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

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 v gas , and a linearly increasing electric field that opposes the movement. A new parameter, the separation ratio Λ = v drift /v gas , 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.Charged gas phase nanoparticles can be subject to drift and separation by means of electric fields. The charge divided by the friction coefficient of the nanoparticle under such fields is defined as electrical mobility and its accurate reckoning is key to the determination of particle size distribution functions. In Aerosol Science, and when dealing with globular particles, a particle's electrical mobility is linked to its diameter, d p , through the well-known equation 1, 2 :where qe is the net charge on the particle (the product of the integer charge state and the unit electron charge), µ is the dynamic viscosity and λ is the mean free path. C c is the Cunningham's correction factor and is a function of the Knudsen number. Most often, mobility based size distribution functions are measured with differential mobility analyzers (DMA) 3 coupled to Condensation Nucleus Counters CNCs 4 , and operated in series as a scanning mobility particle sizer (SMPS) 5,6 . While the SMPS combination has been incredibly successful, there are several shortcomings to its use which could be improved upon employing different techniques. Because the residence time -length divided by sheath velocity-of transmitted particles in a DMA is fixed and independent of particle size, diffusional broadening leads to degradation of instrument resolution for sub 20 nm particles 7,8 . For particles larger than 20 nm, the resolution, defined as Z/ΔZ, is fixed and with values of approximately <10 for most operating commercial devices. This results in adequate resolution but sometimes insufficient -a 90 nm monodisperse distribution is barely distinguishable from a 100 nm monodisperse distribution assuming a resolution of 6-. Similarly, SMPSs typically require sev...

Ion Mobility–Mass Spectrometry Using a Dual-Gated 3D Printed Ion Mobility Spectrometer

“…Tabrizchi et al were able to improve the duty cycles of DT-IMS systems by allowing ions to constantly traverse the drift tube and then periodically disrupting the flow using an “inverse” injection signal, causing notches to appear in the ion stream which move at a rate dependent on the nature of the ions present. , Peak widths were also narrowed due to ion diffusion into the notches; however, this resolution improvement was only seen when high ion fluxes were used . Alternatively, Fernandez-Lima et al were able to achieve resolutions over 50 by reversing the electric field and direction of counter flow gas in a conventional DT-IMS, thereby causing ions to be “trapped” inside the IMS until they were slowly eluted by lowering the electric field. , Nahin et al have done similar work, designing an “inverted” ion mobility system for trapping micrometer-sized particles by opposed electrical and pneumatic fields and then increasing the electric field to separate ions …”

Analysis of Ion Motion and Diffusion Confinement in Inverted Drift Tubes and Trapped Ion Mobility Spectrometry Devices

“…In fact, for linear changes in the slope of the field, an average value of the separation ratio ̅ can be used as a substitute in eq. (17). ̅ can then be related to the scan rate by (See Supporting info):…”

“…Among such systems one can name the most conventional ones, Drift Tube (DTIMS) 5 and Differential Mobility Analyzer (DMA) 6 , which have been available since the 1970s. Recently, many techniques and systems have appeared, including the Transversal Modulated Wave (T-Wave) 7 , Field Asymmetric Ion Mobility Spectrometry (FAIMS) 8 , Overtone Mobility Spectrometer (OMS) 9 , Differential Mobility Spectrometer (DMS) 10 , Radial Opposed Migration of Ion and Aerosol Classifier (ROMIAC) 11 , Fast Integrated Mobility Spectrometer (FIMS) 12 , Structure for Lossless Ion Manipulation (SLIM) 13 , Diffusion Differential Analyzer (DDA) 14 , Trapped Ion Mobility (TIMS) 15,16 and Inverted Drift Tube (IDT) 17 .…”

“…Later on, Bleiholder did a comprehensive study of the trajectory of an ion (or a packet of ions) from a Langevin perspective using non-uniform electric fields in non-stationary gases 29 . The 1D Nernst-Planck balance equation for a distribution of ions of has been previously fully solved by our group showing the diffusion-correction properties of such instruments 17 . Of most significance is the fact that an initial broad distribution can be compressed axially as it travels through the system.…”

“…The IDT, in contrast, works at atmospheric pressure and, as such, cannot make use of RF fields to constrain the ions radially. Under these circumstances, the ion must be kept in constant movement, v⃗ z ion ≠ 0, and a parameter labeled the separation ratio, Λ = v drift / v gas ≤ 1, is used to specify the movement . This parameter plays a key role in the ability of the IDT to resolve different ions.…”