstacles that have limited DT-IMS use for aerosol measurement previously: (1) conventional drift tubes cannot efficiently sample charged particles at ground potential and (2) the sensitivities of commonly used Faraday plate detectors are too low for most aerosols. Obstacle (1) is circumvented by creating a “sample volume” of aerosol for measurement, defined by the streamlines of fluid flow. Obstacle (2) is bypassed by interfacing the end of the drift region with a condensation particle counter. The DT-IMS prototype shows high linearity for arrival time versus inverse electrical mobility (<i>R</i>² &gt; 0.99) over the size range tested (2.2–11.1 nm), and measurements compare well with both analytical and numerical models of device performance. A dimensionless calibration curve linking drift time to inverse electrical mobility is developed. In less than 5 s, it is possible to measure 11.1 nm particles, while 2.2 nm particles are analyzable on a subsecond scale. The transmission efficiency is found to be dependent upon electrostatic deposition for short drift times and upon advective losses for long drift times. Copyright 2014 American Association for Aerosol Research

本研究阐述了一款用于气溶胶颗粒测量的漂移管-离子迁移谱（drift tube-ion mobility spectrometry, DT-IMS）仪器的性能表现。在DT-IMS中，被测颗粒的电迁移率可直接通过其穿过漂移区所需的时间推导得出，颗粒的运动由静电场驱动。因此，电迁移率分布与到达漂移区下游探测器的颗粒的到达时间分布（arrival time distributions, ATDs）直接相关。这款自研仪器解决了此前限制DT-IMS用于气溶胶测量的两大难题：(1) 传统漂移管无法高效采集地电位下的带电颗粒；(2) 常用法拉第盘探测器（Faraday plate detector）的灵敏度过低，难以满足绝大多数气溶胶的检测需求。针对难题(1)，本仪器通过以流体流线界定的气溶胶“采样体积”完成测量，从而规避了该问题；针对难题(2)，本仪器将漂移区末端与凝聚核粒子计数器（condensation particle counter）相连，以此绕过了灵敏度不足的局限。该DT-IMS原型机在测试粒径范围（2.2–11.1 nm）内，到达时间与电迁移率倒数之间呈现出极佳的线性关系（决定系数<i>R</i>² > 0.99），且测量结果与设备性能的解析模型及数值模拟结果均吻合良好。研究还构建了一条关联漂移时间与电迁移率倒数的无量纲校准曲线。该仪器可在5秒内完成11.1 nm颗粒的测量，而2.2 nm颗粒的分析甚至可在亚秒级时间内完成。研究发现，传输效率在短漂移时间下取决于静电沉积效应，而在长漂移时间下则受平流损失影响。2014年美国气溶胶研究协会（American Association for Aerosol Research）版权所有