Enhancing Separation and Constriction of Ion Mobility Distributions in Drift Tubes at Atmospheric Pressure Using Varying Fields.

A linearly decreasing electric field has been previously proven to be effective for diffusional correction of ions in a varying field drift tube (VFDT) system, leading to higher resolving powers compared to a conventional drift tube due to its capacity to narrow distributions midflight. However, the theoretical predictions in resolving power of the VFDT were much higher than what was observed experimentally. The reason behind this discrepancy has been identified as the difference between the theoretically calculated resolving power (spatial) and the experimental one (time). To match the high spatial resolving power experimentally, a secondary high voltage pulse (HVP) at a properly adjusted time is used to provide the ions with enough momentum to increase their drift velocity and hence their time-resolving power. A series of systematic numerical simulations and experimental tests have been designed to corroborate our theoretical findings. The HVP-VFDT atmospheric pressure portable system improves the resolving power from the maximum expected of 60-80 for a regular drift tube to 250 in just 21 cm in length and 7kV, an unprecedent accomplishment.

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

Ion motion in trapped ion mobility spectrometers (TIMS) and inverted drift tubes (IDT) has been investigated. The two-dimensional (2D) axisymmetric analytical solution to the Nernst-Planck equation for constant gas flows and opposed linearly increasing fields is presented for the first time and is used to study the dynamics of ion distributions in the ramp region. It is shown that axial diffusion confinement is possible and that broad packets of ions injected initially into the system can be contracted. This comes at the expense of the generation of a residual radial field that pushes the ions outward. This residual electric field is of significant importance as it hampers sensitivity and resolution when parabolic velocity profiles form. When radio frequency (RF) is employed at low pressures, this radial field affects the stability of ions inside the mobility cell. Trajectories and frequencies for stable motion are determined through the study of Mathieu's equation. Finally, effective resolutions for the ramp and plateau regions of the TIMS instrument are provided. While resolution depends on the inverse of the square root of mobility, when proper parameters are used, resolutions in the thousands can be achieved theoretically for modest distances and large mobilities.

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

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.

Development of Portable Aerosol Mobility Spectrometer for Personal and Mobile Aerosol Measurement.

We describe development of a Portable Aerosol Mobility Spectrometer (PAMS) for size distribution measurement of submicrometer aerosol. The spectrometer is designed for use in personal or mobile aerosol characterization studies and measures approximately 22.5 × 22.5 × 15 cm and weighs about 4.5 kg including the battery. PAMS uses electrical mobility technique to measure number-weighted particle size distribution of aerosol in the 10-855 nm range. Aerosol particles are electrically charged using a dual-corona bipolar corona charger, followed by classification in a cylindrical miniature differential mobility analyzer. A condensation particle counter is used to detect and count particles. The mobility classifier was operated at an aerosol flow rate of 0.05 L/min, and at two different user-selectable sheath flows of 0.2 L/min (for wider size range 15-855 nm) and 0.4 L/min (for higher size resolution over the size range of 10.6-436 nm). The instrument was operated in voltage stepping mode to retrieve the size distribution, which took approximately 1-2 minutes, depending on the configuration. Sizing accuracy and resolution were probed and found to be within the 25% limit of NIOSH criterion for direct-reading instruments (NIOSH 2012). Comparison of size distribution measurements from PAMS and other commercial mobility spectrometers showed good agreement. The instrument offers unique measurement capability for on-person or mobile size distribution measurements of ultrafine and nanoparticle aerosol.

Performance of a newly designed continuous soot monitoring system (COSMOS).

We designed a continuous soot monitoring system (COSMOS) for fully automated, high-sensitivity, continuous measurement of light absorption by black carbon (BC) aerosols. The instrument monitors changes in transmittance across an automatically advancing quartz fiber filter tape using an LED at a 565 nm wavelength. To achieve measurements with high sensitivity and a lower detectable light absorption coefficient, COSMOS uses a double-convex lens and optical bundle pipes to maintain high light intensity and signal data are obtained at 1000 Hz. In addition, sampling flow rate and optical unit temperature are actively controlled. The inlet line for COSMOS is heated to 400 degrees C to effectively volatilize non-refractory aerosol components that are internally mixed with BC. In its current form, COSMOS provides BC light absorption measurements with a detection limit of 0.45 Mm(-1) (0.045 microg m(-3) for soot) for 10 min. The unit-to-unit variability is estimated to be within +/- 1%, demonstrating its high reproducibility. The absorption coefficients determined by COSMOS agreed with those by a particle soot absorption photometer (PSAP) to within 1% (r2 = 0.97). The precision (+/- 0.60 Mm(-1)) for 10 min integrated data was better than that of PSAP and an aethalometer under our operating conditions. These results showed that COSMOS achieved both an improved detection limit and higher precision for the filter-based light absorption measurements of BC compared to the existing methods.