Fast high-resolution nanoDMA measurements with a 25 ms response time electrometer

ABSTRACT Two fast electrometer circuits (1011 and 1012 V/A) are installed in a Faraday cage having a relatively small residence time. Removing readily distinguishable occasional spikes, the root mean square (r.m.s.) noise level at 1012 V/A is 0.11 fA when acquiring data at 1 Hz. This value is close to the expected thermal resistor noise at room temperature (0.09 mV). Both electrometers exhibit a 20 ms flow-related delay, followed by respective half-height rise-times of ∼4 and 25 ms. Fast high-resolution mobility spectra in the 1–2 nm size range are acquired with electrosprayed tetraheptylammonium ions by combining these electrometers with a high-speed DMA. At 1012 V/A, there is no ion mobility peak distortion when acquiring data with discrete voltage steps and dwelling 100 ms at each voltage. With the 1011 V/A electrometer, the DMA voltage VDMA is continuously swept up and down over 600 V in a triangular wave, at up to 1200 V/s. A shift ΔVDMA in the peak center is apparent, with little peak shape distortion. ΔVDMA is symmetric with respect to up or down sweep, and linear with sweep frequency, corresponding approximately to a pure delay Δt = 25 ms. This peak displacement may be offset by adding the correction ΔVDMA = Δt (dVDMA/dt) to the measured peak voltage. Extrapolating the measurements made here over a mobility range Zmax/Zmin of 4 to a much wider mobility range of 300 typical of aerosol studies, we conclude that almost undistorted high-resolution mobility spectra may be acquired in 1.3 s. Copyright © 2017 American Association for Aerosol Research