Fast SEMS size distribution measurements under down-scan operation

Abstract

Particle size distributions obtained with a Scanning Electrical-Mobility Spectrometer (SEMS) are based on the response of charged particles in an applied electric field. A complete operation of SEMS instrument for particle sizing involves the exposure of charged particles to an exponentially increasing voltage (up-scan) followed by an exponentially decreasing voltage (down-scan). Accurate calculation of particle size distributions from SEMS measurements requires knowledge of the instrument transfer function or Kernel as a function of its operating conditions and subsequently, the inversion of an integral equation. A semi-theoretical transfer function calculation approach based on particle arrival-times was shown to accurately characterize fast up-scan SEMS operation. In this paper, we extend the arrival-time calculation approach for SEMS down-scan operation. The combination of a decreasing voltage over time and a parabolic velocity field results in particle trajectories that are fundamentally different from the up-scan operation. However, when smearing due to downstream transport of particles to the detector is considered, the down-scan transfer functions are seen to be a mirror image of the up-scan transfer functions in the mobility space. This suggests that the inversion algorithms optimized for fast up-scan operation could also be applicable for fast down-scan operation. The combination of near real-time up-scan and down-scan transfer functions should enable fast, and accurate size distribution measurements under a range of operating conditions.