Current Monitoring in a Microchannel with Repeated Constrictions for Accurate Detection of Sample Location in Isotachophoresis

We present a new method for accurate detection of sample location in peak mode isotachophoresis (ITP). The technique is based on the design of a microchannel with multiple constrictions and on detecting the passage of the ITP interface through these constrictions. We achieve this by monitoring the electric current across the channel, which exhibits sharp decreases as the ITP interface moves more rapidly through the higher current density constrictions. We show that cross-correlation between the electric current signal and a predefined step function is an effective method for detecting changes in the slope of the electric current curve in real-time and is robust to changes in the composition of the buffers. We demonstrate the use of the technique to deliver sample to a designated location in the channel with an accuracy as low as 50 mu m. Importantly, the method does not require the use of any optics and thus can be used to monitor the location of unlabeled species for application in a variety of ITP assays.