Electronic gate detection for cell or particle counting and sizing in liquids: front-end characteristics, flow-dependent gate impedance, and its remediation

The detection sensitivity and the sizing resolution of electronic gating are inherently limited by fluctuating gate impedance and flow-induced noise. Instabilities of this type, as shown, are due to varying flow patterns of the carrier liquid beyond the gate. Their effects, although largely hidden in dc-operated gating, cause broadening and shift of cell/particle-size distributions under measurement. RF-operated gating, more specifically the demodulation operation, is much more hindered. For an investigation of these effects, a physical model is proposed along with a procedure for the identification of the system parameters. A detector of dedicated concept is used for evaluating the model, and, more specifically, for investigating the impact of configurational and hydrodynamic parameters. Experiments prove that the origin of flow-dependent gate impedance is to be located inside a zone of only a few-micrometer extent at the gate outlet. This is confirmed by the calculated electric field patterns. On such grounds, electrode configurations are proposed that minimize the current density in the zone of hydrodynamic instability and, hence, the flow-induced noise. The same configurations also minimize the impedance of the gate as signal source, facilitating broadband operation, and multifrequency cell impedance measurements.