L. de Juan

Mass and size determination of nanometer particles by means of mobility analysis and focused impaction

Abstract Particles in the size range of a few nanometers are characterized by means of a differential mobility analyzer (DMA) of the Eichler type in tandem with a focusing impactor with electrostatic blowing. One application of the DMA-impactor combination is determination of particle mass m p without knowledge of the relation Z ( d p ) between particle mobility Z and diameter d p . With Z ( d p ) known, also the size and density ρ p of spherical particles can be determined. A mobility versus diameter relation Z ( d p ) for the range of a few nanometers is derived from the literature. It considers the effect of the finite diameter d of gas molecules, neglected in conventional studies. According to this relation, plots of Z −1/2 versus m p 1/3 yield straight lines, and the intersection with the Z −1/2 axis is linearly related to the effective diameter d of the carrier gas molecules. Experimental data recorded with the DMA-impactor combination for silver particles are consistent with this relation. The same applies for values from literature for fullerenes and proteins. For air, d is approximately 0.53 nm , not far from the 0.6 nm estimated by Tammet (1995) . The particle density ρ p is derived from m p and the diameter inferred from Z ( d p ). Measurement of ρ p on silver particles showed that charging devices may introduce contamination leading to formation of an adsorbed layer on the particles, reducing their measured density. The DMA-Impactor combination applied has sufficient resolving power to observe occurrence of different shapes of particles prepared by vapor condensation.

Condensation of supersaturated vapors on monovalent and divalent ions of varying size

Vapor nucleation induced by seed ions of controlled size and charge is studied for several monovalent positive and negative ions as well as divalent cations. Ions from dissolved salts are transferred into a gas via electrospray, purified by differential mobility selection, and introduced into a condensation nucleus counter of the mixing type using supersaturated dibutyl phthalate vapors. The number of drops nucleated on the ions is then measured as a function of the vapor supersaturation S. None of the anions tested is activated. Singly charged cations with mobilities Z from 0.48 up to 0.93 cm2/V/s are activated at values of ln S 30% smaller than predicted by Thomson’s model. All doubly charged cations tested (Z from 0.46 to 1.08 cm2/V/s) fall very near the Thomson curve, independently of their size or of whether the double ionic charge is localized on one metal atom or on two separate monovalent sites.

Improvement of the Resolution of TSI's 3071 DMA via Redesigned Sheath Air and Aerosol Inlets

ABSTRACT The resolution of TSIs 3071 Differential Mobility Analyzer (DMA) is smaller than ideally expected for nondiffusing particles, both at sheath airflow rates Qs in excess of 30 lit/min, and at aerosol over sheath airflow rate ratios Qa/Qs smaller than 10%. The first problem is partly due to a slight widening step following the laminarizing grid, which tends to destabilize the sheath airflow. The second can be associated in part to the recirculation vortex found by Chen et al. (1996) just upstream of the mixing region where the aerosol flow joins the sheath airstream. It is also attributed here to mean flow deceleration for sufficiently small values of Qa/Qs, due to an increase in the flow cross section following the mixing region. These problems are reduced in this study by replacing the DMA piece forming the outer electrode between the laminarization screen and the aerosol inlet into the analyzing region. The new part has a reduced inlet ID to avoid the initial step. It is slightly tapered, ending...

On the current emitted by Taylor cone-jets of electrolytes in vacuo: Implications for liquid metal ion sources

Approximate scaling laws for the charge and size of the drops ejected from the apex of Taylor cones run in the cone-jet mode (electrospray) are now available for highly conducting electrolytes (10−4 S/m<K<1 S/m) electrosprayed at atmospheric pressure. In order to confirm that such laws do also apply to Taylor cones in vacuo, the current versus liquid flow rate curves I(Q) characteristic of a given liquid are investigated both in vacuum and in atmospheric pressure air. Although the sprays of drops differ profoundly in both cases, the two corresponding I(Q) curves are nearly identical for relatively involatile liquids such as tributyl phosphate. A discussion on the possible relation between the behavior of Taylor cones of electrolytes of organic liquids and liquid metal ion sources (K∼106 S/m) is attempted, yielding insights on the role of space charge. However, the electrical conductivity variable which dominates the behavior of liquid cones of electrolytes appears to be irrelevant in liquid metals.

Electrostatic effects in inertial impactors

A focusing impactor is used to study the influence of particle charge q on the capture efficiency versus Stokes number curve η(S). For highly charged particles, image attraction to the collector leads to considerable particle capture even at S = 0. This produces long subcritical tails in the η(S) curve (poor sizing resolution) and reduces the value S * of S at which η = 1/2. A similar behavior, now due to Brownian motion, arises for singly charged ultrafine particles, with a negligible influence of the image force. Both types of tails are reduced or eliminated by a repulsive electric field E between the collector and the nozzle, though at the price of increasing S * . For repelling fields E large enough for the subcritical tails to disappear, S * is a function only of the ratio ZE/U between the electrical and hydrodynamic velocities of the particles. The functional dependence S * (ZE/U) is characterized experimentally for a nearly incompressible thin-plate orifice nozzle flow at Reynolds number Re = 68. Calibration aerosols include singly or doubly charged oil droplets with diameters between 16 and 155 nm. Also polystyrene latex spheres (PSL) 74 nm in diameter generated from a water suspension by electrospray, whose charge q was narrowly controlled in the range 540 > q/e > 390 with a differential mobility analyzer. The analysis of the rate of deposition in the stagnation point region in the limit S = 0 predicts the repulsive fields required to offset the tails, in fair agreement with those observed both for singly and doubly charged as well as highly charged particles. The conclusion is reached that impactors whose detector is an electrometer can attain rather high resolutions in all the range of sizes and charges explored.