Aerosol static electrification and its effects in inductively coupled plasma spectroscopy

Abstract

Reported are variations measured in tertiary aerosol laser scatter signal intensities in response to changes in the concentration of different dissolved solids in the starting solutions, and to varied DC potentials applied to a mesh screen positioned in a spray chamber. The scatter signal from an aerosol derived from a solution containing one of nine different ionic matrices would increase when the concentration of that matrix was increased from 10 to 100 mM, but there was no concentration dependence on the scatter signal when the matrix was non-ionic (sucrose). The tertiary aerosol droplet size distributions for three of these ionic matrices were then measured, and for each increase of the matrix concentration, there was an increased abundance of small droplets in the tertiary aerosol. We attribute the source of this droplet size distribution alteration to be the result of a change in the net charge acquired by the primary droplets at the nebulization step, instigated because of the change in the total concentration of dissolved solids in the starting solution. The net charge acquired by a primary droplet is a critical parameter, because during transport through the spray chamber desolvation can lead to a Coulomb fission event, a process that causes a droplet to split unevenly, releasing numerous new small droplets into the aerosol. We use this argument to explain how a change in the dissolved ionic solids concentration in the starting solution leads to a change in the frequency of Coulomb fission event occurrence within the secondary aerosol. The assumption that primary droplets carry net charge was then put to the test by incorporating a mesh screen into the spray chamber to which DC potentials were then applied. Each starting solution used in this work exhibited a large change in the tertiary aerosol in response to a DC biased mesh screen. In particular, there was decreased transmission for both large and small droplets in each of the tertiary aerosols studied, in response to increased electric field strengths in the spray chamber. These data provide strong evidence that static electrification of aerosols, with subsequent Coulomb fission event occurrence during aerosol transport, are ubiquitous factors in liquid sample introduction methodology for atomic spectroscopy.