Modeling of an Inverted Drift Tube for Improved Mobility Analysis of Aerosol Particles

Minal Nahin,Derek Oberreit,Nobuhiko Fukushima,Carlos Larriba-Andaluz

doi:10.1038/s41598-017-06448-w

Minal Nahin, Derek Oberreit + Show 2 more

Open Access

https://doi.org/10.1038/s41598-017-06448-w

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Abstract

A new mobility particle analyzer, which has been termed Inverted Drift Tube, has been modeled analytically as well as numerically and proven to be a very capable instrument. The basis for the new design have been the shortcomings of the previous ion mobility spectrometers, in particular (a) diffusional broadening which leads to degradation of instrument resolution and (b) inadequate low and fixed resolution (not mobility dependent) for large sizes. To overcome the diffusional broadening and have a mobility based resolution, the IDT uses two varying controllable opposite forces, a flow of gas with velocity vgas, and a linearly increasing electric field that opposes the movement. A new parameter, the separation ratio Λ = vdrift/vgas, is employed to determine the best possible separation for a given set of nanoparticles. Due to the system’s need to operate at room pressure, two methods of capturing the ions at the end of the drift tube have been developed, Intermittent Push Flow for a large range of mobilities, and Nearly-Stopping Potential Separation, with very high separation but limited only to a narrow mobility range. A chromatography existing concept of resolving power is used to differentiate between peak resolution in the IDT and acceptable separation between similar mobility sizes.

Highlights

Charged gas phase nanoparticles can be subject to drift and separation by means of electric fields
Mobility based size distribution functions are measured with differential mobility analyzers (DMA)[3] coupled to Condensation Nucleus Counters CNCs4, and operated in series as a scanning mobility particle sizer (SMPS)[5, 6]
Particles of all mobilities are sampled as a packet at a specific time and are guided by a constant electric field to the detector providing separation that depends on the length and electric field[10]

Summary

Introduction

Charged gas phase nanoparticles can be subject to drift and separation by means of electric fields. With nanoparticles between 1–120 nm, there is the need to 1) increase resolution by correcting diffusion broadening of nanoparticles in the drift cell, 2) increase the maximum fixed resolution or make the resolution directly proportional to particle size (or inverse mobility) and 3) obtain complete unsteady profiles of particles on rapidly varying aerosols. Problems 2 and 3 have been resolved somewhat by the use of Drift Tube Ion Mobility Spectrometers (DT-IMS)[9] In such systems, particles of all mobilities are sampled as a packet at a specific time and are guided by a constant electric field to the detector providing separation that depends on the length and electric field[10]. A new atmospheric pressure instrument is proposed that is based on the DT-IMS, but which uses two varying controllable opposite forces to correct for diffusion broadening while having its resolution be dependent on mobility, and which increases with the size of the particle. Other means of collecting the nanoparticles must be employed in the IDT where the ions cannot be trapped

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