We present a novel concept of filtering based on depletion zone isotachophoresis (dzITP). In the micro/nanofluidic filter, compounds are separated according to isotachophoretic principles and simultaneously released selectively along a nanochannel-induced depletion zone. Thus, a tunable low-pass ionic mobility filter is realized. We demonstrate quantitative control of the release of fluorescent compounds through the filter using current and voltage actuation. Two modes of operation are presented. In continuous mode, supply, focusing, and separation are synchronized with continuous compound release, resulting in trapping of specific compounds. In pulsed mode, voltage pulses result in release of discrete zones. The dzITP filter was used to enhance detection of 6-carboxyfluorescein 4-fold over fluorescein, even though it had 250× lower starting concentration. Moreover, specific high-mobility analytes were extracted and enriched from diluted raw urine, using fluorescein as an ionic mobility cutoff marker and as a tracer for indirect detection. Tunable ionic filtering is a simple but essential addition to the capabilities of dzITP as a versatile toolkit for biochemical assays.