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

ABSTRACTTwo 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 distortio...