AAAR 37th Annual Conference October 14 - October 18, 2019 Oregon Convention Center Portland, Oregon, USA
Abstract View
A Novel High-Resolution Ion Mobility Drift Tube with Diffusion Auto-correction
XI CHEN, Carlos Larriba-Andaluz, IUPUI
Abstract Number: 499 Working Group: Instrumentation and Methods
Abstract Ion Mobility Spectrometers (IMS) are becoming the standard equipment to measure nanoparticle distributions due to their simplicity, reproducibility, and portability. IMS separates nanoparticles through the particle’s electrical mobility Zp – the drift velocity of the particle divided by the electric field driving the movement-, and which is directly related to the size and charge of the entity as well as to the property of the gases. Despite the tremendous success of the differential mobility analyzer-condensation nucleus counter combination (DMA-CPC), there are a number of improvements which could be implemented in ion mobility technology to advance its use not only in aerosol science, but in biomolecular characterization and materials characterization. While one can improve resolution through geometric or external controls, this normally leads to poor signal and sensitivity due to loss of analytes through diffusion. The underlying problem with existing systems therefore arises from the inability to be able to control the diffusion broadening of the analytes that inevitably occurs in the detection cell of choice. Moreover, the resolution of existing devices is not mobility dependent which makes resolution at larger sizes insufficient. Accordingly, there is an urgent need to develop an instrument that can control the diffusion broadening of the signal and with resolution that scales with inverse mobility. Similarly, Scanning Mobility Particle Sizers (SMPS) typically require several minutes to complete voltage scans, and even in faster scanning instruments, particles of different sizes are sampled at different times. This limits information that can be obtained when aerosols are varying rapidly, such as can occur during sampling with an aircraft, near roadways, or from a combustion engine. Further, DMAs require the use of high sheath flow rates, and as such, require modest to high flowrate pumps which must remain stable during operation.
Here we show experimental evidence of a novel Drift Tube Ion Mobility Spectrometer working at atmospheric pressure which achieves resolutions of 100 (Z/ΔZ~100) for particle sizes between 0-30nm obtaining signals in a matter of milliseconds to seconds. We have accomplished this by applying linearly varying electric fields that control diffusion broadening and which allow the pulsing time to be large enough to allow large particle sizes through the pulsing gate. The resolution allows excellent separation between ions and nanoparticles while being able to monitor rapidly varying aerosols quite easily. The length of the tube is approximately 20cm (but can be reduced to 10cm with little to no consequences) and is quite simple and robust, e.g. it does not require polishing electrodes and does not require a very stable sheath flow to operate. Among the different test carried out, we have tested a mixture of 4 different tetraalkylammonium salts ranging from tetrabutylammonium to tetraheptylammonium bromide showing full separation on monomers dimers and trimers, something not possible on regular Differential Mobility Analyzers.